THOR 42: A Test of Magnetic Models for Pre-main-sequence Stars

Abstract

Abstract Precise masses and radii have been reported by Murphy et al. for the components of the THOR 42 binary. Murphy et al. provide evidence that, in their opinion, supports an earlier suggestion by Bell et al. that THOR 42 is a member of the 32 Ori moving group, which, according to Bell et al., has an age of about 24 Myr. From the positions of the THOR 2 components in the mass–radius diagram (MRD), Murphy et al. derive ages of 22–26 Myr using standard (nonmagnetic) models. However, they also find that these models cannot simultaneously reproduce the observed masses, radii, effective temperatures, and luminosities of the assumed coeval components. Their Hertzsprung–Russell diagram (HRD) ages are found to be 2–4 times younger than the MRD ages, and the masses determined from the HRD are found to be ∼50% smaller than the dynamical masses. Here, we attempt to replicate the precise global properties of THOR 24 by applying a model of magnetoconvection. We find that MRD isochrones and HRD isochrones lead to consistent ages provided that the surface magnetic fields have strengths of 300–400 G. However, the ages that our models yield for THOR 24 are in the vicinity of 40 Myr. If this age estimate is correct, it could call into question the membership of THOR 42 in the 32 Ori moving group.