Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997apj...474l..23s&link_type=abstract
Astrophysical Journal Letters v.474, p.L23
Astronomy and Astrophysics
Astronomy
167
Galaxy: Globular Clusters: General, Stars: Abundances, Stars: Evolution, Stars: Horizontal-Branch, Stars: Interiors, Stars: Population Ii
Scientific paper
New noncanonical sequences for the evolution of globular cluster stars have been computed in order to investigate the effects of mixing helium from the hydrogen shell into the envelope during the red giant branch (RGB) phase. The possible occurrence of such mixing, driven by internal rotation, is suggested by the observed abundance variations involving C, N, O, Na, and Al in globular cluster red giants. We find that helium mixing can substantially increase the envelope helium abundance. By increasing the RGB tip luminosity, helium mixing also leads to enhanced mass loss along the RGB. Both of these effects have a potentially large impact on the subsequent horizontal-branch (HB) evolution. In particular, helium mixing produces a bluer HB morphology, thereby making it easier to explain the hot HB population found in various stellar systems, as well as the difference in HB morphology between some second parameter pairs of globular clusters. Helium-mixed sequences show a larger RR Lyrae period shift and predict a smaller age for the metal-poor globular clusters. In addition, such sequences can reproduce the low gravities observed in blue HB stars.
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