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

TL;DR

This study confirms the presence of molecular hydrogen in the AU Mic system using high-resolution UV spectra, suggesting the gas is likely produced by the star itself rather than the disk or planets, with a temperature of 1000-2000 K.

Contribution

First detection of molecular hydrogen in AU Mic using HST-STIS spectra, analyzing its origin and temperature in a young M dwarf system.

Findings

H$_2$ detected in AU Mic system during quiescence and flare

H$_2$ emission likely produced in the star, not the disk or planets

Gas temperature estimated between 1000 and 2000 K

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.