Improving dynamical mass constraints for intermediate-period substellar companions using Gaia DR2

TL;DR

This paper demonstrates how Gaia DR2 data, combined with existing orbital information, can refine the dynamical mass estimates of intermediate-period substellar companions like GJ 758 B, improving mass constraints and revealing age discrepancies.

Contribution

It introduces a method to use Gaia-HIPPARCOS differential proper motion to better constrain the mass of substellar companions with existing orbital data.

Findings

Dynamical mass of GJ 758 B refined to 42.4^{+5.6}_{-5.0} M_Jup.

Significant age tension between star and companion inferred from models.

Gaia-HIPPARCOS data effectively constrains intermediate-separation companion masses.

Abstract

The relationship between luminosity and mass is of fundamental importance for direct imaging studies of brown dwarf and planetary companions to stars. In principle this can be inferred from theoretical mass-luminosity models; however, these relations have not yet been thoroughly calibrated, since there is a lack of substellar companions for which both the brightness and mass have been directly measured. One notable exception is GJ 758 B, a brown dwarf companion in a $\sim$20 AU orbit around a nearby Sun-like star, which has been both directly imaged and dynamically detected through a radial velocity trend in the primary. This has enabled a mass constraint for GJ 758 B of 42$^{+19}_{-7}$ $M_{\rm Jup}$. Here, we note that Gaia is ideally suited for further constraining the mass of intermediate-separation companions such as GJ 758 B. A study of the differential proper motion, $\Delta \mu$, with regards to HIPPARCOS is particularly useful in this context, as it provides a long time baseline for orbital curvature to occur. By exploiting already determined orbital parameters, we show that the dynamical mass can be further constrained to $42.4^{+5.6}_{-5.0}\, M_{\rm Jup}$ through the Gaia-HIPPARCOS $\Delta \mu$ motion. We compare the new dynamical mass estimate with substellar evolutionary models and confirm previous indications that there is significant tension between the isochronal ages of the star and companion, with a preferred stellar age of $\leq 5$ Gyr while the companion is only consistent with very old ages of $\geq 8$ Gyr.