Pre-main-sequence Stars in Taurus: Comparison of Magnetic and Nonmagnetic Model Fits to the Low-mass Stars

Stellar associations should be co-eval, but some associations exhibit apparently discrepant ages: M stars appear younger than FG stars. Among M stars in the Upper Sco association (age 11 Myr), magnetic effects have been shown to slow down evolution of the lowest mass stars, thereby alleviating the age discrepancy. A recent study of pre-main-sequence (PMS) stars in the much younger Taurus star-forming region (SFR) suggest that an age discrepancy may also exist there. Our goal is to compare two approaches to magnetic/nonmagnetic modeling of the Taurus stars. In one approach, Simon et al. concluded that an age discrepancy exists, but it can be resolved by magnetic models if one imposes an "equipartition condition" on the photospheric field of each star. In this paper, we examine an independent sample of seven Taurus PMS stars reported by Rizzuto et al. Our approach to modeling magneto-convection imposes no condition on the photospheric field strength. We find that nonmagnetic isochrones can successfully fit the stars in the Rizzuto et al. sample with ages 1.6-2.4 Myr, without the need to invoke magnetic effects. An upper limit on the vertical field strength on the surfaces of the Rizzuto stars is found to be similar to 300 G for the low-mass secondaries of FF Tau and HP Tau/G2 and similar to 100 G for the other, more massive stars. The Taurus SFR is so large (extending over many square degrees in the sky) that magnetic conditions may plausibly differ from star to star.