Oxygen abundaces in Halo giants. IV - The oxygen-sodium anticorrelation in a sample of 22 bright giants in M13

Details Oxygen abundaces in Halo giants. IV - The oxygen-sodium anticorrelation in a sample of 22 bright giants in M13 Oxygen abundaces in Halo giants. IV - The oxygen-sodium anticorrelation in a sample of 22 bright giants in M13

Oct 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993aj....106.1490k&link_type=abstract

Astronomical Journal (ISSN 0004-6256), vol. 106, no. 4, p. 1490-1507.

Astronomy and Astrophysics

Astronomy

142

Giant Stars, Globular Clusters, Oxygen, Stellar Composition, Stellar Evolution, Abundance, Metallicity, Stellar Envelopes, Stellar Spectra

Scientific paper

We have analyzed high resolution echelle spectra of nine bright M13 giants which, when added to those we have analyzed previously by similar techniques, bring our M13 sample size to 22. The sample is 88 percent complete from the red giant tip to a point one bolometric magnitude fainter, and is presumably representative down to M0(bol) = -2.1. We find that the brightest M13 giants are predominantly super oxygen poor ( ~ -0.4 to -0.8 ) and correspondingly sodium rich. We argue that these super O-poor stars are 'first ascent' giants. The most straightforward way to understand (1) the decline in the average oxygen abundance as M13 stars approach the tip of the giant branch and (2) their anticorrelated oxygen and sodium abundances, is that they are the results of very deep mixing. In the case of the super O-poor stars this idea requires that 90 percent of the oxygen atoms of the envelope be transformed into nitrogen. Since there are significant variations in the oxygen abundances of stars at the same point on the giant branch, such mixing must vary stochastically from star to star. Variable deep mixing is most likely to be the result of variable internal rotation.

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