Fundamental parameters of young brown dwarfs: surface gravity, masses and radii from high-resolution spectroscopy
Subhanjoy Mohanty (Harvard-Smithsonian
Center for Astrophysics, USA)
Ray Jayawardhana (University of Michigan, USA)
Gibor Basri (University of California at Berkeley)
We present an analysis of high resolution optical spectra for a sample of very
young, mid- to late M, low-mass stars and brown dwarfs. Effective temperatures
and surface gravities are derived from a multi-feature spectral analysis,
through comparison with the latest synthetic spectra. Masses, radii and
luminosities are then inferred by combining the temperatures and gravities with
observed photometry, synthetic surface fluxes and distance information.
Crucially, our analysis is independent of theoretical evolutionary models
and isochrones, and thus serves as a test of the latter.
We find that: (1) Our gravities and inferred mass-radius and mass-luminosity
relationships agree remarkably well with the isochrone predictions for the
likely cluster ages for most of the sample. However, (2) the coolest objects
have gravities much lower, and radii and luminosities significantly higher, than
predicted. (3) Moreover, two of our coolest late-M targets appear to have masses
close to the deuterium-fusion boundary, much lower than the evolutionary models
suggest.
These results indicate that (1) current evolutionary models might have
significant uncertainties for very young, cool ultra-low mass objects, arising
from an inadequate treatment of accretion, convection and/or deuterium-burning
effects. Moreover, (2) the faintest late-M objects may be substantially lower
in mass than previously thought: free-floating bodies close to or below the
planetary-mass boundary may not be very rare.