Visible spectroscopic and photometric survey
of Jupiter Trojans: final results on dynamical
families. ∗
Fornasier S.1,2, Dotto E.3, Hainaut O.4, Marzari F.5,
Boehnhardt H.6, De Luise F.3, Barucci M.A.2
October 22, 2018
1 University of Paris 7, France
2 LESIA – Paris Observatory, France.
3 INAF – Osservatorio Astronomico di Roma, Italy;
4 European Southern Observatory, Chile;
5 Dipartimento di Fisica, Università di Padova, Italy;
6 Max-Planck Institute for Solar System Research, Katlenburg-Lindau, Germany
Submitted to Icarus: December 2006
e-mail: sonia.fornasier@obspm.fr
fax: +33145077144, phone: +33145077746
Running head: Investigation of Dynamical Families of Jupiter Trojans
∗Based on observations carried out at the European Southern Observatory (ESO), La
Silla, Chile, ESO proposals 71.C-0650, 73.C-0622, 74.C-0577
http://arxiv.org/abs/0704.0350v1
Send correspondence to:
Sonia Fornasier
LESIA-Observatoire de Paris
Batiment 17
5, Place Jules Janssen
92195 Meudon Cedex
France
e-mail: sonia.fornasier@obspm.fr
fax: +33145077144
phone: +33145077746
Abstract
We present the results of a visible spectroscopic and photometric
survey of Jupiter Trojans belonging to different dynamical families.
The survey was carried out at the 3.5m New Technology Telescope
(NTT) of the European Southern Observatory (La Silla, Chile) in
April 2003, May 2004 and January 2005. We obtained data on 47
objects, 23 belonging to the L5 swarm and 24 to the L4 one. These
data together with those already published by Fornasier et al. (2004a)
and Dotto et al. (2006), acquired since November 2002, constitute a
total sample of visible spectra for 80 objects.
The survey allows us to investigate six families (Aneas, Anchises, Mis-
enus, Phereclos, Sarpedon, Panthoos) in the L5 cloud and four L4 fam-
ilies (Eurybates, Menelaus, 1986 WD and 1986 TS6). The sample that
we measured is dominated by D–type asteroids, with the exception of
the Eurybates family in the L4 swarm, where there is a dominance of
C– and P–type asteroids.
All the spectra that we obtained are featureless with the exception
of some Eurybates members, where a drop–off of the reflectance is
detected shortward of 5200 Å. Similar features are seen in main belt
C–type asteroids and commonly attributed to the intervalence charge
transfer transition in oxidized iron.
Our sample comprises fainter and smaller Trojans as compared to the
literature’s data and allows us to investigate the properties of objects
with estimated diameter smaller than 40–50 km. The analysis of the
spectral slopes and colors versus the estimated diameters shows that
the blue and red objects have indistinguishable size distribution, so
any relationship between size and spectral slopes has been found.
To fully investigate the Trojans population, we include in our anal-
ysis 62 spectra of Trojans available in literature, resulting in a total
sample of 142 objects. Although the mean spectral behavior of L4
and L5 Trojans is indistinguishable within the uncertainties, we find
that the L4 population is more heterogeneous and that it has a higher
abundance of bluish objects as compared to the L5 swarm.
Finally, we perform a statistical investigation of the Trojans’s spectra
property distributions as a function of their orbital and physical pa-
rameters, and in comparison with other classes of minor bodies in the
outer Solar System. Trojans at lower inclination appear significantly
bluer than those at higher inclination, but this effect is strongly driven
by the Eurybates family. The mean colors of the Trojans are similar
to those of short period comets and neutral Centaurs, but their color
distributions are different.
Keywords: Trojan Asteroids – Photometry – Spectroscopy – – Asteroids
families
1 Introduction
Jupiter Trojans are small bodies of the Solar System located in the Jupiter
Lagrangian points L4 and L5. Up to now more than 2000 Trojans have been
discovered, ∼ 1150 belonging to the L4 cloud and ∼ 950 to the L5 one.
The number of L4 Trojans with radius greater than 1 km is estimated to be
around 1.6 ×105 (Jewitt et al., 2000), comparable with the estimated main
belt population of similar size.
The debate about the origin of Jupiter Trojans and how they were trapped
in librating orbits around the Lagrangian points is still open to several possi-
bilities. Considering that Trojans have orbits stable over the age of the Solar
System (Levison et al, 1997, Marzari et al. 2003) their origin must date back
to the early phase of the solar system formation. Some authors (Marzari &
Scholl, 1998a,b; Marzari et al., 2002) suggested that they formed very close
to their present location and were trapped during the growth of Jupiter.
Morbidelli et al. (2005) suggested that Trojans formed in the Kuiper belt
and were subsequently captured in the Jupiter L4 and L5 Lagrangian points
during planetary migration, just after Jupiter and Saturn crossed their mu-
tual 1:2 resonances. In this scenario, Jupiter Troians would give important
clues on the composition and accretion of bodies in the outer regions of the
solar nebula.
Several theoretical studies conclude that Jupiter Trojan clouds are at
least as collisionally evolved as main belt asteroids (Shoemaker et al., 1989;
Binzel & Sauter, 1992; Marzari et al., 1997; Dell’Oro et al., 1998). This
result is supported by the identification of several dynamical families, both
in the L4 and L5 swarm (Shoemaker et al., 1989, Milani, 1993, Beaugé and
Roig, 2001).
Whatever the Trojan origin is, it is plausible to assume that they formed be-
yond the frost line and that they are primitive bodies, are possibly composed
of anhydrous silicates and organic compounds, and possibly still contain ices
in their interior. Several observations of Trojans in the near infrared region
(0.8-2.5 µm) have failed to clearly detect any absorption features indicative
of water ice (Barucci et al, 1994; Dumas et al, 1998; Emery & Brown, 2003,
2004; Dotto et al., 2006). Also in the visible range Trojan spectra appear
featureless (Jewitt & Luu, 1990; Fornasier et al., 2004a, Bendjoya et al.,
2004; Dotto et al., 2006). Up to now only 2 objects (1988 BY1 and 1870
Glaukos) show the possible presence of faint bands (Jewitt & Luu, 1990).
However, these bands are comparable to the peak to peak noise and are not
yet confirmed.
Recently, mineralogical features have been detected in emissivity spectra of
three Trojan asteroids measured by the Spitzer Space Telescope. These fea-
tures are interpreted as indicating the presence of fine-grained silicates on
the surfaces (Emery et al. 2006).
Several questions about Jupiter Trojans’ dynamical origin, physical prop-
erties, composition and link with other groups of minor bodies such as outer
main belt asteroids, cometary nuclei, Centaurs and KBOs are still open.
In order to shed some light on these questions, we have carried out a spectro-
scopic and photometric survey of Jupiter Trojans at the 3.5m New Technol-
ogy Telescope (NTT) of the European Southern Observatory (La Silla, Chile)
and at the 3.5m Telescopio Nazionale Galileo (TNG), La Palma, Spain. In
this paper we present new visible spectroscopic and photometric data, ob-
tained during 7 observing nights, carried out at ESO-NTT on April 2003,
May 2004, and January 2005, for a total of 47 objects belonging to the L5
(23 objects) and L4 (24 objects) swarms. Considering also the results already
published in Fornasier et al. (2004a) and Dotto et al. (2006), obtained in
the framework of the same project, we collected a total sample of 80 Jupiter
Trojan visible spectra, 47 belonging to the L5 clouds and 33 to the L4. This
is the largest homogeneous data set available up to now on these primitive
asteroids.
The principal aim of our survey was the investigation of Jupiter Trojans
belonging to different dynamical families. In fact, since dynamical families
are supposed to be formed from the collisional disruption of parent bodies,
the investigation of the surface properties of small and large family members
can help in understanding the nature of these dynamical groups and might
provide a glimpse of the interior structure of the larger primordial parent
bodies.
We also present an analysis of the visible spectral slopes for all the data in
our survey along with those available in the literature, for a total sample of
142 Trojans.
This enlarged sample allowed us to carry out a significant statistical investi-
gation of the Trojans’ spectral property distributions, as a function of their
orbital and physical parameters, and in comparison with other classes of mi-
nor bodies in the outer Solar System. We also discuss the spectral slope
distribution within the Trojan families.
2 Observations and data reduction
[HERE TABLE 1 AND 2]
The data were obtained in the visible range during 3 different observing
runs at ESO-NTT: 10 and 11 April 2003 for the spectroscopic and pho-
tometric investigation of 6 members of the 4035 1986 WD and 1 member
of 1986 TS6 families; 25 and 26 May 2004 for a spectroscopic survey of L4
Eurybates family; 17, 18, and 19 January 2005 for the spectroscopic and pho-
tometric investigation of 5 Anchises, 6 Misenus, 5 Panthoos, 2 Cloanthus, 2
Sarpedon and 3 Phereclos family members (L5 swarm).
We selected our targets from the list of Jupiter Trojan families provided by
Beaugé and Roig (2001 and P.E.Tr.A. Project at www.daf.on.br/ froig/petra/).
The authors have used a cluster-detection algorithm called Hierarchical Clus-
tering Method (HCM, e.g. Zappalà et al., 1990) to find asteroid families
among Jupiter Trojans starting from a data–base of semi-analytical proper
elements (Beaugé & Roig, 2001). The identification of families is performed
by comparing the mutual distances with a suitable metric in the proper el-
ements’ space. The clustering chain is halted when the mutual distance,
measuring the incremental velocity needed for orbital change after the puta-
tive parent body breakup, is larger than a fixed cut-off value. A lower cutoff
implies a higher statistical significance of the family. Since families in L4 are
on average more robust than those around L5 (Beaugé and Roig, 2001), we
prefer to adopt a cutoff of 100 m/s for the L4 cloud and of 150 m/s for L5.
For the very robust Eurybates family we decided to limit our survey to those
family members defined with a cutoff of 70 m/s.
All the data were acquired using the EMMI instrument, equipped with a
2x1 mosaic of 2048×4096 MIT/LL CCD with square 15µm pixels. For the
spectroscopic investigation during May 2004 and January 2005 runs we used
the grism #1 (150 gr/mm) in RILD mode to cover the wavelength range
4100–9400 Å with a dispersion of 3.1 Å/px (200 Å/mm) at the first order,
while on April 2003 we used a different grism, the #7 (150 gr/mm), covering
the spectral range 5200–9500 Å, with a dispersion of 3.6 Å/px at the first
order. April 2003 and January 2005 spectra were taken through a 1 arcsec
wide slit, while during May 2004 we used a larger slit (1.5 arcsec). The slit
was oriented along the parallactic angle during all the observing runs in order
to avoid flux loss due to the atmospheric differential refraction.
For most objects, the total exposure time was divided into several (usually
2-4) shorter acquisitions. This allowed us to check the asteroid position in
the slit before each acquisition, and correct the telescope pointing and/or
tracking rates if necessary. During each night we also recorded bias, flat–
field, calibration lamp (He-Ar) and several (6-7) spectra of solar analog stars
measured at different airmasses, covering the airmass range of the science
targets. During 17 January 2005, part of the night was lost due to some
technical problems and only 2 solar analog stars were acquired. The ratio
of these 2 stars show minimal variations (less than 1%) in the 5000–8400 Å
range, but higher differences at the edges of this range. For this reason we
omit the spectral region below 4800 Å for most of the asteroids acquired that
night.
The spectra were reduced using ordinary procedures of data reduction as
described in Fornasier et al. (2004a). The reflectivity of each asteroid was
obtained by dividing its spectrum by that of the solar analog star closest
in time and airmass to the object. Spectra were finally smoothed with a
median filter technique, using a box of 19 pixels in the spectral direction for
each point of the spectrum. The threshold was set to 0.1, meaning that the
original value was replaced by the median value if the median value differs
by more than 10% from the original one. The obtained spectra are shown
in Figs. 1–5. In Table 1 and Table 2 we report the circumstances of the
observations and the solar analog stars used respectively for the L5 and L4
family members.
[TABLE 3]
The broadband color data were obtained during the April 2003 and Jan-
uary 2005 runs just before the Trojans’ spectral observation. We used the
RILD mode of EMMI for wide field imaging with the Bessell-type B, V, R,
and I filters (centered respectively at 4139, 5426, 6410 and 7985Å). The ob-
servations were carried out in a 2 × 2 binning mode, yielding a pixel scale
of 0.33 arcsec/pixel. The exposure time varied with the object magnitude:
typically it was about 12-90s in V, 30-180s in B, 12-70s in R and I filters.
The CCD images were reduced and calibrated with a standard method (For-
nasier et al., 2004a), and absolute calibration was obtained through the ob-
servations of several Landolt fields (Landolt, 1992). The instrumental mag-
nitudes were measured using aperture photometry with an integrating radius
typically about three times the average seeing, and sky subtraction was per-
formed using a 5-10 pixels wide annulus around each object.
The results are reported in Table 3. From the visual inspection and the radial
profiles analysis of the images, no coma was detected for any of the observed
Trojans.
On May 2004, as the sky conditions were clear but not photometric, we did
not perform photometry of the Eurybates family targets.
3 Results
[TABLE 4 AND 5]
For each Trojan we computed the slope S of the spectral continuum using
a standard least squared technique for a linear fit in the wavelength range
between 5500 and 8000 Å. The choice of these wavelength limits has been
driven by the spectral coverage of our data. We choose 5500 Å as the lower
limit because of the different instrumental setup used during different ob-
serving runs (with some spectra starting at wavelength ≥ 5200 Å), while
beyond 8000 Å our spectra are generally noisier due to a combination of the
CCD drop-off in sensitivity and the presence of the strong atmospheric water
bands.
The computed slopes and errors are listed in Table 4 and 5. The reported er-
ror bars take into account the 1σ uncertainty of the linear fit plus 0.5%/103Å
attributable to the use of different instruments and solar analog stars (esti-
mated from the different efficiency of the grism used, and from flux losses
due to different slit apertures). In Table 4 and 5 we also report the taxo-
nomic class derived following the Dahlgren & Lagerkvist (1995) classification
scheme.
In the L5 cloud we find 27 D–, 3 DP–, 2 PD–, and 1 P–type objects. In
the L4 cloud we find 10 C–type and 7 P–type objects inside the Eurybates
family, while for the Menelaus, 1986 TS6 and 1986 WD families, including
the data published in Dotto et al. (2006), we get 9 D–, 3 P–, 3C–, and 1
DP–type asteroids.
The majority of the spectra are featureless, although some of the observed
Eurybates’ members show weak spectral absorption features (Fig. 5). These
features are discussed in the following section.
We derived an estimated absolute magnitude H by scaling the measured
V magnitude to r = ∆ = 1 AU and to zero phase assuming G=0.15 (Bowell
et al., 1989). The estimated H magnitude of each Trojan might be skewed
uncertain rotational phase, as the lightcurve amplitudes of Trojans might
vary up to 1 magnitude. In order to investigate possible size dependence in-
side each family, and considering that IRAS diameters are available for very
few objects, we estimate the size using the following relationship:
1329× 10−H/5
where D is the asteroid diameter, p is the geometric albedo, and H is the abso-
lute magnitude. We use H derived from our observations when available, and
from the ASTORB.DAT file (Lowell observatory) for the Eurybates mem-
bers, for which we did not carry out visible photometry. We evaluated the
diameter for an albedo range of 0.03–0.07, assuming a mean albedo of 0.04
for these dark asteroids (Fernandez et al., 2003). The resulting D values are
reported in Tables 4 and 5.
3.1 Dynamical families: L5 swarm
3.1.1 Anchises
[FIGURE 1]
We investigated 5 of the 15 members of the Anchises family (Fig. 1): 1173
Anchises, 23549 1994 ES6, 24452 2000 QU167, 47967 2000 SL298 and 124729
2001 SB173 on 17 January 2005. For 4 out of 5 observed objects we omit
the spectral range below 4800Å due to low S/N ratio and problems with the
solar analog stars. The spectral behavior is confirmed by photometric data
(see Table 3). All the obtained spectra are featureless.
The Anchises family survives at a cutoff corresponding to relative veloc-
ities of 150 m/s. The biggest member, 1173 Anchises, has a diameter of
126 km (IRAS data) and has the lowest spectral slope (3.9 %/103Å) among
the investigated family members. It is classified as P–type, while the other
4 members are all D–types. Anchises was previously observed in the 4000-
7400Å region by Jewitt & Luu (1990), who reported a spectral slope of 3.8
%/103Å, in perfect agreement with the value we found. The three 19-29 km
sized objects have a steeper spectral slope (7.4-9.2 %/103Å), while the small-
est object, 2001 SB173 (spectral slope = 14.78±0.99 %/103Å) is the reddest
one (Table 4).
Even with the uncertainties in the albedo and diameter, a slope–size rela-
tionship is evident among the observed objects, with smaller–fainter members
redder than larger ones (Fig. 7).
3.1.2 Misenus
[FIGURE 2]
For this family we investigated 6 members (11663 1997 GO24, 32794 1989
UE5, 56968 2000 SA92, 99328 2001 UY123, 105685 2000 SC51 and 120453
1988 RE12) out of the 12 grouped at a relative velocity of 150 m/s. The
family survives with the same members also at a stringent cut-off velocity
of 120 m/s. The spectra, together with magnitude color indices transformed
into linear reflectance, are shown in Fig. 2, while the color indices are reported
in Table 3. All the spectra are featureless with different spectral slope values
covering the 4.6–15.9 %/103Å range (Table 4): 1988 RE12 has the lowest
spectral slope and is classified as P–type, 3 objects (11663, 32794 and 2000
SC51) are in the transition region between P– and D– type, with very similar
spectral behavior, while the two other observed members are D–types. Of
these last, 56968 has the highest spectral slope not only inside the family
(15.86 %/103Å) but also inside the whole L5 sample analyzed in this paper.
All the investigated Misenus members are quite faint and have diameters
of a few tens of kilometers. No clear size-slope relationship has been found
inside this family (Fig. 7).
