Stellar Flares versus Luminosity: XUV-induced Atmospheric Escape and Planetary Habitability

TL;DR

This study assesses how stellar XUV radiation and flares influence atmospheric escape in habitable zone planets, revealing that stellar luminosity generally dominates atmospheric erosion, but flares are significant for certain low-mass stars, impacting habitability.

Contribution

It quantifies the relative impact of stellar luminosity and flares on atmospheric escape using observational data and hydrodynamic models, highlighting star-type differences.

Findings

Luminosity-driven escape is the primary atmospheric loss mechanism for most stars.

Flares significantly contribute to atmospheric erosion in about 20% of M4-M10 stars.

M0-M4 stars are most likely to erode their atmospheres, affecting habitability.

Abstract

Space weather plays an important role in the evolution of planetary atmospheres. Observations have shown that stellar flares emit energy in a wide energy range (10^30-10^38 ergs), a fraction of which lies in X-rays and extreme ultraviolet (XUV). These flares heat the upper atmosphere of a planet, leading to increased escape rates, and can result in atmospheric erosion over a period of time. Observations also suggest that primordial terrestrial planets can accrete voluminous H/He envelopes. Stellar radiation can erode these protoatmospheres over time, and the extent of this erosion has implications for the planet's habitability. We use the energy-limited equation to calculate hydrodynamic escape rates from these protoatmospheres irradiated by XUV stellar flares and luminosity. We use the Flare-Frequency Distribution of 492 FGKM stars observed with TESS to estimate atmospheric loss in Habitable Zone planets. We find that for most stars, luminosity-induced escape is the main loss mechanism, with a minor contribution from flares. However, flares dominate the loss mechanism of $\sim$20\% M4-M10 stars. M0-M4 stars are most likely to completely erode both their proto- and secondary atmospheres, and M4-M10 are least likely to erode secondary atmospheres. We discuss the implications of these results on planetary habitability.