Space weather for every flare star in the South
Like the Sun, stars flare and emit coronal mass ejections (CME) that impact planet atmospheres. Cool stars are smaller, dimmer, and redder than the Sun and many frequently emit superflares, events 10-1000X the energies of the largest Solar flares. Cool stars are the most common type of star, making up 75% of the stars in the universe. They also host most of the universe’s Earth-size planets. Stellar flares are thought to create a “flare habitable zone” not unlike the traditional temperature-based habitable zone where planets are too warm or too cold to maintain surface life. In the flare habitable zone, too many stellar flares may dissociate or even strip away Earth-like planetary atmospheres. Too few stellar flares leads to insufficient UV light to power pre-biotic chemistry necessary for life on worlds orbiting cool stars. I use two-minute cadence optical observations from the Evryscope systems and from NASA TESS to answer two key questions as part of the EvryFlare Survey.
(1) How frequent are superflares around each of the nearby cool stars, especially those with rocky planets in the habitable zone? These worlds are the only ones bright and close enough to study the planetary atmosphere with next-generation extremely large telescopes or missions such as the James Webb Space Telescope.
(2) What danger do superflares pose for planetary atmospheres and for surface life on planets orbiting nearby cool stars?
Evryscope is uniquely suited to determine the long-term stellar flaring of nearby stars with planets due to the high cadence of observations (2 minutes) and multi-year span of observations. TESS contributes the smaller events that fall below Evryscope’s detection capabilities from the ground.
Key results from the EvryFlare Survey publications
Across the sky, we report approximately twice the previous largest number of AD-Leo type superflares (i.e. 100X the biggest Solar flares) observed at high-cadence from nearby cool stars. We find 8 unusually-extreme flares with amplitudes of 3+ magnitudes in the optical blue, with the largest reaching 5.6 magnitudes and releasing 10^36 erg. We measure the superflare rate per flare-star and quantify the superflare properties of TESS-planet-search stars as a function of spectral type. We observe 14.6±2% of the stars around which TESS may discover temperate rocky planets emit flares large enough to significantly affect the potential habitability of those planets. We observe 17 stars that may deplete an Earth-like atmosphere via repeated flaring, including one superflare with sufficient energy to photo-dissociate all ozone in an Earth-like atmosphere in a single event.
We observe a decrease in superflare rates and energies as stellar rotation periods increase, observing a possible change in the superflare rates of M-dwarfs at periods corresponding to the spin-down transition from quickly-rotating to slowly-rotating states. We also compare the amplitudes of rotation-induced variability of each flare star in the Evryscope and TESS bands. We find the Evryscope amplitudes of variability are larger than those in TESS data; we observe the effect is correlated with stellar mass and is likely due to the difference between stellar temperatures and starspot temperatures. We measure a median starspot coverage of 13% of the stellar hemisphere and constrain the minimum magnetic field strength consistent with our flare energies and starspot coverage to be 500 G, with later-type stars exhibiting lower values than earlier-types.