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

Atul Mohan; Peter H. Hauschildt; Birgit Fuhrmeister; Surajit Mondal; Vladimir Airapetian; Sven Wedemeyer

doi:10.3847/1538-4357/ade9bf

paper

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

arXiv:2506.19779 · doi:10.3847/1538-4357/ade9bf

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( $ν$ )) of young (< 1 Gyr) M dwarfs (dMs) remain largely unexplored. We present the quiescent mm-S( $ν$ ) of a young dM, ADLeo, observed around 94 GHz using the Northern Extended Millimetre Array (NOEMA). 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 non-thermal emission powered by supra-thermal 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 ( $α_{mm}$ ; $T_{B} (ν) \propto ν^{- α_{mm}}$ ) of AD Leo deviates by a factor of 3 from the $α_{mm}$ - $T_{e f f}$ 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 $\times 1 0^{15} er g s^{- 1} H z^{- 1}$ ) 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.

arXiv comment4 figures, 2 tables

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

Abstract

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