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Enhanced α-elements and He-burning Population II stars

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Abstract

Spectroscopic analysis of Population II stars indicates that oxygen and other α-elements appear to be overabundant with respect to iron (Sneden, 1985; Grattonet al., 1986; Gustafsson, 1987; Gratton, 1987; Gratton and Ortolani, 1989). By use of the Los Alamos Tapes Library, new opacities for Population II stars have been computed, including oxygen, neon, magnesium, and calcium enhanced by a factor five with respect to the ‘solar ratios’ mixture. On this basis, a grid of evolutionary sequences of He-burning models has been computed for low-mass stars covering the He-burning phase from the Zero-Age Horizontal Branch (ZAHB) up to the onset of thermal pulses in an advanced phase of He-shell burning, for the following assumption on the stellar parameters: helium abundance in the envelopeY=0.20 and 0.25; intial chemical composition [Fe/H]=−2.3, and [Fe/H]=−1.3; initial helium-core massM c=0.48 and 0.50 and total mass values fromM=0.56 up toM=0.76M . This choice of parameters was suggested by the opportunity of covering the range of values expected in galactic globular cluster HB stars, when an original abundance of heliumY=0.23 is assumed, as adopted throughout the recent literature. At this conference we present just a short overview of the main results while the large set of computed tracks and a detailed discussion on the topic will be presented elsewhere (Bencivenniet al., 1989).

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References

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Bencivenni, D., Castellani, V., Tornambé, A. et al. Enhanced α-elements and He-burning Population II stars. Astrophys Space Sci 157, 31–40 (1989). https://doi.org/10.1007/BF00637307

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Keywords

  • Burning
  • Helium
  • Neon
  • Evolutionary Sequence
  • Globular Cluster