X-ray Flare Driven Chemistry in Protoplanetary Discs

Author: ORCID icon orcid.org/0000-0002-1566-389X
Waggoner, Abygail, Chemistry - Graduate School of Arts and Sciences, University of Virginia
Cleeves, Ilse, AS-Astronomy (ASTR), University of Virginia

The gas and dust of protoplanetary disks surrounding solar-mass, T Tauri stars are a window into the history of the Solar System and extrasolar planetary systems. The central star plays an important role in shaping disk chemistry and physics, as its UV and X-ray radiation are strong ionizing agents. However, T Tauri stars are also highly variable, as X-ray flaring events temporarily increase disk ionization rates from a few to hundreds of factors times the quiescent rates. My thesis provides the first deep dive into the theoretical and observational consequences of time-variable flare-driven chemistry within protoplanetary disks. I use a combination of disk chemical models and radio observations to determine how individual flares impact molecules on short time scales ($\sim$weeks) and how flares cumulatively impact the molecules available during planet formation. ALMA (Cleeves et al. 2017) and SMA (PI: A. Waggoner) observations of H(13)CO+ 3-2 in the IM Lup protoplanetary disk showed that the line flux approximately doubled for a short period of time while continuum flux remained constant. Enhancement duration is unknown, since observations were spread out over a six-year period, but an X-ray flaring event is the most likely source of H(13)CO+ enhancement. I have also identified tentative HCO+ 1-0 spectral variability in the Molecules with ALMA at Planet forming Scales (MAPS) protoplanetary disks, which may be evidence of flare-driven spatial variability. While a single flare is unlikely to have a long lasting (>1 month) effect on disk chemistry, the cumulative impact of flares over relatively short astrophysical timescales (hundreds of years) could drive chemistry to a new ‘steady state.’ I wrote a stochastic X-ray flare model, XGEN, motivated by observed flare frequency and energy distribution for T Tauri stars. When I incorporated XGEN into the chemical disk model, I found that flares push chemistry to a slightly more complex state, which could help explain two chemical puzzles in disks: the high O2 seen in comets and the low degree of sulfur bearing molecules. Incorporating flare chemistry slightly enhances O2 and tends to form organosulfides in our models. Together, my observational and theoretical work support flares playing a key role in shaping the chemical inventory available to forming planets.

PHD (Doctor of Philosophy)
astrochemistry, chemistry, astronomy, chemical model, planet formation, protoplanetary disk, spectroscopy
Issued Date: