UNC-CH Physics and Astronomy Thesis Defense

Ward Howard

“Investigating exoplanet habitability and the stellar magnetism of cool stars across half the Southern sky via superflares, starspots, and stellar rotation”

Stellar flares are stochastic events that occur when a star’s magnetic field re-connects, releasing intense radiation across the electromagnetic spectrum. Rocky planets in the habitable zones of M-dwarfs are often subjected to superflares, events of at least 10^33 erg and 10-1000X the energy of the largest solar flares. Frequent superflares can erode the ozone layer of an Earth-like atmosphere and allow lethal amounts of UV flux to reach the surface. Conversely, too few flares may result in insufficient UV radiation to power pre-biotic chemistry due to the inherent faintness of M-dwarfs in the UV. Cool stars are often found to exhibit superflares. Cool stars are the most common type of star, and are known to frequently host rocky planets. As a result, they may host most of the universe’s Earth-size planets orbiting in the habitable zones of main sequence stars. My EvryFlare Survey uses observations from the Evryscope array of small telescopes and the Transiting Exoplanet Survey Satellite (TESS) to answer two questions about superflares and their impacts on the habitability of terrestrial planets orbiting cool stars: (1) How frequently are superflares emitted from the nearby cool stars, both in the present and in the first 200 Myr after formation? (2) What impact does superflare UV emission have on planetary atmospheres and surface habitability of planets orbiting cool stars? The EvryFlare Survey has resulted in the detection of 575 superflares from 284 stars. Results include a superflare from Proxima Cen, the nearest host star to a rocky planet in the habitable zone. I used these events to measure a decrease in superflare rates with increasing age, rotation, and starspot coverage. I will discuss the effects of superflares on ozone loss to planetary atmospheres, including one superflare with sufficient energy to photo-dissociate all ozone in an Earth-like atmosphere in a single event. I present the largest-ever survey of simultaneous observations of dozens of M-dwarf superflares with Evryscope and TESS to measure the flare blackbody and estimate UV-C continuum emission. I find superflare temperatures increase with flare energy. The largest and hottest flare briefly reached an estimated 42,000 K. During superflares, I estimate rocky HZ planets orbiting <200 Myr stars typically receive a top-of-atmosphere UV-C flux of ~120 W/m^2 and up to 10^3 W/m^2, 100-1000X the time-averaged XUV flux from Proxima Cen. Finally, I will describe a data analysis project with Robo-AO, exploring the performance of laser guide star adaptive optics systems in the absence of tip-tilt correction.