Helium Mixing in Globular Cluster Stars

TL;DR

This study explores how helium mixing in globular cluster stars affects their evolution, luminosity, and observable properties, potentially explaining various phenomena like HB morphology and RR Lyrae period shifts.

Contribution

It provides detailed evolutionary models demonstrating the effects of helium mixing on globular cluster star properties and discusses observational tests for this scenario.

Findings

Helium mixing leads to higher luminosities on the RGB.

Helium mixing results in bluer and brighter HB stars.

It can mimic age effects and influence RR Lyrae properties.

Abstract

The observed abundance variations in globular cluster red giants indicate that these stars may be mixing helium from the hydrogen shell outward into the envelope, presumably as a result of internal rotation. We have investigated the implications of such helium mixing for both the red- giant-branch (RGB) and horizontal-branch (HB) phases by computing a number of noncanonical evolutionary sequences for different assumed mixing depths and mass loss rates. We find that the helium-mixed models evolve to higher luminosities during the RGB phase and consequently lose more mass than their canonical counterparts. This enhanced mass loss together with the higher envelope helium abundances of the helium-mixed models produces a markedly bluer and somewhat brighter HB morphology. As a result, helium mixing can mimic age as a 2nd parameter and can reduce the ages of the metal-poor globular clusters derived from the luminosity difference between the HB and the main-sequence turnoff. Helium mixing might also lead to a larger RR Lyrae period shift and to a steeper slope for the RR Lyrae luminosity - metallicity relation if the mixing is more extensive at low metallicities, as suggested by the observed abundance variations. We discuss the implications of helium mixing for a number of other topics including the low gravities of the blue HB stars, the origin of the extreme HB stars, and the evolutionary status of the sdO stars. A variety of observational tests are presented to test this helium-mixing scenario.