The Effects of Starspots on Spectroscopic Mass Estimates of Young Stars

Magnetic fields and mass accretion processes create cold and hot spots on the surface of young stars, which affect the global temperatures measured on the sources. Previous studies have reported this problem on individual stars, but none has performed a large-scale study on this subject. We present an iSHELL high-resolution K-band spectroscopic survey of 42 young stars in Taurus-Aurigae and Ophiuchus star-forming regions. We use a magnetic radiative transfer code to measure the infrared temperature of the stars and their magnetic field strengths. We combine our infrared observations with optical measurements and find significant temperature differences between optical and IR values. The measured temperature differences increase with stellar temperature but also with magnetic field strength. We suggest that thermal inhomogeneities on the surface of the young stars (i.e., starspots) produce the observed temperature differences. To further understand the effects of the spots, we analyze a sub-sample of 25 stars that have dynamical masses measured with ALMA. We then use magnetic evolutionary models to compute stellar masses from optical and IR temperatures and compare them to the ALMA-derived masses. In the range of stellar masses analyzed (0.1-1.3 $\rm M_\odot$), neither IR nor optical temperature perfectly reproduces the stellar dynamical masses. However, on average, the IR temperatures provide more precise and accurate stellar masses than optical values