Confronting Substellar Theoretical Models with Stellar Ages

TL;DR

This study tests substellar models using a brown dwarf binary with a well-determined age and mass, revealing potential underpredictions of luminosity by current models, which could impact broader astrophysical research.

Contribution

It provides the first precise mass, luminosity, and age measurements for a field brown dwarf binary, highlighting possible systematic errors in substellar evolutionary models.

Findings

Substellar models may underpredict brown dwarf luminosity by 2-3 times.

HD 130948BC is a unique benchmark with known mass, luminosity, and age.

Systematic errors in models could affect initial mass function and exoplanet radius predictions.

Abstract

By definition, brown dwarfs never reach the main-sequence, cooling and dimming over their entire lifetime, thus making substellar models challenging to test because of the strong dependence on age. Currently, most brown dwarfs with independently determined ages are companions to nearby stars, so stellar ages are at the heart of the effort to test substellar models. However, these models are only fully constrained if both the mass and age are known. We have used the Keck adaptive optics system to monitor the orbit of HD 130948BC, a brown dwarf binary that is a companion to the young solar analog HD 130948A. The total dynamical mass of 0.109+/-0.003 Msun shows that both components are substellar, and the ensemble of available age indicators from the primary star suggests an age comparable to the Hyades, with the most precise age being 0.79 Gyr based on gyrochronology. Therefore, HD 130948BC is unique among field L and T dwarfs as it possesses a well-determined mass, luminosity, and age. Our results indicate that substellar evolutionary models may underpredict the luminosity of brown dwarfs by as much as a factor of ~2-3x. The implications of such a systematic error in evolutionary models would be far-reaching, for example, affecting determinations of the initial mass function and predictions of the radii of extrasolar gas-giant planets. This result is largely based on the reliability of stellar age estimates, and the case study of HD 130948A highlights the difficulties in determining the age of an arbitrary field star, even with the most up-to-date chromospheric activity and gyrochronology relations. In order to better assess the potential systematic errors present in substellar models, more refined age estimates for HD 130948A and other stars with binary brown dwarf companions (e.g., eps Ind Bab) are critically needed.