**FULL TITLE**
ASP Conference Series, Vol. **VOLUME**, **YEAR OF PUBLICATION**
**NAMES OF EDITORS**
Outstanding Issues in Our Understanding of L, T, and Y
Dwarfs
J. Davy Kirkpatrick
Infrared Processing and Analysis Center, California Institute of
Technology
Abstract.
Since the discovery of the first L dwarf 19 years ago and the discovery of the
first T dwarf 7 years after that, we have amassed a large list of these objects,
now numbering almost six hundred. Despite making headway in understand-
ing the physical chemistry of their atmospheres, some important issues remain
unexplained. Three of these are the subject of this paper:
(1) What is the role of “second parameters” such as gravity and metallicity
in shaping the emergent spectra of L and T dwarfs? Can we establish a robust
classification scheme so that objects with unusual values of log(g) or [M/H],
unusual dust content, or unresolved binarity are easily recognized?
(2) Which physical processes drive the unusual behavior at the L/T tran-
sition? Which observations can be obtained to better confine the problem?
(3) What will objects cooler than T8 look like? How will we know a Y
dwarf when we first observe one?
1. Introduction
In the last thirteen years, the sample of L and T dwarfs has grown from a
paltry 1 (GD 165B, Becklin & Zuckerman 1988) to an astounding 586 at the
time of this writing (see http://www.DwarfArchives.org). As the sample has
grown, additional follow-up has further added to our knowledge of these objects.
With this newfound knowledge major deficiencies have begun to show in our
understanding of the physics shaping the spectra of L and T dwarfs. In this
paper I discuss three important questions being asked by low mass star and
brown dwarf researchers today. First, what role do “secondary” parameters such
as gravity, metallicity, dust content, and unresolved binarity play in shaping
emergent spectra? Second, why has the transition between L dwarfs and T
dwarfs been so difficult to model? Third, what will objects cooler than type T8
look like?
2. Issue #1: Gravity, Metallicity, and Other “Secondary Parameter”
Effects
Astronomers have come to expect that spectral type can be used as a proxy for
temperature. While this is true for main sequence stars classified from types
O through M, is the same true for L and T dwarfs? Using measurements of
trigonometric parallaxes (see Dahn et al. 2002; Tinney et al. 2003; Vrba et al.
http://arxiv.org/abs/0704.1522v1
http://www.DwarfArchives.org
2004) and bolometric luminosities (see Reid et al. 1999; Golimowski et al. 2004),
it can be shown that there is an amazingly linear correlation between optical
spectral type and effective temperature for the entire L dwarf class. Effective
temperature appears constant from early- through mid-T, then again shows a
drop as a function of type for later T dwarfs. This is illustrated in the upper
panel of Figure 1.
Figure 1. Effective temperature plotted against spectral type. The top
panel shows the relation between temperature and optical spectral type; the
bottom panel shows temperature versus near-infrared type. See Kirkpatrick
(2005) for details.
The same correlation, however, is not seen for near-infrared L and T dwarf
classifications. This is illustrated in the lower panel of Figure 1. Here the
effective temperatures are essentially constant, albeit with a large scatter of
±200 K, from mid-L through mid-T types. This is somewhat surprising given
that the near-infrared classification is loosely based on the optical classifica-
tion (Reid et al. 2001; Testi et al. 2001; Geballe et al. 2002). However, these
systems used only spectral indices for classification and not anchor points (pri-
mary spectral standards) to serve as on-sky comparisons. This differs from the
methodology used in the optical, where anchor points were selected and checked
for self consistency across the entire wavelength region used for classification,
not just over the areas used for measuring spectral indices (Kirkpatrick et al.
1999). In all fairness, the optical behavior of L dwarfs is much less complicated
than the near-infrared behavior, as evidenced by the sometimes non-monotnic
behavior of H- and K
-band fluxes and shapes as a function of type (e.g., Fig.
3 of Geballe et al. 2002 and Fig. 10 of McLean et al. 2003), which is further
reflected in the large spectral type error bars seen for some objects in the lower
panel of Figure 1. Until very recently, setting up near-infrared L-dwarf spectral
standards would have been difficult because of this scatter and the relatively
sparse sampling of 1.0-2.5 µm spectra for each L dwarf subclass.
What causes the large dispersion of morphologies seen in the near-infrared?
As the optical spectra of L and T dwarfs do not show this degree of variation,
we can use them to probe the influence of other parameters such as gravity,
metallicity, and dust on the spectral shape at longer wavelengths.
2.1. Exploring the Effects
Low Gravity The effects of lower gravity have been well studied in the opti-
cal for late-M dwarfs. The earliest studies of gravity-dependent features were
based on young objects in the Pleiades (Steele & Jameson 1995; Mart́ın et al.