No other data on the Misenus family members are available in the literature,
so we do not know if the large gap between the spectral slope of 56968 and
those of the other 5 investigated objects is real or it could be filled by other
members not yet observed. If real, 56968 can be an interloper inside the
family.
3.1.3 Panthoos
[FIGURE 3]
The Panthoos family has 59 members for a relative velocity cutoff of 150
m/s. We obtained new spectroscopic and photometric data of 5 members:
4829 Sergestus, 30698 Hippokoon, 31821 1999 RK225, 76804 2000 QE and
111113 2001 VK85 (Fig. 3). Three objects presented by Fornasier et al.
(2004a) as belonging to the Astyanax family (23694 1997 KZ3, 32430 2000
RQ83, 30698 Hippokoon) and one to the background population (24444 2000
OP32) are now included among the members of the Panthoos family. Peri-
odic updates of the proper elements can change the family membership. In
particular the Astyanax group disappeared in the latest revision of dynami-
cal families, and its members are now in the Panthoos family within a cutoff
of 150m/s. The Panthos family survives also a cutoff of 120 m/s, with 7
members, and 90 m/s, with 6 members.
We observed 30698 Hippokoon during two different runs (on 9 Nov. 2002
and on 18 Jan. 2005), and both spectral slopes and colors are in agreement
inside the error bars (see Table 3, Table 4, and Fornasier et al., 2004a). No
other data on the Panthoos family are available in the literature.
The analysis of the 8 members (for 24444 only photometry is available)
show featureless spectra with slopes that seem to slightly increase as the
asteroid size decreases (Table 4 and Fig. 7). However, all the members have
dimensions very similar within the uncertainties, making it difficult for any
slope-size relationship to be studied. The largest member, 4829 Sergestus, is
a PD–type with a slope of about 5 %/103Å, while all the other investigated
members are D–types.
3.1.4 Cloantus
[FIGURE 4]
We observed only 2 out of 8 members of the Cloantus family (5511 Cloan-
thus and 51359 2000 SC17, see Fig. 4) as grouped at a cutoff corresponding
to relative velocities of 150 m/s. This family survives at a stringent cutoff
and 3 members (including the two that we observed) also survive for relative
velocities of 60 m/s. Both of the observed objects are D–types with very
similar, featureless, reddish spectra (Table 4 and Fig. 7). 5511 Cloanthus
was observed also by Bendjoya et al. (2004), who found a slope of 13.0±0.1
%/103Å in the 5000-7500 Å wavelength range, while we measure a value of
10.84±0.15 %/103Å. Our spectrum has a higher S/N ratio than the spectrum
by Bendjoya et al. (2004), and it is perfectly matched by our measured color
indices that confirm the spectral slope. This difference cannot be caused by
the slightly different spectral ranges used to measure the slope, but could
possibly be due to heterogeneous surface composition.
3.1.5 Phereclos
The Phereclos family comprises 15 members at a cutoff of 150 m/s. The
family survives with 8 members also at a cutoff of 120m/s. We obtained
spectroscopic and photometric data of 3 members (9030 1989 UX5, 11488
1988 RM11 and 31820 1999 RT186, see Fig 4), that, together with the 4
spectra (2357 Phereclos, 6998 Tithonus, 9430 1996 HU10, 18940 2000QV49)
already presented by Fornasier et al. (2004a), allow us to investigate about
half of the Phereclos family population defined at a cutoff of 150m/s. The
spectral slope of these objects, all classified as D–type except one PD–type
(11488), varies from 5.3 to 11.3 %/103Å (Table 4). The size of the fam-
ily members ranges from about 20 km in diameter for 31820 to 95 km for
2357, but we do not observe any clear slope-diameter relationship (Fig. 7 and
Table 4).
3.1.6 Sarpedon
We obtained new spectroscopic and photometric data of 2 members of the
Sarpedon family (48252 2001 TL212 and 84709 2002 VW120), whose spectra
and magnitude color indices are reported in Fig. 4 and Table 4. Including
the previous observations (Fornasier et al., 2004a) of 4 other members (2223
Sarpedon, 5130 Ilioneus, 17416 1988 RR10, and 25347 1999 RQ116), we have
measurements of 6 of the 21 members of this family dynamically defined at a
cutoff of 150 m/s. All the 6 aforementioned objects, except 25347, constitute
a robust clustering which survives up to 90 m/s with 9 members. The cluster
which contains (2223) Sarpedon was also recognized as a family by Milani
(1993).
All the 6 investigated members have very similar colors (see Table 3) and
spectral behavior. The spectral slope (Fig. 7) varies over a very restricted
range, from 9.6 to 11.6 %/103Å (Table 4), despite a significant variation of
the estimated size (from the 18 km of 17416 to the 105 km of 2223). Con-
sequently, the surface composition of the Sarpedon family members appears
to be very homogeneous.
3.2 Dynamical families: L4 swarm
3.2.1 Eurybates
[FIGURE 5]
Eurybates family members were observed in May 2004. The selection of
the targets was made on the basis of a very stringent cutoff, corresponding
to relative velocities of 70 m/s, that gives a family population of 28 objects.
We observed 17 of these members (see Table 2) that constitute a very robust
clustering in the space of the proper elements: all the members we studied,
except 2002 CT22, survive at a cutoff of 40 m/s.
The spectral behavior of these objects (Fig. 5) is quite homogeneous with
10 asteroids classified as C–type and 7 as P–type. The spectral slopes (Ta-
ble 5) range from neutral to moderately red (from -0.5 to 4.6 %/103Å). The
slopes of six members are close to zero (3 slightly negative) with solar-like col-
ors. The asteroids 18060, 24380, 24420, and 39285, all classified as C–types,
clearly show a drop off of reflectance for wavelength shorter than 5000–5200
Å. The presence of the same feature in the spectra of 2 other members (1996
RD29 and 28958) is less certain due to the lower S/N ratio. This absorp-
tion is commonly seen on main belt C–type asteroids (Vilas 1994; Fornasier
et al. 1999), where is due to the intervalence charge transfer transitions
(IVCT) in oxidized iron, and is often coupled with other visible absorption
features related to the presence of aqueous alteration products (e.g. phyl-
losilicates, oxides, etc). These IVCTs comprise multiple absorptions that are
not uniquely indicative of phyllosilicates, but are present in the spectrum
of any object containing Fe2+ and Fe3+ in its surface material (Vilas 1994).
Since no other phyllosilicate absorption features are present in the C-type
spectra of the Eurybates family, there is no evidence that aqueous alteration
processes occurred on the surface of these bodies.
In Fig. 8 we show the spectral slopes versus the estimated diameters for
the Eurybates family members. All the observed objects, except the largest
member (3548) that has a diameter of about 70 km and exhibit a neutral (∼
solar-like) spectral slope, are smaller than ∼ 40 km and present both neutral
and moderately red colors. The spectral slopes are strongly clustered around
S = 2%/103Å, with higher S values restricted to smaller objects (D< 25
3.2.2 1986 WD
[FIGURE 6]
We investigated 6 out of 17 members of the 4035 1986 WD family that is
dynamically defined at a cutoff of 130 m/s (Fig. 6 and Table 2). Three of our
targets (4035, 6545 and 11351) were already observed by Dotto et al. (2006):
for 6545 and 11351 there is a good consistency between our spectra and
those already published. 4035 was observed also by Bendjoya et al. (2004):
all the spectra are featureless, but Bendjoya et al. (2004) obtain a slope of
8.8 %/103Å, comparable to the one here presented, while Dotto et al. (2006)
found a higher value (see Table 5). This could be interpreted as due to the
different rotational phases seen in the three observations, and could indicate
some inhomogeneities on the surface of 4035.
The observed family members show heterogeneous behaviors (Fig. 8),
with spectral slopes ranging from neutral values for the smaller members
(24341 and 14707) to reddish ones for the 3 members with size bigger than
50 km (4035, 6545, and 11351). For this family, it seems that a size-slope
relationship exists, with smaller members having solar colors and spectral
slopes increasing with the object’ sizes.
3.2.3 1986 TS6
The 1986 TS6 family includes 20 objects at a cut-off of 100 m/s. We present
new spectroscopy and photometry of a single member, 12921 1998 WZ5
(Fig. 6). The spectrum we present here is flat and featureless, with a spectral
slope of 4.6±0.8%/103Å. Dotto et al. (2006) presented a spectrum obtained
a month after our data (in May 2003) that has a very similar spectral slope
3.7± 0.8%/103Å. Previously, 12917 1998 TG16, 13463 Antiphos, 12921 1998
WZ5, 15535 2000 AT177, 20738 1999 XG191, and 24390 2000 AD177 were
included in the Makhoan family. Refined proper elements now place all of
these bodies in the 1986 TS6 family.
In Fig. 8 we report the spectral slopes vs. estimated diameters of the
6 observed members. The family shows different spectral slopes with the
presence of both P–type (12921 and 13463) and D–type asteroids (12917,
15535, 20738, and 24390). Due to the very similar diameters, a slope-size
relationship is not found.
[FIGURE 7 AND 8]
4 Discussion
The spectra of Jupiter Trojan members of dynamical families show a range
of spectral variation from C– to D–type asteroids. With the exception of the
L4 Eurybates family, all the observed objects have featureless spectra, and
we cannot find any spectral bands which could help in the identification of
minerals present on their surfaces. The lack of detection of any mineralogy
diagnostic feature might indicate the formation of a thick mantle on the Tro-
jan surfaces. Such a mantle could be formed by a phase of cometary activity
and/or by space weathering processes as demonstrated by laboratory exper-
iments on originally icy surfaces (Moore et al., 1983; Thompson et al., 1987;
Strazzulla et al., 1998; Hudson & Moore, 1999).
A peculiar case is constituted by the Eurybates family, which shows a pre-
ponderance of C–type objects and a total absence of D–types. Moreover,
this is the only family in which some members exhibit spectral features at
wavelengths shorter than 5000–5200 Å, most likely due to the intervalence
charge transitions in materials containing oxidized iron (Vilas 1994).
4.1 Size vs spectral slope distribution:
Individual families
The plots of spectral slopes vs. diameters are shown in Fig. 7 and 8. A
relationship between spectral slopes and diameters seems to exist for only
three of the nine families we studied. In the Anchises and Panthoos families,
smaller objects have redder spectra, while for the 1986 WD family larger
objects have the redder spectra.
Moroz et al. (2004) have shown that ion irradiation on natural complex
hydrocarbons gradually neutralizes the spectral slopes of these red organic
solids. If the process studied by Moroz et al. (2004) occurred on the surface of
Jupiter Trojans, the objects having redder spectra have to be younger than
those characterized by bluish-neutral spectra. In this scenario the largest
and spectrally reddest objects of the 1986 WD family could come from the
interior of the parent body and expose fresh material. In the case of the
Anchises and Panthoos families the spectrally reddest members, being the
smallest, could come from the interior of the parent body, or alternatively
could be produced by more recent secondary fragmentations. In particular,
small family members may be more easily resurfaced, as significant collisions
(an impactor having a size greater than a few percent of the target), as well
as seismic shaking and recoating by fresh dust, may occur frequently at small
sizes.
[FIGURE 9]
4.2 Size vs slope distribution:
The Trojan population as a whole
[TABLE 6]
As compared to the data available in literature, our sample strongly con-
tributed to the analysis of fainter and smaller Trojans, with estimated di-
ameters smaller than 50 km. Jewitt & Luu (1990), analyzing a sample of
32 Trojans, found that the smaller objects were redder than the bigger ones.
However, our data play against the existence of a possible color-dimension
trend. In fact, the spectral slope’s range of the objects smaller than 50 km
is similar to that of the larger Trojans, as shown in Fig. 9.
The Eurybates family strongly contributes to the population of small
spectrally neutral objects, filling the region of bodies with mean diameter
D<40 km and with spectral slopes smaller than 3 %/103Å.
In order to carry out a complete analysis of the spectroscopic and pho-
tometric characteristics of the whole available data set on Jupiter Trojans,
we considered all the visible spectra published in the literature: Jewitt &
Luu (1990, 32 objects), Fitzimmons et al. (1994, 3 objects), Bendjoya et
al. (2004, 34 objects), Fornasier et al. (2004a, 26 L5 objects), and Dotto et
al. (2006, 24 L4 Trojans). We also add several Trojans spectra (11 L4 and
3 L5 Trojans) from the files available on line (Planetary data System archive,
pdssbn.astro.umd.edu, and www.daf.on.br/∼lazzaro/S3OS2-Pub/s3os2.htm)
from the SMASS I, SMASS II and S3OS2 surveys (Xu et al., 1995; Bus &
Binzel, 2003; Lazzaro et al., 2004). Including all these data, we compile a
sample of 142 different Trojans, 68 belonging to the L5 cloud and 74 belong-
ing to the L4. We performed the taxonomic classification of this enlarged
sample, on the basis of the Dahlgren and Lagerkvist (1995) scheme, by ana-
lyzing spectral slopes computed in the range 5500-8000 Å. Different authors,
of course, considered different spectral ranges for their own slope gradient
evaluations: Jewitt & Luu (1990) and Fitzimmons et al. (1994) use the
4000-7400 Å and Bendjoya et al. (2004, Table 2) used a slightly different
ranges around 5200-7500 Å. Since all the cited papers show spectra with lin-
ear featureless trends, the different wavelength ranges used for the spectral
gradient computation by Bendjoya et al. (2004) and Jewitt & Luu (1990)
are not expected to influence the obtained slopes.
In order to search for a dependency of the spectral slope distribution with
the size of the objects, all observations (from this paper as well as from the
literature) were combined. The objects were isolated in 5 size bins (smaller
than 25 km, 25–50 km, 50–75 km, 75-100 km and larger than 100 km). Each
bin contains between 20 and 50 objects. These subsamples are large enough
to be compared using classical statistical tests: the t-test, which estimates
if the mean values are compatible, the f-test, which checks if the widths of
the distributions are compatible (even if they have different means), and
the KS test, which compares directly the full distributions. A probability is
computed for each test; a small probability indicates that the tested distri-
butions are not compatible, i.e. the objects are not randomly extracted from
the same population, while a large probability value has no meaning (i.e. it
is not possible to assure that both samples come from the same population,
we can just say in that case that they are not incompatible). In order to
quantify the probability levels that we consider as significant, the same tests
were run on randomized distributions (see Hainaut & Delsanti 2002 for the
method). Since probability lower than 0.04-0.05 does not appear in these
randomized distributions, we consider that values smaller than 0.05 indicate
a significant incompatibility.
Each sub-sample was compared with the four others – the results are sum-
marized in Table 6. The average slope of the 5 bins are all compatible among
each other. The only marginally significant result is that the width of the
slope distribution among the larger objects (diam. > 100 km) is narrower
than that of all the smaller objects.
This narrower color distribution could be due to the aging processes affect-
ing the surface of bigger objects, which are supposed to be older. The wider
color distribution of small members is possibly related to the different ages
of their surfaces: some of them could be quite old, while some other could
have been recently refreshed.
4.3 Spectral slopes and L4/L5 Clouds
[HERE FIGURES 10 AND 11]
Considering only the Trojan observations reported in this paper, the aver-
age slope is 8.84±3.03%/103Å for the L5 population, and 4.57±4.01%/103Å
for the L4.
Considering now all the spectra available in the literature, the 68 L5
Trojans have an average slope of 9.15±4.19%/103Å, and the 78 L4 objects,
6.10±4.48%/103Å. Performing the same statistical tests as above, it appears
that these two populations are significantly different. In particular, the av-
erage slopes are incompatible at the 10−5 level.
Nevertheless, as described in Section 3.2.1, the Eurybates family members
have quite different spectral characteristics than the other objects and con-
stitute a large subset of the whole sample. Indeed, comparing their distribu-
tion with the whole populations, they are found significantly different at the
10−10 level. In other words, the Eurybates family members do not constitute
a random subset of the other Trojans.
Once excluded the Eurybates family, the remaining 61 Trojans from the
L4 swarm have an average slope of 7.33±4.24%/103Å. The very slight dif-
ference of average slope between the L5 and remaining L4 objects is very
marginally significant (probability of 1.6%), and the shape and width of the
slope distributions are compatible with each other.
The taxonomic classification we have performed shows that the majority
(73.5%) of the observed L5 Trojans (Fig. 10) are D–type (slope > 7 %/103
Å) with featureless reddish spectra, 11.8% are DP/PD –type (slope between
5 and 7 %/103 Å), 10.3% are P–type, and only 3 objects are classified as
C–type (4.4%).
In the L4 swarm (Fig. 11), even though the D–type still dominate the
population (48.6%), the spectral types are more heterogeneous as compared
to the L5 cloud, with a higher percentage of neutral-bluish objects: 20.3%
are P–type, 8.1% are DP/PD-type, 12.2% are C–type, and 10.8% of the
bodies have negative spectral slope. The higher percentage of C– and P–
type as compared to the L5 swarm is strongly associated with the presence
of the very peculiar Eurybates family. Among 17 observed members 10 are
classified as C–types (among which 3 have negative spectral slopes) and 7 are
P–types. Considering the 57 asteroids that compose the L4 cloud without
the Eurybates family, we find percentages of P, and PD/DP –types very
similar to those of the L5 cloud (14.0% and 10.5% respectively), a smaller
percentage of D–types (63.2%) and of the C–types (3.5%), and the presence
of a 8.8% Trojans with negative spectral slopes.
The visible spectra of the Eurybates members are very similar to those of
C–type main belt asteroids, Chiron-like Centaurs, and cometary nuclei. This
similarity is compatible with three different scenarios: the family could have
been produced by the fragmentation of a parent body very different from
all the other Jupiter Trojans (in which case the origin of such a peculiar
parent must still be assessed); this could be a very old family where space
weathering processes have covered any differences in composition among the
family members and flattened all the spectra; this could be a young family
where space weathering processes occurred within time scales smaller than
the age of the family. In the last two cases the Eurybates family would give
the first observational evidence of spectra flattened owing to space weathering
processes. This would then imply, according to the results of Moroz et al.
