Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996aj....112.1517s&link_type=abstract
Astronomical Journal v.112, p.1517
Astronomy and Astrophysics
Astronomy
136
Globular Clusters: General, Stars: Evolution
Scientific paper
Results of a systematic survey of Al, Mg, and Eu abundances of stars near the tip of the giant branch in globular clusters and in the halo field are reported. Mg abundances were obtained independently from spectral synthesis of the 5136 Å MgH features and from EW analysis of five Mg I features. A comparison of the two sets of Mg abundances shows good agreement. Within the errors, the field stars exhibit no star-to-star differences in 0, Mg, or Al at any [Fe/H] metallicity. Among the moderately (- 1.0> [Fe/H]> 1.9) and very metal-poor (-1.9>[Fe/H]> -3.0) halo field giants, [Na/Fe] is found to have star-to-star differences. The giants in the globular cluster M71 ([Fe/H] =-0.8) are also found to have star-to-star differences in Na but not in Mg, Al. It is concluded that either Al and Na are created in different nucleosynthetic processes, or the NeNa cycle of deep mixing can occur without the NO or MgAl cycles. Star-to-star differences in Al abundances for bright giants in M92 ([Fe/H]= -2.2), M13 ([Fe/H] - 1.5), and MS ([Fe/H] = -1.2) have been detected. The Al-Na correlation and the Al-O anti-correlation observed in these clusters are consistent with the predictions of the deep mixing theory (Langer & Hoffman 1995). M13, which has the largest spread in Al, Na, and 0 abundances, has star-to-star differences in the Mg abundance that are anti-correlated with Al. The [Al/Mg] ratio is consistent with the hypothesis that Mg is being processed into Al. However, if only 25Mg and 26Mg are expected to be converted to 27Al, the implied ratios of these isotopes to 24Mg would he considerably larger than the measured Mg isotopic ratios in the few halo field giants which have been studied. The abundances, [Eu/Fe], of the r-process element Eu are found to be positive over the entire metallicity range. No correlations (χ2v=0.64) between Eu and the deep mixing elements Al, Mg, O, and Na are found, ruling out a neutron-rich environment as the origin of the star-to-star abundance differences of these elements. The evidence presented here, when combined with results presented in the literature, imply that the star-to-star abundance differences observed among stars in the clusters M13, MS, and M92 are due to deep mixing of NeNa, MgAl, and CNO processed material from within the stars themselves.
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