Masses and Implications for Ages of Low-Mass Pre-Main Sequence Stars in
  Taurus and Ophiuchus
  The accuracy of masses of pre-main sequence (PMS) stars derived from their
locations on the Hertzsprung-Russell Diagram (HRD) can be tested by comparison
with accurate and precise masses determined independently. We present 29 single
stars in the Taurus star-forming region (SFR) and 3 in the Ophiuchus SFR with
masses measured dynamically to a precision of at least $10 \%$. Our results
include 9 updated mass determinations and 3 that have not had their dynamical
masses published before. This list of stars with fundamental, dynamical masses,
M$_{dyn}$, is drawn from a larger list of 39 targets in the Taurus SFR and 6 in
the Ophiuchus SFR. Placing the stars with accurate and precise dynamical masses
on HRDs that do not include internal magnetic fields underestimates the mass
compared to M$_{dyn}$ by about $30 \%$. Placing them on an HRD that does
include magnetic fields yields mass estimates in much better agreement with
M$_{dyn}$, with an average difference between M$_{dyn}$ and the estimated track
mass of $0.01\pm0.02$~\msun. The ages of the stars, 3--10 MY on tracks that
include magnetic fields, is older than the 1--3 MY indicated by the
non-magnetic models. The older ages of T Tauri stars predicted by the magnetic
models increase the time available for evolution of their disks and formation
of the giant gas exoplanets. The agreement between our M$_{dyn}$ values and the
masses on the magnetic field tracks provides indirect support for these older
ages.
