Presentation #515.03D in the session “Stellar Rotation, Variability, and Flares 2”.
I performed the EvryFlare Survey using observations from the Evryscope array of small telescopes and the Transiting Exoplanet Survey Satellite (TESS) to answer two questions: (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? Stellar flares are stochastic events that occur when a star’s magnetic field re-connects, releasing radiation across the electromagnetic spectrum. Rocky planets in the habitable zones (HZ) of M-dwarfs are often subjected to superflares, events of at least 1033 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 often 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 HZ planets. The EvryFlare Survey detected hundreds of superflares from hundreds of nearby cool stars, allowing me to measure the dependence of superflare rates on stellar mass, age, rotation, and starspot coverage. The EvryFlare survey detected the first superflare from Proxima Cen, the nearest host star to a rocky planet in the habitable zone. I will discuss the effects of superflares on ozone loss to planetary atmospheres, including that of Proxima b. I present the largest-ever survey of simultaneous observations of dozens of M-dwarf superflares with Evryscope and TESS. Statistical scaling laws between energy and temperature demonstrate for the first time in a large sample of M-dwarf superflares that higher-energy events are hotter events. I quantify the amount of time superflares emit at very high temperatures, and determine the UV-C flux reaching the HZ of young stars reaches values up to 103 W m-2, 1000X the time-averaged XUV flux from Proxima Cen.