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

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.

arXiv comment13 pages, 4 figures, accepted to ApJ

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Cited by in corpus (4)

• Red Supergiants in the JWST Era. I: Near-IR Photometric Diagnostics
• A spectroscopic follow-up for Gaia19bld
• Searching for Galactic Red Supergiants with Gaia RVS Spectra
• There Are (super)Giants in the Sky: Searching for Misidentified Massive Stars in Algorithmically-Selected Quasar Catalogs