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arXiv · 1904.10421

EvryFlare I: Long-term Evryscope Monitoring of Flares from the Cool Stars Across Half the Southern Sky

Ward S. Howard·Hank Corbett·Nicholas M. Law·Jeffrey K. Ratzloff·Amy L. Glazier·Octavi Fors·Daniel del Ser·Joshua Haislip
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Abstract

We search for superflares from 4,068 cool stars in 2+ years of Evryscope photometry, focusing on those with high-cadence data from both Evryscope and TESS. The Evryscope array of small telescopes observed 575 flares from 284 stars, with a median energy of 10^34.0 erg. Since 2016, Evryscope has enabled the detection of rare events from all stars observed by TESS through multi-year, high-cadence continuous observing. We report ~2X the previous largest number of 10^34 erg high-cadence flares from nearby cool stars. We find 8 flares with amplitudes of 3+ g' magnitudes, with the largest reaching 5.6 magnitudes and releasing 10^36.2 erg. We observe a 10^34 erg superflare from TOI-455 (LTT 1445), a mid-M with a rocky planet candidate. We measure the superflare rate per flare-star and quantify the average flaring of active stars as a function of spectral type, including superflare rates, FFDs, and typical flare amplitudes in g'. We confirm superflare morphology is broadly consistent with magnetic re-connection. We estimate starspot coverage necessary to produce superflares, and hypothesize maximum-allowed superflare energies and waiting-times between flares corresponding to 100% coverage of the stellar hemisphere. We observe decreased flaring at high galactic latitudes. We explore the effects of superflares on ozone loss to planetary atmospheres: we observe 1 superflare with sufficient energy to photo-dissociate all ozone in an Earth-like atmosphere in one event. We find 17 stars that may deplete an Earth-like atmosphere via repeated flaring. Of the 1822 stars around which TESS may discover temperate rocky planets, we observe 14.6% +/- 2% emit large flares.

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Ward S. Howard, Hank Corbett, Nicholas M. Law, Jeffrey K. Ratzloff, Amy L. Glazier, Octavi Fors, Daniel del Ser, Joshua Haislip. 2019-04-23. EvryFlare I: Long-term Evryscope Monitoring of Flares from the Cool Stars Across Half the Southern Sky. https://doi.org/10.3847/1538-4357%2Fab2767

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