Spitzer Spectroscopy of low-mass stars and brown dwarfs

Abstract

We present the first results of a spectroscopic survey of M, L, and T dwarfs being conducted with the Infrared Spectrograph onboard the Spitzer Space Telescope. We observed GJ 1001 A (M3.5 V), DENIS 0255-4700 (L8.0 V), and epsilon Ind Ba/Bb (T1/T6 V). As expected from the predictions of model atmospheres, the spectra show absorption features of H(sub 2)O, CH(sub 4) at 7.8 micron, and NH(sub 3) at 10.5 micron. This is the first detection of the 7.8 micron CH(sub 4) band and the first secure detection of NH(sub 3) in the spectra of brown dwarfs. Finally, we compute the bolometric luminosities of three of the dwarfs in our sample by combining 0.6-4.1 micron spectra with the Spitzer spectra and find good agreement with previous results that use L' -and M' -band photometry to account for the flux emitted between 2.5 and 5 micron.

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SPITZER SPECTROSCOPY OF LOW-MASS STARS AND BROWN DWARFS
M.C. Cushing1, M.S. Marley2, and T.L. Roellig2
1SETI Institute, NASA Ames Research Center MS 245-3, Moffett Field, CA 94043
2NASA Ames Research Center MS 245-3, Moffett Field, CA 94043
Abstract
We present the first results of a spectroscopic survey
of M, L, and T dwarfs being conducted with the Infrared
Spectrograph onboard the Spitzer Space Telescope. We
observed GJ 1001 A (M3.5 V), DENIS 0255−4700 (L8.0
V), and ǫ Ind Ba/Bb (T1/T6 V). As expected from the
predictions of model atmospheres, the spectra show ab-
sorption features of H2O, CH4 at 7.8 µm, and NH3 at
10.5 µm. This is the first detection of the 7.8 µm CH4
band and the first secure detection of NH3 in the spec-
tra of brown dwarfs. Finally, we compute the bolometric
luminosities of three of the dwarfs in our sample by com-
bining 0.6−4.1 µm spectra with the Spitzer spectra and
find good agreement with previous results that use L′-
and M′-band photometry to account for the flux emitted
between 2.5 and 5 µm.
Key words: stars: late-type – stars: low-mass, brown dwarfs
– infrared: stars
1. Introduction
In the last decade, a large population of very low-mass
(VLM) stars and brown dwarfs (BDs), collectively known
as ultracool dwarfs, has been discovered in wide-field sur-
veys such as the Two Micron All Sky Survey (2MASS),
the Sloan Digital Sky Survey (SDSS) and the Deep Near-
Infrared Southern Sky Survey (DENIS) (e.g., Kirkpatrick
et al. 2000, Knapp et al. 2004). These objects have been
studied primarily in the optical and near-infrared because
the spectral energy distributions (SEDs) of ultracool dwar-
fs peak at ∼1 µm and these wavelengths are easily acces-
sible from the ground. Although we have learned a great
deal about VLM stars and BDs from these observations
(e.g., Burrows et al. 2001), there are strong motivations
to observe them at longer wavelengths.
The 2.5 to 20 µm spectral region contains a number of
molecular absorption bands (H2O, CO, CH4, NH3) that
are powerful diagnostics of the atmospheric physics and
chemistry of ultracool dwarfs. For example, the detection
of the 4.7 µm fundamental band of CO in the spectrum
of Gl 229 B (Noll et al. 1997) indicates the abundance of
CO is ∼1000 times greater than thermochemical equilib-
rium calculations predict. Unfortunately there are only a
limited number of ultracool dwarf spectra at 2.5 > λ > 5
µm (Noll et al. 1997, Noll et al. 2000) and there are no
ultracool dwarf spectra at λ > 5 µm.
The launch of the Spitzer Space Telescope (Werner
et al. 2004), which is sensitive in the mid-infrared (3.6
to 160 µm), has therefore opened a heretofore untapped
wavelength range in the study of ultracool dwarfs. To take
advantage of this unique opportunity, the science team of
the Infrared Spectrograph (IRS; Houck et al. 2004) cre-
ated the ‘Dim Suns’ Guaranteed Time Observing (GTO)
program which aims to observe a sample of M, L and T
dwarfs with the IRS. In this paper, we present the first
results from this survey (see also Roellig et al. 2004).
2. Spitzer spectra and model comparisons
We observed GJ 1001 A (M3.5 V), DENIS 0255−4700
(L8.0 V) and the T1/T6 binary system ǫ Ind Ba/Bb using
the Short-Low module of the IRS. This module covers
from 5.5 to 14 µm at a resolving power of R ≡ λ/∆λ ≈
100. We also observed GJ 1001A and DENIS 0255−4700
with the Long-Low module that covers from 14 to 38 µm
at R ≈ 100 but do not report on these results here (see
Roellig et al. 2004).
Figure 1 shows the 5−14 µm spectra of the three dwar-
fs. The ǫ Ind Ba/Bb spectrum is a composite of the two
components since the spatial resolution of the IRS is not
high enough to resolve the individual components. The
spectrum of GJ 1001 A is relatively featureless except for
the ‘kink’ at ∼6.5 µm due to H2O absorption. The spec-
trum of DENIS 0255−4700 exhibits H2O absorption as
well but also exhibits strong CH4absorption at ∼7.8 µm.
This is the first detection of this CH4band in the spec-
trum of an ultracool dwarf. Finally the spectrum of ǫ Ind
Ba/Bb shows even stronger H2O and CH4bands but also
exhibits an NH3 band at ∼10.5 µm. Although NH3 has
been predicted to be present in the atmospheres of BDs
for some time (Tsuji 1964; Marley et al. 1996; Feggley &
Lodders 1996; Allard et al. 2001; Burrows, Sudarsky, &
Lunine 2003), this is the first secure detection of NH3in
the spectrum of a BD.
We have also compared model spectra to the data in
order to determine how well they qualitatively match the
data. The best fitting models are superposed on the spec-
tra shown in Figure 1. The models have been smoothed
to the resolving power of the Spitzer spectra (R ≈ 100)
Proc. 13th Cool Stars Workshop, Hamburg, 5–9 July 2004 (ESA SP-560, Jan. 2005, F. Favata, G. Hussain & B. Battrick eds.)