1996) and ρ Ophiuchi (Luhman et al. 1997). At these ages (in the 1-100 Myr
range) late-M dwarfs are substellar and are still contracting to their final radii;
this means that they are both larger and less massive than old stars of the
same effective temperature and hence will have lower surface gravity. Gravity-
dependent features in the optical spectra of M dwarfs are well known as these
are also the features used to distinguish luminosity classes (M dwarfs vs. M gi-
ants) on the MK classification scheme. In the far red portion of the spectrum,
lower gravity results in weaker alkali lines and weaker hydride bands. At later
M types, TiO and VO bandstrengths are stronger at lower gravity as well, and
this occurs because of the gravity (pressure) dependent nature of condensation
(Lodders & Fegley 2002).
The study of low-gravity effects in L dwarfs, on the other hand, has had a
brief history because few young cluster brown dwarfs with L spectral types are
known. Recent discoveries of field L dwarfs believed to be young, low-gravity
brown dwarfs have, however, recently been reported (Kirkpatrick et al. 2006;
Cruz et al. 2007; Cruz et al., this volume).
With these optically typed, low-gravity late-M and L dwarfs, we can explore
the effects of low gravity in the near-infrared. Figure 2 shows the 2MASS J−K
colors for optically typed late-M through late-L dwarfs. Small dots with error
bars are field dwarf fiducials, and objects with optical signatures of low gravity
are denoted by triangles. Note that low-gravity objects tend to be redder in
than normal dwarfs of the same type. An example of one low-gravity L0
in discussed in detail in Kirkpatrick et al. (2006). Features at J-band showing
the hallmarks of low-gravity include retarded condensation of VO and weaker
alkali lines. The redder J −K
colors are believed to be due to a weakening of
the collision-induced absorption by H2, which is a dominant opacity source at
H− and particularly K
−bands.
Figure 2. J − K
colors for a collection of M7-L8 dwarfs with optically
determined spectral types. Each bin represents a full integral subtype: “M7”
includes M7 and M7.5 dwarfs, “M8” includes M8 and M8.5 dwarfs, etc. For
each group of objects, the median color in the group is plotted highest in the
bin; colors falling farther from the median are plotted progressively farther
down the y-axis. Grey circles show the color locations of objects optically
classified as subdwarfs, and grey triangles show the color locations of objects
whose optical spectra show the hallmarks of lower gravity.
Low Metallicity The earliest studies illustrating effects of low metallicity on the
spectra of ultra-cool dwarfs were by Gizis (1997), Schweitzer et al. (1999), and
Lépine et al. (2003). Lower metallicity means that fewer metal+metal molecules
will be formed relative to metal+hydrogen molecules. Thus in far optical spectra,
absorption by metal hydrides will be increased relative to that of metallic oxides.
How are the spectra of low-Z dwarfs affected at the transition from late-M
to early-L? The hallmark of the M/L transition is the appearance of condensates,
or rather the weakening of certain bands in the optical spectra as those molecules
disappear into condensates. Metallicity is expected to play an important role
in condensation (e.g., Lodders & Fegley 2002), with low-Z meaning that fewer
heavier elements will be present to form condensates in the first place.
Burgasser et al. (2007) have compiled a list of known subdwarfs and extreme
subdwarfs of type M7 and later. (See Burgasser et al., this volume, for in-depth
discussion of spectroscopic features.) Using optically classified objects from this
list and new additions from Kirkpatrick et al. (in prep.) we find that they
tend to have bluer J − K
colors, in some cases extremely bluer colors, than
normal late-M and L dwarfs of the same optical type (circles in Figure 2). This
discrepancy is believed to be largely due to collision-induced absorption by H2,
which is a far more dominant absorber at these wavelengths due to the decrease
in abundance of metal species.
Dust Content Despite the fact that low gravity tends to redden the near-
infrared spectra and low metallicity tends to make them bluer, not all anoma-
lously red near-infrared spectra appear to have low gravity nor do the anoma-
lously blue ones appear to have low metallicity.
An example of the first kind is 2MASS J22443167+2043433, whose color
of J −K
=2.45±0.16 makes it one of the reddest L dwarfs known. The optical
spectrum of this object looks like that of a normal L6.5 dwarf, but the near-
infrared spectrum shows enhanced flux at H- and K
-bands and weak K I lines
at J-band (McLean et al. (2003)). It is possible that this is a late-L analog
to the low-gravity, early-L dwarfs discussed above. In this case the optical
spectrum may be mimicking the spectrum of a normal slightly earlier L dwarf
via weakening of the 7665/7699 Å K I resonance doublet, which is the primary
shaper of the spectrum in the far red. However, it is also possible that thicker
dust clouds and veiling could produce these same effects, although the physical
explanation of the thicker dust relies on further undetermined physics (e.g.,
higher metallicity).
