The Astrophysical Journal, 601:L191–L194, 2004 February 1
䉷 2004. The American Astronomical Society. All rights reserved. Printed in U.S.A.

EVIDENCE OF ICY GRAINS IN COMET C/2002 T7 (LINEAR) AT 3.52 AU
Hideyo Kawakita
Gunma Astronomical Observatory, 6860-86 Nakayama, Takayama, Gunma 377-0702, Japan; kawakita@astron.pref.gunma.jp

Jun-ichi Watanabe and Takafumi Ootsubo
National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

Ryosuke Nakamura
Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sengen 2-1-1, Tsukuba 305-8505, Japan

Tetsuharu Fuse and Naruhisa Takato
Subaru Telescope, National Astronomical Observatory of Japan, 650 North A‘ohoku Place, Hilo, HI 96720

andSho Sasaki and Takanori Sasaki
Department of Earth and Planetary Science, University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Received 2003 October 14; accepted 2003 December 16; published 2004 January 27

ABSTRACT
We present evidence of icy grains in the coma of comet C/2002 T7 (LINEAR) at 3.52 AU from the Sun. This
comet will approach the Sun in the spring of 2004, and it is expected to be very bright near its perihelion passage.
The comet was observed using the Subaru Telescope with the Cooled Infrared Spectrograph and Camera for
OHS (CISCO) on 2003 September 14.6 (UT). The near-infrared (J, H, K) spectrum was extracted from the nearnucleus region (1250 km # 1250 km at the comet), and it showed clear absorption features at 1.5 and 2.05 mm
that originated from water ice grains. The calculated reflectance spectrum, based on the intimate mixture model
for water ice grains and astronomical silicate grains (the diameters are 5 and 0.5 mm, respectively), can reproduce
the observed reflectance spectrum of the comet up to 2.1 mm. The poor fit for the wavelength region longer than
2.1 mm is probably indicative of other grain species to be included in the model. Furthermore, the absence ofthe 1.65 mm feature of crystalline water ice may indicate that the water ice was in an amorphous state during
the observation.
Subject headings: comets: general — comets: individual (C/2002 T7 (LINEAR)) — solar system: formation

length range 2.5–3.5 mm). Furthermore, A’Hearn, Dwek, &Tokunaga (1984) pointed out that the 3 mm absorption featurein comet Bowell was inconsistent with model calculations forwater ice grains. Thus, previous searches for the water ice featurearound 3 mm might be inconclusive. The first secure detectionof water ice absorption features at 1.5 and 2.05 mm was reported
by Davies et al. (1997) in comet Hale-Bopp at 7 AU from theSun. Although, these near-infrared features could not be detectedin the same comet at 4.6 AU, the Infrared Space Observatory(ISO) could detect the emission band features originating fromwater ice near 44 and 65 mm in comet Hale-Bopp at 2.9 AU
with the long wavelength spectrometer (Lellouc et al. 1998).
Lellouch et al. (1998) also reported a possible absorption featurenear 3 mm. The 65 mm feature of crystalline water ice was also
detected by ISO with ISOPHOT in comet Hale-Bopp at 4.6–
4.9 AU from the Sun (Grün et al. 2001).
In this Letter, we report the firm detection of near-infrared
water ice absorption features at 1.5 and 2.05 mm in comet C/
2002 T7 (LINEAR) at 3.52 AU from the Sun. This comet will
be close to the Sun in the spring of 2004, and this apparition
is considered to be the first travel from the Oort Cloud to theinner solar system (Marsden 2003). This comet is expected tobe very bright near its perihelion passage.