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246M. C. Cushing et al.
Figure1. 5 to 14 µm spectra of GJ 1001 A (blue/solid, M3.5 V), DENIS 0255−4700 (red/solid, L8.0 V) and ǫ Ind Ba/Bb
(magenta/solid, T1/T6 V). Absorption bands of H2O, CH4, and NH3 are indicated. Overplotted on the spectra are the best
fitting model spectra (black/dashed).
and resampled onto the same wavelength grid. The model
for GJ 1001 A is an Ames-DUSTY model (Allard et al.
2001) with Teff= 3200 K and logg =5.0. The parameters
were determined from an analysis of the optical and near-
infrared spectra of GJ 1001 A by Leggett et al. (2002).
The best fitting model (by eye) for DENIS 0255−4700 is a
CLOUDY model (Marley et al. 2002) with Teff= 1400 K,
logg = 4.5 and fsed=3. These models treat the sedimen-
tation of condensates as described in Ackerman & Marley
(2001). Finally the model for ǫ Ind Ba/Bb is a composite
CLEAR model (Marley et al. 2002). The CLEAR models
include condensate sedimentation in the chemical equilib-
rium calculations but ignore the cloud opacity. Based on
the estimates of Lbolfor each component (McCaughrean
et al. 2004) and the estimated age of ǫ Ind A (Lachaume
et al. 1999), we used the evolutionary models of Marley
et al. (in preparation) to determine the Teff, logg, and
R for each component and then synthesized a compos-
ite CLEAR spectrum based on these parameters. On the
whole, the models fit the data well. Note however the spec-
trum of DENIS 0255−4700 dips below the model at ∼ 10.5
µm. This dip could be caused by NH3absorption and/or
the silicate cloud (Saumon et al. 2003). More spectra of
dwarfs at L/T transition will be required to determine the
cause of this dip.
3. SEDs and bolometric luminosities
Four of the ultracool dwarfs in our sample also have pub-
lished optical and near-infrared spectra. Figure 2 shows
the 0.6−15 µm spectra Gl 229 A (M1.0 V), 2MASS
0746+2000AB (L3.0 V), DENIS 0255−4700 (L8.0 V), and
2MASS 0559−1404 (T5.0 V). The optical spectra of the
dwarfs are from Kirkpatrick et al. (1991), Rutledge et
al.(2000), Mart´ ın et al. (1999), Burgasser et al. (2003)
while the infrared spectra are from Cushing (2004). The
variations in spectral morphology reflect the changes in at-
mospheric chemistry that occur with decreasing effective
temperatures.
In the M and early-type L dwarfs, H2O, CO, and N2
are the dominent O, C, and N bearing molecules, respec-
tively. In addition, the refractory elements such as Ti, Fe,
Mg, Si remain in a gaseous state. Therefore, the spectra of
the M and early-type L dwarfs are dominated by TiO and
VO in the optical, and H2O and CO in the infrared. In the
mid- to late-type L dwarfs, CH4begins to replaces CO as
the dominate carbon bearing gas. The near-infrared bands
of CH4 are the defining characteristic of the T spectral
class (Burgasser et al. 2002, Geballe et al. 2002). However,
since the 3.3 and 7.8 µm fundamental bands of CH4are
intrinsically ∼100 times stronger than the near-infrared
bands, they are present in the spectra of the hotter late-
type L dwarfs such as DENIS 0255−4700. In addition, the

