A Unified Near‐Infrared Spectral Classification Scheme for T Dwarfs

Abstract

A revised near infrared classification scheme for T dwarfs is presented, based on and superseding prior schemes developed by Burgasser et al. and Geballe et al., and defined following the precepts of the MK Process. Drawing from two large spectroscopic libraries of T dwarfs identified largely in the Sloan Digital Sky Survey and the Two Micron All Sky Survey, nine primary spectral standards and five alternate standards spanning spectral types T0 to T8 are identified that match criteria of spectral character, brightness, absence of a resolved companion and accessibility from both northern and southern hemispheres. The classification of T dwarfs is formally made by the direct comparison of near infrared spectral data of equivalent resolution to the spectra of these standards. Alternately, we have redefined five key spectral indices measuring the strengths of the major H 2 O and CH 4 bands in the 1-2.5 µm region that may be used as a proxy to direct spectral comparison. Two methods of determining T spectral type using these indices are outlined and yield equivalent results. These classifications are also equivalent to those from prior schemes, implying that no revision of existing spectral type trends is required. The one-dimensional scheme presented here provides a first step toward the observational characterization of the lowest luminosity brown dwarfs currently known. Future extensions to incorporate spectral variations arising from differences in photospheric dust content, gravity and metallicity are briefly discussed. A compendium of all currently known T dwarfs with updated classifications is presented.

2MASS 0213+3648 C: A wide T3 benchmark companion to an an active, old M dwarf binary

“…The individual indices and the derived spectral types are shown in Table 2. We also compared visually with the standards of Burgasser et al (2006), finding a best fit of T3 [note the T3 standard of Burgasser et al (2006)] was found to be a binary by Liu, Dupuy & Leggett (2010), see later discussion on the comparison with the newer standard). As this compares well with the spectral types derived from the indices, we adopt this as our final spectral type.…”

“…Fig. 9 also shows a comparison with the young (∼100 Myr) T3 GU Psc b (Naud et al 2014), the young (∼300 Myr) T2.5 HN Peg B (Luhman et al 2007), the original T3 standard (2MASS J120956.13−100400.8, Burgasser et al 2006), and the alternative T3 standard (SDSS J120602.51+281328.7) suggested by Liu et al (2010) after 2MASS J120956.13−100400.8 was found to be a binary. We note that 2MASS J0213+3648 C has a narrower J band peak and deeper J-band water absorption than SDSS J120602.51+281328.7 but matches both HN Peg B and 2MASS J120956.13−100400.8 well in this regime, the H and K bands the spectrum are a better fit to SDSS J120602.51+281328.7.…”

“…Our spectrum of 2MASS J02132062+3648506C, we classify this object as a T3 ± 0.5. Shown for comparison are the spectra for the young (∼100 Myr) T3.5 GU Psc b (Naud et al 2014), the young (∼300 Myr) T2.5 HN Peg B (Luhman et al 2007), the alternative T3 dwarf standard SDSSJ120602.51+281328.7 (Chiu et al 2006) [as suggested by Liu et al (2010)], the previous T3 standard 2MASS J120956.13−100400.8 (Burgasser et al 2004(Burgasser et al , 2006; now known to be a binary) and the blue T dwarf (Burningham et al 2010;Marocco et al 2015). All spectra are smoothed to similar resolutions and normalized to the J-band peak.…”

“…These are for the trigonometric parallax M J = 13.424 ± 0.483 mag, M H = 13.153 ± 0.483 mag and M K = 13.196 ± 0.483 mag and M J = 14.321 ± 0.639 mag, Table 2. Spectral classification of 2MASS J0213+3648 C. Shown are the spectral indices (Burgasser et al 2006) and the implied spectral classifications from polynomial relations (Burgasser 2007) Figure 10. An updated version of the benchmark companion age-spectral type plot from Bowler et al (2015).…”

Spectrophotometric characterization of high proper motion sources fromWISE

“…Using the tabulated coefficients of Table 14 in Dupuy & Liu (2012) and the 2MASS and WISE Table 2. Main spectral indexes for T dwarfs as given in Burgasser et al (2006) Object…”