2025ApJ...989...20M

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)$\,\sim \,{\nu }^{-{\alpha }_{{\rm{mm}}}}$) 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. ), 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 × 10¹⁵ 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.