Orbital characterization of GJ1108A system, and comparison of dynamical mass with model-derived mass for resolved binaries

TL;DR

This study characterizes the orbit of the young low-mass binary GJ1108A, compares its dynamical mass with model predictions, and discusses implications for stellar evolutionary models and age estimates.

Contribution

It provides the first orbital solution for GJ1108Aab using combined astrometry and radial velocities, and evaluates the accuracy of evolutionary models for low-mass binaries.

Findings

Dynamical masses are higher than model predictions assuming Columba group age.

The system is likely older than the Columba group, affecting mass estimates.

Current evolutionary models generally reproduce low-mass binary masses without significant offsets.

Abstract

We report an orbital characterization of GJ1108Aab that is a low-mass binary system in pre-main-sequence phase. Via the combination of astrometry using adaptive optics and radial velocity measurements, an eccentric orbital solution of $e$=0.63 is obtained, which might be induced by the Kozai-Lidov mechanism with a widely separated GJ1108B system. Combined with several observed properties, we confirm the system is indeed young. Columba is the most probable moving group, to which the GJ1108A system belongs, although its membership to the group has not been established. If the age of Columba is assumed for GJ1108A, the dynamical masses of both GJ1108Aa and GJ1108Ab ($M_{\rm dynamical,GJ1108Aa}=0.72\pm0.04 M_{\odot}$ and $M_{\rm dynamical,GJ1108Ab}=0.30\pm0.03 M_{\odot}$) are more massive than what an evolutionary model predicts based on the age and luminosities. We consider the discrepancy in mass comparison can attribute to an age uncertainty; the system is likely older than stars in Columba, and effects that are not implemented in classical models such as accretion history and magnetic activity are not preferred to explain the mass discrepancy. We also discuss the performance of the evolutionary model by compiling similar low-mass objects in evolutionary state based on the literature. Consequently, it is suggested that the current model on average reproduces the mass of resolved low-mass binaries without any significant offsets.