(2004), that its primordial composition was rich in complex hydrocarbons.
The knowledge of the age of the Eurybates family is therefore a fundamental
step to investigate the nature and the origin of the parent body, and to assess
the effect of space weathering processes on the surfaces of its members.
The present sample of Jupiter Trojans suggests a more heterogeneous
composition of the L4 swarm as compared to the L5 one. As previously
noted by Bendjoya et al. (2004), the L4 swarm contains a higher percentage
of C– and P–type objects. This result is enhanced by members of the Eu-
rybates family, but remains even when these family members are excluded.
Moreover, the dynamical families belonging to the L4 cloud are more robust
than those of the L5 one, surviving with densely populated clustering even
at low relative velocity cut-off. We therefore could argue that the L4 cloud
is more collisionally active than the L5 swarm. Nevertheless, we still cannot
intepret this in terms of the composition of the two populations, since we
cannot exclude that as yet unobserved C– and P–type families are present
in the L5 cloud.
4.4 Orbital Elements
[HERE FIGURE 12 and TABLES 7 and 8 ]
We analyzed the spectral slope as a function of the Trojans’ orbital el-
ements. As an illustration, Fig. 12 shows the B − R color distribution as
a function of the orbital elements. In order to investigate variations with
orbital parameters, the Trojan population is divided in 2 sub samples: those
with the considered orbital element lower than the median value, and those
with the orbital element higher than the median (by construction, the two
subsamples have the same size). Taking a as an example, half the Trojans
have a < 5.21AU, and half have a larger than this value.
The mean color, the color dispersion, and the color distribution of the
2 subsamples are compared using the three statistical tests mentioned in
Section 4.2. The method is discussed in details in Hainaut & Delsanti (2002).
The tests are repeated for all color and spectral slope distributions. The
results are the following.
• q, perihelion distance: the color distribution of the Trojans with small
q is marginally broader than that of Trojans with larger q. This result
is not very strong (5%), and is dominated by the red-end of the visible
wavelength. Removing the Eurybates from the sample maintains the
result, at the same weak level.
• e, eccentricity: the distribution shows a similar result, also at the weak
5% significance. The objects with larger e have broader color distribu-
tion than those with lower e. This result is entirely dominated by the
Eurybates’ contribution.
• i, inclination: objects with smaller inclination are significantly bluer
than those with larger i. This result is observed at all wavelengths. It
is worth noting that this is contrary to what is usually observed on
other Minor Bodies in the Outer Solar System survey (MBOSSes),
where objects with high i, or more generally, high excitation E =√
e2 + sin2 i, are bluer (Hainaut & Delsanti, 2002; Doressoundiram et
al., 2005). This can also be visually appreciated in Fig. 12. This result
is also completely dominated by the Eurybates’ contribution. The non-
Eurybates Trojans do not display this trend.
• E =
e2 + sin2 i, orbital excitation: the objects with small E are also
significantly bluer than those with high E. This result is also com-
pletely dominated by the Eurybates’ contribution. The non-Eurybates
Trojans do not display this trend.
In summary this analysis shows that the Eurybates sub-sample of the
Trojans is well separated in orbital elements and in colors.
For the other Minor Bodies in the outer Solar System, the relation be-
tween color and inclination–orbital excitation (objects with a higher orbital
excitation tend to be bluer) is interpreted as a relation between excitation
and surface aging/rejuvenating processes (Doressoudiram et al., 2005). The
Eurybates family has low excitation and neutral-blue colors, suggesting that
the aging/rejuvenating processes affecting them are different from the other
objects. This could be due to different surface compositions, different irradi-
ation processes, or different collisional properties – which would be natural
for a collisional family.
5 Comparison with other outer Solar System
minor bodied
5.1 Introduction and methods
[HERE FIGURES 13 AND 14]
The statistical tests set described in section 4.2 has also been applied
to compare the colors and the spectral slopes distribution of the Trojans
with those of the other minor bodies in the outer Solar System taken from
the updated, on-line version of the Hainaut & Delsanti (2002) database.
Figure 13, as an example, displays the (R-I) vs (V-R) diagrams, while Fig. 14
shows the (B-V) and (V-R) color distributions, as well as the spectral slope
distribution of the different classes of objects. The tests were performed
on all the color indices derived from filters in the visible (UBVRI) and near
infrared range (JHK) but in Table 7 and 8 we summarize the most significant
results.
In order to “calibrate” the significant probabilities, additional artificial
classes are also compared: first, the objects which have an even internal
number in the database with the odd ones. As this internal number is purely
arbitrary, both classes are statistically indistinguishable. The other tested
pair is the objects with a “1999” designation versus the others. Again, this
selection criterion is arbitrary, so the pseudo-classes it generates are sub-
sample of the total population, and should be indistinguishable. However, as
many more objects have been discovered in all the other years than during
that specific year, the size of these sub-samples are very different. This
permits us to estimate the sensitivity of the tests on sample of very different
sizes. Some of the tests found the arbitrary populations incompatible at the
5% level, so we use 0.5% as a conservative threshold for statistical significance
of the distribution incompatibility
5.2 Results
Table 7 and Fig. 14 clearly show that the Trojans’ colors distribution is
different as compare to that of Centaurs, TNOs and comets. Trojans are at
the same time bluer, and their distribution is narrower than all the other
populations. Using the statistical tests (see Table 8), we can confirm the
significance of these results.
• The average colors of the Trojans are significantly different from those
of all the other classes of objects (t-test), with the notable exception of
the short period comet nuclei. Refining the test to the Eurybates/non-
Eurybates, it appears that the Eurybates have marginally different
mean colors, while the non-Eurybates average colors are indistinguish-
able from those of the comets.
• Considering the full shape of the distribution (KS test), we obtain the
same results: the Trojans colors distributions are significantly differ-
ent from those of all the other classes, with the exception of the SP
comets, which are compatible. Again, this result becomes stronger
separating the Eurybates: their distributions are different from those
of the comets, while the non-Eurybates ones are indistinguishable.
• The results when considering the widths of the color distributions (f-
test) are slightly different. Classes of objects with different mean colors
could still have the same distribution width. This could suggest that a
similar process (causing the width of the distribution) is in action, but
reached a different equilibrium point (resulting in different mean val-
ues). This time, all classes are incompatible with the Trojans, including
the comets, with strong statistical significance.
In order to further explore possible similarities between Trojans and other
classes, the comparisons were also performed with the neutral Centaurs.
These were selected with S < 20%/103Å); this cut-off line falls in the gap
between the ”neutral” and ”red” Centaurs (Peixinho et al., 2003, Fornasier
et al., 2004b).
The t-Test (mean color) only reveals a very moderate incompatibility be-
tween the Trojans and neutral Centaurs, at the 5% level, i.e. only marginally
significant. On the other hand, the f-Test gives some strong incompatibilities
in various colors (moderate in B− V and H −K, very strong in R− I), but
the two populations are compatible for most of the other colors. Similarly,
only the R − I KS-test reveals a strong incompatibility. It should also be
noted that only 18 neutral Centaurs are known in the database. In summary,
while the Trojans and neutral Centaurs have fairly similar mean colors, their
color distributions are also different.
6 Conclusions
From 2002, we carried out a spectroscopic and photometric survey of Jupiter
Trojans, with the aim of investigating the members of dynamical families.
In this paper we present new data on 47 objects belonging to several dy-
namical families: Anchises (5 members), Cloanthus (2 members), Misenus (6
members), Phereclos (3 members), Sarpedon (2 members) and Panthoos (5
members) from the L5 swarm; Eurybates (17 members), 1986 WD (6 mem-
bers), and Menelaus (1 member) for the L4 swarm. Together with the data
already published by Fornasier et al. (2004a) and Dotto et al. (2006), taken
within the same observing program, we have a total sample of 80 Trojans,
the largest homogeneous data set available to date on these primitive aster-
oids. The main results coming from the observations presented here, and
from the analysis including previously published visible spectra of Trojans,
are the following:
• Trojans’ visible spectra are mostly featureless. However, some mem-
bers of the Eurybates family show a UV drop-off in reflectivity for
wavelength shorter than 5000–5200 Å that is possibly due to interva-
lence charge transfer transitions (IVCT) in oxidized iron.
• The L4 Eurybates family strongly differs from all the other families
in that it is dominated by C– and P–type asteroids. Also its spectral
slope distribution is significantly different when compared to that of
the other Trojans (at the 10−10 level).
This family is very peculiar and is dynamically very strong, as it sur-
vives also at a very stringent cutoff (40 m/s). Further observations in
the near-infrared region are strongly encouraged to look for possible
absorption features due to water ice or to material that experienced
aqueous alteration.
• The average spectral slope for the L5 Trojans is 9.15±4.19%/103Å, and
6.10±4.48%/103Å for the L4 objects. Excluding the Eurybates, the L4
average slope values becomes 7.33±4.24%/103Å. The slope distribu-
tions of the L5 and of the non-Eurybates L4 are indistinguishable.
• Both L4 and L5 clouds are dominated by D–type asteroids, but the L4
swarm has an higher presence of C– and P–type asteroids, even when
the Eurybates family is excluded, and appears more heterogeneous in
composition as compared to the L5 one.
• We do not find any size versus spectral slope relationship inside the
whole Trojans population.
• The Trojans with higher orbital inclination are significantly redder than
those with lower i. While this trend is the opposite of that observed
for other distant minor bodies, this effect is entirely dominated by the
Eurybates family.
• Comparing the Trojans colors with those of other distant minor bod-
ies, they are the bluest of all classes, and their colors distribution is the
narrowest. This difference is mostly due to the Eurybates family. In
fact, if we consider only the Trojan population without the Eurybates
members, their average colors and overall distributions are not distin-
guishable from that of the short period comets. However, the widths
of their color distributions are not compatible. The similarity in the
overall color distributions might be caused by the small size of the short
period comet sample rather than by a physical analogy. The Trojans
average colors are also fairly similar to those of the neutral Centaurs,
but the overall distributions are not compatible.
After this study, we have to conclude that Trojans have peculiar charac-
teristics very different from those of all the other populations of the outer
Solar System.
Unfortunately, we still cannot assess if this is due to differences in the physi-
cal nature, or in the aging/rejuvenating processes which modified the surface
materials in different way at different solar distances. Further observations,
mainly in V+NIR spectroscopy and polarimetry, are absolutely needed to
better investigate the nature of Jupiter Trojans and to definitively assess if a
genetical link might exist with Trans-Neptunian Objects, Centaurs and short
period comets.
Acknowledgments
We thank Beaugé and Roig for kindly providing us with updated Trojan
family list, and R.P. Binzel and J.P. Emery for their useful comments in the
reviewing process.
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Tables
Table 1: Observing conditions of the investigated L5 asteroids. For each
object we report the observational date and universal time, total exposure
time, number of acquisitions with exposure time of each acquisition, airmass,
and the observed solar analogs with their airmass.
Obj Date UT Texp (s) nexp air. Solar An. (air.)
Anchises
1173 17 Jan 05 06:06 60 1×60s 1.42 HD76151 (1.48)
23549 17 Jan 05 07:20 480 2×240s 1.60 HD76151 (1.48)
24452 17 Jan 05 07:54 960 4×240s 1.44 HD76151 (1.48)
47967 17 Jan 05 05:34 800 2×400s 1.38 HD76151 (1.48)
2001 SB173 17 Jan 05 06:28 1200 2×600s 1.35 HD76151 (1.48)
Cloanthus
5511 19 Jan 05 06:04 960 4×240s 1.26 HD76151 (1.12)
51359 19 Jan 05 04:13 660 1×660s 1.36 HD76151 (1.12)
Misenus
11663 17 Jan 05 05:13 400 1×400s 1.21 HD44594 (1.12)
32794 18 Jan 05 03:13 1800 2×900s 1.39 HD28099 (1.44)
56968 17 Jan 05 04:31 400 2×400s 1.21 HD44594 (1.12)
1988 RE12 18 Jan 05 04:12 2000 2×1000s 1.31 HD28099 (1.44)
2000 SC51 18 Jan 05 06:09 1320 2×660s 1.16 HD44594 (1.17)
2001 UY123 18 Jan 05 06:46 1320 2×660s 1.32 HD44594 (1.17)
Phereclos
9030 18 Jan 05 08:19 1000 1×1000s 1.37 HD44594 (1.17)
11488 19 Jan 05 03:31 1320 2×660s 1.99 HD76151 (1.12)
31820 19 Jan 05 07:02 1320 2×660s 1.35 HD76151 (1.11)
Sarpedon
48252 18 Jan 05 02:32 1320 2×660s 1.30 HD28099 (1.44)
84709 19 Jan 05 05:35 1320 2×660s 1.34 HD76151 (1.12)
Panthoos
4829 17 Jan 05 08:37 720 3×240s 1.45 HD76151 (1.48)
30698 18 Jan 05 01:54 1320 2×660s 1.73 HD28099 (1.44)
31821 18 Jan 05 05:27 1320 2×660s 1.35 HD28099 (1.44)
76804 17 Jan 05 03:35 1800 3×600s 1.38 HD44594 (1.12)
2001 VK85 18 Jan 05 07:31 2000 2×1000s 1.23 HD44594 (1.17)
Table 2: Observing conditions of the investigated L4 asteroids. For each
object we report the observational date and universal time, total exposure
time, number of acquisitions with exposure time of each acquisition, airmass,
and the observed solar analogs with their airmass.
Obj Date UT Texp (s) nexp air. Solar An. (air.)
Eurybates
3548 25 May 04 05:14 600 2×300s 1.02 SA107-684 (1.19)
9818 26 May 04 00:13 780 1×780s 1.19 SA102-1081(1.15)
13862 25 May 04 03:35 1200 2×600s 1.09 SA107-998 (1.15)
18060 25 May 04 02:47 1500 2×750s 1.07 SA107-998 (1.15)
24380 25 May 04 06:53 780 1×780s 1.18 SA107-684 (1.19)
24420 25 May 04 08:49 900 1×900s 1.59 SA112-1333 (1.17)
24426 26 May 04 00:13 1440 2×720s 1.13 SA107-684 (1.17)
28958 26 May 04 07:14 1800 2×900s 1.35 SA107-684 (1.17)
39285 25 May 04 05:40 2700 3×900s 1.09 SA107-684 (1.19)
43212 25 May 04 07:39 2340 3×780s 1.39 SA110-361 (1.15)
53469 25 May 04 02:05 1800 2×900s 1.04 SA107-998 (1.15)
65150 26 May 04 01:59 3600 4×900s 1.07 SA102-1081 (1.20)
65225 26 May 04 03:40 3600 4×900s 1.04 SA107-684 (1.17)
1996RD29 26 May 04 05:12 2700 3×900s 1.10 SA107-684 (1.17)
2000AT44 25 May 04 04:14 1800 2×900s 1.04 SA107-684 (1.19)
2002CT22 26 May 04 00:49 2400 4×600s 1.08 SA102-1081 (1.15)
2002EN68 26 May 04 08:10 1800 2×900s 1.62 SA107-684 (1.17)
1986 WD
4035 10 Apr 03 03:28 600 1×600s 1.09 SA107-684 (1.15)
6545 10 Apr 03 02:39 900 1×900s 1.16 SA107-684 (1.15)
11351 10 Apr 03 09:21 900 1×900s 1.28 SA107-684 (1.15)
14707 11 Apr 03 08:11 1200 1×1200s 1.15 SA107-684 (1.15)
24233 11 Apr 03 02:29 1200 1×1200s 1.39 SA107-684 (1.37)
24341 11 Apr 03 05:47 900 1×900s 1.16 SA107-684 (1.17)
1986 TS6
12921 10 Apr 03 07:33 900 1×900s 1.39 SA107-684 (1.40)
Table 3: Visible photometric observations of L4 and L5 Trojans (ESO-NTT
EMMI): for each object, date, computed V magnitude, B-V, V-R and V-
I colors are reported. The given UT is for the V filter acquisition. The
observing photometric sequence (V-R-B-I) took a few minutes.
