ASASSN-16ae: A Powerful White-Light Flare on an Early-L Dwarf

TL;DR

This paper reports the discovery of a powerful white-light flare on an old early-L dwarf, suggesting magnetic activity persists in such low-mass, aged stars, with implications for stellar magnetic activity understanding.

Contribution

It presents the first detection of a significant flare on an early-L dwarf, expanding knowledge of magnetic activity in low-mass, old stars.

Findings

Flare energy exceeds 4.9×10^{33} ergs, among the strongest for M dwarf flares.

SDSS0533 is likely part of the thick disk, indicating an old stellar population.

Magnetic activity may persist in late-type, old stars, contrary to previous assumptions.

Abstract

We report the discovery and classification of SDSS~J053341.43+001434.1 (SDSS0533), an early-L dwarf first discovered during a powerful $\Delta V < -11$ magnitude flare observed as part of the ASAS-SN survey. Optical and infrared spectroscopy indicate a spectral type of L0 with strong H$\alpha$ emission and a blue NIR spectral slope. Combining the photometric distance, proper motion, and radial velocity of SDSS0533 yields three-dimensional velocities of $(U,V,W)=(14\pm13,-35\pm14,-94\pm22)$~km~s$^{-1}$, indicating that it is most likely part of the thick disk population and probably old. The three detections of SDSS0533 obtained during the flare are consistent with a total $V$-band flare energy of at least $4.9\times10^{33}$~ergs (corresponding to a total thermal energy of at least $E_{\rm tot}>3.7\times10^{34}$~erg), placing it among the strongest detected M dwarf flares. The presence of this powerful flare on an old L0 dwarf may indicate that stellar-type magnetic activity persists down to the end of the main sequence and on older ML transition dwarfs.