An example of the second kind is 2MASS J17210390+3344160, whose J−K
color of 1.14±0.03 is at least a half magnitude bluer than a normal, optically
classified L3 dwarf. One possible explanation for this object is that it has lower
metallicity than a standard field L dwarf, but not as low as the subdwarfs whose
optical spectra clearly identify them as low-Z. In this case the near-infrared re-
gion (because of collision induced absorption by H2) may be a more sensitive
indicator of metallicity than any of the optical diagnostics. An alternate expla-
nation is a reduction of condensates in the photosphere, leading to reduced flux
at H- and K-bands relative to a normal L dwarf. This could be caused either
by more efficient sedimentation (the physical process for which is unknown)
or by reduced metallicity. Both scenarios are discussed at greater length in
Cruz et al. (2007). For additional examples of blue L dwarfs and further discus-
sion on possible physical scenarios, see Knapp et al. (2004), Chiu et al. (2006),
and Cruz et al. (2007).
Unresolved Binarity Another factor contributing to the larger scatter in near-
infrared types may be unresolved binarity. Kirkpatrick et al. (in prep.) syn-
thesize composite binaries using single dwarfs with types from late-M through
late-T, and show that the optical spectral composites are virtually indistinguish-
able from spectra of single objects. The largest deviations between the synthetic
binaries and the spectra of single standards occur for late-L primaries with mid-
to late-T secondaries. Looper et al. (in prep.) have done a similar analysis in
the near-infrared. The largest deviations occur for the same set of hypothetical
binaries, but the discrepancies are sometimes large enough to lead to peculiar
looking spectra. See also Burgasser (2007) for other synthetic binary analysis in
the near-infrared.
2.2. Disentangling these Effects through Classification
One of our future challenges is finding spectral features that can reliably dis-
tinguish between these four effects. Ideally we should establish standard near-
infrared sequences of normal L dwarfs and optical/near-infrared sequences of L
subdwarfs and low-gravity L dwarfs. L subdwarfs may be separable into sev-
eral sequences to parallel overall metal content, but the construction of such
sequences is hampered by a severe lack of current examples from which to draw
standards. Also, nature should preclude L subdwarfs of lowest metallicity from
currently existing in the Milky Way, as brown dwarfs of such low-Z (age≈10
Gyr) will have long ago cooled to type T or later.
Figure 3 shows what a slice at L7 might look like for three of these near-
infrared spectral sequences. The top spectrum shows an L7 that is anomalously
red, due either to dust effects, low-gravity, or both. The bottom spectrum shows
an L7 that is anomalously blue, probably due to low-metallicity effects. In the
middle is a normal L7. The extreme nature of the variations is obvious.
Inroads have already been made into formulating these new spectral se-
quences and their classification schemes. Gorlova et al. (2003) and McGovern
(2005) have obtained near-infrared spectra of young late-M dwarfs in clusters
of various ages to quantify the effects of lower gravity at these temperatures.
Allers et al. (2007) has devised a pair of indices at near-infrared wavelengths
that can measure spectral type independent of gravity, and gravity independent
of reddening, for late-M and early-L dwarfs.
3. Issue #2: The L/T Transition
Trigonometric parallax measurements have revealed another effect not predicted
a priori by the models. This is the ∼1.3 mag brightening of J-band seen between
late-L and mid-T. This so called “J-band bump” is illustrated in Figure 4. High-
resolution imaging studies of objects in the L/T transition region have revealed
that the fraction of binaries with types between L7 and T3.5 is twice that seen
at earlier or later types (Burgasser et al. 2006b). If all objects residing in the
bump were binaries, then it might be possible to split the overluminosity of these
joint systems between the components and substantially reduce the magnitude
of the bump, as suggested by Liu et al. (2006) and Burgasser et al. (2006b).
However, not all objects occupying the bump have been successfully split.
The biggest offender, 2MASS J0559−1404 (see Figure 4), appears single to 0.′′05
with HST imaging (Burgasser et al. 2003c) and to 0.′′04 imaging with laser guide
star adaptive optics at Keck (Gelino & Kulkarni 2005). Even if it eventually
proves to be a tight, equal-magnitude binary, this reduces its absolute J-band
magnitude by only 0.75 mag, meaning that the J-band bump still has an ampli-
tude of 0.5-0.6 mag. On-going high-resolution radial velocity measurements as
Figure 3. Overplot of three objects typed as ∼L7 in J-band. Shown are an
unusually red L7 (top, light grey), a normal L7 (middle, dark grey), and a
blue L7 (bottom, black). The spectra, from top to bottom, come from Looper
(priv. comm.), Cruz (priv. comm.), and Burgasser et al. (2003a). The blue L7
has only partial wavelength coverage in this wavelength regime. All spectra
have been normalized at 1.2 µm.
well as on-going astrometric monitoring of 2MASS J0559−1404 will eventually
reveal the truth about this object.