Object date UT V B-V V-R V-I
1986 WD
4035 10 Apr 03 03:11 16.892±0.031 0.752±0.040 0.473±0.042 0.926±0.055
4035 10 Apr 03 04:22 16.981±0.031 0.752±0.040 0.495±0.042 0.945±0.055
6545 10 Apr 03 02:22 17.558±0.031 0.734±0.041 0.499±0.042 0.935±0.055
11351 10 Apr 03 09:03 18.407±0.032 0.739±0.044 0.498±0.044 0.900±0.057
14707 11 Apr 03 06:46 18.666±0.031 0.751±0.041 0.401±0.033 0.804±0.055
14707 11 Apr 03 08:37 18.873±0.031 0.754±0.041 0.424±0.033 0.790±0.056
24233 11 Apr 03 01:33 18.894±0.034 0.704±0.051 0.481±0.037 0.899±0.058
24341 11 Apr 03 05:05 19.376±0.032 0.713±0.043 0.369±0.035 0.759±0.057
1986 TS6
12921 10 Apr 03 07:12 18.393±0.031 0.673±0.040 0.421±0.042 0.786±0.055
L5 cut off 150m/s
Anchises
1173 17 Jan 05 05:54 16.595±0.024 0.811±0.034 0.402±0.035 0.805±0.038
23549 17 Jan 05 07:09 18.969±0.050 0.800±0.071 0.485±0.068 0.872±0.075
24452 17 Jan 05 07:48 18.757±0.043 0.872±0.056 0.441±0.056 0.847±0.066
47967 17 Jan 05 05:27 19.382±0.044 0.899±0.058 0.489±0.069 0.965±0.075
2001 SB173 17 Jan 05 06:20 19.882±0.043 0.992±0.060 0.503±0.064 0.927±0.078
Cloanthus
5511 19 Jan 05 05:52 17.968±0.020 0.906±0.027 0.442±0.027 0.968±0.032
51359 19 Jan 05 03:54 19.631±0.102 0.864±0.201 0.447±0.131 0.885±0.164
Misenus
11663 17 Jan 05 05:05 18.473±0.022 0.837±0.030 0.409±0.030 0.872±0.039
32794 18 Jan 05 03:07 19.685±0.038 0.923±0.065 0.393±0.056 0.879±0.057
56968 17 Jan 05 04:18 18.596±0.026 0.986±0.040 0.494±0.033 1.003±0.036
1988 RE12 18 Jan 05 04:00 20.892±0.081 0.826±0.132 0.388±0.108 0.871±0.106
2000 SC51 18 Jan 05 06:03 19.876±0.038 1.016±0.055 0.444±0.059 0.896±0.056
2001 UY123 18 Jan 05 06:41 19.869±0.047 0.890±0.058 0.537±0.056 0.971±0.063
Phereclos
9030 18 Jan 05 08:14 18.397±0.020 0.887±0.024 0.493±0.027 0.973±0.028
11488 19 Jan 05 02:57 18.931±0.066 0.868±0.101 0.430±0.079 0.848±0.084
31820 19 Jan 05 06:39 20.041±0.077 0.889±0.093 0.520±0.091 0.916±0.123
Sarpedon
48252 18 Jan 05 02:25 19.878±0.060 0.949±0.100 0.467±0.093 0.903±0.090
84709 19 Jan 05 05:10 19.862±0.068 0.855±0.087 0.462±0.090 1.010±0.094
Panthoos
4829 17 Jan 05 08:18 18.430±0.029 0.851±0.050 0.420±0.039 0.792±0.052
30698 18 Jan 05 01:45 19.353±0.036 – 0.472±0.042 0.865±0.047
31821 18 Jan 05 05:21 19.328±0.076 0.980±0.111 0.440±0.097 0.901±0.108
76804 17 Jan 05 03:21 19.471±0.065 0.803±0.082 0.446±0.070 0.889± 0.080
2001 VK85 18 Jan 05 07:23 20.179±0.038 0.822±0.063 0.462±0.048 1.020±0.050
Table 4: L5 families. We report for each target the absolute magnitude H
and the estimated diameter (diameters marked by ∗ are taken from IRAS
data), the spectral slope S computed between 5500 and 8000 Å and the
taxonomic class (T) derived following Dahlgren & Lagerkvist (1995) classi-
fication scheme. The asteroids marked with a were observed by Fornasier
et al. (2004a), and their spectral slope values have been recomputed in the
5500-8000 Å wavelength range; asteroids 23694, 30698 and 32430, previously
Astyanax members, have been reassigned to the Panthoos family due to re-
fined proper elements.
Obj H D (km) S (%/103Å) T
Anchises
1173 8.99 ∗126+11
3.87±0.70 P
23549 12.04 26+4
8.49±0.88 D
24452 11.85 29+5
7.42±0.70 D
47967 12.15 25+4
9.21±0.78 D
2001 SB173 12.77 19+3
14.78±0.99 D
Cloanthus
5511 10.43 55+8
10.84±0.65 D
51359 12.25 24+6
12.63±1.30 D
Misenus
11663 10.95 44+7
6.91±0.70 DP
32794 12.77 19+3
6.59±0.88 DP
56968 11.72 30+5
15.86±0.71 D
1988 RE12 13.20 16+2
4.68±1.20 P
2000 SC51 12.69 20+3
6.54±0.98 DP
2001 UY123 12.75 19+3
8.28±0.88 D
Phereclos
a2357 8.86 ∗95+4
9.91±0.68 D
a6998 11.43 34+5
11.30±0.75 D
9030 11.14 40+6
10.35±0.76 D
a9430 11.47 35+5
10.02±0.90 D
11488 11.82 29+5
5.37±0.92 PD
a18940 11.81 29+4
7.13±0.75 D
31820 12.63 20+3
7.53±0.80 D
Sarpedon
a2223 9.25 ∗95+4
10.20±0.65 D
a5130 9.85 71+11
10.45±0.65 D
a17416 12.83 18+3
10.80±0.90 D
a25347 11.59 33+5
10.11±0.83 D
48252 12.84 18+3
9.62±0.82 D
84709 12.70 19+3
11.64±0.84 D
Panthoos
4829 11.16 39+6
5.03±0.70 PD
a23694 11.61 32+5
8.20±0.72 D
30698 12.14 25+4
8.23±1.00 D
a30698 12.27 25+4
9.08±0.82 D
a32430 12.23 25+4
8.12±1.00 D
31821 11.99 27+4
10.58±0.82 D
76804 12.16 25+4
7.29±0.71 D
2001 VK85 12.79 19+3
14.39±0.81 D
Table 5: L4 Families. We report for each target the absolute magnitude H
and the estimated diameter (diameters marked by ∗ are taken from IRAS
data, while absolute magnitudes marked by ∗∗ are taken from the astorb.dat
file of the Lowell Observatory), the spectral slope S computed between 5500
and 8000 Å, and the taxonomic class (T) derived following Dahlgren &
Lagerkvist (1995) classification scheme. The asteroids marked with a were
observed by Dotto et al. (2006), and their spectral slope values have been
recomputed in the 5500-8000 Å wavelength range.
Obj H D (km) S (%/103Å) T
Eurybates
3548 9.50∗∗ ∗72+4
-0.18±0.57 C
9818 11.00∗∗ 42+6
2.12±0.72 P
13862 11.10∗∗ 40+6
1.59±0.70 C
18060 11.10∗∗ 40+6
2.86±0.60 P
24380 11.20∗∗ 38+6
0.34±0.65 C
24420 11.50∗∗ 33+5
1.65±0.70 C
24426 12.50∗∗ 21+3
4.64±0.80 P
28958 12.10∗∗ 25+4
-0.04±0.80 C
39285 12.90∗∗ 17+3
0.25±0.69 C
43212 12.30∗∗ 23+4
1.19±0.78 C
53469 11.80∗∗ 29+4
0.17±0.80 C
65150 12.90∗∗ 17+3
4.14±0.70 P
65225 12.80∗∗ 18+3
0.97±0.85 C
1996RD29 13.06∗∗ 16+3
2.76±0.89 P
2000AT44 12.16∗∗ 24+3
-0.53±0.83 C
2002CT22 12.04∗∗ 26+4
2.76±0.73 P
2002EN68 12.30∗∗ 23+3
3.60±0.98 P
1986 WD
4035 9.72 ∗68+5
9.78±0.61 D
a4035 9.30∗∗ ∗68+5
15.19±0.61 D
6545 10.42 55+8
11.32±0.63 D
a6545 10.00∗∗ 66+10
9.88±0.56 D
11351 10.88 44+7
10.26±0.67 D
a11351 10.50∗∗ 53+8
10.44±0.61 D
14707 11.25 38+6
−9.4 -1.06±1.00 C
24233 11.58 33+5
−8.0 6.37±0.67 DP
24341 11.99 27+4
-0.26±0.71 C
1986 TS6
12917 11.61 32+5
10.98±0.68 D
12921 11.12 40+6
4.63±0.75 P
a12921 10.70∗∗ 48+7
3.74±1.00 P
13463 11.27 37+6
4.37±0.65 P
15535 10.70 48+7
10.67±0.65 D
20738 11.67 31+5
8.84±0.70 D
24390 11.80 29+5
9.53±0.62 D
Table 6: Results of the statistical analysis on the spectral slope distribution as a function of the diameters. For each
test bin, the average slope and the dispersion are listed; the size of the sample is reported in parenthesis. For each pair
of subsamples, the probability that both are randomly extracted from the same global sample is listed, as estimated
by the t-, f- and ks-test, respectively. Low probability indicates significant differences between the subsamples.
Diameter range 0–25 km 25–50 km 50–75 km 75–100 km >100 km
S average±σ 7.17±4.79 (22) 6.92±4.69 (48) 8.91±4.68 (26) 6.74±5.85 (21) 7.87±2.88 (21)
(%/103Å)
0–25 0.842 0.876 0.579 0.213 0.903 0.575 0.792 0.370 0.775 0.551 0.017 0.494
25–50 0.088 0.985 0.150 0.897 0.216 0.519 0.286 0.011 0.275
50–75 0.176 0.289 0.469 0.344 0.019 0.440
75–100 0.442 0.001 0.469
Table 7: Mean color indices and spectral slope of different classes of minor bodies of the outer Solar System. For each
class the number of objects considered is also listed.
Color Plutinos Cubewanos Centaurs Scattered Comets Trojans
B-V 36 87 29 33 2 74
0.895± 0.190 0.973± 0.174 0.886± 0.213 0.875± 0.159 0.795± 0.035 0.777± 0.091
V-R 38 96 30 34 19 80
0.568± 0.106 0.622± 0.126 0.573± 0.127 0.553± 0.132 0.441± 0.122 0.445± 0.048
V-I 34 64 25 25 7 80
1.095± 0.201 1.181± 0.237 1.104± 0.245 1.070± 0.220 0.935± 0.141 0.861± 0.090
V-J 10 14 11 8 1 12
2.151± 0.302 1.750± 0.456 1.904± 0.480 2.041± 0.391 1.630± 0.000 1.551± 0.120
V-H 3 7 11 4 1 12
2.698± 0.083 2.173± 0.796 2.388± 0.439 2.605± 0.335 1.990± 0.000 1.986± 0.177
V-K 2 5 9 2 1 12
2.763± 0.000 2.204± 1.020 2.412± 0.396 2.730± 0.099 2.130± 0.000 2.125± 0.206
R-I 34 64 25 26 8 80
0.536± 0.135 0.586± 0.148 0.548± 0.150 0.517± 0.102 0.451± 0.059 0.416± 0.057
J-H 11 17 21 11 1 12
0.403± 0.292 0.370± 0.297 0.396± 0.112 0.348± 0.127 0.360± 0.000 0.434± 0.064
H-K 10 16 20 10 1 12
-0.034± 0.171 0.084± 0.231 0.090± 0.142 0.091± 0.136 0.140± 0.000 0.139± 0.041
Slope 38 91 30 34 8 80
(%/103Å) 19.852± 10.944 25.603± 13.234 20.601± 13.323 18.365± 12.141 10.722± 6.634 7.241± 3.909
Table 8: Statistical tests performed to compare the color and slope distributions of different classes of minor bodies
(Plt= Plutinos, Resonant TNOs; QB1= Cubiwanos, Classical TNOs; Cent= Centaurs; Scat= scattered TNOs; Com=
Short Period Comet nuclei) with those of Trojans. The first five columns consider all the Trojans, the second five only
the Eurybates family, the third five only the non-Eurybates family Trojans. For each color, the first line shows the
number of objects used for the comparison (2nd is the number of Trojans), and the second line reports the probability
resulting from the test. A very low value indicates that the two compared distributions are not statistically compatible.
Probabilities are in boldface when the size of the samples is large enough for the value to be meaningful.
f-test
Color All Trojans Only Eurybates Only NON-Eurybates
Plt QB1 Cent Scat Com Plt QB1 Cent Scat Com Plt QB1 Cent Scat Com
B-V 36 74 83 74 29 74 33 74 2 74 36 14 83 14 29 14 33 14 2 14 36 60 83 60 29 60 33 60 2 60
0.000 0.000 0.000 0.000 0.600 0.001 0.001 0.000 0.005 0.722 0.000 0.000 0.000 0.000 0.598
V-R 38 80 92 80 30 80 34 80 19 80 38 17 92 17 30 17 34 17 19 17 38 63 92 63 30 63 34 63 19 63
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
R-I 34 80 62 80 25 80 26 80 8 80 34 17 62 17 25 17 26 17 8 17 34 63 62 63 25 63 26 63 8 63
0.000 0.000 0.000 0.000 0.773 0.000 0.000 0.000 0.001 0.335 0.000 0.000 0.000 0.000 0.185
Slope 38 80 87 80 30 80 34 80 8 80 38 17 87 17 30 17 34 17 8 17 38 63 87 63 30 63 34 63 8 63
0.000 0.000 0.000 0.000 0.020 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
t-test
Color All Trojans Only Eurybates Only NON-Eurybates
Plt QB1 Cent Scat Com Plt QB1 Cent Scat Com Plt QB1 Cent Scat Com
B-V 36 74 83 74 29 74 33 74 2 74 36 14 83 14 29 14 33 14 2 14 36 60 83 60 29 60 33 60 2 60
0.001 0.000 0.012 0.002 0.608 0.000 0.000 0.001 0.000 0.139 0.003 0.000 0.025 0.006 0.858
V-R 38 80 92 80 30 80 34 80 19 80 38 17 92 17 30 17 34 17 19 17 38 63 92 63 30 63 34 63 19 63
0.000 0.000 0.000 0.000 0.916 0.000 0.000 0.000 0.000 0.083 0.000 0.000 0.000 0.000 0.532
R-I 34 80 62 80 25 80 26 80 8 80 34 17 62 17 25 17 26 17 8 17 34 63 62 63 25 63 26 63 8 63
0.000 0.000 0.000 0.000 0.154 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.001 0.000 0.502
Slope 38 80 87 80 30 80 34 80 8 80 38 17 87 17 30 17 34 17 8 17 38 63 87 63 30 63 34 63 8 63
0.000 0.000 0.000 0.000 0.185 0.000 0.000 0.000 0.000 0.008 0.000 0.000 0.000 0.000 0.404
KS-test
Color All Trojans Only Eurybates Only NON-Eurybates
Plt QB1 Cent Scat Com Plt QB1 Cent Scat Com Plt QB1 Cent Scat Com
B-V 36 74 83 74 29 74 33 74 2 74 36 14 83 14 29 14 33 14 2 14 36 60 83 60 29 60 33 60 2 60
0.001 0.000 0.001 0.004 0.330 0.002 0.000 0.035 0.000 0.065 0.003 0.000 0.002 0.047 0.468
V-R 38 80 92 80 30 80 34 80 19 80 38 17 92 17 30 17 34 17 19 17 38 63 92 63 30 63 34 63 19 63
0.000 0.000 0.000 0.000 0.040 0.000 0.000 0.000 0.000 0.008 0.000 0.000 0.000 0.000 0.056
R-I 34 80 62 80 25 80 26 80 8 80 34 17 62 17 25 17 26 17 8 17 34 63 62 63 25 63 26 63 8 63
0.000 0.000 0.000 0.000 0.201 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.587
Slope 38 80 87 80 30 80 34 80 8 80 38 17 87 17 30 17 34 17 8 17 38 63 87 63 30 63 34 63 8 63
0.000 0.000 0.000 0.000 0.088 0.000 0.000 0.000 0.000 0.002 0.000 0.000 0.000 0.000 0.211
Figure captions
Fig. 1 - Reflectance spectra of 5 Anchises family members (L5 swarm).
The photometric color indices are also converted to relative reflectance and
overplotted on each spectrum. Spectra and photometry are shifted by 0.5 in
reflectance for clarity.
Fig. 2 - Reflectance spectra of 6 Misenus family members (L5 swarm).
The photometric color indices are also converted to relative reflectance and
overplotted on each spectrum. Spectra and photometry are shifted by 0.5 in
reflectance for clarity.
Fig. 3 - Reflectance spectra of 5 Panthoos family members (L5 swarm).
The photometric color indices are also converted to relative reflectance and
overplotted on each spectrum. Spectra and photometry are shifted by 0.5 in
reflectance for clarity. For asteroid 30698, the B-V color is missing as a B
filter measurement was not available.
Fig. 4 - Reflectance spectra of 2 Cloantus, 3 Phereclos and 2 Sarpedon
family members (L5 swarm). The photometric color indices are also con-
verted to relative reflectance and overplotted on each spectrum. Spectra and
photometry are shifted by 1.0 in reflectance for clarity.
Fig. 5 - Reflectance spectra of the 17 Eurybates family members (L4
swarm). Spectra are shifted by 0.5 in reflectance for clarity.
Fig. 6 - Reflectance spectra of the 6 1986 WD family members and 12921,
which is a member of the 1986 TS6 family (all belonging to the L4 swarm).
Spectra are shifted by 1.0 in reflectance for clarity.
Fig. 7 - Plot of the spectral slope versus the estimated diameter for the
families observed in the L5 swarm.
Fig. 8 - Plot of the spectral slope versus the estimated diameter for the
families observed in the L4 swarm.
Fig. 9 - Plot of the observed spectral slopes versus the estimated diameter
for the whole population of Jupiter Trojans investigated by us and available
from the literature. The errors on slopes and diameters are not plotted to
avoid confusion.
Fig. 10 - Histogram of L5 Trojans taxonomic classes.
Fig. 11 - Histogram of L4 Trojans taxonomic classes (Neg indicates ob-
jects with negative spectral slope).
Fig. 12 - Color distributions as functions of the absolute magnitude
M(1, 1), the inclination i [degrees], the orbital semi-major axis a [AU], the
perihelion distance q [AU], the eccentricity e, and the orbital energy E (see
text for definition). We include all the available colors for distant minor bod-
ies (TNOs, Centaurs, and cometary nuclei, see Hainaut & Delsanti 2002).
The Plutinos (resonant TNOs) are red filled triangles, Cubiwanos (classical
TNOs) are pink filled circles, Centaurs are green open triangles, Scattered
TNOs are blue open circles, and Trojans are cyan filled triangles.
Fig. 13 - V −R versus R−I color-color diagram for the observed Trojans
and all distant minor bodies available in the updated Hainaut & Delsanti
(2002) database. The solid symbols are for the Trojans (square for Eurby-
bates, triangles for others). The open symbols are used as following: tri-
angles for Plutinos, circles for Cubiwanos, squares for Centaurs, pentagons
for Scattered, and starry square for Comets. The continuous line represents
the ”reddening line”, that is the locus of objects with a linear reflectivity
spectrum. The star symbol represents the Sun.
Fig. 14 - Cumulative function and histograms of the B − V and V − R
color distributions and of the spectral slope for all the considered classes of
objects. The dotted line marks the solar colors.
