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Detection of H2 in AU Mic: An Extremely Cold Star Spot?

Flagg, Laura; Johns-Krull, Christopher; France, Kevin; Herczeg, Gregory; Najita, Joan; Youngblood, Allison; Carvalho, Adolfo; Carpenter, John; Kenyon, Scott J.; Newton, Elisabeth R.

We have detected molecular hydrogen in the AU Mic system using high-resolution FUV spectra from HST-STIS. We measured the temperature of the gas at ~1000 K. Based on the velocities and dispersion of the H2 lines, we believe that it is unlikely to be from the disk or the planet, making star spots the most likely origin of this gas. However, the temperature of this gas is significantly below the temperature of the photosphere (~3800 K) and the predicted temperature of its star spots. Possible sources include a cold star spot or cold layer between the photosphere and the chromosphere, equivalent to the CO-mosphere in solar type stars.  We also discuss non-thermal heating as the result of a flare.

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