Furthermore, some of the L/T transition binaries that have been split show
the J-band brightness reversal themselves. Gizis et al. (2003) found the first sys-
tem in which the secondary was brighter than the primary, although this observa-
tion was done not at J-band but at the F1042W filter (∼ Y -band) of HST. Since
then three other systems have been published (Liu et al. 2006; Burgasser et al.
2006b; Cruz et al. 2004). A fifth binary with brightness reversals in its compo-
nents has been recently reported by Looper et al. (in prep.) and for this system
the secondary is brighter than the primary by 0.5 mag at J . In summary, the
“J-band bump” is real with an amplitude of at least half a mag.
Regardless of the exact amplitude of the bump, we have come to learn
that the relative lack of single objects at early-T is probably indicative of
rapid evolution from late-L to mid-T (Burgasser et al. 2006b). The bump it-
self my be due to a 1µm analog of the well-known 5µm “holes” in Jupiter’s
atmosphere (Gillett et al. 1969; Westphal 1969). Using a scenario described in
Ackerman & Marley (2001), it may be the formation of holes in the cloud cov-
erage that is responsible for the extra flux near 1µm (Burgasser et al. 2002b).
These holes would be relatively opacity- and cloud-free windows allowing pho-
tons from deeper, warmer layers to escape. Orton et al. (1996) has shown that
only ∼1% of Jupiter’s atmosphere contributes to the 5µm hole phenomenon, so
the extent of cloud break-up may not have to be severe at the L/T boundary to
account for the extra 1µm flux.
Other possible explanations, including the gravity-dependent L/T transi-
tion model of Tsuji & Nakajima (2003) and the sudden downpour model of
Knapp et al. (2004), are discussed at greater length in Knapp et al. (2004) and
Kirkpatrick (2005). See Barman (this volume) for further discussion of this and
other outstanding issues being targetted by theorists.
4. Issue #3: Finding “Y” Dwarfs
Current spectroscopic classifications for brown dwarfs run as late as T8, which
corresponds to Teff ≈ 750K (Vrba et al. 2004; Golimowski et al. 2004). A total of
three objects, all discovered using 2MASS (Burgasser et al. 2002a; Tinney et al.
2005), are typed either in the optical or near-infrared as T8 dwarfs (Burgasser et al.
2003b, 2006b). The two with measured trigonometric parallaxes lie at distances
of only 5.7 and 9.1 pc (Vrba et al. 2004). This together with the fact that their
limiting H-band brightnesses, 15.5< H <15.8, fall below the SNR=7 level of
2MASS (Skrutskie et al. 2006), suggest that we are missing later dwarfs simply
because the detectability limit of 2MASS does not enable large enough volumes
to be sampled for later types.
What, then, might dwarfs cooler than T8 look like and what should they
be called? If later objects are discovered with spectroscopic morphologies very
similar to T dwarfs, the numbering should continue to T9, T10, or even be-
yond. Only when a distinct change in spectroscopic morphology – such as the
disappearance in the optical of the oxide bands at M/L boundary or the ap-
pearance shortward of 2.5 µm of methane at the L/T boundary – should a new
class be introduced. Finding these objects, which are commonly referred to
as “Y dwarfs” after the suggestion of Kirkpatrick et al. (1999) and Kirkpatrick
(2000), are one of the goals of current and planned surveys (see Pinfield et al.
and Leggett contributions, this volume).
Nature will eventually reveal what the spectra of Y dwarfs look like. In the
meantime we are left to theoretical predictions to divine what the trigger at the
T/Y boundary might be. Figure 5 shows a sequence of 0.6-2.5 µm model spectra
below Teff=800K from Burrows et al. (2003). Inspection of these models suggests
several possible triggers, among which are (1) the appearance of ammonia in
the near-infrared near Teff ≈600K, (2) the disappearance of the alkali lines near
Teff ≈500K, and (3) the end of the blueward trend of J − Ks color with type
Figure 4. Absolute J-band magnitude versus spectral type, adapted from
Burgasser et al. (2006b). Optical types are shown for L dwarfs and near-
infrared types for T dwarfs. Objects known to be close doubles are encircled,
two of which are shown both with their joint type and magnitude as well their
individual component’s types and magnitudes (filled dots). The location of
2MASS J05591914−1404488 is singled out.
near Teff ≈350K. Interestingly, the onset of water clouds near Teff ≈400-500K
has no appreciable affect on the spectra.
