Temperature-column mass structures for the models of each star. We shade the approximate temperature ranges over which the core and wings of Lyα forms in purple (≈8000–25,000 K for the core, and ≈4000–7500 K for the wings), the actual range of temperatures varies by star and is determined by where the optical depth, τ (λ), equals to one. — astro-ph.SR
Lyman-α (Lyα) 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 Lyα profile, requiring reconstructions that introduce systematic uncertainties. We present intrinsic Lyα profiles for 12 high radial velocity 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 (Teff).
The depth of self-reversal, driven by non-local thermodynamic equilibrium (LTE) effects, decreases with decreasing Teff, 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 Teff, providing empirical constraints for stellar atmosphere models. A comparison of Lyα 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 Lyα emission in K and M dwarfs, reducing uncertainties in reconstructions and improving models of stellar UV emission relevant to exoplanetary studies.
Sarah Peacock, Travis S. Barman, R. O. Parke Loyd, Adam C. Schneider, Allison Youngblood, Kenneth G. Carpenter, Evgenya L. Shkolnik
Comments: 21 pages, 11 figures, 3 tables, accepted to ApJ
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2509.09029 [astro-ph.SR] (or arXiv:2509.09029v1 [astro-ph.SR] for this version)
https://doi.org/10.48550/arXiv.2509.09029
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Submission history
From: Sarah Peacock
[v1] Wed, 10 Sep 2025 21:57:26 UTC (2,080 KB)
https://arxiv.org/abs/2509.09029
Astrobiology