H-alpha as a Luminosity Class Diagnostic for K- and M-type Stars

TL;DR

This study identifies H-alpha absorption as a new spectroscopic indicator of luminosity class in K- and M-type stars, useful for distinguishing stellar populations in galaxies.

Contribution

The paper introduces H-alpha as a novel, metallicity-insensitive diagnostic for luminosity class in cool stars, extending its applicability to late M types.

Findings

H-alpha absorption correlates with luminosity class and stellar radius.

The behavior is explained by density-dependent hydrogen level overpopulation.

Potential for distinguishing stellar populations in extragalactic studies.

Abstract

We have identified the H-alpha absorption feature as a new spectroscopic diagnostic of luminosity class in K- and M-type stars. From high-resolution spectra of 19 stars with well-determined physical properties (including effective temperatures and stellar radii), we measured equivalent widths for H-alpha and the Ca II triplet and examined their dependence on both luminosity class and stellar radius. H-alpha shows a strong relation with both luminosity class and radius that extends down to late M spectral types. This behavior in H-alpha has been predicted as a result of the density-dependent overpopulation of the metastable 2S level in hydrogen, an effect that should become dominant for Balmer line formation in non-LTE conditions. We conclude that this new metallicity-insensitive diagnostic of luminosity class in cool stars could serve as an effective means of discerning between populations such as Milky Way giants and supergiant members of background galaxies.