Acknowledgments. The author would like to acknowledge insightful dis-
cussions with Katelyn Allers, Kelle Cruz, Dagny Looper, and Adam Burgasser
during the preparation of this manuscript.
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http://arxiv.org/abs/astro-ph/0611505

  Since the discovery of the first L dwarf 19 years ago and the discovery of
the first T dwarf 7 years after that, we have amassed a large list of these
objects, now numbering almost six hundred. Despite making headway in
understanding the physical chemistry of their atmospheres, some important
issues remain unexplained. Three of these are the subject of this paper: (1)
What is the role of "second parameters" such as gravity and metallicity in
shaping the emergent spectra of L and T dwarfs? Can we establish a robust
classification scheme so that objects with unusual values of log(g) or [M/H],
unusual dust content, or unresolved binarity are easily recognized? (2) Which
physical processes drive the unusual behavior at the L/T transition? Which
observations can be obtained to better confine the problem? (3) What will
objects cooler than T8 look like? How will we know a Y dwarf when we first
observe one?

Introduction
In the last thirteen years, the sample of L and T dwarfs has grown from a
paltry 1 (GD 165B, Becklin & Zuckerman 1988) to an astounding 586 at the
time of this writing (see http://www.DwarfArchives.org). As the sample has
grown, additional follow-up has further added to our knowledge of these objects.
With this newfound knowledge major deficiencies have begun to show in our
understanding of the physics shaping the spectra of L and T dwarfs. In this
paper I discuss three important questions being asked by low mass star and
brown dwarf researchers today. First, what role do “secondary” parameters such
as gravity, metallicity, dust content, and unresolved binarity play in shaping
emergent spectra? Second, why has the transition between L dwarfs and T
dwarfs been so difficult to model? Third, what will objects cooler than type T8
look like?
2. Issue #1: Gravity, Metallicity, and Other “Secondary Parameter”
Effects
Astronomers have come to expect that spectral type can be used as a proxy for
temperature. While this is true for main sequence stars classified from types
O through M, is the same true for L and T dwarfs? Using measurements of
trigonometric parallaxes (see Dahn et al. 2002; Tinney et al. 2003; Vrba et al.
http://arxiv.org/abs/0704.1522v1
http://www.DwarfArchives.org
2004) and bolometric luminosities (see Reid et al. 1999; Golimowski et al. 2004),
it can be shown that there is an amazingly linear correlation between optical
spectral type and effective temperature for the entire L dwarf class. Effective
temperature appears constant from early- through mid-T, then again shows a
drop as a function of type for later T dwarfs. This is illustrated in the upper
panel of Figure 1.
Figure 1. Effective temperature plotted against spectral type. The top
panel shows the relation between temperature and optical spectral type; the
bottom panel shows temperature versus near-infrared type. See Kirkpatrick
(2005) for details.
The same correlation, however, is not seen for near-infrared L and T dwarf
classifications. This is illustrated in the lower panel of Figure 1. Here the
effective temperatures are essentially constant, albeit with a large scatter of
±200 K, from mid-L through mid-T types. This is somewhat surprising given
that the near-infrared classification is loosely based on the optical classifica-
tion (Reid et al. 2001; Testi et al. 2001; Geballe et al. 2002). However, these
systems used only spectral indices for classification and not anchor points (pri-
mary spectral standards) to serve as on-sky comparisons. This differs from the
methodology used in the optical, where anchor points were selected and checked
for self consistency across the entire wavelength region used for classification,
not just over the areas used for measuring spectral indices (Kirkpatrick et al.
1999). In all fairness, the optical behavior of L dwarfs is much less complicated
than the near-infrared behavior, as evidenced by the sometimes non-monotnic
behavior of H- and K
-band fluxes and shapes as a function of type (e.g., Fig.
3 of Geballe et al. 2002 and Fig. 10 of McLean et al. 2003), which is further
reflected in the large spectral type error bars seen for some objects in the lower
panel of Figure 1. Until very recently, setting up near-infrared L-dwarf spectral
standards would have been difficult because of this scatter and the relatively
sparse sampling of 1.0-2.5 µm spectra for each L dwarf subclass.
What causes the large dispersion of morphologies seen in the near-infrared?
As the optical spectra of L and T dwarfs do not show this degree of variation,
we can use them to probe the influence of other parameters such as gravity,
metallicity, and dust on the spectral shape at longer wavelengths.
2.1. Exploring the Effects
Low Gravity The effects of lower gravity have been well studied in the opti-
cal for late-M dwarfs. The earliest studies of gravity-dependent features were
based on young objects in the Pleiades (Steele & Jameson 1995; Mart́ın et al.
