A Search for Substellar Companions Around Pre-Main Sequence Stars Using Infrared Spectroscopy

Abstract

Observing and characterizing newly-formed planets around young stars is important for developing planet formation and evolution theory. However, given the challenges involved in detecting young planetary systems, current models are primarily based on observed systems that are billions of years old. In this dissertation, I present a survey to characterize pre-main sequence stars, and detect or confirm young substellar companions using the radial velocity (RV) method. This survey used the Immersion Grating Infrared Spectrograph (IGRINS), which was deployed as a visiting instrument at the 4.3-m Lowell Discovery Telescope (LDT). Infrared spectroscopy is preferable to optical because it is less affected by RV variability triggered by stellar activity effects, such as starspots. The initial survey consisted of 70 pre-main sequence stars with ages <5 Myr in the relatively nearby Taurus star-forming region. I measured the projected rotational velocity (vsini) of these pre-main sequence stars, and related the rotation (and other properties) to young star evolution. Additionally, I investigated a subset of 10 targets to look for substellar companions. The RV survey is generally sensitive to hot Jupiters with mass >6 MJ and brown dwarfs within ∼1 AU. No companions were detected in this sample, however, upper limits on occurrence rates were estimated based on injection/recovery simulations, and are consistent with multiple theories of formation and evolution. Correlations between stellar properties and RV scatter indicate that rotation, stellar activity, and disk accretion are possible contributing factors to RV variability. The average infrared RV scatter is roughly half the typical optical RV scatter for very young stars, confirming the potential of infrared RV surveys to detect young substellar companions.