1. INTRODUCTION

The existence of an icy grain halo in the cometary coma has
been debated for a long time. Since the icy grains are considered
to be dirty ice particles (not pure water ice) and can absorb
the incident solar flux, the icy grains evaporate and vanish ina short time around 1 AU from the Sun (Hanner 1981; Mukai
1986; Mukai et al. 1989). It is difficult to detect water ice
absorption features in the reflectance spectrum of the coma
when a comet is near 1 AU. In order to detect water ice features,
we need to observe the comet at farther than ∼2.5 AU (Hanner1981), where H2O ice does not evaporate significantly andwhere the sublimation of highly volatile species like CO candrive the coma activity. Furthermore, the size of the icy grain
halo is expected to be spatially small, so we should extract thereflectance spectra within small region close to the nucleus.
Thus, the high spatial resolution and the high signal-to-noise
ratio are required for the detection of the icy grain halo.
Water ice has spectral features near 1.5, 2.05, and 3 mm in
the near-infrared region (e.g., Lellouch et al. 1998; Davies etal. 1997). Because the 3 mm feature is strongest among thesefeatures, this band feature was the target for searching water
ice features in early attempts (Campins, Rieke, & Lebofsky
1983; Hanner 1984). Campins et al. (1983) showed that thereflectivity near 3.25 mm was lower than expected from thesolar spectrum in comet Bowell at 3.4–3.5 AU from the Sun.
A similar result was obtained by Hanner (1984) in comet Cernis
at 3.3 AU. However, 1.5 and 2.05 mm features were not detected, and other molecular ices might have contributed to the
3 mm absorption feature (C i O, O i H, and N i H groups all
have fundamental vibration absorption features in the wave-

2. OBSERVATION AND DATA ANALYSIS

A near-infrared spectroscopic observation of comet C/2002
T7 (LINEAR) was performed using the 8 m Subaru Telescopewith Cooled Infrared Spectrograph and Camera for OHS(CISCO; Motohara et al. 2002) on 2002 September 14.6 (UT).
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Vol. 601

TABLE 1
Observational Parameters
UT DateObjectTotal Integration
(s)

ra
(AU)

Db
(AU)

Air Mass

Grism

2003 Sep 14.61 . . . . . .
2003 Sep 14.62 . . . . . .
2003 Sep 14.65 . . . . . .

C/LINEAR
C/LINEARSAO 59433
SAO 59433

1200
1200
80
80

3.52
3.52
…
…

3.45
3.45
…
…

1.13
1.10
1.14
1.16

JH
wK
JH
wK

a
b

Heliocentric distance.
Geocentric distance.

The heliocentric and geocentric distances of the comet were3.52 and 3.45 AU, respectively.
In order to obtain a wide-range spectrum in the near-infrared
region, the grisms named “JH ” and “wK ”(wide-K) were used.
At the JH grism setting, both J and H bands (1.05–1.8 mm)
were covered simultaneously, and a part of the H band and allof the K band (1.8–2.45 mm) were covered in a single exposure
at the wK grism setting. The slit size was 108  # 0⬙. 5 in ourobservation ,and it corresponds to the spectral resolving powerof R p 400 at the JH setting and R p 600 at the wK setting.
The typical seeing size at the observation was about 0⬙. 3 in the
K band.
The total integration time for C/LINEAR was 1200 s foreach (JH or wK) setting. We put the comet on the slit at two
different positions (A and B) to subtract sky background emissions by the (A-B) operation. The nodding angle was 20⬙ forthe comet observations. For the cancellation of telluric absorption features, an A0 star (SAO 59433) was observed justafter the cometary observation. The difference in the air mass
during the observations of the comet and the reference star was
smaller than 0.1 for each grism setting. The reference star was
also observed at the A and B positions (the nodding angle was10⬙). Observational parameters are listed in Table 1.
We used the NOAO IRAF astronomical software package
to reduce the near-infrared spectra obtained by CISCO. The
TWODSPEC package was used for the reduction. First of all,dark subtraction and flat-fielding were applied to all frames. The
OH sky emission lines were used for the wavelength calibration.
We extracted the cometary spectra within the area of 0⬙. 5 #0⬙. 5 near the nucleus because icy grains in the coma rapidly
evaporated after they ejected from the nucleus. A sensitivity
calibration was done as follows. First, the cometary spectra were

Fig. 1.—Spectrum of comet C/2002 T7 (LINEAR) along with synthesized
spectra for pure H2O crystalline icy grains (0.1, 1.0, and 10.0 mm, in diameter).
The absorption by H2O ice is clearly shown in the cometary spectrum. However, the agreement between the cometary spectrum and the pure H2O spectrum
is not good in the J band.

divided by spectra of the reference star to cancel the telluric
absorption features. Then the ratio spectra was multiplied by themodel spectrum of the A0 star and finally divided by the synthesized solar spectrum (Kurucz 1994). Thus, we can get thenear-infrared spectrum of the reflectance (or albedo) in comet
C/LINEAR. Note that the albedo obtained here is just a relativevalue. Since the wavelength ranges of the JH and the wK spectra
are slightly overlapped near 1.8 mm (by ∼0.02 mm), we finallyscaled these spectra to be consistent with each other in the overlapped wavelength region.
3. DISCUSSION

