2025ApJ...992...87P

Lyman-alpha (Lyα) is the most prominent ultraviolet (UV) 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 Lyα profile, requiring reconstructions that introduce systematic uncertainties. We present intrinsic Lyα profiles for 12 high-radial-velocity (RV) K and M dwarfs, where Doppler shifting minimizes ISM contamination, allowing direct measurements of ∼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 Lyα emission profiles on effective temperature (T_eff). The depth of self-reversal, driven by non-local thermodynamic equilibrium 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 ∼20% Lyα variability confined to the line core-implying that studies relying on reconstructions may underestimate temporal variability. We find strong correlations between Lyα flux, peak-to-trough ratio, and hydrogen departure coefficients with T_eff, providing empirical constraints for stellar atmosphere models. A comparison of Lyα flux in the habitable zone shows measured values for high-RV 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 Lyα emission in K and M dwarfs, reducing uncertainties in reconstructions and improving models of stellar UV emission relevant to exoplanetary studies.