Bibcode
Marocco, F.; Jones, H. R. A.; Day-Jones, A. C.; Pinfield, D. J.; Lucas, P. W.; Burningham, B.; Zhang, Z. H.; Smart, R. L.; Gomes, J. I.; Smith, L.
Referencia bibliográfica
Monthly Notices of the Royal Astronomical Society, Volume 449, Issue 4, p.3651-3692
Fecha de publicación:
6
2015
Número de citas
56
Número de citas referidas
49
Descripción
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 (0.30-2.48 μm) 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 of 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)
× 10-3 objects per cubic parsec in the T1-T4.5 range.
We notice that there seems to be an excess of objects in the L-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-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.
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