Astron. J., 137, 3345-3357 (2009/February-0)
Young L dwarfs identified in the field: a preliminary low-gravity, optical spectral sequence from L0 to L5.
CRUZ K.L., KIRKPATRICK J.D. and BURGASSER A.J.
Abstract (from CDS):
We present an analysis of 23 L dwarfs whose optical spectra display unusual features. Twenty-one were uncovered during our search for nearby, late-type objects using the Two Micron All Sky Survey, while two were identified in the literature. The unusual spectral features, notably weak FeH molecular absorption and weak Na I and K I doublets, are attributable to low gravity and indicate that these L dwarfs are young, low-mass brown dwarfs. We use these data to expand the spectral classification scheme for L0- to L5-type dwarfs to include three gravity classes. Most of the low-gravity L dwarfs have southerly declinations and distance estimates within 60 pc. Their implied youth, on-sky distribution, and distances suggest that they are members of nearby, intermediate-age (∼10-100 Myr), loose associations such as the β Pictoris moving group, the Tucana/Horologium Association, and the AB Doradus moving group. At an age of 30 Myr and with effective temperatures from 1500 to 2400 K, evolutionary models predict masses of 11-30 MJupiter for these objects. One object, 2M 0355+11, with J - Ks= 2.52±0.03, is the reddest L dwarf found in the field and its late spectral type and spectral features indicative of a very low gravity suggest it might also be the lowest mass field L dwarf. However, before ages and masses can be confidently adopted for any of these low-gravity L dwarfs, additional kinematic observations are needed to confirm cluster membership.
open clusters and associations: general - stars: fundamental parameters - stars: late-type - stars: low-mass, brown dwarfs
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