Role of non-thermal processes in the quiescent and active millimeter spectrum of a young M dwarf

Abstract

Millimeter (mm) emission from F - M dwarfs (cool stars) primarily traces chromospheric activity, with thermal emission thought to dominate in quiescence. Despite the high chromospheric activity, the quiescent mm spectral fluence (mm-S($\nu$)) of young (< 1 Gyr) M dwarfs (dMs) remain largely unexplored. We present the quiescent mm-S($\nu$) of a young dM, ADLeo, observed around 94 GHz using the Northern Extended Millimetre Array (NOEMA). The observed quiescent mm-S($\nu$) exceeds the thermal flux density from a 1D chromospheric model, constrained by optical-UV spectroscopic data, by up to a factor of 7. This indicates a quasi-steady non-thermal emission powered by supra-thermal electrons unlike in old (> 1 Gyr) cool stars, whose quiescent mm-S($\nu$) generally agree with 1D thermal models. The mm-brightness temperature spectral index ($\alpha_{mm}$; $T_B(\nu)\propto \nu^{- \alpha_{mm}}$) of AD Leo deviates by a factor of 3 from the $\alpha_{mm}$ - $T_{eff}$ scaling law for old sun-like stars (Mohan, A., et al., 2022), while UV Ceti, an older M6V star, follows the trend. Also, we report a double-hump flare with second-scale variability in flux density and spectral index, and a frequency-rising nature with brightness increasing with frequency. The flare resemble certain solar events, but is unlike the second-scale events reported in dMs. The non-thermal flare humps suggest multiple injections of accelerated electrons. The mean flare luminosity (2 - 5 $\times 10^{15} erg s^{-1} Hz^{-1}$) and duration ($18\pm 2$ s) are comparable to flares reported in AU Mic and Proxima Cen, but 100 - 1000 times weaker than the minutes-long dM flares observed by the South Pole Telescope.