NICS-TNG Low-Resolution 0.85–2.45 micron Spectra of L Dwarfs: A Near-Infrared Spectral Classification Scheme for Faint Dwarfs

Abstract

We present complete near-infrared (0.85-2.45 µm), low-resolution (∼100) spectra of a sample of 26 disk L-dwarfs with reliable optical spectral type classification. The observations have been obtained with NICS at the TNG using a prism-based optical element (the Amici device) that provides a complete spectrum of the source on the detector. Our observations show that low-resolution near-infrared spectroscopy can be used to determine the spectral classification of L-dwarfs 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 spectral types that can be used to accurately classify L-dwarfs earlier than L6.

A low-resolution near-infrared spectral library of M-, L-, and T-dwarfs

“…Wilking et al (2003) showed that the spectral index they define for the classification of young M-type brown dwarfs has values closer to those of dwarfs rather than to giants (or luminosity class III). Other authors have highlighted the differences between young objects, field dwarfs, and giants (e.g., Kirkpatrick et al 2006;Allers et al 2007), A discussion of this problem is beyond the scope of this paper; we note, however, that, despite the difference in surface gravity, field M-and L-type dwarfs can be used, and indeed have been used, as comparison standards in classifying young embedded very low-mass stars and brown dwarfs Testi et al 2002). The computation of the spectral indices in the previous section for young objects in the SpeX Prism library are indeed consistent with their published spectral types (see Appendix A).…”

“…With the possible exception of a few spectra of the lower signal to noise ratio, the library of Amici field dwarf spectra can be used as a source of templates to derive spectral types for faint or embedded cool objects for which higher resolution infrared spectroscopy or optical spectroscopy is impractical. This method was applied with success in deriving spectral classifications of young embedded brown dwarfs Testi et al 2002;Bouvier et al 2008). a For M-and L-dwarfs we list the optical spectral types (from the reference in Col. 4), for T-dwarfs we list the NIR spectral types (from Burgasser et al 2004).…”

“…Low-resolution near-infrared spectroscopy is especially useful to the spectral classification of embedded cool objects, such as young brown dwarfs in star-forming regions (e.g., Testi et al 2002;Natta et al 2002). Other authors have discussed the issue of using dwarfs (or luminosity class V) as spectral standards for young pre-main-sequence objects (which have spectral properties of subgiants, or luminosity class IV).…”

“…The combination of cool atmospheres and environment extinction prevents the use of optical classification schemes in many young star-forming regions. As an example, Natta et al (2002) and Testi et al (2002) used low-resolution nearinfrared spectroscopic classification methods to classify embedded young brown dwarfs in the ρ-Ophiuchi star-forming region.…”

Performance of the VLT Planet Finder SPHERE

“…The libraries of field brown dwarfs used for the L-type spectra are taken from Testi et al (2001), while the T-type spectra are from Looper et al (2007), Burgasser et al (2004) and Burgasser et al (2006). The flux ratio between the fluxes of the two IRDIS band (H2/H3) in the different spectral types ranged from a minimum value of 0.85 (L4-type, the flattest one) to a maximum of 7.46 (T8-type).…”