The Na 8200 Angstrom Doublet as an Age Indicator in Low-Mass Stars

TL;DR

This paper demonstrates that the Na 8200 Angstrom doublet can serve as an effective age indicator for low-mass M dwarfs, especially distinguishing stars younger or older than 100 million years, aiding in stellar age estimation.

Contribution

The study introduces a new method using the Na 8200 Angstrom doublet as an age indicator for M dwarfs, supported by observational data and analysis of temperature, gravity, and metallicity effects.

Findings

Na doublet EW distinguishes stars older/younger than 100 Myr.

Method is effective for stars with (V-K_s) >= 5.0.

Metallicity influences the reliability of the age indicator.

Abstract

We investigate the use of the gravity sensitive neutral sodium (NaI) doublet at 8183 Angstroms 8195 Angstroms (Na 8200 Angstrom doublet) as an age indicator for M dwarfs. We measured the Na doublet equivalent width (EW) in giants, old dwarfs, young dwarfs, and candidate members of the Beta Pic moving group using medium resolution spectra. Our Na 8200 Angstrom doublet EW analysis shows that the feature is useful as an approximate age indicator in M-type dwarfs with (V-K_s) >= 5.0, reliably distinguishing stars older and younger than 100 Myr. A simple derivation of the dependence of the Na EW on temperature and gravity supports the observational results. An analysis of the effects of metallicity show that this youth indicator is best used on samples with similar metallicity. The age estimation technique presented here becomes useful in a mass regime where traditional youth indicators are increasingly less reliable, is applicable to other alkali lines, and will help identify new-low mass members in other young clusters and associations.