Dust Opacity Underestimated in Brown Dwarf Atmopsheres

Testing Model Atmospheres for Young Very Low Mass Stars and Brown Dwarfs in the Infrared: Evidence for Significantly Underestimated Dust Opacities

Authors:

Tottle et al

Abstract:

We test state-of-the-art model atmospheres for young very low-mass stars and brown dwarfs in the infrared, by comparing the predicted synthetic photometry over 1.2-24 {\mu}m to the observed photometry of M-type spectral templates in star-forming regions. We find that (1) in both early and late young M types, the model atmospheres imply effective temperatures (Teff) several hundred Kelvin lower than predicted by the standard Pre-Main Sequence spectral type-Teff conversion scale (where the latter is based on theoretical evolutionary models). It is only in the mid-M types that the two temperature estimates agree. (2) The Teff discrepancy in the early M types (corresponding to stellar masses above 0.6 Msol at a few Myr) probably arises from remaining uncertainties in the treatment of atmospheric convection. The agreement in the mid-M types implies a reasonably good atmospheric modeling of H2O opacities, which dominate in the infrared at these spectral types. Conversely, the Teff discrepancy in the late M types is likely due to an underestimation of dust opacity in the atmospheric models, as well as an attendant overestimation of H2O opacity, due to insufficient backwarming by dust. While magnetic field/activity effects can cause Teff in M types to be lower than evolutionary models predict, this cannot explain the magnitude or the spectral type-dependency of the above Teff discrepancy. (3) The empirical and model-atmosphere J-band bolometric corrections are both roughly flat, and similar to each other, over the M-type Teff range. Thus the model atmospheres yield reasonably accurate bolometric luminosities (Lbol), but tend to overestimate radii (due to underestimated Teff), and underestimate age and mass for the early and (especially) late young M types compared to evolutionary theory; we demonstrate this by placing a large sample of young Cha I/Taurus sources on an HR diagram.