The Largest M Dwarf Flares from ASAS-SN
Drout, Maria R.; Kochanek, C. S.; Madore, Barry F.; van Saders, Jennifer L.; Stanek, K. Z.; Shappee, Benjamin J.; Brown, Jonathan S.; Dong, Subo; Seibert, Mark; Simonian, Gregory V. A.; Holoien, T. W. -S.; Prieto, J. L.; Johnson, Sean; Schmidt, Sarah J.; Frank, Stephan; Seidel, Marja K.
Germany, United States, China, Chile
Abstract
The All-sky Automated Survey for Supernovae (ASAS-SN) is the only project in existence to scan the entire sky in optical light approximately every day, reaching a depth of g ∼ 18 mag. Over the course of its first 4 yr of transient alerts (2013-2016), ASAS-SN observed 53 events classified as likely M dwarf flares. We present follow-up photometry and spectroscopy of all 53 candidates, confirming flare events on 47 M dwarfs, one K dwarf, and one L dwarf. The remaining four objects include a previously identified T Tauri star, a young star with outbursts, and two objects too faint to confirm. A detailed examination of the 49 flare star light curves revealed an additional six flares on five stars, resulting in a total of 55 flares on 49 objects ranging in V-band contrast from ΔV = -1 to -10.2 mag. Using an empirical flare model to estimate the unobserved portions of the flare light curve, we obtain lower limits on the V-band energy emitted during each flare, spanning {log}({E}V/{erg})=32-35, which are among the most energetic flares detected on M dwarfs. The ASAS-SN M dwarf flare stars show a higher fraction of Hα emission, as well as stronger Hα emission, compared to M dwarfs selected without reference to activity, consistent with belonging to a population of more magnetically active stars. We also examined the distribution of tangential velocities, finding that the ASAS-SN flaring M dwarfs are likely to be members of the thin disk and are neither particularly young nor old.