1996) and ρ Ophiuchi (Luhman et al. 1997). At these ages (in the 1-100 Myr
range) late-M dwarfs are substellar and are still contracting to their final radii;
this means that they are both larger and less massive than old stars of the
same effective temperature and hence will have lower surface gravity. Gravity-
dependent features in the optical spectra of M dwarfs are well known as these
are also the features used to distinguish luminosity classes (M dwarfs vs. M gi-
ants) on the MK classification scheme. In the far red portion of the spectrum,
lower gravity results in weaker alkali lines and weaker hydride bands. At later
M types, TiO and VO bandstrengths are stronger at lower gravity as well, and
this occurs because of the gravity (pressure) dependent nature of condensation
(Lodders & Fegley 2002).
The study of low-gravity effects in L dwarfs, on the other hand, has had a
brief history because few young cluster brown dwarfs with L spectral types are
known. Recent discoveries of field L dwarfs believed to be young, low-gravity
brown dwarfs have, however, recently been reported (Kirkpatrick et al. 2006;
Cruz et al. 2007; Cruz et al., this volume).
With these optically typed, low-gravity late-M and L dwarfs, we can explore
the effects of low gravity in the near-infrared. Figure 2 shows the 2MASS J−K
colors for optically typed late-M through late-L dwarfs. Small dots with error
bars are field dwarf fiducials, and objects with optical signatures of low gravity
are denoted by triangles. Note that low-gravity objects tend to be redder in
than normal dwarfs of the same type. An example of one low-gravity L0
in discussed in detail in Kirkpatrick et al. (2006). Features at J-band showing
the hallmarks of low-gravity include retarded condensation of VO and weaker
alkali lines. The redder J −K
colors are believed to be due to a weakening of
the collision-induced absorption by H2, which is a dominant opacity source at
H− and particularly K
−bands.
Figure 2. J − K
colors for a collection of M7-L8 dwarfs with optically
determined spectral types. Each bin represents a full integral subtype: “M7”
includes M7 and M7.5 dwarfs, “M8” includes M8 and M8.5 dwarfs, etc. For
each group of objects, the median color in the group is plotted highest in the
bin; colors falling farther from the median are plotted progressively farther
down the y-axis. Grey circles show the color locations of objects optically
classified as subdwarfs, and grey triangles show the color locations of objects
whose optical spectra show the hallmarks of lower gravity.
Low Metallicity The earliest studies illustrating effects of low metallicity on the
spectra of ultra-cool dwarfs were by Gizis (1997), Schweitzer et al. (1999), and
Lépine et al. (2003). Lower metallicity means that fewer metal+metal molecules
will be formed relative to metal+hydrogen molecules. Thus in far optical spectra,
absorption by metal hydrides will be increased relative to that of metallic oxides.
How are the spectra of low-Z dwarfs affected at the transition from late-M
to early-L? The hallmark of the M/L transition is the appearance of condensates,
or rather the weakening of certain bands in the optical spectra as those molecules
disappear into condensates. Metallicity is expected to play an important role
in condensation (e.g., Lodders & Fegley 2002), with low-Z meaning that fewer
heavier elements will be present to form condensates in the first place.
Burgasser et al. (2007) have compiled a list of known subdwarfs and extreme
subdwarfs of type M7 and later. (See Burgasser et al., this volume, for in-depth
discussion of spectroscopic features.) Using optically classified objects from this
list and new additions from Kirkpatrick et al. (in prep.) we find that they
tend to have bluer J − K
colors, in some cases extremely bluer colors, than
normal late-M and L dwarfs of the same optical type (circles in Figure 2). This
discrepancy is believed to be largely due to collision-induced absorption by H2,
which is a far more dominant absorber at these wavelengths due to the decrease
in abundance of metal species.
Dust Content Despite the fact that low gravity tends to redden the near-
infrared spectra and low metallicity tends to make them bluer, not all anoma-
lously red near-infrared spectra appear to have low gravity nor do the anoma-
lously blue ones appear to have low metallicity.
An example of the first kind is 2MASS J22443167+2043433, whose color
of J −K
=2.45±0.16 makes it one of the reddest L dwarfs known. The optical
spectrum of this object looks like that of a normal L6.5 dwarf, but the near-
infrared spectrum shows enhanced flux at H- and K
-bands and weak K I lines
at J-band (McLean et al. (2003)). It is possible that this is a late-L analog
to the low-gravity, early-L dwarfs discussed above. In this case the optical
spectrum may be mimicking the spectrum of a normal slightly earlier L dwarf
via weakening of the 7665/7699 Å K I resonance doublet, which is the primary
shaper of the spectrum in the far red. However, it is also possible that thicker
dust clouds and veiling could produce these same effects, although the physical
explanation of the thicker dust relies on further undetermined physics (e.g.,
higher metallicity).