Figure 1 shows the relative reflectance spectrum of C/LINEAR
on 2003 September 14.6 (UT). The cometary spectrum is normalized to unity at the shortest wavelength. In this figure, data
points were averaged for each of the 5 pixels (nearly the sameas the spectral resolving element), and the spectrum is plottedwith Ⳳ3 j error bars. The data are removed in the wavelength
range where the telluric absorptions were strong and where theerror bars are large. Our spectrum is quite similar to that observedin comet Hale-Bopp at 7 AU (Davies et al. 1997). Both 1.5 and2.05 mm absorption features of water ice are recognized, and thereflectance (where l 1 2.2 mm) decreases for longer wavelength.
The synthesized reflectance spectra of pure crystalline water
ice grains are overplotted in the same figure. The grain sizes
are 0.1, 1, and 10 mm in diameter. These spectra were produced
by Hapke’s bidirectional reflectance model (Hapke 1993),
which was also used in Davies et al. (1997) to model thereflectance spectrum of the icy grains in comet Hale-Bopp. The
optical constants of the crystalline water ice are from Quirico,
Douté, & Schmitt (1999). These water ice reflectance spectra
were scaled appropriately to compare with the observation. If
we consider H and K bands only, the pure water ice grains
with 1 mm size can reasonably explain the observation. However, the color in the J band must be slightly reddish, and thiswavelength dependence cannot be explained by the pure water
ice. This fact demonstrates the importance of the J-band spectrum for investigating the dirty materials in icy grains. In thecase of comet Hale-Bopp, unfortunately, Davies et al. (1997)
observed only H- and K-band spectra. If there was a J-band
spectrum of comet Hale-Bopp in addition to the H- and Kband spectra, it would give stronger constraints to the dirty
materials in icy grains.
Figure 2 shows the synthesized spectrum produced by the
intimate mixtures of water ice grains and astronomical silicate
grains (Draine 1985). In the intimate mixtures, individual components exist as separate grains that are in turn thoroughly
mixed at the granular scale (Davies et al. 1997). Davies et al.
also used the spatial mixture model, in which the total reflectance is modeled by a linear combination of the reflectance ofeach component weighted by its spatial extent. The spatial
mixture model represents a surface having distinctive albedo

No. 2, 2004

EVIDENCE OF ICY GRAINS IN COMET C/LINEAR

Fig. 2.—Spectrum of comet C/2002 T7 (LINEAR) along with a synthesized
spectrum for the intimate mixture of water ice and astronomical silicate grains.
The mass ratio is assumed to be 100 : 10. The size of the icy grain is 5 mm,
while the size of silicate dust is 0.5 mm. The color of the cometary grains in
the J band is illustrated well by the synthesized spectrum. However, the shallow
absorption near 2.2 mm is not understood well.

areas existing on a scale smaller than the spatial resolution of
the observations. We consider that the intimate mixture model
is better for modeling cometary dust comae because a real
cometary grain seems to consist of silicate grains together withsome kinds of ices and seems to be mixed at the granular scale.
Therefore, only the intimate mixture model is used here.
We simply assumed that the mass ratio between the water
ice and the astronomical silicate is 100 : 10 and that the sizesof grains are 5 and 0.5 mm in diameter, respectively. The resultant spectra can explain the observation well up to ∼2.1 mm.
The observed albedo, where l 1 2.1 mm, is lower than themodeled spectrum, and a 2.2 mm shallow absorption featureexists. Again, we note that Davies et al. (1997) found thecomparable solutions for both intimate and spatial mixtures incomet Hale-Bopp, and the spatial mixture also showed the poor
fitting for l 1 2.2 mm.
Some additional materials seem to be necessary to explain
the lower reflectance at l 1 2.1 mm. Even if we tried to fit theobservation to the model by considering ammonia, methanol,
methane, or ethane ice grains, our attempts would have failed
in the above Hapke’s model. However, with respect to theshallow absorption at 2.2 mm, it may be attributed to the ammonia hydrate ices (not pure ammonia ice) as demonstrated byBrown & Calvin (2000) for Pluto’s satellite Charon (unfortunately, the refractive indexes of ammonia hydrate ice at therelevant wavelengths are not presently available). A more sophisticated grain model (with a more complicated grain size
distribution and grain composition) and the optical constantsfor various kinds of ices at the relevant wavelength region seemto be required to reproduce the observation. Regarding thelower reflectivity at l 1 2.1 mm, it may be possible to explain
the observed low reflectance by hydrate silicates. Recently,
Kanno et al. (2003) found a 3 mm absorption of hydrated silicates on the D-type asteroid Irmintraud. It is well known thatbare cometary nuclei are classified as D-type asteroids with
their dark and reddish spectra in the visible and near-infrared
wavelengths (e.g., Licandro et al. 2003). While comets are
generally supposed to have anhydrous compositions as a resultof low temperatures, we cannot rule out the possibility thathydrated silicates are responsible for the observed feature beyond 2.1 mm.
Anyway, our simple model is enough to demonstrate the

