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Astron. Astrophys. 333, 926-941 (1998)


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The puzzling MgAl anticorrelation in globular-cluster red giants: primordial plus deep mixing scenario?

P.A. Denissenkov 1, 2, 3, G.S. Da Costa 1, J.E. Norris 1 and A. Weiss 3

1 Mount Stromlo and Siding Spring Observatories, Institute of Advanced Studies, The Australian National University, Private Bag, Weston Creek P.O., ACT 2611, Australia
2 Astronomical Institute of the St. Petersburg University, Bibliotechnaja Pl. 2, Petrodvorets, 198904 St. Petersburg, Russia
3 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, D-85740 Garching, Germany

Received 22 July 1997 / Accepted 26 November 1997

Abstract

Star-to-star abundance variations of C, N, O, Na and Al in globular-cluster red giants have been recently supplemented by the finding that [Mg/Fe] is depleted in stars with extremely large [Al/Fe] (Shetrone 1996a). To find out which of the magnesium isotopes is responsible for the observed depletion of [Mg/Fe] Shetrone (1996b) also undertook an isotopic analysis of Mg and found that it is 24 Mg which is depleted in Al-rich giants. On the other hand, Norris & Da Costa (1995) demonstrated that even in the massive globular cluster [FORMULA]  Cen which has intrinsic spreads in both [Fe/H] and the abundances of the s-process elements, [O/Fe] anticorrelates with [Na/Fe] and [Al/Fe] as in "normal" monometallic clusters. These new spectroscopic results allow us to test current models of stellar evolution and nucleosynthesis, as well as those of the formation and chemical enrichment of globular clusters. In an effort to explain self-consistently these observations we have considered two possibilities: (1) a deep mixing scenario which assumes that in red giants some kind of (extra)mixing transports products of nuclear reactions from the hydrogen burning shell (HBS) to the base of the convective envelope; and (2) a combination of primordial and deep mixing scenarios. It is shown that (1) cannot account for the anticorrelation of [Mg/Fe] vs. [Al/Fe] without additional ad hoc assumptions, among which we identify a strong but still undetected low energy resonance in the reaction 24 Mg(p, [FORMULA] Al, and episodical increases of the HBS temperature up to the value [FORMULA] K. In (2) intermediate mass AGB stars are assumed to produce the decreased 24 Mg and increased 25 Mg initial abundances in some globular-cluster low mass stars and Al is synthesized at the expense of 25 Mg in the HBS and transported to the surface of the red giant by extramixing. We discuss advantages and deficiencies of both scenarios and propose some observational tests.

Key words: stars: abundances – stars: evolution – stars: interiors – globular clusters: general

Send offprint requests to: A. Weiss (weiss@mpa-garching.mpg.de)

© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998

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