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

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.

ApJ in press

References in corpus (6)

• Spectroscopic and Photometric evidence of two stellar populations in the Galactic Globular Cluster NGC6121 (M4)
• Heavy element abundances in giant stars of the globular clusters M4 and M5
• The M4 Core Project with HST -- II. Multiple Stellar Populations at the Bottom of the Main Sequence
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Cited by in corpus (5)

• The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. XVI. The helium abundance of multiple populations
• Four stellar populations and extreme helium variation in the massive outer-halo globular cluster NGC 2419
• Globular cluster chemistry in fast rotating dwarf stars belonging to intermediate age open clusters
• Stellar population astrophysics (SPA) with the TNG: Measurement of the He I 10830Å line in the open cluster Stock 2
• Omega Centauri: weak MgH-band in red giants directly trace the helium content