Dynamical Masses of Young Stars II: Young Taurus Binaries Hubble~4, FF~Tau, and HP~Tau/G3

TL;DR

This study measures the dynamical masses of three young Taurus binary stars using VLBI parallaxes and compares these with stellar models, revealing systematic discrepancies and suggesting revisions to pre-main sequence evolutionary tracks.

Contribution

First dynamical mass measurements for these Taurus binaries with high precision, highlighting model inaccuracies and age estimation issues for young convective stars.

Findings

Dynamical masses agree with models within 1-5% uncertainties.

Hubble~4 may be a triple system due to spectral inconsistencies.

Model ages for secondaries are younger than primaries and older than associated G-type star.

Abstract

One of the most effective ways to test stellar evolutionary models is to measure dynamical masses for binary systems at a range of temperatures. In this paper, we present orbits of three young K+M binary systems in Taurus (Hubble~4, FF~Tau, and HP~Tau/G3) with VLBI parallaxes. We obtained precision astrometry with Keck-II/NIRC2, optical photometry with HST/WFC3, and low-resolution optical spectra with WIFeS on the ANU 2.3 m telescope. We fit orbital solutions and dynamical masses with uncertainties of 1-5% for the three binary systems. The spectrum, photometry, and mass for Hubble~4 are inconsistent with a binary system, suggesting that it may be a triple system where the primary component consists of two stars. For HP~Tau/G3 and FF~Tau, model masses derived from SED determined component temperatures and luminosities agree with the dynamical masses, with a small offset towards larger model masses. We find model ages for the primary components of these systems of $\sim$3 Myr, but find that the secondaries appear younger by a factor of two. These estimates also disagree with the age of the physically associated G-type star HP~Tau/G2, which is older ($\sim$5 Myr) according to the same models. This discrepancy is equivalent to a luminosity under-prediction of 0.1-0.2 dex, or a temperature over-prediction of 100-300 K, for K/M-type stars at a given model age. We interpret this as further evidence for a systematic error in pre-main sequence evolutionary tracks for convective stars. Our results reinforce that the ages of young populations determined from the locus of M-type members on the HR-diagram may require upward revision.