Semi-empirical Modeling of the Photosphere, Chromosphere, Transition Region, and Corona of the M-dwarf Host Star GJ 832

TL;DR

This paper develops a semi-empirical model of the star GJ 832's atmosphere, accurately predicting its EUV emission crucial for understanding exoplanet atmospheric photochemistry and mass loss.

Contribution

It presents the first detailed semi-empirical atmospheric model for GJ 832, including a synthetic spectrum that matches UV to optical observations and predicts EUV flux reliably.

Findings

The transition region of GJ 832 is hotter than that of the quiet Sun.

The model's EUV luminosity agrees with other estimation methods.

Unobserved EUV flux of GJ 832 is comparable to the active Sun.

Abstract

Stellar radiation from X-rays to the visible provides the energy that controls the photochemistry and mass loss from exoplanet atmospheres. The important extreme ultraviolet (EUV) region (10--91.2~nm) is inaccessible and should be computed from a reliable stellar model. It is essential to understand the formation regions and physical processes responsible for the various stellar emission features in order to predict how the spectral energy distribution varies with age and activity levels. We compute a state-of-the-art semi-empirical atmospheric model and the emergent high-resolution synthetic spectrum of the moderately active M2~V star GJ~832 as the first of a series of models for stars with different activity levels. Using non-LTE radiative transfer techniques and including many molecular lines, we construct a one-dimensional simple model for the physical structure of the star's chromosphere, chromosphere-corona transition region, and corona. The synthesized spectrum for this model fits the continuum and lines across the UV to optical spectrum. Particular emphasis is given to the emission lines at wavelengths shorter than 300~nm observed with {\em HST}, which have important effects on the photochemistry in the exoplanet atmospheres. The FUV line ratios indicate that the transition region of GJ~832 is more biased to hotter material than that of the quiet Sun. The excellent agreement of our computed EUV luminosity with that obtained by two other techniques indicates that our model predicts reliable EUV emission from GJ~832. We find that unobserved EUV flux of GJ~832, which heats the outer atmospheres of exoplanets and drives their mass loss, is comparable to the active Sun.