EvryFlare. III. Temperature Evolution and Habitability Impacts of Dozens of Superflares Observed Simultaneously by Evryscope and TESS

Abstract

Superflares may provide the dominant source of biologically relevant UV radiation to rocky habitable-zone M-dwarf planets (M-Earths), altering planetary atmospheres and conditions for surface life. The combined line and continuum flare emission has usually been approximated by a 9000 K blackbody. If superflares are hotter, then the UV emission may be 10 timeshigher than predicted from the optical. However, it is unknown for how long M-dwarf superflares reach temperatures above 9000 K. Only a handful of M-dwarf superflares have been recorded with multiwavelength high-cadence observations. We double the total number of events in the literature using simultaneous Evryscope and Transiting Exoplanet Survey Satellite observations to provide the first systematic exploration of the temperature evolution of M-dwarf superflares. We also increase the number of superflaring M dwarfs with published time-resolved blackbody evolution by ∼10×. We measure temperatures at 2 minutes cadence for 42 superflares from 27 K5-M5 dwarfs. We find superflare peak temperatures (defined as the mean of temperatures corresponding to flare FWHM) increase with flare energy and impulse. We find the amount of time flares emit at temperatures above 14,000 K depends on energy. We discover that 43% of the flares emit above 14,000 K, 23% emit above 20,000 K and 5% emit above 30,000 K. The largest and hottest flare briefly reached 42,000 K. Some do not reach 14,000 K. During superflares, we estimate M-Earths 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 , 100-1000 timesthe timeaveraged X-ray and UV flux from Proxima Cen.

Stellar flares detected with the Next Generation Transit Survey

“…However, while these studies have shown that the optical spectrum of many flares can be broadly approximated with a 9000-10 000 K blackbody, the flare temperature has been observed to change both between and within individual flare events to values outside this range (e.g. Howard et al 2020a). We note that while this limits the use of a single temperature model, we currently cannot predict these changes in the flare temperature, preventing the use of more complex models.…”

Spots and Flares in Hot Main Sequence Stars Observed by Kepler, K2, and TESS

“…If the flux distribution in the flare corresponds to a blackbody with temperature higher than about 10,000 K, the true energies will be larger. Thus, Schmitt et al (2019) adopt a blackbody flux distribution corresponding to 12,000 K, while Froning et al (2019) and Howard et al (2020) use 40,000 K. From Table 3 the range in flare energy is 10 33.5 to 10 36 erg with a mean of 10 34.5 erg, but these may be just lower limits.…”

The extreme-ultraviolet stellar characterization for atmospheric physics and evolution (ESCAPE) mission concept

“…34 Different emission lines and continua vary by different factors on all timescales, while connections between the flare energy and frequency distribution of white light flares (such as those observed with Kepler/K2 and T ESS) and UV flares remain poorly constrained by observation and theory. [35][36][37][38] Estimates of the EUV response of the Great AD Leo Flare of 1985 39 observed with the International Ultraviolet Explorer, IU E, indicate that the atmospheres of planets orbiting very active stars with frequent flares never achieve a steady state because the timescales for atmospheric recovery are much longer than the time between successive flares. 40 EU V E provided constraints on flare frequencies for a small sample of young, active stars, 41 Characterization of the EUV flare frequency and spectral variability on stellar evolutionary timescales requires (1) directly collecting EUV spectra over long enough temporal baselines to monitor variability and (2) a broad sample of stars with a range of masses and ages (activity levels, e.g., West et al 2015).…”