The Active Chromospheres of Lithium-Rich Red Giant Stars

TL;DR

This study surveys chromospheric activity in nearly 300 red giant stars using near-infrared spectra, revealing a strong link between lithium richness and increased chromospheric activity, with implications for stellar evolution and binary interactions.

Contribution

It provides the first large-scale spectroscopic evidence connecting lithium enrichment with chromospheric activity in red giants, highlighting the role of binary interactions.

Findings

Li-rich giants show stronger 10830 absorption, indicating higher chromospheric activity.

Most Li-poor stars exhibit weak 10830 features, suggesting lower activity.

Li-rich giants may have a higher binary fraction, influencing lithium and activity levels.

Abstract

We have gathered near-infrared $zyJ$-band high resolution spectra of nearly 300 field red giant stars with known lithium abundances in order to survey their \species{He}{i} $\lambda$10830 absorption strengths. This transition is an indicator of chromospheric activity and/or mass loss in red giants. The majority of stars in our sample reside in the red clump or red horizontal branch based on their $V-J,M_V$ color-magnitude diagram and their Gaia \teff, \logg\ values. Most of our target stars are Li-poor in the sense of having normally low Li abundances, defined here as \eps{Li}~$<$~1.25. Over 90\% of these Li-poor stars have weak $\lambda$10830 features. But more than half of the 83 Li-rich stars (\eps{Li}~$>$~1.25) have strong $\lambda$10830 absorptions. These large $\lambda$10830 lines signal excess chromospheric activity in Li-rich stars; there is almost no indication of significant mass loss. The Li-rich giants also may have a higher binary fraction than do Li-poor stars, based on their astrometric data. It appears likely that both residence on the horizontal branch and present or past binary interaction play roles in the significant Li-He connection established in this survey.