A large spectroscopic sample of L and T dwarfs from UKIDSS LAS: peculiar objects, binaries, and space density

Abstract

We present the spectroscopic analysis of a large sample of late-M, L, and T dwarfs from the United Kingdom Deep Infrared Sky Survey. Using the YJHK photometry from the Large Area Survey and the red-optical photometry from the Sloan Digital Sky Survey we selected a sample of 262 brown dwarf candidates and we have followed-up 196 of them using the echelle spectrograph X-shooter on the Very Large Telescope. The large wavelength coverage (0.30 − 2.48µm) and moderate resolution (R∼ 5000 − 9000) of X-shooter allowed us to identify peculiar objects including 22 blue L dwarfs, 2 blue T dwarfs, and 2 low gravity M dwarfs. Using a spectral indices-based technique we identified 27 unresolved binary candidates, for which we have determined the spectral type of the potential components via spectral deconvolution. The spectra allowed us to measure the equivalent width of the prominent absorption features and to compare them to atmospheric models. Cross-correlating the spectra with a radial velocity standard, we measured the radial velocity for our targets, and we determined the distribution of the sample, which is centred at -1.7±1.2 km s −1 with a dispersion of 31.5 km s −1 . Using our results we estimated the space density of field brown dwarfs and compared it with the results of numerical simulations. Depending on the binary fraction, we found that there are (0.85±0.55)×10 −3 to (1.00±0.64)×10 −3 objects per cubic parsec in the L4-L6.5 range, (0.73 ± 0.47) × 10 −3 to (0.85 ± 0.55) × 10 −3 objects per cubic parsec in the L7-T0.5 range, and (0.74 ± 0.48) × 10 −3 to (0.88 ± 0.56) × 10 −3objects per cubic parsec in the T1-T4.5 range. We notice that there seem to be an excess of objects in the L to T transition with respect to the late T dwarfs, a discrepancy that could be explained assuming a higher binary fraction than expected for the L to T transition, or that objects in the high-mass end and low-mass end of this regime form in different environments, i.e. following different Initial Mass Functions.

The Effect of Atmospheric Cooling on Vertical Velocity Dispersion and Density Distribution of Brown Dwarfs∗

“…Using various assumptions for the initial mass function (IMF), birth rate, and cooling models (Burrows et al 1997; Baraffe et al 2003), the author confirms the relatively low number density of mid-L dwarfs, even for shallow IMFs. (2) Deacon & Hambly (2006) develop a similar calculation to model the present-day luminosity function (PDLF), which has been used to rule out extreme (halo-type) birth rates (Day-Jones et al 2013) and confirm the low space density of L/T transition dwarfs (Marocco et al 2015). The increasing number of late-T field dwarfs has been seen in several studies (e.g., Metchev et al 2008;Kirkpatrick et al 2012;Burningham et al 2013), which is seemingly a pile-up of cool dwarfs.…”

The VLT-FLAMES Tarantula Survey

“…This approach has been widely used (see e.g. Marocco et al 2015;Manick et al 2015;Robertson & Mahadevan 2014, and references therein), with our methodology being similar to that used by Marocco et al (2015).…”

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

“…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.…”