4000 5000 6000 7000 8000 9000
Figure 1:
4000 5000 6000 7000 8000 9000
Figure 2:
4000 5000 6000 7000 8000 9000
Figure 3:
Figure 4:
Figure 5:
Figure 6:
Figure 7:
Figure 8:
Figure 9:
Figure 10:
Figure 11:
M(1,1) E
a [AU] q [AU]
Figure 12:
Figure 13:
Figure 14:
	Introduction
	Observations and data reduction
	Results
	Dynamical families: L5 swarm
	Anchises
	Misenus
	Panthoos
	Cloantus
	Phereclos
	Sarpedon
	Dynamical families: L4 swarm
	Eurybates
	1986 WD
	1986 TS6
	Discussion
	Size vs spectral slope distribution:Individual families
	Size vs slope distribution:  The Trojan population as a whole
	Spectral slopes and L4/L5 Clouds
	Orbital Elements
	Comparison with other outer Solar System minor bodied
	Introduction and methods
	Results
	Conclusions

  We present the results of a visible spectroscopic and photometric survey of
Jupiter Trojans belonging to different dynamical families carried out at the
ESO-NTT telescope. We obtained data on 47 objects, 23 belonging to the L5 swarm
and 24 to the L4 one. These data together with those already published by
Fornasier et al. (2004a) and Dotto et al. (2006), constitute a total sample of
visible spectra for 80 objects. The survey allows us to investigate six
families (Aneas, Anchises, Misenus, Phereclos, Sarpedon, Panthoos) in the L5
cloud and four L4 families (Eurybates, Menelaus, 1986 WD and 1986 TS6). The
sample that we measured is dominated by D--type asteroids, with the exception
of the Eurybates family in the L4 swarm, where there is a dominance of C- and
P-type asteroids. All the spectra that we obtained are featureless with the
exception of some Eurybates members, where a drop--off of the reflectance is
detected shortward of 5200 A. Similar features are seen in main belt C-type
asteroids and commonly attributed to the intervalence charge transfer
transition in oxidized iron. Our sample comprises fainter and smaller Trojans
as compared to the literature's data and allows us to investigate the
properties of objects with estimated diameter smaller than 40--50 km. The
analysis of the spectral slopes and colors versus the estimated diameters shows
that the blue and red objects have indistinguishable size distribution. We
perform a statistical investigation of the Trojans's spectra property
distributions as a function of their orbital and physical parameters, and in
comparison with other classes of minor bodies in the outer Solar System.
Trojans at lower inclination appear significantly bluer than those at higher
inclination, but this effect is strongly driven by the Eurybates family.

Introduction
Jupiter Trojans are small bodies of the Solar System located in the Jupiter
Lagrangian points L4 and L5. Up to now more than 2000 Trojans have been
discovered, ∼ 1150 belonging to the L4 cloud and ∼ 950 to the L5 one.
The number of L4 Trojans with radius greater than 1 km is estimated to be
around 1.6 ×105 (Jewitt et al., 2000), comparable with the estimated main
belt population of similar size.
The debate about the origin of Jupiter Trojans and how they were trapped
in librating orbits around the Lagrangian points is still open to several possi-
bilities. Considering that Trojans have orbits stable over the age of the Solar
System (Levison et al, 1997, Marzari et al. 2003) their origin must date back
to the early phase of the solar system formation. Some authors (Marzari &
Scholl, 1998a,b; Marzari et al., 2002) suggested that they formed very close
to their present location and were trapped during the growth of Jupiter.
Morbidelli et al. (2005) suggested that Trojans formed in the Kuiper belt
and were subsequently captured in the Jupiter L4 and L5 Lagrangian points
during planetary migration, just after Jupiter and Saturn crossed their mu-
tual 1:2 resonances. In this scenario, Jupiter Troians would give important
clues on the composition and accretion of bodies in the outer regions of the
solar nebula.
Several theoretical studies conclude that Jupiter Trojan clouds are at
least as collisionally evolved as main belt asteroids (Shoemaker et al., 1989;
Binzel & Sauter, 1992; Marzari et al., 1997; Dell’Oro et al., 1998). This
result is supported by the identification of several dynamical families, both
in the L4 and L5 swarm (Shoemaker et al., 1989, Milani, 1993, Beaugé and
Roig, 2001).
Whatever the Trojan origin is, it is plausible to assume that they formed be-
yond the frost line and that they are primitive bodies, are possibly composed
of anhydrous silicates and organic compounds, and possibly still contain ices
in their interior. Several observations of Trojans in the near infrared region
(0.8-2.5 µm) have failed to clearly detect any absorption features indicative
of water ice (Barucci et al, 1994; Dumas et al, 1998; Emery & Brown, 2003,
2004; Dotto et al., 2006). Also in the visible range Trojan spectra appear
featureless (Jewitt & Luu, 1990; Fornasier et al., 2004a, Bendjoya et al.,
2004; Dotto et al., 2006). Up to now only 2 objects (1988 BY1 and 1870
Glaukos) show the possible presence of faint bands (Jewitt & Luu, 1990).
However, these bands are comparable to the peak to peak noise and are not
yet confirmed.
Recently, mineralogical features have been detected in emissivity spectra of
three Trojan asteroids measured by the Spitzer Space Telescope. These fea-
tures are interpreted as indicating the presence of fine-grained silicates on
the surfaces (Emery et al. 2006).
Several questions about Jupiter Trojans’ dynamical origin, physical prop-
erties, composition and link with other groups of minor bodies such as outer
main belt asteroids, cometary nuclei, Centaurs and KBOs are still open.
In order to shed some light on these questions, we have carried out a spectro-
scopic and photometric survey of Jupiter Trojans at the 3.5m New Technol-
ogy Telescope (NTT) of the European Southern Observatory (La Silla, Chile)
and at the 3.5m Telescopio Nazionale Galileo (TNG), La Palma, Spain. In
this paper we present new visible spectroscopic and photometric data, ob-
tained during 7 observing nights, carried out at ESO-NTT on April 2003,
May 2004, and January 2005, for a total of 47 objects belonging to the L5
(23 objects) and L4 (24 objects) swarms. Considering also the results already
published in Fornasier et al. (2004a) and Dotto et al. (2006), obtained in
the framework of the same project, we collected a total sample of 80 Jupiter
Trojan visible spectra, 47 belonging to the L5 clouds and 33 to the L4. This
is the largest homogeneous data set available up to now on these primitive
asteroids.
The principal aim of our survey was the investigation of Jupiter Trojans
belonging to different dynamical families. In fact, since dynamical families
are supposed to be formed from the collisional disruption of parent bodies,
the investigation of the surface properties of small and large family members
can help in understanding the nature of these dynamical groups and might
provide a glimpse of the interior structure of the larger primordial parent
bodies.
We also present an analysis of the visible spectral slopes for all the data in
our survey along with those available in the literature, for a total sample of
142 Trojans.
This enlarged sample allowed us to carry out a significant statistical investi-
gation of the Trojans’ spectral property distributions, as a function of their
orbital and physical parameters, and in comparison with other classes of mi-
nor bodies in the outer Solar System. We also discuss the spectral slope
distribution within the Trojan families.
2 Observations and data reduction
[HERE TABLE 1 AND 2]
The data were obtained in the visible range during 3 different observing
runs at ESO-NTT: 10 and 11 April 2003 for the spectroscopic and pho-
tometric investigation of 6 members of the 4035 1986 WD and 1 member
of 1986 TS6 families; 25 and 26 May 2004 for a spectroscopic survey of L4
Eurybates family; 17, 18, and 19 January 2005 for the spectroscopic and pho-
tometric investigation of 5 Anchises, 6 Misenus, 5 Panthoos, 2 Cloanthus, 2
Sarpedon and 3 Phereclos family members (L5 swarm).
We selected our targets from the list of Jupiter Trojan families provided by
Beaugé and Roig (2001 and P.E.Tr.A. Project at www.daf.on.br/ froig/petra/).
The authors have used a cluster-detection algorithm called Hierarchical Clus-
tering Method (HCM, e.g. Zappalà et al., 1990) to find asteroid families
among Jupiter Trojans starting from a data–base of semi-analytical proper
elements (Beaugé & Roig, 2001). The identification of families is performed
by comparing the mutual distances with a suitable metric in the proper el-
ements’ space. The clustering chain is halted when the mutual distance,
measuring the incremental velocity needed for orbital change after the puta-
tive parent body breakup, is larger than a fixed cut-off value. A lower cutoff
implies a higher statistical significance of the family. Since families in L4 are
on average more robust than those around L5 (Beaugé and Roig, 2001), we
prefer to adopt a cutoff of 100 m/s for the L4 cloud and of 150 m/s for L5.
For the very robust Eurybates family we decided to limit our survey to those
family members defined with a cutoff of 70 m/s.
All the data were acquired using the EMMI instrument, equipped with a
2x1 mosaic of 2048×4096 MIT/LL CCD with square 15µm pixels. For the
spectroscopic investigation during May 2004 and January 2005 runs we used
the grism #1 (150 gr/mm) in RILD mode to cover the wavelength range
4100–9400 Å with a dispersion of 3.1 Å/px (200 Å/mm) at the first order,
while on April 2003 we used a different grism, the #7 (150 gr/mm), covering
the spectral range 5200–9500 Å, with a dispersion of 3.6 Å/px at the first
order. April 2003 and January 2005 spectra were taken through a 1 arcsec
wide slit, while during May 2004 we used a larger slit (1.5 arcsec). The slit
was oriented along the parallactic angle during all the observing runs in order
to avoid flux loss due to the atmospheric differential refraction.
For most objects, the total exposure time was divided into several (usually
2-4) shorter acquisitions. This allowed us to check the asteroid position in
the slit before each acquisition, and correct the telescope pointing and/or
tracking rates if necessary. During each night we also recorded bias, flat–
field, calibration lamp (He-Ar) and several (6-7) spectra of solar analog stars
measured at different airmasses, covering the airmass range of the science
targets. During 17 January 2005, part of the night was lost due to some
technical problems and only 2 solar analog stars were acquired. The ratio
of these 2 stars show minimal variations (less than 1%) in the 5000–8400 Å
range, but higher differences at the edges of this range. For this reason we
omit the spectral region below 4800 Å for most of the asteroids acquired that
night.
The spectra were reduced using ordinary procedures of data reduction as
described in Fornasier et al. (2004a). The reflectivity of each asteroid was
obtained by dividing its spectrum by that of the solar analog star closest
in time and airmass to the object. Spectra were finally smoothed with a
median filter technique, using a box of 19 pixels in the spectral direction for
each point of the spectrum. The threshold was set to 0.1, meaning that the
original value was replaced by the median value if the median value differs
by more than 10% from the original one. The obtained spectra are shown
in Figs. 1–5. In Table 1 and Table 2 we report the circumstances of the
observations and the solar analog stars used respectively for the L5 and L4
family members.
[TABLE 3]
The broadband color data were obtained during the April 2003 and Jan-
uary 2005 runs just before the Trojans’ spectral observation. We used the
RILD mode of EMMI for wide field imaging with the Bessell-type B, V, R,
and I filters (centered respectively at 4139, 5426, 6410 and 7985Å). The ob-
servations were carried out in a 2 × 2 binning mode, yielding a pixel scale
of 0.33 arcsec/pixel. The exposure time varied with the object magnitude:
typically it was about 12-90s in V, 30-180s in B, 12-70s in R and I filters.
The CCD images were reduced and calibrated with a standard method (For-
nasier et al., 2004a), and absolute calibration was obtained through the ob-
servations of several Landolt fields (Landolt, 1992). The instrumental mag-
nitudes were measured using aperture photometry with an integrating radius
typically about three times the average seeing, and sky subtraction was per-
formed using a 5-10 pixels wide annulus around each object.
The results are reported in Table 3. From the visual inspection and the radial
profiles analysis of the images, no coma was detected for any of the observed
Trojans.
On May 2004, as the sky conditions were clear but not photometric, we did
not perform photometry of the Eurybates family targets.
3 Results
[TABLE 4 AND 5]
For each Trojan we computed the slope S of the spectral continuum using
a standard least squared technique for a linear fit in the wavelength range
between 5500 and 8000 Å. The choice of these wavelength limits has been
driven by the spectral coverage of our data. We choose 5500 Å as the lower
limit because of the different instrumental setup used during different ob-
serving runs (with some spectra starting at wavelength ≥ 5200 Å), while
beyond 8000 Å our spectra are generally noisier due to a combination of the
CCD drop-off in sensitivity and the presence of the strong atmospheric water
bands.
The computed slopes and errors are listed in Table 4 and 5. The reported er-
ror bars take into account the 1σ uncertainty of the linear fit plus 0.5%/103Å
attributable to the use of different instruments and solar analog stars (esti-
mated from the different efficiency of the grism used, and from flux losses
due to different slit apertures). In Table 4 and 5 we also report the taxo-
nomic class derived following the Dahlgren & Lagerkvist (1995) classification
scheme.
In the L5 cloud we find 27 D–, 3 DP–, 2 PD–, and 1 P–type objects. In
the L4 cloud we find 10 C–type and 7 P–type objects inside the Eurybates
family, while for the Menelaus, 1986 TS6 and 1986 WD families, including
the data published in Dotto et al. (2006), we get 9 D–, 3 P–, 3C–, and 1
DP–type asteroids.
The majority of the spectra are featureless, although some of the observed
Eurybates’ members show weak spectral absorption features (Fig. 5). These
features are discussed in the following section.
We derived an estimated absolute magnitude H by scaling the measured
V magnitude to r = ∆ = 1 AU and to zero phase assuming G=0.15 (Bowell
et al., 1989). The estimated H magnitude of each Trojan might be skewed
uncertain rotational phase, as the lightcurve amplitudes of Trojans might
vary up to 1 magnitude. In order to investigate possible size dependence in-
side each family, and considering that IRAS diameters are available for very
few objects, we estimate the size using the following relationship:
1329× 10−H/5
where D is the asteroid diameter, p is the geometric albedo, and H is the abso-
lute magnitude. We use H derived from our observations when available, and
from the ASTORB.DAT file (Lowell observatory) for the Eurybates mem-
bers, for which we did not carry out visible photometry. We evaluated the
diameter for an albedo range of 0.03–0.07, assuming a mean albedo of 0.04
for these dark asteroids (Fernandez et al., 2003). The resulting D values are
reported in Tables 4 and 5.
3.1 Dynamical families: L5 swarm
3.1.1 Anchises
[FIGURE 1]
We investigated 5 of the 15 members of the Anchises family (Fig. 1): 1173
Anchises, 23549 1994 ES6, 24452 2000 QU167, 47967 2000 SL298 and 124729
2001 SB173 on 17 January 2005. For 4 out of 5 observed objects we omit
the spectral range below 4800Å due to low S/N ratio and problems with the
solar analog stars. The spectral behavior is confirmed by photometric data
(see Table 3). All the obtained spectra are featureless.
The Anchises family survives at a cutoff corresponding to relative veloc-
ities of 150 m/s. The biggest member, 1173 Anchises, has a diameter of
126 km (IRAS data) and has the lowest spectral slope (3.9 %/103Å) among
the investigated family members. It is classified as P–type, while the other
4 members are all D–types. Anchises was previously observed in the 4000-
7400Å region by Jewitt & Luu (1990), who reported a spectral slope of 3.8
%/103Å, in perfect agreement with the value we found. The three 19-29 km
sized objects have a steeper spectral slope (7.4-9.2 %/103Å), while the small-
est object, 2001 SB173 (spectral slope = 14.78±0.99 %/103Å) is the reddest
one (Table 4).
Even with the uncertainties in the albedo and diameter, a slope–size rela-
tionship is evident among the observed objects, with smaller–fainter members
redder than larger ones (Fig. 7).
3.1.2 Misenus
[FIGURE 2]
For this family we investigated 6 members (11663 1997 GO24, 32794 1989
UE5, 56968 2000 SA92, 99328 2001 UY123, 105685 2000 SC51 and 120453
1988 RE12) out of the 12 grouped at a relative velocity of 150 m/s. The
family survives with the same members also at a stringent cut-off velocity
of 120 m/s. The spectra, together with magnitude color indices transformed
into linear reflectance, are shown in Fig. 2, while the color indices are reported
in Table 3. All the spectra are featureless with different spectral slope values
covering the 4.6–15.9 %/103Å range (Table 4): 1988 RE12 has the lowest
spectral slope and is classified as P–type, 3 objects (11663, 32794 and 2000
SC51) are in the transition region between P– and D– type, with very similar
spectral behavior, while the two other observed members are D–types. Of
these last, 56968 has the highest spectral slope not only inside the family
(15.86 %/103Å) but also inside the whole L5 sample analyzed in this paper.
All the investigated Misenus members are quite faint and have diameters
of a few tens of kilometers. No clear size-slope relationship has been found
inside this family (Fig. 7).
No other data on the Misenus family members are available in the literature,
so we do not know if the large gap between the spectral slope of 56968 and
those of the other 5 investigated objects is real or it could be filled by other
members not yet observed. If real, 56968 can be an interloper inside the
family.
3.1.3 Panthoos
[FIGURE 3]
The Panthoos family has 59 members for a relative velocity cutoff of 150
m/s. We obtained new spectroscopic and photometric data of 5 members:
4829 Sergestus, 30698 Hippokoon, 31821 1999 RK225, 76804 2000 QE and
111113 2001 VK85 (Fig. 3). Three objects presented by Fornasier et al.
(2004a) as belonging to the Astyanax family (23694 1997 KZ3, 32430 2000
RQ83, 30698 Hippokoon) and one to the background population (24444 2000
OP32) are now included among the members of the Panthoos family. Peri-
odic updates of the proper elements can change the family membership. In
particular the Astyanax group disappeared in the latest revision of dynami-
cal families, and its members are now in the Panthoos family within a cutoff
of 150m/s. The Panthos family survives also a cutoff of 120 m/s, with 7
members, and 90 m/s, with 6 members.
