Smith, Graeme H., Dupree, Andrea K., and Strader, Jay. 2014. "A Study of the ?10830 He I Line Among Red Giants in Messier 13." Publications of the Astronomical Society of the Pacific 126:901-913. https://doi.org/10.1086/678542
Two properties of Messier 13 are pertinent to the study of mass loss among metal-poor stars and the chemical evolution of globular clusters: (1) an extended blue horizontal branch, which seems to demand mass loss from red giant progenitor stars and possibly an enhanced helium abundance, and (2) 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 λ10830 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 λ3883 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.