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

TL;DR

This study evaluates infrared model atmospheres for young low-mass stars and brown dwarfs, revealing underestimated dust opacities and temperature discrepancies that impact derived stellar properties.

Contribution

It identifies significant underestimation of dust opacity in current models and quantifies temperature and luminosity discrepancies for young low-mass stars and brown dwarfs.

Findings

Model atmospheres underestimate dust opacity in late M types.

Effective temperatures are several hundred Kelvin lower than standard scales.

Bolometric luminosities are accurate, but mass and age estimates are underestimated.

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 (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 0.4 Msol at ages of a few Myr) probably arises from remaining uncertainties in the treatment of atmospheric convection within the atmospheric models, whereas in the late M types it is likely due to an underestimation of dust opacity. (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 lead to underestimations of mass and age relative to evolutionary expectations (especially in the late M types) due to lower Teff. We demonstrate this for a large sample of young Cha I and Taurus sources. (4) The trends in the atmospheric model J-K colors, and their deviations from the data, are similar at Pre-Main Sequence and Main Sequence ages, suggesting that the model dust opacity errors we postulate here for young ages also apply at field ages.