Role of Nonthermal Processes in the Quiescent and Active Millimeter Spectrum of a Young M Dwarf

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 ( ν )) of young (<1 Gyr) M dwarfs (dMs) remains largely unexplored. We present the quiescent mm- S ( ν ) of a young dM, AD Leo, observed around 94 GHz using the Northern Extended Millimetre Array. The observed quiescent mm- S ( ν ) 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 nonthermal emission powered by suprathermal electrons, unlike in old (>1 Gyr) cool stars, whose quiescent mm- S ( ν ) generally agree with 1D thermal models. The mm-brightness temperature spectral index ( α mm ; T B ( ν ) ∼ ν − α mm ) of AD Leo deviates by a factor of 3 from the α mm – T eff scaling law for old Sun-like stars (A. Mohan 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 resembles certain solar events, but is unlike the second-scale events reported in dMs. The nonthermal flare humps suggest multiple injections of accelerated electrons. The mean flare luminosity (∼2–5 × 10 15 erg s −1 Hz −1 ) and duration (18 ± 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.