The White-light Superflares from Cool Stars in GWAC Triggers

Abstract

M-type stars are the ones that flare most frequently, but how big their maximum flare energy can reach is still unknown. We present 163 flares from 162 individual M2 through L1-type stars that triggered the GWAC, with flare energies ranging from 10 ^32.2 to 10 ^36.4 erg. The flare amplitudes range from △ G = 0.84 to ∼10 mag. Flare energy increases with stellar surface temperature ( T _eff ) but both △ G and equivalent duration ${\mathrm{log}}_{10}(\mathrm{ED})$ seem to be independent of T _eff . Combining periods detected from light curves of TESS and K2, spectra from LAMOST, the Sloan Digital Sky Survey, the 2.16 m telescope, and the Gaia DR3 data, we found that these GWAC flare stars are young. For the stars that have spectra, we found that these stars are in or very near the saturation region, and ${\mathrm{log}}_{10}({L}_{{\rm{H}}\alpha }/{L}_{\mathrm{bol}})$ is lower for M7–L1 stars than for M2–M6 stars. We also studied the relation between GWAC flare bolometric energy E _bol and stellar hemispherical area S , and found that ${\mathrm{log}}_{10}{E}_{\mathrm{bol}}$ (in erg) increases with increasing S (in square centimeters), and the maximum flare energy ${\mathrm{log}}_{10}{E}_{\mathrm{bol},\ \max }\geqslant {\mathrm{log}}_{10}S+14.25$ . For M7–L1 stars, there seem to be other factors limiting their maximum flare energies in addition to the stellar hemispherical area.

Keywords

• Optical flares
• Red dwarf flare stars