Evryscope and K2 Constraints on TRAPPIST-1 Superflare Occurrence and Planetary Habitability

Abstract

The nearby ultracool dwarf TRAPPIST-1 possesses several Earth-sized terrestrial planets, three of which have equilibrium temperatures that may support liquid surface water, making it a compelling target for exoplanet characterization. TRAPPIST-1 is an active star with frequent flaring, with implications for the habitability of its planets. Superflares (stellar flares whose energy exceeds 10 33 erg) can completely destroy the atmospheres of a cool star's planets, allowing ultraviolet radiation and highenergy particles to bombard their surfaces. However, ultracool dwarfs emit little ultraviolet flux when quiescent, raising the possibility of frequent flares being necessary for prebiotic chemistry that requires ultraviolet light. We combine Evryscope and Kepler observations to characterize the high-energy flare rate of TRAPPIST-1. The Evryscope is an array of 22 small telescopes imaging the entire Southern sky in g' every two minutes. Evryscope observations, spanning 170 nights over 2 years, complement the 80-day continuous short-cadence K2 observations by sampling TRAPPIST-1's long-term flare activity. We update TRAPPIST-1's superflare rate, finding a cumulative rate of 4.2 +1.9 −0.2 superflares per year. We calculate the flare rate necessary to deplete ozone in the habitable-zone planets' atmospheres, and find that TRAPPIST-1's flare rate is insufficient to deplete ozone if present on its planets. In addition, we calculate the flare rate needed to provide enough ultraviolet flux to power prebiotic chemistry. We find TRAPPIST-1's flare rate is likely insufficient to catalyze some of the Earthlike chemical pathways thought to lead to RNA synthesis, and flux due to flares in the biologically relevant UV-B band is orders of magnitude less for any TRAPPIST-1 planet than has been experienced by Earth at any time in its history.

Activity of TRAPPIST-1 analogue stars observed with TESS

“…And since α ≈ 2 corresponds to 1 − α ≈ −1 exponent, the border of the abiogenesis zone is approximately parallel to the FFD of TRAPPIST-1, hinting that they do not intersect, not even at higher energies. These strengthen the findings of Glazier et al (2020), that the current flare rate of TRAPPIST-1 is unlikely to cause ozone depletion or initiate abiogenesis.…”

“…Is seems that the flaring rate of TRAPPIST-1 is not sufficient to fully destroy the possible ozone layer of its planets, nor to initiate abiogenesis via UV radiation. Glazier et al (2020) reached a similar conclusion, using an upper limit for the superflare rate from Evryscope observations.…”

“…According to Rimmer et al (2018), the lower limit on the FFD where abiogenesis is possible is also a power law, with −1 exponent. Their original relation used U-band energy, but we adopt the formula of Glazier et al (2020) with E bol (using blackbody conversion factors from Osten & Wolk 2015), thus giving:…”

No Such Thing as a Simple Flare: Substructure and QPPs Observed in a Statistical Sample of 20 Second Cadence TESS Flares

“…Habitability impacts of optical stellar flares are usually estimated from the integrated energy of the flare (e.g. Howard et al 2018;Günther et al 2020;Glazier et al 2020). However, the time-resolved properties of flares can provide more detailed constraints on planetary atmospheres (Segura et al 2010;Loyd et al 2018) and surface habitability (Ranjan et al 2017;O'Malley-James & Kaltenegger 2019b,a).…”

“…Optical flares are described by a rapid-rise, exponential decay template empirically determined byDavenport et al (2014). This template or ones derived from it are routinely used to fit flare light curves (e.g Vida et al 2017;Schmitt et al 2019;Günther et al 2020;Glazier et al 2020)…”