The relative emission from chromospheres and coronae: dependence on spectral type and age

TL;DR

This study analyzes how stellar chromospheric and coronal emissions vary with spectral type and age, revealing distinct patterns that influence exoplanet habitability and atmospheric evolution.

Contribution

It provides the first large-scale comparison of Lyman-alpha and X-ray emissions across 79 stars, identifying how these emissions depend on stellar age and spectral type.

Findings

Chromospheric and coronal emissions follow specific patterns with stellar age.

M dwarf stars have significantly lower Lyman-alpha fluxes compared to warmer stars at the same X-ray flux.

Emission ratios vary with stellar age and temperature, affecting exoplanet atmospheres.

Abstract

Extreme-ultraviolet and X-ray emissions from stellar coronae drive mass loss from exoplanet atmospheres, and ultraviolet emission from stellar chromospheres drives photo-chemistry in exoplanet atmospheres. Comparisons of the spectral energy distributions of host stars are, therefore, essential for understanding the evolution and habitability of exoplanets. The large number of stars observed with the MUSCLES, Mega-MUSCLES, and other recent HST observing programs has provided for the first time a large sample (79 stars) of reconstructed Lyman-alpha fluxes that we compare with X-ray fluxes to identify significant patterns in the relative emission from these two atmospheric regions as a function of stellar age and effective temperature. We find that as stars age on the main sequence, the emissions from their chromospheres and coronae follow a pattern in response to the amount of magnetic heating in these atmospheric layers. A single trendline slope describes the pattern of X-ray vs. Lyman-alpha emission for G and K dwarfs, but the different trendlines for M dwarf stars show that the Lyman-alpha fluxes of M stars are significantly smaller than warmer stars with the same X-ray flux. The X-ray and Lyman-alpha luminosities divided by the stellar bolometric luminosities show different patterns depending on stellar age. The L(Lyman-alpha)/L(bol) ratios increase smoothly to cooler stars of all ages, but the L(X)/L(bol) ratios show different trends. For older stars, the increase in coronal emission with decreasing T(eff) is much steeper than chromospheric emission. We suggest a fundamental link between atmospheric properties and trendlines relating coronal and chromospheric heating,