An example of the second kind is 2MASS J17210390+3344160, whose J−K
color of 1.14±0.03 is at least a half magnitude bluer than a normal, optically
classified L3 dwarf. One possible explanation for this object is that it has lower
metallicity than a standard field L dwarf, but not as low as the subdwarfs whose
optical spectra clearly identify them as low-Z. In this case the near-infrared re-
gion (because of collision induced absorption by H2) may be a more sensitive
indicator of metallicity than any of the optical diagnostics. An alternate expla-
nation is a reduction of condensates in the photosphere, leading to reduced flux
at H- and K-bands relative to a normal L dwarf. This could be caused either
by more efficient sedimentation (the physical process for which is unknown)
or by reduced metallicity. Both scenarios are discussed at greater length in
Cruz et al. (2007). For additional examples of blue L dwarfs and further discus-
sion on possible physical scenarios, see Knapp et al. (2004), Chiu et al. (2006),
and Cruz et al. (2007).
Unresolved Binarity Another factor contributing to the larger scatter in near-
infrared types may be unresolved binarity. Kirkpatrick et al. (in prep.) syn-
thesize composite binaries using single dwarfs with types from late-M through
late-T, and show that the optical spectral composites are virtually indistinguish-
able from spectra of single objects. The largest deviations between the synthetic
binaries and the spectra of single standards occur for late-L primaries with mid-
to late-T secondaries. Looper et al. (in prep.) have done a similar analysis in
the near-infrared. The largest deviations occur for the same set of hypothetical
binaries, but the discrepancies are sometimes large enough to lead to peculiar
looking spectra. See also Burgasser (2007) for other synthetic binary analysis in
the near-infrared.
2.2. Disentangling these Effects through Classification
One of our future challenges is finding spectral features that can reliably dis-
tinguish between these four effects. Ideally we should establish standard near-
infrared sequences of normal L dwarfs and optical/near-infrared sequences of L
subdwarfs and low-gravity L dwarfs. L subdwarfs may be separable into sev-
eral sequences to parallel overall metal content, but the construction of such
sequences is hampered by a severe lack of current examples from which to draw
standards. Also, nature should preclude L subdwarfs of lowest metallicity from
currently existing in the Milky Way, as brown dwarfs of such low-Z (age≈10
Gyr) will have long ago cooled to type T or later.
Figure 3 shows what a slice at L7 might look like for three of these near-
infrared spectral sequences. The top spectrum shows an L7 that is anomalously
red, due either to dust effects, low-gravity, or both. The bottom spectrum shows
an L7 that is anomalously blue, probably due to low-metallicity effects. In the
middle is a normal L7. The extreme nature of the variations is obvious.
Inroads have already been made into formulating these new spectral se-
quences and their classification schemes. Gorlova et al. (2003) and McGovern
(2005) have obtained near-infrared spectra of young late-M dwarfs in clusters
of various ages to quantify the effects of lower gravity at these temperatures.
Allers et al. (2007) has devised a pair of indices at near-infrared wavelengths
that can measure spectral type independent of gravity, and gravity independent
of reddening, for late-M and early-L dwarfs.
3. Issue #2: The L/T Transition
Trigonometric parallax measurements have revealed another effect not predicted
a priori by the models. This is the ∼1.3 mag brightening of J-band seen between
late-L and mid-T. This so called “J-band bump” is illustrated in Figure 4. High-
resolution imaging studies of objects in the L/T transition region have revealed
that the fraction of binaries with types between L7 and T3.5 is twice that seen
at earlier or later types (Burgasser et al. 2006b). If all objects residing in the
bump were binaries, then it might be possible to split the overluminosity of these
joint systems between the components and substantially reduce the magnitude
of the bump, as suggested by Liu et al. (2006) and Burgasser et al. (2006b).
However, not all objects occupying the bump have been successfully split.
The biggest offender, 2MASS J0559−1404 (see Figure 4), appears single to 0.′′05
with HST imaging (Burgasser et al. 2003c) and to 0.′′04 imaging with laser guide
star adaptive optics at Keck (Gelino & Kulkarni 2005). Even if it eventually
proves to be a tight, equal-magnitude binary, this reduces its absolute J-band
magnitude by only 0.75 mag, meaning that the J-band bump still has an ampli-
tude of 0.5-0.6 mag. On-going high-resolution radial velocity measurements as
Figure 3. Overplot of three objects typed as ∼L7 in J-band. Shown are an
unusually red L7 (top, light grey), a normal L7 (middle, dark grey), and a
blue L7 (bottom, black). The spectra, from top to bottom, come from Looper
(priv. comm.), Cruz (priv. comm.), and Burgasser et al. (2003a). The blue L7
has only partial wavelength coverage in this wavelength regime. All spectra
have been normalized at 1.2 µm.
well as on-going astrometric monitoring of 2MASS J0559−1404 will eventually
reveal the truth about this object.