We observed 30698 Hippokoon during two different runs (on 9 Nov. 2002
and on 18 Jan. 2005), and both spectral slopes and colors are in agreement
inside the error bars (see Table 3, Table 4, and Fornasier et al., 2004a). No
other data on the Panthoos family are available in the literature.
The analysis of the 8 members (for 24444 only photometry is available)
show featureless spectra with slopes that seem to slightly increase as the
asteroid size decreases (Table 4 and Fig. 7). However, all the members have
dimensions very similar within the uncertainties, making it difficult for any
slope-size relationship to be studied. The largest member, 4829 Sergestus, is
a PD–type with a slope of about 5 %/103Å, while all the other investigated
members are D–types.
3.1.4 Cloantus
[FIGURE 4]
We observed only 2 out of 8 members of the Cloantus family (5511 Cloan-
thus and 51359 2000 SC17, see Fig. 4) as grouped at a cutoff corresponding
to relative velocities of 150 m/s. This family survives at a stringent cutoff
and 3 members (including the two that we observed) also survive for relative
velocities of 60 m/s. Both of the observed objects are D–types with very
similar, featureless, reddish spectra (Table 4 and Fig. 7). 5511 Cloanthus
was observed also by Bendjoya et al. (2004), who found a slope of 13.0±0.1
%/103Å in the 5000-7500 Å wavelength range, while we measure a value of
10.84±0.15 %/103Å. Our spectrum has a higher S/N ratio than the spectrum
by Bendjoya et al. (2004), and it is perfectly matched by our measured color
indices that confirm the spectral slope. This difference cannot be caused by
the slightly different spectral ranges used to measure the slope, but could
possibly be due to heterogeneous surface composition.
3.1.5 Phereclos
The Phereclos family comprises 15 members at a cutoff of 150 m/s. The
family survives with 8 members also at a cutoff of 120m/s. We obtained
spectroscopic and photometric data of 3 members (9030 1989 UX5, 11488
1988 RM11 and 31820 1999 RT186, see Fig 4), that, together with the 4
spectra (2357 Phereclos, 6998 Tithonus, 9430 1996 HU10, 18940 2000QV49)
already presented by Fornasier et al. (2004a), allow us to investigate about
half of the Phereclos family population defined at a cutoff of 150m/s. The
spectral slope of these objects, all classified as D–type except one PD–type
(11488), varies from 5.3 to 11.3 %/103Å (Table 4). The size of the fam-
ily members ranges from about 20 km in diameter for 31820 to 95 km for
2357, but we do not observe any clear slope-diameter relationship (Fig. 7 and
Table 4).
3.1.6 Sarpedon
We obtained new spectroscopic and photometric data of 2 members of the
Sarpedon family (48252 2001 TL212 and 84709 2002 VW120), whose spectra
and magnitude color indices are reported in Fig. 4 and Table 4. Including
the previous observations (Fornasier et al., 2004a) of 4 other members (2223
Sarpedon, 5130 Ilioneus, 17416 1988 RR10, and 25347 1999 RQ116), we have
measurements of 6 of the 21 members of this family dynamically defined at a
cutoff of 150 m/s. All the 6 aforementioned objects, except 25347, constitute
a robust clustering which survives up to 90 m/s with 9 members. The cluster
which contains (2223) Sarpedon was also recognized as a family by Milani
(1993).
All the 6 investigated members have very similar colors (see Table 3) and
spectral behavior. The spectral slope (Fig. 7) varies over a very restricted
range, from 9.6 to 11.6 %/103Å (Table 4), despite a significant variation of
the estimated size (from the 18 km of 17416 to the 105 km of 2223). Con-
sequently, the surface composition of the Sarpedon family members appears
to be very homogeneous.
3.2 Dynamical families: L4 swarm
3.2.1 Eurybates
[FIGURE 5]
Eurybates family members were observed in May 2004. The selection of
the targets was made on the basis of a very stringent cutoff, corresponding
to relative velocities of 70 m/s, that gives a family population of 28 objects.
We observed 17 of these members (see Table 2) that constitute a very robust
clustering in the space of the proper elements: all the members we studied,
except 2002 CT22, survive at a cutoff of 40 m/s.
The spectral behavior of these objects (Fig. 5) is quite homogeneous with
10 asteroids classified as C–type and 7 as P–type. The spectral slopes (Ta-
ble 5) range from neutral to moderately red (from -0.5 to 4.6 %/103Å). The
slopes of six members are close to zero (3 slightly negative) with solar-like col-
ors. The asteroids 18060, 24380, 24420, and 39285, all classified as C–types,
clearly show a drop off of reflectance for wavelength shorter than 5000–5200
Å. The presence of the same feature in the spectra of 2 other members (1996
RD29 and 28958) is less certain due to the lower S/N ratio. This absorp-
tion is commonly seen on main belt C–type asteroids (Vilas 1994; Fornasier
et al. 1999), where is due to the intervalence charge transfer transitions
(IVCT) in oxidized iron, and is often coupled with other visible absorption
features related to the presence of aqueous alteration products (e.g. phyl-
losilicates, oxides, etc). These IVCTs comprise multiple absorptions that are
not uniquely indicative of phyllosilicates, but are present in the spectrum
of any object containing Fe2+ and Fe3+ in its surface material (Vilas 1994).
Since no other phyllosilicate absorption features are present in the C-type
spectra of the Eurybates family, there is no evidence that aqueous alteration
processes occurred on the surface of these bodies.
In Fig. 8 we show the spectral slopes versus the estimated diameters for
the Eurybates family members. All the observed objects, except the largest
member (3548) that has a diameter of about 70 km and exhibit a neutral (∼
solar-like) spectral slope, are smaller than ∼ 40 km and present both neutral
and moderately red colors. The spectral slopes are strongly clustered around
S = 2%/103Å, with higher S values restricted to smaller objects (D< 25
3.2.2 1986 WD
[FIGURE 6]
We investigated 6 out of 17 members of the 4035 1986 WD family that is
dynamically defined at a cutoff of 130 m/s (Fig. 6 and Table 2). Three of our
targets (4035, 6545 and 11351) were already observed by Dotto et al. (2006):
for 6545 and 11351 there is a good consistency between our spectra and
those already published. 4035 was observed also by Bendjoya et al. (2004):
all the spectra are featureless, but Bendjoya et al. (2004) obtain a slope of
8.8 %/103Å, comparable to the one here presented, while Dotto et al. (2006)
found a higher value (see Table 5). This could be interpreted as due to the
different rotational phases seen in the three observations, and could indicate
some inhomogeneities on the surface of 4035.
The observed family members show heterogeneous behaviors (Fig. 8),
with spectral slopes ranging from neutral values for the smaller members
(24341 and 14707) to reddish ones for the 3 members with size bigger than
50 km (4035, 6545, and 11351). For this family, it seems that a size-slope
relationship exists, with smaller members having solar colors and spectral
slopes increasing with the object’ sizes.
3.2.3 1986 TS6
The 1986 TS6 family includes 20 objects at a cut-off of 100 m/s. We present
new spectroscopy and photometry of a single member, 12921 1998 WZ5
(Fig. 6). The spectrum we present here is flat and featureless, with a spectral
slope of 4.6±0.8%/103Å. Dotto et al. (2006) presented a spectrum obtained
a month after our data (in May 2003) that has a very similar spectral slope
3.7± 0.8%/103Å. Previously, 12917 1998 TG16, 13463 Antiphos, 12921 1998
WZ5, 15535 2000 AT177, 20738 1999 XG191, and 24390 2000 AD177 were
included in the Makhoan family. Refined proper elements now place all of
these bodies in the 1986 TS6 family.
In Fig. 8 we report the spectral slopes vs. estimated diameters of the
6 observed members. The family shows different spectral slopes with the
presence of both P–type (12921 and 13463) and D–type asteroids (12917,
15535, 20738, and 24390). Due to the very similar diameters, a slope-size
relationship is not found.
[FIGURE 7 AND 8]
4 Discussion
The spectra of Jupiter Trojan members of dynamical families show a range
of spectral variation from C– to D–type asteroids. With the exception of the
L4 Eurybates family, all the observed objects have featureless spectra, and
we cannot find any spectral bands which could help in the identification of
minerals present on their surfaces. The lack of detection of any mineralogy
diagnostic feature might indicate the formation of a thick mantle on the Tro-
jan surfaces. Such a mantle could be formed by a phase of cometary activity
and/or by space weathering processes as demonstrated by laboratory exper-
iments on originally icy surfaces (Moore et al., 1983; Thompson et al., 1987;
Strazzulla et al., 1998; Hudson & Moore, 1999).
A peculiar case is constituted by the Eurybates family, which shows a pre-
ponderance of C–type objects and a total absence of D–types. Moreover,
this is the only family in which some members exhibit spectral features at
wavelengths shorter than 5000–5200 Å, most likely due to the intervalence
charge transitions in materials containing oxidized iron (Vilas 1994).
4.1 Size vs spectral slope distribution:
Individual families
The plots of spectral slopes vs. diameters are shown in Fig. 7 and 8. A
relationship between spectral slopes and diameters seems to exist for only
three of the nine families we studied. In the Anchises and Panthoos families,
smaller objects have redder spectra, while for the 1986 WD family larger
objects have the redder spectra.
Moroz et al. (2004) have shown that ion irradiation on natural complex
hydrocarbons gradually neutralizes the spectral slopes of these red organic
solids. If the process studied by Moroz et al. (2004) occurred on the surface of
Jupiter Trojans, the objects having redder spectra have to be younger than
those characterized by bluish-neutral spectra. In this scenario the largest
and spectrally reddest objects of the 1986 WD family could come from the
interior of the parent body and expose fresh material. In the case of the
Anchises and Panthoos families the spectrally reddest members, being the
smallest, could come from the interior of the parent body, or alternatively
could be produced by more recent secondary fragmentations. In particular,
small family members may be more easily resurfaced, as significant collisions
(an impactor having a size greater than a few percent of the target), as well
as seismic shaking and recoating by fresh dust, may occur frequently at small
sizes.
[FIGURE 9]
4.2 Size vs slope distribution:
The Trojan population as a whole
[TABLE 6]
As compared to the data available in literature, our sample strongly con-
tributed to the analysis of fainter and smaller Trojans, with estimated di-
ameters smaller than 50 km. Jewitt & Luu (1990), analyzing a sample of
32 Trojans, found that the smaller objects were redder than the bigger ones.
However, our data play against the existence of a possible color-dimension
trend. In fact, the spectral slope’s range of the objects smaller than 50 km
is similar to that of the larger Trojans, as shown in Fig. 9.
The Eurybates family strongly contributes to the population of small
spectrally neutral objects, filling the region of bodies with mean diameter
D<40 km and with spectral slopes smaller than 3 %/103Å.
In order to carry out a complete analysis of the spectroscopic and pho-
tometric characteristics of the whole available data set on Jupiter Trojans,
we considered all the visible spectra published in the literature: Jewitt &
Luu (1990, 32 objects), Fitzimmons et al. (1994, 3 objects), Bendjoya et
al. (2004, 34 objects), Fornasier et al. (2004a, 26 L5 objects), and Dotto et
al. (2006, 24 L4 Trojans). We also add several Trojans spectra (11 L4 and
3 L5 Trojans) from the files available on line (Planetary data System archive,
pdssbn.astro.umd.edu, and www.daf.on.br/∼lazzaro/S3OS2-Pub/s3os2.htm)
from the SMASS I, SMASS II and S3OS2 surveys (Xu et al., 1995; Bus &
Binzel, 2003; Lazzaro et al., 2004). Including all these data, we compile a
sample of 142 different Trojans, 68 belonging to the L5 cloud and 74 belong-
ing to the L4. We performed the taxonomic classification of this enlarged
sample, on the basis of the Dahlgren and Lagerkvist (1995) scheme, by ana-
lyzing spectral slopes computed in the range 5500-8000 Å. Different authors,
of course, considered different spectral ranges for their own slope gradient
evaluations: Jewitt & Luu (1990) and Fitzimmons et al. (1994) use the
4000-7400 Å and Bendjoya et al. (2004, Table 2) used a slightly different
ranges around 5200-7500 Å. Since all the cited papers show spectra with lin-
ear featureless trends, the different wavelength ranges used for the spectral
gradient computation by Bendjoya et al. (2004) and Jewitt & Luu (1990)
are not expected to influence the obtained slopes.
In order to search for a dependency of the spectral slope distribution with
the size of the objects, all observations (from this paper as well as from the
literature) were combined. The objects were isolated in 5 size bins (smaller
than 25 km, 25–50 km, 50–75 km, 75-100 km and larger than 100 km). Each
bin contains between 20 and 50 objects. These subsamples are large enough
to be compared using classical statistical tests: the t-test, which estimates
if the mean values are compatible, the f-test, which checks if the widths of
the distributions are compatible (even if they have different means), and
the KS test, which compares directly the full distributions. A probability is
computed for each test; a small probability indicates that the tested distri-
butions are not compatible, i.e. the objects are not randomly extracted from
the same population, while a large probability value has no meaning (i.e. it
is not possible to assure that both samples come from the same population,
we can just say in that case that they are not incompatible). In order to
quantify the probability levels that we consider as significant, the same tests
were run on randomized distributions (see Hainaut & Delsanti 2002 for the
method). Since probability lower than 0.04-0.05 does not appear in these
randomized distributions, we consider that values smaller than 0.05 indicate
a significant incompatibility.
Each sub-sample was compared with the four others – the results are sum-
marized in Table 6. The average slope of the 5 bins are all compatible among
each other. The only marginally significant result is that the width of the
slope distribution among the larger objects (diam. > 100 km) is narrower
than that of all the smaller objects.
This narrower color distribution could be due to the aging processes affect-
ing the surface of bigger objects, which are supposed to be older. The wider
color distribution of small members is possibly related to the different ages
of their surfaces: some of them could be quite old, while some other could
have been recently refreshed.
4.3 Spectral slopes and L4/L5 Clouds
[HERE FIGURES 10 AND 11]
Considering only the Trojan observations reported in this paper, the aver-
age slope is 8.84±3.03%/103Å for the L5 population, and 4.57±4.01%/103Å
for the L4.
Considering now all the spectra available in the literature, the 68 L5
Trojans have an average slope of 9.15±4.19%/103Å, and the 78 L4 objects,
6.10±4.48%/103Å. Performing the same statistical tests as above, it appears
that these two populations are significantly different. In particular, the av-
erage slopes are incompatible at the 10−5 level.
Nevertheless, as described in Section 3.2.1, the Eurybates family members
have quite different spectral characteristics than the other objects and con-
stitute a large subset of the whole sample. Indeed, comparing their distribu-
tion with the whole populations, they are found significantly different at the
10−10 level. In other words, the Eurybates family members do not constitute
a random subset of the other Trojans.
Once excluded the Eurybates family, the remaining 61 Trojans from the
L4 swarm have an average slope of 7.33±4.24%/103Å. The very slight dif-
ference of average slope between the L5 and remaining L4 objects is very
marginally significant (probability of 1.6%), and the shape and width of the
slope distributions are compatible with each other.
The taxonomic classification we have performed shows that the majority
(73.5%) of the observed L5 Trojans (Fig. 10) are D–type (slope > 7 %/103
Å) with featureless reddish spectra, 11.8% are DP/PD –type (slope between
5 and 7 %/103 Å), 10.3% are P–type, and only 3 objects are classified as
C–type (4.4%).
In the L4 swarm (Fig. 11), even though the D–type still dominate the
population (48.6%), the spectral types are more heterogeneous as compared
to the L5 cloud, with a higher percentage of neutral-bluish objects: 20.3%
are P–type, 8.1% are DP/PD-type, 12.2% are C–type, and 10.8% of the
bodies have negative spectral slope. The higher percentage of C– and P–
type as compared to the L5 swarm is strongly associated with the presence
of the very peculiar Eurybates family. Among 17 observed members 10 are
classified as C–types (among which 3 have negative spectral slopes) and 7 are
P–types. Considering the 57 asteroids that compose the L4 cloud without
the Eurybates family, we find percentages of P, and PD/DP –types very
similar to those of the L5 cloud (14.0% and 10.5% respectively), a smaller
percentage of D–types (63.2%) and of the C–types (3.5%), and the presence
of a 8.8% Trojans with negative spectral slopes.
The visible spectra of the Eurybates members are very similar to those of
C–type main belt asteroids, Chiron-like Centaurs, and cometary nuclei. This
similarity is compatible with three different scenarios: the family could have
been produced by the fragmentation of a parent body very different from
all the other Jupiter Trojans (in which case the origin of such a peculiar
parent must still be assessed); this could be a very old family where space
weathering processes have covered any differences in composition among the
family members and flattened all the spectra; this could be a young family
where space weathering processes occurred within time scales smaller than
the age of the family. In the last two cases the Eurybates family would give
the first observational evidence of spectra flattened owing to space weathering
processes. This would then imply, according to the results of Moroz et al.
(2004), that its primordial composition was rich in complex hydrocarbons.
The knowledge of the age of the Eurybates family is therefore a fundamental
step to investigate the nature and the origin of the parent body, and to assess
the effect of space weathering processes on the surfaces of its members.
The present sample of Jupiter Trojans suggests a more heterogeneous
composition of the L4 swarm as compared to the L5 one. As previously
noted by Bendjoya et al. (2004), the L4 swarm contains a higher percentage
of C– and P–type objects. This result is enhanced by members of the Eu-
rybates family, but remains even when these family members are excluded.
Moreover, the dynamical families belonging to the L4 cloud are more robust
than those of the L5 one, surviving with densely populated clustering even
at low relative velocity cut-off. We therefore could argue that the L4 cloud
is more collisionally active than the L5 swarm. Nevertheless, we still cannot
intepret this in terms of the composition of the two populations, since we
cannot exclude that as yet unobserved C– and P–type families are present
in the L5 cloud.
