AU Microscopii in the FUV: Observations in Quiescence, During Flares, and Implications for AU Mic b and c

TL;DR

This study presents detailed FUV observations of AU Mic, analyzing stellar flares and radiation effects on its exoplanets, with implications for atmospheric mass loss and future JWST studies.

Contribution

First detailed FUV spectral analysis of AU Mic, quantifying flare energies, emission features, and planetary atmospheric mass-loss rates, informing future exoplanet atmospheric studies.

Findings

Detected 13 flares with energies up to 10^31 ergs.

Identified 120 emission features across 10^4-10^7 K temperatures.

Estimated planetary mass-loss rates up to 10^14 g/s during flares.

Abstract

High energy X-ray and ultraviolet (UV) radiation from young stars impacts planetary atmospheric chemistry and mass loss. The active $\sim 22$ Myr M dwarf AU Mic hosts two exoplanets orbiting interior to its debris disk. Therefore, this system provides a unique opportunity to quantify the effects of stellar XUV irradiation on planetary atmospheres as a function of both age and orbital separation. In this paper we present over 5 hours of Far-UV (FUV) observations of AU Mic taken with the Cosmic Origins Spectrograph (COS; 1070-1360 Angstrom) on the Hubble Space Telescope (HST). We provide an itemization of $120$ emission features in the HST/COS FUV spectrum and quantify the flux contributions from formation temperatures ranging from $10^4-10^7$ K. We detect 13flares in the FUV white-light curve with energies ranging from $10^{29} - 10^{31}$ ergs. The majority of the energy in each of these flares is released from the transition region between the chromosphere and the corona. There is a 100$\times$ increase in flux at continuum wavelengths $\lambda < 1100$ Angstrom in each flare which may be caused by thermal Bremsstrahlung emission. We calculate that the baseline atmospheric mass-loss rate for AU Mic b is $\sim 10^8$ g s$^{-1}$, although this rate can be as high as $\sim 10^{14}$ g s$^{-1}$ during flares with $L_\textrm{flare} \simeq 10^{33}$ erg s$^{-1}$. Finally, we model the transmission spectra for AU Mic b and c with a new panchromatic spectrum of AU Mic c and motivate future JWST observations of these planets.