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Fig. 3.—Close-up of Fig. 2 in the H band. The absence of the 1.65 mm
absorption feature of crystalline ice may indicate that the cometary ice wasin an amorphous state.

existence of water ice grains in the coma of C/LINEAR at
3.52 AU. The size of water ice grains is assumed to be 5 mm
in our model. Is this size reasonable? We can check the validity of the grain size obtained from the model independently
by comparing the lifetime of the grain in the coma at
3.52 AU with the aperture size used here. The lifetime of theicy grain for the sublimation should be longer than the traveling time of icy grains from the nucleus to the edge of theaperture used (0⬙. 5 # 0⬙. 5 at the sky). According to the heterogeneous icy grain model (Mukai 1986), the 5 mm sized
dirty ice grain (80% of water ice, 10% of magnetite, and 10%
of silicate) has a lifetime of ∼3 # 10 4 s at 3.5 AU fromthe Sun. Of course, if the grains consist of pure water ice,the lifetime of the grains is much longer than the lifetime ofthe dirty icy grains (i.e., the lifetime is about 10 8 s for 5 mm
sized pure water ice grains even at 1 AU). If we assume an
expansion speed of 100 m s⫺1 for the icy grains, the size ofthe icy grain halo is expected to be about 6000 km. Because
the observed spectrum was extracted from an aperture of1250 km # 1250 km at the comet, the 5 mm sized icy grains
could easily survive in the area within the aperture (on theother hand, the size of the halo of 1 mm sized dirty ice grains
is about a few tens of kilometers only). This estimation isconsistent with our detection of the icy grains.
Finally, we note the absence of the 1.65 mm feature of thecrystalline water ice in our observation (Fig. 3). The 1.65 mm
feature was also absent in the near-infrared spectrum of cometHale-Bopp at 7 AU (Davies et al. 1997). Davies et al. (1997)
suggested that the absence of the 1.65 mm feature means thatthe water ice was in an amorphous state during their observation. The amorphous water ice changes its phase from amorphous to cubic at T ∼ 140 K. Therefore, the absence of the
1.65 mm feature may indicate that the grain temperature waslower than 140 K. The equilibrated blackbody temperature at
3.52 AU from the Sun is 149 K, while the equilibrated temperature for the pure water ice grain (10 mm size) is estimatedto be 70 K (Mukai et al. 1989) at the same heliocentric distance.
Therefore, the icy grains observed here might have little darkmaterials, and their absolute reflectance in the visible wavelength region may be high enough to achieve an equilibrated
temperature of lower than 140 K. Note that the change in the
shapes of the absorption features between 1.0 and 2.7 mm wasreported by Grundy & Schmitt (1998), although their report
was for the hexagonal water ice (not the cubic water ice). WithL194

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higher temperature, the shapes of the absorption features become more smoothed. However, if we take this effect into
account, we consider that the 1.65 mm feature should be recognized in our spectrum in the case of an icy grain temperatureof 149 K (the blackbody temperature) during our observation.
In future work, the development of a more sophisticated icy
grain model will help us to reveal the composition of the ice,and a comparison between the near-nucleus spectrum and theoff-nucleus spectra will provide us with information on the
sublimation of icy grains (i.e., the change in the grain size),on the dynamical properties of grains, and so on.

Vol. 601

The authors thank R. Furusho for her fruitful discussions
and an anonymous referee for providing valuable commentsthat greatly improved the manuscript. This work is financially
supported by the Ministry of Education, Science, and Cultureof Japan under grants 15740127 (H. Kawakita) and 15654078(J. Watanabe). This report is based on data collected at theSubaru Telescope, which is operated by the National Astronomical Observatory of Japan.

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