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NIR low­resolution spectroscopy of L­dwarfs:
an efficient classification scheme for faint dwarfs
Leonardo Testi 1 , Francesca D'Antona 2 , Francesca Ghinassi 3 , Javier Licandro 3 ,
Antonio Magazz`u 3;4 , Antonella Natta 1 , and Ernesto Oliva 1;3
1 Osservatorio Astrofisico di Arcetri
2 Osservatorio Astronomico di Roma
3 Centro Galileo Galilei and Telescopio Nazionale Galileo
4 Osservatorio Astrofisico di Catania
Abstract. We present complete near infrared (0.85--2.45 ¯m), low­resolution (¸100)
spectra of a sample of 26 disk L­dwarfs with reliable optical spectral type classification.
The observations have been obtained with a prism­based optical element (the Amici
device) that provides a complete spectrum of the source on the detector. Our obser­
vations show that low­resolution near­infrared spectroscopy can be used to determine
the spectral classification of L­dwarfs in a fast but accurate way. We present a library
of spectra that can be used as templates for spectral classification of faint dwarfs. We
also discuss a set of near infrared spectral indices well correlated with the optical spec­
tral types that can be used to accurately classify L­dwarfs earlier than L6. Our results
show that with the VLT/NIRMOS instrument we will be able to extend the study of
objects below the deuterium burning limit to all young clusters and associations within
¸1 kpc.
1 Introduction
The latest years have witnessed the discovery of numerous brown dwarfs close
to the Sun, in nearby clusters and associations, and in binaries. The strategy
of the optical and near--IR imaging surveys (2MASS [1], [2], the Sloan Digital
Sky Survey [3], and DENIS [4], [5]) has been so successful that two new spectral
classes (L and T) have been added to the previous types, to help to classify very
cool stellar objects.
For L dwarfs, in spite of the remaining uncertainties in model atmospheres
for such cool objects, (e.g. [6]), it has been possible to derive a detailed spectral
classification system in 9 subclasses from the systematic changes observed in
selected spectral features ([1], [7]) . This spectral classification has been devel­
oped in the red part of the optical spectrum: the beginning of the L type is set
by the weakening of the TiO and VO bands, while the appearence of the CH 4
bands signals the transition to the T type. However, the optical spectral confir­
mation and classification of a candidate DENIS or 2MASS L­dwarf requires up
to ¸1 hr of integration time with a low resolution (¸1000) optical spectrograph
at a large (10m­class) telescope, depending on the spectral type and magnitude
of the candidate. This prevents the applicability of the optical classification to
deeper surveys.

2 Leonardo Testi et al.
Given that L­dwarfs emit most of their radiation in the near infrared bands
from 1 to 2.5 ¯m, the advantage of longer wavelengths is obvious. At present,
near IR spectra are available for a handful of objects (14) and very recently it has
been attemped to establish a near­infrared classification scheme from full (0.9 to
2.5 ¯m) UKIRT spectra with resolution ¸ 500­1000 [8]; each spectrum required
integration times between 1 and 4 hours, depending on the spectral type of the
star. With a 4­m class telescope the time demand is comparable (or higher) to
that required for the optical classification, and thus prohibitive for large surveys.
It is clear that intermediate­ and high­resolution spectroscopy, while necessary
for investigating photospheric properties of selected objects, is not suitable for
candidate confirmation and classification of large, deep surveys.
In this contribution we discuss a different approach, based on very low­
resolution near infrared spectroscopy using a high throughput device. In [9] we
presented complete near­infrared low­resolution (¸100) spectra of a sample of 26
L dwarfs with reliable optical spectral classification ([2]). All spectra have been
obtained with a prism­based optical element (the Amici device), which provided
a complete near­infrared spectrum of each star in less than 15 min on source at
the italian Telescopio Nazionale Galileo (TNG), a 3.56­m telescope. In Sect. 2
and 3 we present our new observations and the proposed low­resolution near
infrared classification scheme and in Sect. 4 we discuss the great promises of the
next generation VLT low resolution near infrared spectrograph (NIRMOS) for
this type of studies.
2 Observations and results
The observational data were collected at the 3.56m TNG with the Near Infrared
Camera and Spectrograph (NICS), a cryogenic focal reducer designed as near­
infrared common­user instrument for that telescope. The instrument is equipped
with a Rockwell 1024 2 HAWAII near infrared array detector. Among the many
imaging and spectroscopic observing modes ([10]), NICS offers a unique, high
throughput, very low resolution mode with an approximately constant resolving
power of ¸50, when the 1 00 wide slit is used. In this mode a prism­based optical
element, the Amici device, is used to obtain on the detector a complete 0.85--
2.45 ¯m long slit spectrum of the astronomical source ([11]).
The 26 L­dwarfs in our sample cover in an approximately uniform way the
optically defined spectral types ranging from L0 to L8. All the selected sources
are brighter than mKs ¸ 14:4, with 3 exceptions with mKs =14.5--14.8. The
sources were observed during the commissioning of NICS in several observing
runs in the December 2000 to February 2001 period. We used the 0. 0 5 wide slit
and the resulting spectra have an effective resolution of ¸100 across the entire
spectral range. Integration times on source varied from 2 to 15 minutes depending
on the source brightness. Wavelength calibration was performed using an Argon
lamp and the deep telluric absorption features. The telluric absorption was was
then removed by dividing each of the object spectra by an A0 reference star
spectrum observed at similar airmass. Finally, flux normalization was done using

NIR classification of L­dwarfs 3
Fig. 1. 0.85--2.45¯m low­resolution near­infrared spectra for all the L­Dwarfs in [9].
All spectra have been normalized by the average flux between 1.235 and 1.305 ¯m
and a constant shift has been added to each to separate them vertically. Each
spectrum is labeled with the 2MASS name (the 2MASSJ prefix has been omitted)
and the optical spectral type ([2]). For comparison, the spectrum of the T­dwarf
2MASSI J0559191+140448 ([12]) at R=25, also obtained with the Amici device, is
shown at the bottom right.

