Measurement of starspot properties by forward modeling IGRINS spectra of LkCa4

Cool star light curves often show periodic variability indicative of rotational modulation from starspots entering and exiting the visible stellar disk.  T-Tauri stars exhibit some of the greatest modulations.  We computed the interquartile range of K2 cycle 2 light curves for 1658 young stellar objects or candidate young stellar objects towards Ophiuchus and Upper Scorpius.  The typical interquartile range is 1-15%.  The modulation amplitude only indicates the presence of longitudinally asymmetric starspots and therefore cannot uniquely constrain the starspot properties (Neff et al. 1995, Harrison et al. 2011).  Spectroscopy offers a way to constrain the the longitudinally symmetric component of starspots, through the spectral signature of multiple photospheric components.  We extended the modular likelihood framework, Starfish (Czekala et al. 2015), to include a two-component photosphere parameterized by an areal filling factor $f$ of starspots with a temperature contrast $\delta T_{\rm eff}$.  We applied the spectroscopic forward modeling framework to ~40 unique echelle orders of IGRINS spectroscopy of LkCa4 with R~45,000 in H- and K- bands.  A fit to each order with one photospheric component yields derived effective Temperatures in the range 4100-3300 K, with lower temperatures derived in K-band than H-band.  The two-component photospheric models yield measurements for $\delta T_{eff}$ ~1000 K and fill factors of cool spots >50\%.  The importance of starspots needs to be empirically evaluated to assess systematic biases of pre-main sequence stellar evolutionary models.