The largest M dwarfs flares from ASAS-SN

• Main Authors: Schmidt, Sarah J, Shappee, Benjamin J, van Saders, Jennifer L, Stanek, K. Z, Brown, Jonathan S, Kochanek, C. S, Dong, Subo, Drout, Maria R, Frank, Stephen, Holoien, Thomas W. -S, Johnson, Sean, Madore, Barry F, Prieto, Jose L, Seibert, Mark, Seidel, Marja K, Simonian, Gregory V. A
• Format: Journal Article
• Language: English
• Published: 12-09-2018
• Subjects: Physics - Solar and Stellar Astrophysics
• DOI: 10.48550/arxiv.1809.04510

The All-Sky Automated Survey for Supernovae (ASAS-SN) is the only project in
existence to scan the entire sky in optical light every $\sim$day, reaching a
depth of $g\sim18$ mag. Over the course of its first four years 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 TT 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 $\Delta V = -1$ to $-10.2$ mags. 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
$\log(E_V/{\rm ergs})=32$ to $35$, which are among the most energetic flares
detected on M dwarfs. The ASAS-SN M-dwarf flare stars show a higher fraction of
H$\alpha$ emission as well as stronger H$\alpha$ 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.