The Largest M Dwarf Flares from ASAS-SN

• Published in: The Astrophysical journal Vol. 876; no. 2; pp. 115 - 133
• 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, Stephan, Holoien, T. W.-S., Johnson, Sean, Madore, Barry F., Prieto, J. L., Seibert, Mark, Seidel, Marja K., Simonian, Gregory V. A.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 10-05-2019; IOP Publishing
• Subjects: Astrophysics; Emissions; Flare stars; H alpha line; Light curve; Outbursts; Red dwarf stars; Sky surveys (astronomy); Spectroscopy; stars: chromospheres; stars: late-type; stars: low-mass; Stellar flares; Supernovae; T Tauri stars
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ab148d

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 -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 emission, as well as stronger H 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.