4.4 Orbital Elements
[HERE FIGURE 12 and TABLES 7 and 8 ]
We analyzed the spectral slope as a function of the Trojans’ orbital el-
ements. As an illustration, Fig. 12 shows the B − R color distribution as
a function of the orbital elements. In order to investigate variations with
orbital parameters, the Trojan population is divided in 2 sub samples: those
with the considered orbital element lower than the median value, and those
with the orbital element higher than the median (by construction, the two
subsamples have the same size). Taking a as an example, half the Trojans
have a < 5.21AU, and half have a larger than this value.
The mean color, the color dispersion, and the color distribution of the
2 subsamples are compared using the three statistical tests mentioned in
Section 4.2. The method is discussed in details in Hainaut & Delsanti (2002).
The tests are repeated for all color and spectral slope distributions. The
results are the following.
• q, perihelion distance: the color distribution of the Trojans with small
q is marginally broader than that of Trojans with larger q. This result
is not very strong (5%), and is dominated by the red-end of the visible
wavelength. Removing the Eurybates from the sample maintains the
result, at the same weak level.
• e, eccentricity: the distribution shows a similar result, also at the weak
5% significance. The objects with larger e have broader color distribu-
tion than those with lower e. This result is entirely dominated by the
Eurybates’ contribution.
• i, inclination: objects with smaller inclination are significantly bluer
than those with larger i. This result is observed at all wavelengths. It
is worth noting that this is contrary to what is usually observed on
other Minor Bodies in the Outer Solar System survey (MBOSSes),
where objects with high i, or more generally, high excitation E =√
e2 + sin2 i, are bluer (Hainaut & Delsanti, 2002; Doressoundiram et
al., 2005). This can also be visually appreciated in Fig. 12. This result
is also completely dominated by the Eurybates’ contribution. The non-
Eurybates Trojans do not display this trend.
• E =
e2 + sin2 i, orbital excitation: the objects with small E are also
significantly bluer than those with high E. This result is also com-
pletely dominated by the Eurybates’ contribution. The non-Eurybates
Trojans do not display this trend.
In summary this analysis shows that the Eurybates sub-sample of the
Trojans is well separated in orbital elements and in colors.
For the other Minor Bodies in the outer Solar System, the relation be-
tween color and inclination–orbital excitation (objects with a higher orbital
excitation tend to be bluer) is interpreted as a relation between excitation
and surface aging/rejuvenating processes (Doressoudiram et al., 2005). The
Eurybates family has low excitation and neutral-blue colors, suggesting that
the aging/rejuvenating processes affecting them are different from the other
objects. This could be due to different surface compositions, different irradi-
ation processes, or different collisional properties – which would be natural
for a collisional family.
5 Comparison with other outer Solar System
minor bodied
5.1 Introduction and methods
[HERE FIGURES 13 AND 14]
The statistical tests set described in section 4.2 has also been applied
to compare the colors and the spectral slopes distribution of the Trojans
with those of the other minor bodies in the outer Solar System taken from
the updated, on-line version of the Hainaut & Delsanti (2002) database.
Figure 13, as an example, displays the (R-I) vs (V-R) diagrams, while Fig. 14
shows the (B-V) and (V-R) color distributions, as well as the spectral slope
distribution of the different classes of objects. The tests were performed
on all the color indices derived from filters in the visible (UBVRI) and near
infrared range (JHK) but in Table 7 and 8 we summarize the most significant
results.
In order to “calibrate” the significant probabilities, additional artificial
classes are also compared: first, the objects which have an even internal
number in the database with the odd ones. As this internal number is purely
arbitrary, both classes are statistically indistinguishable. The other tested
pair is the objects with a “1999” designation versus the others. Again, this
selection criterion is arbitrary, so the pseudo-classes it generates are sub-
sample of the total population, and should be indistinguishable. However, as
many more objects have been discovered in all the other years than during
that specific year, the size of these sub-samples are very different. This
permits us to estimate the sensitivity of the tests on sample of very different
sizes. Some of the tests found the arbitrary populations incompatible at the
5% level, so we use 0.5% as a conservative threshold for statistical significance
of the distribution incompatibility
5.2 Results
Table 7 and Fig. 14 clearly show that the Trojans’ colors distribution is
different as compare to that of Centaurs, TNOs and comets. Trojans are at
the same time bluer, and their distribution is narrower than all the other
populations. Using the statistical tests (see Table 8), we can confirm the
significance of these results.
• The average colors of the Trojans are significantly different from those
of all the other classes of objects (t-test), with the notable exception of
the short period comet nuclei. Refining the test to the Eurybates/non-
Eurybates, it appears that the Eurybates have marginally different
mean colors, while the non-Eurybates average colors are indistinguish-
able from those of the comets.
• Considering the full shape of the distribution (KS test), we obtain the
same results: the Trojans colors distributions are significantly differ-
ent from those of all the other classes, with the exception of the SP
comets, which are compatible. Again, this result becomes stronger
separating the Eurybates: their distributions are different from those
of the comets, while the non-Eurybates ones are indistinguishable.
• The results when considering the widths of the color distributions (f-
test) are slightly different. Classes of objects with different mean colors
could still have the same distribution width. This could suggest that a
similar process (causing the width of the distribution) is in action, but
reached a different equilibrium point (resulting in different mean val-
ues). This time, all classes are incompatible with the Trojans, including
the comets, with strong statistical significance.
In order to further explore possible similarities between Trojans and other
classes, the comparisons were also performed with the neutral Centaurs.
These were selected with S < 20%/103Å); this cut-off line falls in the gap
between the ”neutral” and ”red” Centaurs (Peixinho et al., 2003, Fornasier
et al., 2004b).
The t-Test (mean color) only reveals a very moderate incompatibility be-
tween the Trojans and neutral Centaurs, at the 5% level, i.e. only marginally
significant. On the other hand, the f-Test gives some strong incompatibilities
in various colors (moderate in B− V and H −K, very strong in R− I), but
the two populations are compatible for most of the other colors. Similarly,
only the R − I KS-test reveals a strong incompatibility. It should also be
noted that only 18 neutral Centaurs are known in the database. In summary,
while the Trojans and neutral Centaurs have fairly similar mean colors, their
color distributions are also different.
6 Conclusions
From 2002, we carried out a spectroscopic and photometric survey of Jupiter
Trojans, with the aim of investigating the members of dynamical families.
In this paper we present new data on 47 objects belonging to several dy-
namical families: Anchises (5 members), Cloanthus (2 members), Misenus (6
members), Phereclos (3 members), Sarpedon (2 members) and Panthoos (5
members) from the L5 swarm; Eurybates (17 members), 1986 WD (6 mem-
bers), and Menelaus (1 member) for the L4 swarm. Together with the data
already published by Fornasier et al. (2004a) and Dotto et al. (2006), taken
within the same observing program, we have a total sample of 80 Trojans,
the largest homogeneous data set available to date on these primitive aster-
oids. The main results coming from the observations presented here, and
from the analysis including previously published visible spectra of Trojans,
are the following:
• Trojans’ visible spectra are mostly featureless. However, some mem-
bers of the Eurybates family show a UV drop-off in reflectivity for
wavelength shorter than 5000–5200 Å that is possibly due to interva-
lence charge transfer transitions (IVCT) in oxidized iron.
• The L4 Eurybates family strongly differs from all the other families
in that it is dominated by C– and P–type asteroids. Also its spectral
slope distribution is significantly different when compared to that of
the other Trojans (at the 10−10 level).
This family is very peculiar and is dynamically very strong, as it sur-
vives also at a very stringent cutoff (40 m/s). Further observations in
the near-infrared region are strongly encouraged to look for possible
absorption features due to water ice or to material that experienced
aqueous alteration.
• The average spectral slope for the L5 Trojans is 9.15±4.19%/103Å, and
6.10±4.48%/103Å for the L4 objects. Excluding the Eurybates, the L4
average slope values becomes 7.33±4.24%/103Å. The slope distribu-
tions of the L5 and of the non-Eurybates L4 are indistinguishable.
• Both L4 and L5 clouds are dominated by D–type asteroids, but the L4
swarm has an higher presence of C– and P–type asteroids, even when
the Eurybates family is excluded, and appears more heterogeneous in
composition as compared to the L5 one.
• We do not find any size versus spectral slope relationship inside the
whole Trojans population.
• The Trojans with higher orbital inclination are significantly redder than
those with lower i. While this trend is the opposite of that observed
for other distant minor bodies, this effect is entirely dominated by the
Eurybates family.
• Comparing the Trojans colors with those of other distant minor bod-
ies, they are the bluest of all classes, and their colors distribution is the
narrowest. This difference is mostly due to the Eurybates family. In
fact, if we consider only the Trojan population without the Eurybates
members, their average colors and overall distributions are not distin-
guishable from that of the short period comets. However, the widths
of their color distributions are not compatible. The similarity in the
overall color distributions might be caused by the small size of the short
period comet sample rather than by a physical analogy. The Trojans
average colors are also fairly similar to those of the neutral Centaurs,
but the overall distributions are not compatible.
After this study, we have to conclude that Trojans have peculiar charac-
teristics very different from those of all the other populations of the outer
Solar System.
Unfortunately, we still cannot assess if this is due to differences in the physi-
cal nature, or in the aging/rejuvenating processes which modified the surface
materials in different way at different solar distances. Further observations,
mainly in V+NIR spectroscopy and polarimetry, are absolutely needed to
better investigate the nature of Jupiter Trojans and to definitively assess if a
genetical link might exist with Trans-Neptunian Objects, Centaurs and short
period comets.
Acknowledgments
We thank Beaugé and Roig for kindly providing us with updated Trojan
family list, and R.P. Binzel and J.P. Emery for their useful comments in the
reviewing process.
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Tables
Table 1: Observing conditions of the investigated L5 asteroids. For each
object we report the observational date and universal time, total exposure
time, number of acquisitions with exposure time of each acquisition, airmass,
and the observed solar analogs with their airmass.
Obj Date UT Texp (s) nexp air. Solar An. (air.)
Anchises
1173 17 Jan 05 06:06 60 1×60s 1.42 HD76151 (1.48)
23549 17 Jan 05 07:20 480 2×240s 1.60 HD76151 (1.48)
24452 17 Jan 05 07:54 960 4×240s 1.44 HD76151 (1.48)
47967 17 Jan 05 05:34 800 2×400s 1.38 HD76151 (1.48)
2001 SB173 17 Jan 05 06:28 1200 2×600s 1.35 HD76151 (1.48)
Cloanthus
5511 19 Jan 05 06:04 960 4×240s 1.26 HD76151 (1.12)
51359 19 Jan 05 04:13 660 1×660s 1.36 HD76151 (1.12)
Misenus
11663 17 Jan 05 05:13 400 1×400s 1.21 HD44594 (1.12)
32794 18 Jan 05 03:13 1800 2×900s 1.39 HD28099 (1.44)
56968 17 Jan 05 04:31 400 2×400s 1.21 HD44594 (1.12)
1988 RE12 18 Jan 05 04:12 2000 2×1000s 1.31 HD28099 (1.44)
2000 SC51 18 Jan 05 06:09 1320 2×660s 1.16 HD44594 (1.17)
2001 UY123 18 Jan 05 06:46 1320 2×660s 1.32 HD44594 (1.17)
Phereclos
9030 18 Jan 05 08:19 1000 1×1000s 1.37 HD44594 (1.17)
11488 19 Jan 05 03:31 1320 2×660s 1.99 HD76151 (1.12)
31820 19 Jan 05 07:02 1320 2×660s 1.35 HD76151 (1.11)
Sarpedon
48252 18 Jan 05 02:32 1320 2×660s 1.30 HD28099 (1.44)
84709 19 Jan 05 05:35 1320 2×660s 1.34 HD76151 (1.12)
Panthoos
4829 17 Jan 05 08:37 720 3×240s 1.45 HD76151 (1.48)
30698 18 Jan 05 01:54 1320 2×660s 1.73 HD28099 (1.44)
31821 18 Jan 05 05:27 1320 2×660s 1.35 HD28099 (1.44)
76804 17 Jan 05 03:35 1800 3×600s 1.38 HD44594 (1.12)
2001 VK85 18 Jan 05 07:31 2000 2×1000s 1.23 HD44594 (1.17)
Table 2: Observing conditions of the investigated L4 asteroids. For each
object we report the observational date and universal time, total exposure
time, number of acquisitions with exposure time of each acquisition, airmass,
and the observed solar analogs with their airmass.
Obj Date UT Texp (s) nexp air. Solar An. (air.)
Eurybates
3548 25 May 04 05:14 600 2×300s 1.02 SA107-684 (1.19)
9818 26 May 04 00:13 780 1×780s 1.19 SA102-1081(1.15)
13862 25 May 04 03:35 1200 2×600s 1.09 SA107-998 (1.15)
18060 25 May 04 02:47 1500 2×750s 1.07 SA107-998 (1.15)
24380 25 May 04 06:53 780 1×780s 1.18 SA107-684 (1.19)
24420 25 May 04 08:49 900 1×900s 1.59 SA112-1333 (1.17)
24426 26 May 04 00:13 1440 2×720s 1.13 SA107-684 (1.17)
28958 26 May 04 07:14 1800 2×900s 1.35 SA107-684 (1.17)
39285 25 May 04 05:40 2700 3×900s 1.09 SA107-684 (1.19)
43212 25 May 04 07:39 2340 3×780s 1.39 SA110-361 (1.15)
53469 25 May 04 02:05 1800 2×900s 1.04 SA107-998 (1.15)
65150 26 May 04 01:59 3600 4×900s 1.07 SA102-1081 (1.20)
65225 26 May 04 03:40 3600 4×900s 1.04 SA107-684 (1.17)
1996RD29 26 May 04 05:12 2700 3×900s 1.10 SA107-684 (1.17)
2000AT44 25 May 04 04:14 1800 2×900s 1.04 SA107-684 (1.19)
2002CT22 26 May 04 00:49 2400 4×600s 1.08 SA102-1081 (1.15)
2002EN68 26 May 04 08:10 1800 2×900s 1.62 SA107-684 (1.17)
1986 WD
4035 10 Apr 03 03:28 600 1×600s 1.09 SA107-684 (1.15)
6545 10 Apr 03 02:39 900 1×900s 1.16 SA107-684 (1.15)
11351 10 Apr 03 09:21 900 1×900s 1.28 SA107-684 (1.15)
14707 11 Apr 03 08:11 1200 1×1200s 1.15 SA107-684 (1.15)
24233 11 Apr 03 02:29 1200 1×1200s 1.39 SA107-684 (1.37)
24341 11 Apr 03 05:47 900 1×900s 1.16 SA107-684 (1.17)
1986 TS6
12921 10 Apr 03 07:33 900 1×900s 1.39 SA107-684 (1.40)
Table 3: Visible photometric observations of L4 and L5 Trojans (ESO-NTT
EMMI): for each object, date, computed V magnitude, B-V, V-R and V-
I colors are reported. The given UT is for the V filter acquisition. The
observing photometric sequence (V-R-B-I) took a few minutes.
Object date UT V B-V V-R V-I
1986 WD
4035 10 Apr 03 03:11 16.892±0.031 0.752±0.040 0.473±0.042 0.926±0.055
4035 10 Apr 03 04:22 16.981±0.031 0.752±0.040 0.495±0.042 0.945±0.055
6545 10 Apr 03 02:22 17.558±0.031 0.734±0.041 0.499±0.042 0.935±0.055
11351 10 Apr 03 09:03 18.407±0.032 0.739±0.044 0.498±0.044 0.900±0.057
14707 11 Apr 03 06:46 18.666±0.031 0.751±0.041 0.401±0.033 0.804±0.055
14707 11 Apr 03 08:37 18.873±0.031 0.754±0.041 0.424±0.033 0.790±0.056
24233 11 Apr 03 01:33 18.894±0.034 0.704±0.051 0.481±0.037 0.899±0.058
24341 11 Apr 03 05:05 19.376±0.032 0.713±0.043 0.369±0.035 0.759±0.057
1986 TS6
12921 10 Apr 03 07:12 18.393±0.031 0.673±0.040 0.421±0.042 0.786±0.055
L5 cut off 150m/s
Anchises
1173 17 Jan 05 05:54 16.595±0.024 0.811±0.034 0.402±0.035 0.805±0.038
23549 17 Jan 05 07:09 18.969±0.050 0.800±0.071 0.485±0.068 0.872±0.075
24452 17 Jan 05 07:48 18.757±0.043 0.872±0.056 0.441±0.056 0.847±0.066
47967 17 Jan 05 05:27 19.382±0.044 0.899±0.058 0.489±0.069 0.965±0.075
2001 SB173 17 Jan 05 06:20 19.882±0.043 0.992±0.060 0.503±0.064 0.927±0.078
Cloanthus
5511 19 Jan 05 05:52 17.968±0.020 0.906±0.027 0.442±0.027 0.968±0.032
51359 19 Jan 05 03:54 19.631±0.102 0.864±0.201 0.447±0.131 0.885±0.164
Misenus
11663 17 Jan 05 05:05 18.473±0.022 0.837±0.030 0.409±0.030 0.872±0.039
32794 18 Jan 05 03:07 19.685±0.038 0.923±0.065 0.393±0.056 0.879±0.057
56968 17 Jan 05 04:18 18.596±0.026 0.986±0.040 0.494±0.033 1.003±0.036
1988 RE12 18 Jan 05 04:00 20.892±0.081 0.826±0.132 0.388±0.108 0.871±0.106
2000 SC51 18 Jan 05 06:03 19.876±0.038 1.016±0.055 0.444±0.059 0.896±0.056
2001 UY123 18 Jan 05 06:41 19.869±0.047 0.890±0.058 0.537±0.056 0.971±0.063
Phereclos
9030 18 Jan 05 08:14 18.397±0.020 0.887±0.024 0.493±0.027 0.973±0.028
11488 19 Jan 05 02:57 18.931±0.066 0.868±0.101 0.430±0.079 0.848±0.084
31820 19 Jan 05 06:39 20.041±0.077 0.889±0.093 0.520±0.091 0.916±0.123
Sarpedon
48252 18 Jan 05 02:25 19.878±0.060 0.949±0.100 0.467±0.093 0.903±0.090
84709 19 Jan 05 05:10 19.862±0.068 0.855±0.087 0.462±0.090 1.010±0.094
Panthoos
4829 17 Jan 05 08:18 18.430±0.029 0.851±0.050 0.420±0.039 0.792±0.052
30698 18 Jan 05 01:45 19.353±0.036 – 0.472±0.042 0.865±0.047
31821 18 Jan 05 05:21 19.328±0.076 0.980±0.111 0.440±0.097 0.901±0.108
76804 17 Jan 05 03:21 19.471±0.065 0.803±0.082 0.446±0.070 0.889± 0.080
2001 VK85 18 Jan 05 07:23 20.179±0.038 0.822±0.063 0.462±0.048 1.020±0.050
Table 4: L5 families. We report for each target the absolute magnitude H
and the estimated diameter (diameters marked by ∗ are taken from IRAS
data), the spectral slope S computed between 5500 and 8000 Å and the
taxonomic class (T) derived following Dahlgren & Lagerkvist (1995) classi-
fication scheme. The asteroids marked with a were observed by Fornasier
et al. (2004a), and their spectral slope values have been recomputed in the
5500-8000 Å wavelength range; asteroids 23694, 30698 and 32430, previously
Astyanax members, have been reassigned to the Panthoos family due to re-
fined proper elements.
