The Mysterious Affair of the H${}_2$ in AU Mic

Abstract

Molecular hydrogen is the most abundant molecule in the Galaxy and plays important roles for planets, their circumstellar environments, and many of their host stars. We have confirmed the presence of molecular hydrogen in the AU Mic system using high-resolution FUV spectra from HST-STIS during both quiescence and a flare. AU Mic is a $\sim$23 Myr M dwarf which hosts a debris disk and at least two planets. We estimate the temperature of the gas at 1000 to 2000 K, consistent with previous detections. Based on the radial velocities and widths of the H${}_2$ line profiles and the response of the H${}_2$ lines to a stellar flare, the H${}_2$ line emission is likely produced in the star, rather than in the disk or the planet. However, the temperature of this gas is significantly below the temperature of the photosphere ($\sim$3650 K) and the predicted temperature of its star spots ($\gtrsim$2650 K). We discuss the possibility of colder star spots or a cold layer in the photosphere of a pre-main sequence M dwarf.

arXiv commentaccepted to ApJ, 20 pages, many figures

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Cited by in corpus (3)

• AU Microscopii in the FUV: Observations in Quiescence, During Flares, and Implications for AU Mic b and c
• The Variable Detection of Atmospheric Escape around the young, Hot Neptune AU Mic b
• High CO/H2 ratios supports an exocometary origin for a CO-rich debris disk