Connecting the Dots: Investigating Planet Formation and Composition Through Observations of Carbon and Oxygen Species in Stars, Disks, and Planets

Abstract

What physical processes and sources of material contribute to exoplanet compositions? Specifically, what roles do the protoplanetary disk composition and structure, and host star abundances play in the different stages of planet formation? In this thesis, beginning with a brief literature review in Chapter 2, I trace oxygen and carbon species through these stages to inform how, when, and where planets form. In Chapter 3 I describe a study of the molecular emission from the warm inner disks of T Tauri stars, where terrestrial planets likely form. I report moderate correlations between HCN emission strength and both stellar accretion rate (measured from UV or optical excess emission associated with accretion) and X-ray luminosity. These correlations point towards accretion related processes being an important source of disk atmosphere heating, and suggests that efficient H₂O formation and/or UV dissociation of N₂ (both also associated with higher stellar accretion rates) may aid in the production of HCN. Studies following mine have further connected the abundance of HCN versus H₂O to the growth and migration of planetesimals in the disk, which helps control the formation of both giant and terrestrial planets. I shift to an already-formed exoplanet in Chapter 4, where I present optical photometry of the best-observed transiting super-Earth GJ 1214b with the goal of constraining the short-wavelength slope of its transmission spectrum. Most previous observations suggested a flat spectrum from the near-IR to the optical, corresponding to a low-scale-height, high-molecular-weight atmosphere. My observations are in general agreement with these findings, keeping the "door open" for a H₂O-rich atmosphere for GJ 1214b, which other published g-band observations appeared to contradict. Chapters 5-7 of my thesis focus on measuring stellar abundances, particularly C/O ratios, in transiting (mostly) hot Jupiter exoplanet host stars from high resolution optical spectroscopy. Host star abundances may indicate the precursor materials present in the disk and available for incorporation into planets. In hot Jupiters, the C/O ratio affects the partitioning of C in the major observable molecules, making C and O diagnostic of temperature structure and composition. I also demonstrate that extra caution is necessary in deriving carbon and oxygen abundances, especially for cool and metal-rich stars. Though exoplanetary C/O ratios are still uncertain, the more precise abundance analysis possible right now for their host stars can help constrain their formation environments and current compositions. I summarize my graduate school research in Chapter 8, and discuss the next steps I will take in my postdoctoral career.