4 Leonardo Testi et al.
Fig. 2. Top panel: system relative efficiency (including atmosphere). Bottom panel:
Amici spectra of three of the dwarfs shown in Figure 1.
a theoretical A0 star spectrum smoothed to the appropriate resolution. Four of
the targets, which are also among the fainter in our sample, were observed in
unfavorable weather conditions resulting in a poor compensation of the deep
atmospheric features and noisier spectra.
The final spectra are shown in Figure 1. The objects are shown from top to
bottom and from left to right in order of increasing optical spectral type from
L0 to L8. The spectra have been normalized by the average flux in the 1.235--
1.305 ¯m region, a constant offset has been added to each one to avoid overlap.
The spectra show the same general features described in [6] and [8]. In our low
resolution spectra the atomic lines of Na I and K I and the FeH lines in the
J­band are not resolved, although their blended absorption features are clearly
seen in the early type dwarfs. The spectra are dominated by the H 2 O features at
¸0.95, ¸1.15, ¸1.40, ¸1.85, and ¸2.4 ¯m. TiO, near 0.85 ¯m, FeH, at 1.00 ¯m,
and CO, longward of 2.3 ¯m, are visible in some of the spectra, depending on
spectral type and signal to noise.
3 The NIR classification scheme
Despite their low­resolution, the spectra of Figure 1 allow us to identify a set of
spectral indices that can be used to define a near­infrared spectral classification
scheme which is well correlated with the widely used optical classification scheme
of [1] and [7]. A first attempt in this direction has already been taken by [8]. The
main conclusion of their study is that while the J­band atomic lines are only
weakly correlated with the optical spectral types, it is possible to define indices
based on the H 2 O wings which are well correlated with the optical types (at least
up to L6). For all the stars in our sample we computed the three indices that [8]
found to be the most correlated with the optical spectral type: K1 (see also [13]),
H 2 O A , and H 2 O B , all related to the strength or slope of the water absorption

NIR classification of L­dwarfs 5
Fig. 3. Correlations between optical spectral types and the near­infrared spectral in­
dices. The top three panels show the H2O A , H2O B , and K1 indices calculated for the
stars in our sample, the dotted lines show the linear fits of [8]. The bottom six pan­
els show the new indices defined in [9]. The four sources with poor telluric correction
spectra are shown as crosses.
features. In the top panels of Fig. 3 we show the datapoints from our sample
compared with the fits reported by [8]; our spectra are generally consistent with
their fits. Note that, as in [8], the K1 index can be used only for types earlier
than L5; moreover our data indicate saturation at late spectral types also for
H 2 O B , while H 2 O A shows a very large scatter. It is possible that this behaviour
of the H 2 O A and H 2 O B indices may be caused by the lower resolution of our
spectra.
In [9] we also defined six additional indices which are best suited for low­
resolution, complete spectra. Two of the new indices (sHJ and sKJ) are based on
the slope of the continuum, and can be reliably defined using our spectra because
the entire spectral range is observed simultaneously in the same atmospheric
conditions, without the need of a problematic intercalibration of various spectral
segments. All the other indices measure the slope of the water line wings. They
have been defined trying to avoid as much as possible the spectral regions affected
by the worse telluric absorption. In Fig. 2 we show the relative system efficiency
(including atmosphere), three of the spectra of Fig. 1, representative of the
extreme classes (L0.5 and L8) plus the low­resolution (R¸25) spectrum of a
T­dwarf. In Figure 3 the value of all the six indices are plotted against the
optical spectral type of each star. The sH 2 O J index is a measure of the strength

6 Leonardo Testi et al.
of the water absorption feature at 1.1 ¯m and, although it shows a very nice
correlation with the optical spectral type in our data, it should be used with care
as it may be seriously affected by a poor correction of the telluric absorption.
With only few exceptions, all stars with good spectra show a tight correlation
between the newly defined spectral indices and the optical spectral type.
4 The VLT promise
Our work has shown that complete 0.85--2.45 ¯m low­resolution (¸100) spec­
tra are a powerful tool for identification and and spectral classification of L
dwarfs from large, deep surveys, where the number and magnitudes of poten­
tial candidates make other techniques prohibitive, even on large telescopes. The
VLT/NIRMOS instrument (see the contribution of A. Moorwood in this vol­
ume) will allow to perform large area near infrared imaging surveys and low
resolution multi­object near infrared followup spectroscopy. We estimate that
the NIRMOS will allow to measure in 1 hr the 0:9 \Gamma 1:7 ¯m spectrum of faint
(m J ? 21) dwarfs, and classify them using our sHJ, sH 2 O J and sH 2 O H1 indices.
A new era of survey for cool dwarfs in clusters and associations will be open by
this VLT/NIRMOS as it will allow to study substellar objects as those presented
in the contributions by Roche et al. and Barrado y Navascu'es et al. in clusters
and associations as distant as 1.2 kpc.
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