The 10830 A He I Line Among M13 Red Giants

TL;DR

This study investigates the He I 10830 A line in M13 red giants to understand mass loss and chemical enrichment, finding evidence of stellar winds but little support for helium abundance variations from IM-AGB star activity.

Contribution

The paper provides spectroscopic evidence on He I 10830 A in M13 red giants, challenging the idea of helium enrichment from intermediate-mass AGB stars.

Findings

Detection of fast stellar winds in some giants.

Limited evidence for helium abundance differences.

Results do not strongly support IM-AGB star enrichment hypothesis.

Abstract

Two properties of Messier 13 are pertinent to the study of mass loss among metal-poor stars and the chemical evolution of globular clusters: (i) an extended blue horizontal branch, which seems to demand mass loss from red giant progenitor stars and possibly an enhanced helium abundance, and (ii) the presence of internal abundance inhomogeneities of elements in the mass range from C to Al. A popular explanation for this second phenomenon is that M13 was self-enriched by intermediate-mass asymptotic giant branch (IM-AGB) stars of a type that may also have been able to instigate helium enrichment. Spectra of the 10830 A absorption feature produced by He I have been obtained by using the NIRSPEC spectrometer on the Keck 2 telescope for seven red giants in M13 chosen to have a range in 3883 CN band strengths, oxygen, and sodium abundances. Whereas these spectra do reveal the presence of fast winds among some M13 red giants, they provide little support for helium abundance differences of the type that might have been generated by a burst of IM-AGB star activity within the M13 protocluster.