The Significance of Stellar Flares on Exoplanet Atmospheres and Habitability

Stellar flaring is an important topic of concern for estimating the true habitability of planets around cool stars, where the habitable zone is in close proximity to high fluxes that are potentially harmful for the orbiting planets. Flares occur when magnetic reconnection events heat localized regions of a stellar surface resulting in elevated fluxes across most wavelengths, including the Extreme-Ultraviolet (EUV, 100-912 Å) region. Unfortunately, the current lack of EUV-capable observatories and obscuring effects by the ISM make direct observations of host star EUV fluxes difficult. We use the PHOENIX atmosphere code to model the EUV region during quiescent and flaring events using GALEX and EUVE photometry and archival HST Ultraviolet data as guidance. Our approach of tuning temperature structures and using non-LTE radiative transfer is similar to that previously used to model Solar flares and our models have revealed subtle differences between quiescent and flare model predictions that may be hidden by lower-resolution models or broadband-integrated flux comparisons. Here we quantify EUV flux changes during flaring events and investigate specific emission lines to determine if flares are an important contributor to long term EUV-driven mass loss and their potential impact when modeling exoplanet atmospheres.