Disentangling spectra of young stars: Unveiling the accretion spectrum with Gaussian Processes

The study of the spectra of young, actively accreting stars can constrain their stellar properties and offer insight on how pre-main sequence stars evolve. Spectroscopic observations simultaneously measure the sum of the intrinsic photospheric flux and any emission resulting from hot gas shocking as it accretes onto the star from its protoplanetary disk. These accretion signatures "veil," or fill in, the stellar photosphere and complicate measurements of stellar properties like effective temperature and metallicity. In practice, the veiling has traditionally been assumed to be a simple, constant continuum excess. We develop a statistically rigorous analytical framework using Gaussian Processes to analyze time series photometry and high resolution optical spectra of the T Tauri star LkCa 15. We identify veiling features which selectively fill in photospheric lines, challenging the traditional assumption of a smooth veiling continuum, and implying that detailed spectroscopic measurements are necessary to accurately characterize the accretion rate onto the star. Moreover, the K2 mission has recently revealed LkCa 15 to be a “dipper star,” a member of a class of photometric variables that dim by a factor of two or more over a several day time scale. As many as 30% of young stars are expected to exhibit the dipper phenomenon, which is believed to be the result of obscuration by magnetospherically accreting material from the protoplanetary disk. By better characterizing the accretion spectrum and the spectroscopic signatures of the dimming events, we will enable a more accurate determination of LkCa 15’s fundamental stellar properties. Via our open source Gaussian Process framework, we aspire to provide a framework for detailed characterization of accretion in young stars.