Talk abstract
Testing Theory with Dynamical Masses and Orbits of Ultracool Binaries
Abstract
Direct mass measurements are essential to test the evolutionary and
atmospheric models that underpin studies of very low-mass objects. I
will present results from our program to test models using precise
dynamical masses (as good as 2%) for ultracool binaries, based on IR
parallaxes from CFHT, near-IR spectroscopy, and Keck laser guide star
AO astrometry for a sample of over 30 objects since 2005. In just the
last 2 years, we have more than tripled the number of late-M, L, and T
dwarf binaries with dynamical masses. We find that the temperatures
predicted by evolutionary models for most field binaries of known mass
are discrepant with those derived from fitting the observed spectra
with model atmospheres, indicating systematic errors of ~200 K (or
30--40% in radius). For the rare field binaries in triple systems that
have age determinations from their solar-type primaries, we find that
evolutionary models systematically underpredict luminosities by a
factor of ~2 at a given mass, which means that model-based substellar
mass determinations (e.g., for the low-mass IMF) may be systematically
overestimating masses. Finally, we have employed our large sample of
binary orbits to carry out a novel test of the earliest evolutionary
stages, by using the distribution of orbital eccentricities to
distinguish between competing models of brown dwarf formation.
atmospheric models that underpin studies of very low-mass objects. I
will present results from our program to test models using precise
dynamical masses (as good as 2%) for ultracool binaries, based on IR
parallaxes from CFHT, near-IR spectroscopy, and Keck laser guide star
AO astrometry for a sample of over 30 objects since 2005. In just the
last 2 years, we have more than tripled the number of late-M, L, and T
dwarf binaries with dynamical masses. We find that the temperatures
predicted by evolutionary models for most field binaries of known mass
are discrepant with those derived from fitting the observed spectra
with model atmospheres, indicating systematic errors of ~200 K (or
30--40% in radius). For the rare field binaries in triple systems that
have age determinations from their solar-type primaries, we find that
evolutionary models systematically underpredict luminosities by a
factor of ~2 at a given mass, which means that model-based substellar
mass determinations (e.g., for the low-mass IMF) may be systematically
overestimating masses. Finally, we have employed our large sample of
binary orbits to carry out a novel test of the earliest evolutionary
stages, by using the distribution of orbital eccentricities to
distinguish between competing models of brown dwarf formation.