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