Furthermore, some of the L/T transition binaries that have been split show
the J-band brightness reversal themselves. Gizis et al. (2003) found the first sys-
tem in which the secondary was brighter than the primary, although this observa-
tion was done not at J-band but at the F1042W filter (∼ Y -band) of HST. Since
then three other systems have been published (Liu et al. 2006; Burgasser et al.
2006b; Cruz et al. 2004). A fifth binary with brightness reversals in its compo-
nents has been recently reported by Looper et al. (in prep.) and for this system
the secondary is brighter than the primary by 0.5 mag at J . In summary, the
“J-band bump” is real with an amplitude of at least half a mag.
Regardless of the exact amplitude of the bump, we have come to learn
that the relative lack of single objects at early-T is probably indicative of
rapid evolution from late-L to mid-T (Burgasser et al. 2006b). The bump it-
self my be due to a 1µm analog of the well-known 5µm “holes” in Jupiter’s
atmosphere (Gillett et al. 1969; Westphal 1969). Using a scenario described in
Ackerman & Marley (2001), it may be the formation of holes in the cloud cov-
erage that is responsible for the extra flux near 1µm (Burgasser et al. 2002b).
These holes would be relatively opacity- and cloud-free windows allowing pho-
tons from deeper, warmer layers to escape. Orton et al. (1996) has shown that
only ∼1% of Jupiter’s atmosphere contributes to the 5µm hole phenomenon, so
the extent of cloud break-up may not have to be severe at the L/T boundary to
account for the extra 1µm flux.
Other possible explanations, including the gravity-dependent L/T transi-
tion model of Tsuji & Nakajima (2003) and the sudden downpour model of
Knapp et al. (2004), are discussed at greater length in Knapp et al. (2004) and
Kirkpatrick (2005). See Barman (this volume) for further discussion of this and
other outstanding issues being targetted by theorists.
4. Issue #3: Finding “Y” Dwarfs
Current spectroscopic classifications for brown dwarfs run as late as T8, which
corresponds to Teff ≈ 750K (Vrba et al. 2004; Golimowski et al. 2004). A total of
three objects, all discovered using 2MASS (Burgasser et al. 2002a; Tinney et al.
2005), are typed either in the optical or near-infrared as T8 dwarfs (Burgasser et al.
2003b, 2006b). The two with measured trigonometric parallaxes lie at distances
of only 5.7 and 9.1 pc (Vrba et al. 2004). This together with the fact that their
limiting H-band brightnesses, 15.5< H <15.8, fall below the SNR=7 level of
2MASS (Skrutskie et al. 2006), suggest that we are missing later dwarfs simply
because the detectability limit of 2MASS does not enable large enough volumes
to be sampled for later types.
What, then, might dwarfs cooler than T8 look like and what should they
be called? If later objects are discovered with spectroscopic morphologies very
similar to T dwarfs, the numbering should continue to T9, T10, or even be-
yond. Only when a distinct change in spectroscopic morphology – such as the
disappearance in the optical of the oxide bands at M/L boundary or the ap-
pearance shortward of 2.5 µm of methane at the L/T boundary – should a new
class be introduced. Finding these objects, which are commonly referred to
as “Y dwarfs” after the suggestion of Kirkpatrick et al. (1999) and Kirkpatrick
(2000), are one of the goals of current and planned surveys (see Pinfield et al.
and Leggett contributions, this volume).
Nature will eventually reveal what the spectra of Y dwarfs look like. In the
meantime we are left to theoretical predictions to divine what the trigger at the
T/Y boundary might be. Figure 5 shows a sequence of 0.6-2.5 µm model spectra
below Teff=800K from Burrows et al. (2003). Inspection of these models suggests
several possible triggers, among which are (1) the appearance of ammonia in
the near-infrared near Teff ≈600K, (2) the disappearance of the alkali lines near
Teff ≈500K, and (3) the end of the blueward trend of J − Ks color with type
Figure 4. Absolute J-band magnitude versus spectral type, adapted from
Burgasser et al. (2006b). Optical types are shown for L dwarfs and near-
infrared types for T dwarfs. Objects known to be close doubles are encircled,
two of which are shown both with their joint type and magnitude as well their
individual component’s types and magnitudes (filled dots). The location of
2MASS J05591914−1404488 is singled out.
near Teff ≈350K. Interestingly, the onset of water clouds near Teff ≈400-500K
has no appreciable affect on the spectra.
Acknowledgments. The author would like to acknowledge insightful dis-
cussions with Katelyn Allers, Kelle Cruz, Dagny Looper, and Adam Burgasser
during the preparation of this manuscript.
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