Dynamical Masses of Young Stars I: Discordant Model Ages of Upper Scorpius

TL;DR

This study uses orbital monitoring and Bayesian analysis to determine the ages and masses of binary stars in Upper Scorpius, revealing a discrepancy in model ages between G- and M-type stars.

Contribution

It provides new orbital and age measurements for binary stars in Upper Scorpius, highlighting inconsistencies in stellar evolution models across different spectral types.

Findings

G-type binaries have ages ~11.5 Myr, consistent with the region's age.

M-type binaries have younger ages ~7 Myr, indicating model discrepancies.

Luminosity and temperature predictions in models are inconsistent for M-type stars.

Abstract

We present the results of a long term orbit monitoring program, using sparse aperture masking observations taken with NIRC2 on the Keck-II telescope, of seven G to M-type members of the Upper Scorpius subgroup of the Sco-Cen OB association. We present astrometry and derived orbital elements of the binary systems we have monitored, and also determine the age, component masses, distance and reddening for each system using the orbital solutions and multi-band photometry, including Hubble Space Telescope photometry, and a Bayesian fitting procedure. We find that the models can be forced into agreement with any individual system by assuming an age, but that age is not consistent across the mass range of our sample. The G-type binary systems in our sample have model ages of ~11.5 Myr, which is consistent with the latest age estimates for Upper Scorpius, while the M-type binary systems have significantly younger model ages of ~7 Myr. Based on our fits, this age discrepancy in the models corresponds to a luminosity under-prediction of 0.8-0.15 dex, or equivalently an effective temperature over-prediction of 100-300 K for M-type stars at a given premain-sequence age. We also find that the M-type binary system RXJ 1550.0-2312 has an age (~16 Myr) and distance (~90 pc) indicating that it is either a nearby young binary system or a member of the Upper-Centaurus-Lupus subgroup with a 57% probability of membership.