Stellar flares observed in the Evryscope science images. Note the cosmic ray also in the right panel

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.

The multi-year Evryscope light curve (star brightness versus time) of a flare star, L 173-39. Evryscope flares are highlighted in blue, while TESS flares are highlighted in red. This flare star demonstrates how the Evryscope light curve complements the TESS light curve. While 56 days of TESS observations captures the frequent flares of lower energy, long-term Evryscope observations capture the rare, high-energy flares.

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.

 

The largest flares seen in our sample of Evryscope light curves. The largest flare to the top left emitted almost the maximum amount of energy an M-dwarf flare can physically release. The x-axis of each flare is the time in hours and the y-axis is the amount the stellar brightness increased during the flare.

Stellar rotation-induced variability in Evryscope (blue) and TESS (red) light curves. Each light curve is phase-folded to the stellar rotation period. We find the amplitude of rotation-induced variability is less in TESS data than Evryscope data as a function of stellar mass and measures starspot versus stellar temperatures.