Obj H D (km) S (%/103Å) T
Anchises
1173 8.99 ∗126+11
3.87±0.70 P
23549 12.04 26+4
8.49±0.88 D
24452 11.85 29+5
7.42±0.70 D
47967 12.15 25+4
9.21±0.78 D
2001 SB173 12.77 19+3
14.78±0.99 D
Cloanthus
5511 10.43 55+8
10.84±0.65 D
51359 12.25 24+6
12.63±1.30 D
Misenus
11663 10.95 44+7
6.91±0.70 DP
32794 12.77 19+3
6.59±0.88 DP
56968 11.72 30+5
15.86±0.71 D
1988 RE12 13.20 16+2
4.68±1.20 P
2000 SC51 12.69 20+3
6.54±0.98 DP
2001 UY123 12.75 19+3
8.28±0.88 D
Phereclos
a2357 8.86 ∗95+4
9.91±0.68 D
a6998 11.43 34+5
11.30±0.75 D
9030 11.14 40+6
10.35±0.76 D
a9430 11.47 35+5
10.02±0.90 D
11488 11.82 29+5
5.37±0.92 PD
a18940 11.81 29+4
7.13±0.75 D
31820 12.63 20+3
7.53±0.80 D
Sarpedon
a2223 9.25 ∗95+4
10.20±0.65 D
a5130 9.85 71+11
10.45±0.65 D
a17416 12.83 18+3
10.80±0.90 D
a25347 11.59 33+5
10.11±0.83 D
48252 12.84 18+3
9.62±0.82 D
84709 12.70 19+3
11.64±0.84 D
Panthoos
4829 11.16 39+6
5.03±0.70 PD
a23694 11.61 32+5
8.20±0.72 D
30698 12.14 25+4
8.23±1.00 D
a30698 12.27 25+4
9.08±0.82 D
a32430 12.23 25+4
8.12±1.00 D
31821 11.99 27+4
10.58±0.82 D
76804 12.16 25+4
7.29±0.71 D
2001 VK85 12.79 19+3
14.39±0.81 D
Table 5: L4 Families. We report for each target the absolute magnitude H
and the estimated diameter (diameters marked by ∗ are taken from IRAS
data, while absolute magnitudes marked by ∗∗ are taken from the astorb.dat
file of the Lowell Observatory), the spectral slope S computed between 5500
and 8000 Å, and the taxonomic class (T) derived following Dahlgren &
Lagerkvist (1995) classification scheme. The asteroids marked with a were
observed by Dotto et al. (2006), and their spectral slope values have been
recomputed in the 5500-8000 Å wavelength range.
Obj H D (km) S (%/103Å) T
Eurybates
3548 9.50∗∗ ∗72+4
-0.18±0.57 C
9818 11.00∗∗ 42+6
2.12±0.72 P
13862 11.10∗∗ 40+6
1.59±0.70 C
18060 11.10∗∗ 40+6
2.86±0.60 P
24380 11.20∗∗ 38+6
0.34±0.65 C
24420 11.50∗∗ 33+5
1.65±0.70 C
24426 12.50∗∗ 21+3
4.64±0.80 P
28958 12.10∗∗ 25+4
-0.04±0.80 C
39285 12.90∗∗ 17+3
0.25±0.69 C
43212 12.30∗∗ 23+4
1.19±0.78 C
53469 11.80∗∗ 29+4
0.17±0.80 C
65150 12.90∗∗ 17+3
4.14±0.70 P
65225 12.80∗∗ 18+3
0.97±0.85 C
1996RD29 13.06∗∗ 16+3
2.76±0.89 P
2000AT44 12.16∗∗ 24+3
-0.53±0.83 C
2002CT22 12.04∗∗ 26+4
2.76±0.73 P
2002EN68 12.30∗∗ 23+3
3.60±0.98 P
1986 WD
4035 9.72 ∗68+5
9.78±0.61 D
a4035 9.30∗∗ ∗68+5
15.19±0.61 D
6545 10.42 55+8
11.32±0.63 D
a6545 10.00∗∗ 66+10
9.88±0.56 D
11351 10.88 44+7
10.26±0.67 D
a11351 10.50∗∗ 53+8
10.44±0.61 D
14707 11.25 38+6
−9.4 -1.06±1.00 C
24233 11.58 33+5
−8.0 6.37±0.67 DP
24341 11.99 27+4
-0.26±0.71 C
1986 TS6
12917 11.61 32+5
10.98±0.68 D
12921 11.12 40+6
4.63±0.75 P
a12921 10.70∗∗ 48+7
3.74±1.00 P
13463 11.27 37+6
4.37±0.65 P
15535 10.70 48+7
10.67±0.65 D
20738 11.67 31+5
8.84±0.70 D
24390 11.80 29+5
9.53±0.62 D
Table 6: Results of the statistical analysis on the spectral slope distribution as a function of the diameters. For each
test bin, the average slope and the dispersion are listed; the size of the sample is reported in parenthesis. For each pair
of subsamples, the probability that both are randomly extracted from the same global sample is listed, as estimated
by the t-, f- and ks-test, respectively. Low probability indicates significant differences between the subsamples.
Diameter range 0–25 km 25–50 km 50–75 km 75–100 km >100 km
S average±σ 7.17±4.79 (22) 6.92±4.69 (48) 8.91±4.68 (26) 6.74±5.85 (21) 7.87±2.88 (21)
(%/103Å)
0–25 0.842 0.876 0.579 0.213 0.903 0.575 0.792 0.370 0.775 0.551 0.017 0.494
25–50 0.088 0.985 0.150 0.897 0.216 0.519 0.286 0.011 0.275
50–75 0.176 0.289 0.469 0.344 0.019 0.440
75–100 0.442 0.001 0.469
Table 7: Mean color indices and spectral slope of different classes of minor bodies of the outer Solar System. For each
class the number of objects considered is also listed.
Color Plutinos Cubewanos Centaurs Scattered Comets Trojans
B-V 36 87 29 33 2 74
0.895± 0.190 0.973± 0.174 0.886± 0.213 0.875± 0.159 0.795± 0.035 0.777± 0.091
V-R 38 96 30 34 19 80
0.568± 0.106 0.622± 0.126 0.573± 0.127 0.553± 0.132 0.441± 0.122 0.445± 0.048
V-I 34 64 25 25 7 80
1.095± 0.201 1.181± 0.237 1.104± 0.245 1.070± 0.220 0.935± 0.141 0.861± 0.090
V-J 10 14 11 8 1 12
2.151± 0.302 1.750± 0.456 1.904± 0.480 2.041± 0.391 1.630± 0.000 1.551± 0.120
V-H 3 7 11 4 1 12
2.698± 0.083 2.173± 0.796 2.388± 0.439 2.605± 0.335 1.990± 0.000 1.986± 0.177
V-K 2 5 9 2 1 12
2.763± 0.000 2.204± 1.020 2.412± 0.396 2.730± 0.099 2.130± 0.000 2.125± 0.206
R-I 34 64 25 26 8 80
0.536± 0.135 0.586± 0.148 0.548± 0.150 0.517± 0.102 0.451± 0.059 0.416± 0.057
J-H 11 17 21 11 1 12
0.403± 0.292 0.370± 0.297 0.396± 0.112 0.348± 0.127 0.360± 0.000 0.434± 0.064
H-K 10 16 20 10 1 12
-0.034± 0.171 0.084± 0.231 0.090± 0.142 0.091± 0.136 0.140± 0.000 0.139± 0.041
Slope 38 91 30 34 8 80
(%/103Å) 19.852± 10.944 25.603± 13.234 20.601± 13.323 18.365± 12.141 10.722± 6.634 7.241± 3.909
Table 8: Statistical tests performed to compare the color and slope distributions of different classes of minor bodies
(Plt= Plutinos, Resonant TNOs; QB1= Cubiwanos, Classical TNOs; Cent= Centaurs; Scat= scattered TNOs; Com=
Short Period Comet nuclei) with those of Trojans. The first five columns consider all the Trojans, the second five only
the Eurybates family, the third five only the non-Eurybates family Trojans. For each color, the first line shows the
number of objects used for the comparison (2nd is the number of Trojans), and the second line reports the probability
resulting from the test. A very low value indicates that the two compared distributions are not statistically compatible.
Probabilities are in boldface when the size of the samples is large enough for the value to be meaningful.
f-test
Color All Trojans Only Eurybates Only NON-Eurybates
Plt QB1 Cent Scat Com Plt QB1 Cent Scat Com Plt QB1 Cent Scat Com
B-V 36 74 83 74 29 74 33 74 2 74 36 14 83 14 29 14 33 14 2 14 36 60 83 60 29 60 33 60 2 60
0.000 0.000 0.000 0.000 0.600 0.001 0.001 0.000 0.005 0.722 0.000 0.000 0.000 0.000 0.598
V-R 38 80 92 80 30 80 34 80 19 80 38 17 92 17 30 17 34 17 19 17 38 63 92 63 30 63 34 63 19 63
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
R-I 34 80 62 80 25 80 26 80 8 80 34 17 62 17 25 17 26 17 8 17 34 63 62 63 25 63 26 63 8 63
0.000 0.000 0.000 0.000 0.773 0.000 0.000 0.000 0.001 0.335 0.000 0.000 0.000 0.000 0.185
Slope 38 80 87 80 30 80 34 80 8 80 38 17 87 17 30 17 34 17 8 17 38 63 87 63 30 63 34 63 8 63
0.000 0.000 0.000 0.000 0.020 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
t-test
Color All Trojans Only Eurybates Only NON-Eurybates
Plt QB1 Cent Scat Com Plt QB1 Cent Scat Com Plt QB1 Cent Scat Com
B-V 36 74 83 74 29 74 33 74 2 74 36 14 83 14 29 14 33 14 2 14 36 60 83 60 29 60 33 60 2 60
0.001 0.000 0.012 0.002 0.608 0.000 0.000 0.001 0.000 0.139 0.003 0.000 0.025 0.006 0.858
V-R 38 80 92 80 30 80 34 80 19 80 38 17 92 17 30 17 34 17 19 17 38 63 92 63 30 63 34 63 19 63
0.000 0.000 0.000 0.000 0.916 0.000 0.000 0.000 0.000 0.083 0.000 0.000 0.000 0.000 0.532
R-I 34 80 62 80 25 80 26 80 8 80 34 17 62 17 25 17 26 17 8 17 34 63 62 63 25 63 26 63 8 63
0.000 0.000 0.000 0.000 0.154 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.001 0.000 0.502
Slope 38 80 87 80 30 80 34 80 8 80 38 17 87 17 30 17 34 17 8 17 38 63 87 63 30 63 34 63 8 63
0.000 0.000 0.000 0.000 0.185 0.000 0.000 0.000 0.000 0.008 0.000 0.000 0.000 0.000 0.404
KS-test
Color All Trojans Only Eurybates Only NON-Eurybates
Plt QB1 Cent Scat Com Plt QB1 Cent Scat Com Plt QB1 Cent Scat Com
B-V 36 74 83 74 29 74 33 74 2 74 36 14 83 14 29 14 33 14 2 14 36 60 83 60 29 60 33 60 2 60
0.001 0.000 0.001 0.004 0.330 0.002 0.000 0.035 0.000 0.065 0.003 0.000 0.002 0.047 0.468
V-R 38 80 92 80 30 80 34 80 19 80 38 17 92 17 30 17 34 17 19 17 38 63 92 63 30 63 34 63 19 63
0.000 0.000 0.000 0.000 0.040 0.000 0.000 0.000 0.000 0.008 0.000 0.000 0.000 0.000 0.056
R-I 34 80 62 80 25 80 26 80 8 80 34 17 62 17 25 17 26 17 8 17 34 63 62 63 25 63 26 63 8 63
0.000 0.000 0.000 0.000 0.201 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.587
Slope 38 80 87 80 30 80 34 80 8 80 38 17 87 17 30 17 34 17 8 17 38 63 87 63 30 63 34 63 8 63
0.000 0.000 0.000 0.000 0.088 0.000 0.000 0.000 0.000 0.002 0.000 0.000 0.000 0.000 0.211
Figure captions
Fig. 1 - Reflectance spectra of 5 Anchises family members (L5 swarm).
The photometric color indices are also converted to relative reflectance and
overplotted on each spectrum. Spectra and photometry are shifted by 0.5 in
reflectance for clarity.
Fig. 2 - Reflectance spectra of 6 Misenus family members (L5 swarm).
The photometric color indices are also converted to relative reflectance and
overplotted on each spectrum. Spectra and photometry are shifted by 0.5 in
reflectance for clarity.
Fig. 3 - Reflectance spectra of 5 Panthoos family members (L5 swarm).
The photometric color indices are also converted to relative reflectance and
overplotted on each spectrum. Spectra and photometry are shifted by 0.5 in
reflectance for clarity. For asteroid 30698, the B-V color is missing as a B
filter measurement was not available.
Fig. 4 - Reflectance spectra of 2 Cloantus, 3 Phereclos and 2 Sarpedon
family members (L5 swarm). The photometric color indices are also con-
verted to relative reflectance and overplotted on each spectrum. Spectra and
photometry are shifted by 1.0 in reflectance for clarity.
Fig. 5 - Reflectance spectra of the 17 Eurybates family members (L4
swarm). Spectra are shifted by 0.5 in reflectance for clarity.
Fig. 6 - Reflectance spectra of the 6 1986 WD family members and 12921,
which is a member of the 1986 TS6 family (all belonging to the L4 swarm).
Spectra are shifted by 1.0 in reflectance for clarity.
Fig. 7 - Plot of the spectral slope versus the estimated diameter for the
families observed in the L5 swarm.
Fig. 8 - Plot of the spectral slope versus the estimated diameter for the
families observed in the L4 swarm.
Fig. 9 - Plot of the observed spectral slopes versus the estimated diameter
for the whole population of Jupiter Trojans investigated by us and available
from the literature. The errors on slopes and diameters are not plotted to
avoid confusion.
Fig. 10 - Histogram of L5 Trojans taxonomic classes.
Fig. 11 - Histogram of L4 Trojans taxonomic classes (Neg indicates ob-
jects with negative spectral slope).
Fig. 12 - Color distributions as functions of the absolute magnitude
M(1, 1), the inclination i [degrees], the orbital semi-major axis a [AU], the
perihelion distance q [AU], the eccentricity e, and the orbital energy E (see
text for definition). We include all the available colors for distant minor bod-
ies (TNOs, Centaurs, and cometary nuclei, see Hainaut & Delsanti 2002).
The Plutinos (resonant TNOs) are red filled triangles, Cubiwanos (classical
TNOs) are pink filled circles, Centaurs are green open triangles, Scattered
TNOs are blue open circles, and Trojans are cyan filled triangles.
Fig. 13 - V −R versus R−I color-color diagram for the observed Trojans
and all distant minor bodies available in the updated Hainaut & Delsanti
(2002) database. The solid symbols are for the Trojans (square for Eurby-
bates, triangles for others). The open symbols are used as following: tri-
angles for Plutinos, circles for Cubiwanos, squares for Centaurs, pentagons
for Scattered, and starry square for Comets. The continuous line represents
the ”reddening line”, that is the locus of objects with a linear reflectivity
spectrum. The star symbol represents the Sun.
Fig. 14 - Cumulative function and histograms of the B − V and V − R
color distributions and of the spectral slope for all the considered classes of
objects. The dotted line marks the solar colors.
4000 5000 6000 7000 8000 9000
Figure 1:
4000 5000 6000 7000 8000 9000
Figure 2:
4000 5000 6000 7000 8000 9000
Figure 3:
Figure 4:
Figure 5:
Figure 6:
Figure 7:
Figure 8:
Figure 9:
Figure 10:
Figure 11:
M(1,1) E
a [AU] q [AU]
Figure 12:
Figure 13:
Figure 14:
	Introduction
	Observations and data reduction
	Results
	Dynamical families: L5 swarm
	Anchises
	Misenus
	Panthoos
	Cloantus
	Phereclos
	Sarpedon
	Dynamical families: L4 swarm
	Eurybates
	1986 WD
	1986 TS6
	Discussion
	Size vs spectral slope distribution:Individual families
	Size vs slope distribution:  The Trojan population as a whole
	Spectral slopes and L4/L5 Clouds
	Orbital Elements
	Comparison with other outer Solar System minor bodied
	Introduction and methods
	Results
	Conclusions