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Spitzer spectroscopy of low-mass stars and brown dwarfs
247
Figure2. 0.6 to 15 µm spectra of Gl 229 A (M1.0 V), 2MASS 0746+2000AB (L0.5 V), DENIS 0255−4700 (L8.0 V), and
2MASS 0559−1404 (T5.0 V). Prominent absorption bands of H2O, CO, CH4, and NH3, as well as K I lines, are shown.
refractory elements have condensed out of the gas remov-
ing the TiO and VO opacity. As a result, the atmosphere
becomes more transparent in the optical and the K I reso-
nance doublet at 7700˚ A becomes highly pressure broad-
ened. Finally the spectra of the T dwarfs exhibit strong
H2O and CH4bands throughout their spectra as CO has
been fully reduced to CH4 and the leftover O has gone
into H2O. It is also at these temperatures that N2reduces
to NH3, as is evident by the NH3 band at ∼10.5 µm in
the spectrum of 2MASS 0559−1404 (T5.0 V).
The broad wavelength coverage of the spectra shown
in Figure 2 also allows us to constrain the bolomeric lumi-
nosities of three of the ultracool dwarfs (DENIS 0255−4700
lacks a measured parallax). To construct spectra suitable
for integration over all wavelengths, we first linearly in-
terpolate from zero flux at zero wavelength to the flux
densities at the bluest wavelengths of the spectra. Since
the M dwarf emits a significant fraction of its energy in
the optical, we used published U-, B- and V -band pho-
tometry to first extend its spectrum blueward to 0.36 µm
(e.g., Leggett et al. 1996). The gaps in each spectrum are
removed by linear interpolation between the flux densities
at the gap edges. Finally, we extend a Rayleigh-Jeans tail
from the reddest wavelength of each spectrum to infinity.
This is the same technique employed by Golimowski et al.
(2004) except that they use L′- and M′-band photometry
to account for the flux emitted between 2.5 and 5 µm.
Integrating over the final spectra, we find our bolometric
luminosity estimates agree within the errors of those of
Golimowski et al. (2004).
Acknowledgements
This work is based in part on observations made with the
Spitzer Space Telescope, which is operated by the Jet Propul-
sion Laboratory, California Institute of Technology, under
NASA contract 1407, the Infrared Telescope Facility, which is
operated by the University of Hawaii under Cooperative Agree-
ment no. NCC 5-538 with the National Aeronautics and Space
Administration, Office of Space Science, Planetary Astronomy
Program, and the Subaru Telescope, which is operated by the
National Astronomical Observatory of Japan. M.C.C. is sup-
ported by a Spitzer Fellowship.
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