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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. D'Antona2, E. Oliva1,3, A. MagazzЫ3,4, F. Ghinassi3, J. Licandro3, 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 R~100 complete near-infrared spectra from 0.8 to 2.4 Л m 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.

C.

1. Why NIR low-resolution spectroscopy?
ь M, L and, especially, T dwarfs radiate mostly in the near infrared ь Optical spectroscopic classification is highly demanding, even at 8-10m class telescopes ь Only in the NIR it will be possible to classify embedded young sources ь The broad features (mainly due to H2O and CH4) that can be used for NIR classification do not require high-resolution spectroscopy ь Confirmation and classification of brown dwarf candidates using high throughput low-resolution (R=100) NIR spectroscopy at a 3.5m telescope can be competitive with optical and NIR medium-resolution (R=500-1000) spectroscopy 2. The Amici device at a large (8-10m) telescope using traditional grating or grism based instrumentation 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 A0 L0 calibrated by means of A0 stars observations and models. 2.5 Л m

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).

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.

0.8 Л m

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 Я-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 (AV=2 mag); a deeply embedded object (AV=8 mag); and a very low-mass, embedded, late-M spectral type object (M~8-12 Mjup, AV=9 mag, M8.5). See Testi et al. (2002) and Natta et al. (2002) for more details.

M6-M7.5; T~2600-2700

M6-M8; T~2500-2700

M8-M9; T~2300-2500

6. References
Baffa et al. 2001, A&A 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, A&A, submitted Oliva 2000, Mem. Soc. Astron. Italiana, 71, 861 Testi et al. 2001, ApJ 552, L147 Testi et al. 2002, ApJ 571, L155