The 10830 Angstrom Helium Line Among Evolved Stars in the Globular Cluster M4

TL;DR

This study investigates the helium 10830 A line in stars of globular cluster M4, revealing temperature-dependent line strength and evidence of stellar outflows, but no clear link to chemical abundance variations.

Contribution

It provides the first detailed spectroscopic analysis of the He I 10830 A line across different stellar evolutionary stages in M4, highlighting the role of temperature and chromospheric conditions.

Findings

He I line strength correlates with effective temperature.

Hotter stars show strong He I absorption and outflows.

No clear correlation between He I line and Na/Al abundances.

Abstract

Helium is a pivotal element in understanding multiple main sequences and extended horizontal branches observed in some globular clusters. Here we present a spectroscopic study of helium in the nearby globular cluster M4. We have obtained spectra of the chromospheric He I 10830 A line in 16 red horizontal branch, red giant branch, and asymptotic giant branch stars. Clear He I absorption or emission is present in most of the stars. Effective temperature is the principal parameter that correlates with 10830 A line strength. Stars with T_eff < 4450 K do not exhibit the helium line. Red horizontal branch stars, which are the hottest stars in our sample, all have strong He I line absorption. A number of these stars show very broad 10830 A lines with shortward extensions indicating outflows as high as 80-100 km/s and the possibility of mass loss. We have also derived [Na/Fe] and [Al/Fe] abundances to see whether these standard tracers of "second generation" cluster stars are correlated with He I line strength. Unlike the case for our previous study of Omega Cen, no clear correlation is observed. This may be because the sample does not cover the full range of abundance variations found in M4, or simply because the physical conditions in the chromosphere, rather than the helium abundance, primarily determine the He I 10830 A line strength. A larger sample of high-quality He I spectra of both "first" and "second" generation red giants within a narrow range of T_eff and luminosity is needed to test for the subtle spectroscopic variations in He I expected in M4.