📝 SummarXiv

Abstract

Lyman-$\alpha$ (Ly$\alpha$) is the most prominent ultraviolet emission line in low-mass stars, playing a crucial role in exoplanet atmospheric photochemistry, heating, and escape. However, interstellar medium (ISM) absorption typically obscures most of the Ly$\alpha$ profile, requiring reconstructions that introduce systematic uncertainties. We present intrinsic Ly$\alpha$ profiles for 12 high radial velocity K and M dwarfs, where Doppler shifting minimizes ISM contamination, allowing direct measurements of $\sim$50-95\% of the line flux. Our sample spans the K-to-M spectral transition, enabling us to constrain the dependence of self-reversals in Ly$\alpha$ emission profiles on effective temperature ($T_{eff}$). The depth of self-reversal, driven by non-local thermodynamic equilibrium (LTE) effects, decreases with decreasing $T_{eff}$, with M dwarfs exhibiting little to none. Two stars, Ross 1044 and Ross 451, were observed over multiple days, revealing $\sim$20\% Ly$\alpha$ variability confined to the line core - implying that studies relying on reconstructions may underestimate temporal variability. We find strong correlations between Ly$\alpha$ flux, peak-to-trough ratio, and hydrogen departure coefficients with $T_{eff}$, providing empirical constraints for stellar atmosphere models. A comparison of Ly$\alpha$ flux in the habitable zone shows measured values for high radial velocity stars less than the reconstructed values for the rest of the sample, likely due to the older ages of the high-RV stars and/or overestimated reconstructed fluxes due to model deficiency (e.g., neglecting self-reversal). Our results establish an empirical foundation for Ly$\alpha$ emission in K and M dwarfs, reducing uncertainties in reconstructions and improving models of stellar UV emission relevant to exoplanetary studies.