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1
An efficient low-resolution NIR classification
scheme for M, L, and T dwarfs and its application
to young brown dwarfs L. Testi1, A. Natta1, F.
DAntona2, E. Oliva1,3, A. Magazzù3,4, F.
Ghinassi3, J. Licandro3, C. Baffa1, G.
Comoretto1, S. Gennari1 (1INAF-Osservatorio di
Arcetri, 2INAF-Osservatorio di Roma, 3Centro
Galileo Galilei e Telescopio Nazionale Galileo,
4INAF-Osservatorio di Catania)
We present the preliminary results of a programme
aimed at defining a low-resolution near-infrared
spectral classification scheme for faint M, L,
and T-dwarfs. The method is based on the global
shape of R100 complete near-infrared spectra
from 0.8 to 2.4µm as obtained through a
high-throughput prism-based optical element, the
Amici device, mounted inside the NICS instrument
at the TNG 3.5m telescope. We present the results
for the L-type dwarfs, and sample spectra for the
M and T-dwarfs range. A preliminary application
of the method to the classification of young
embedded brown-dwarf candidates is also
discussed. The method is shown to be accurate and
competitive the high system throughput coupled
with the possibility of obtaining in a single
shot the complete spectrum of the objects make
the NICS/TNG system more efficient than existing
large telescopes.
3. Amici spectra of field dwarfs Shown below are
a sample of field dwarfs spectra obtained for our
programme, from M4.5 through T8. The most
conspicuous features in the spectra are the water
absorption bands and for the T-dwarfs the methane
absorption bands. Depending on signal to noise
and spectral type, the spectra also show blended
absorption features from CO, TiO, FeH and KI. The
spectra show the evolution of the various
features with the spectral type, as expected. Our
proposed classification is mainly based on the
strength and shape of the water and methane
bands, which strongly affect the global
appearance of the spectrum. Each spectrum
requires 5 to 15 min of integration time at the
TNG, depending on source brightness. The sample
has been selected from Henry et al. (1994),
Kirkpatrick et al. (1995 1999 2000), and
Burgasser et al. (2002).

• 1. Why NIR low-resolution spectroscopy?
• M, L and, especially, T dwarfs radiate mostly in
  the near infrared
• Optical spectroscopic classification is highly
  demanding, even at 8-10m class telescopes
• Only in the NIR it will be possible to classify
  embedded young sources
• The broad features (mainly due to H2O and CH4)
  that can be used for NIR classification do not
  require high-resolution spectroscopy
• Confirmation and classification of brown dwarf
  candidates using high throughput low-resolution
  (R100) NIR spectroscopy at a 3.5m telescope can
  be competitive with optical and NIR
  medium-resolution (R500-1000) spectroscopy at a
  large (8-10m) telescope using traditional grating
  or grism based instrumentation

2. The Amici device The Amici device is a prism
based, high-throughput optical element that
produces a complete near infrared low-resolution
long slit spectrum on the NICS detector (Baffa et
al. 2001 Oliva et al. 2000). The device offer an
approximately constant resolving power across the
near infrared range, when coupled with a 0.5
arcsec wide slit, as used for this programme, the
resolution is 100. At this resolution OH lines
cannot be used for wavelength calibration that is
performed using the telluric absorption features
and arc spectra. System response is calibrated by
means of A0 stars observations and models.
4. Classification of field L-dwarfs As a first
step, we derived a spectral classification scheme
for L-type field dwarfs, to be compared with both
the well established optical classification
methods and the proposed near infrared
classification schemes. For a sample of 26 disk
dwarfs with known optical classification, we
obtained Amici spectra and defined a set of
indices useful for spectral classification. The
indices are defined on the basis of the continuum
shape and the water bands wings, using portions
of the spectra that are less affected by telluric
absorption, as shown in the figure above. In the
top panel we show the system relative efficiency,
compared with two L-dwarfs spectra of the extreme
spectral types. The grey shaded areas are the
regions of the spectra that have been used to
derive the spectral indices. We found that our
indices are well correlated with the optical
spectral types, and the classification method we
derive is more efficient and as accurate that
those based on optical or higher resolution
infrared spectroscopy (Testi et al. 2001). In
the figure below we show the correlations between
the values of the spectral indices computed from
our spectra and the optical spectral type in
Kirkpatrick et al. (1999 2000). The top panels
show the values of the indices defined by Reid et
al. (2001) and Tokunaga Kobayashi (1999),
computed for the stars in our sample. The bottom
six panels show the behaviour of the new indices
optimized for use with low-resolution
spectroscopy. The dotted lines show the fits from
Reid et al. (2001), while the dashed lines are
the best fits for our own indices.
5. Application to young embedded BDs One of our
primary goals in deriving a NIR low-resolution
spectral classification scheme for M, L, and
T-dwarfs was the possibility of applying the
classification to young embedded brown dwarfs.
Shown below is our determination of spectral type
and effective temperatures of two very low mass
objects in the ?-Oph molecular cloud core (Natta
et al. 2002) based on our preliminary procedure.
The procedure works as follows first we obtain
an estimate of the spectral type and extinction
based on the comparison with field dwarfs
spectra, then we derive the effective temperature
estimate by comparison with model atmospheres
with appropriate surface gravity (Log(g)3.5).
The comparison between the observed young brown
dwarfs candidates and reddened field dwarfs and
model spectra are shown below for a few extreme
cases an object with relatively low extinction
(AV2 mag) a deeply embedded object (AV8 mag)
and a very low-mass, embedded, late-M spectral
type object (M8-12 Mjup, AV9 mag, M8.5). See
Testi et al. (2002) and Natta et al. (2002) for
more details.
6. References Baffa et al. 2001, AA 378,
722 Burgasser et al. 2002, ApJ 564, 421 Henry,
Kirkpatrick, Simons 1994, AJ 108,
1437 Kirkpatrick et al. 1999, ApJ 519,
802 Kirkpatrick et al. 2000, AJ 120, 447 Natta et
al. 2002, AA, submitted Oliva 2000, Mem. Soc.
Astron. Italiana, 71, 861 Testi et al. 2001, ApJ
552, L147 Testi et al. 2002, ApJ 571, L155