Masses and Implications for Ages of Low-mass Pre-main-sequence Stars in Taurus and Ophiuchus

The accuracy of masses of pre-main-sequence 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 ± 0.02 M _⊙. The ages of the stars, 3-10 MY on tracks that include magnetic fields, is older than the 1-3 MY indicated by the nonmagnetic 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.