The O-Na and Mg-Al anticorrelations in turn-off and early subgiants in globular clusters ^*

¹ Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
² Osservatorio Astronomico di Trieste, Italy
³ Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ Dipartimento di Fisica, Università di Pisa, Italy
⁵ Istituto di Astrofisica Spaziale, CNR, Italy
⁶ Osservatorio Astronomico di Roma, Italy
⁷ Dipartimento di Astronomia, Università di Padova, Italy
⁸ European Southern Observatory
⁹ Department of Astronomy, University of Aarhus, Denmark
¹⁰ The University of Texas at Austin, USA
¹¹ Observatoire de Meudon, France
¹² Osservatorio Astronomico di Collurania, Italy

Corresponding author: R. G. Gratton, gratton@pd.astro.it

Received: 18 December 2000
Accepted: 26 January 2001

Abstract

High dispersion spectra () for a quite large number of stars at the main sequence turn-off and at the base of the giant branch in NGC 6397 and NGC 6752 were obtained with the UVES on Kueyen (VLT UT2). The [Fe/H] values we found are and for NGC 6397 and NGC 6752 respectively, where the first error bars refer to internal and the second ones to systematic errors (within the abundance scale defined by our analysis of 25 subdwarfs with good Hipparcos parallaxes). In both clusters the [Fe/H]'s obtained for TO-stars agree perfectly (within a few percent) with that obtained for stars at the base of the RGB. The value we obtain for NGC 6397 is quite low, but it agrees with previous results obtained for giants in this cluster. Moreover, the star-to-star scatter in both O and Fe is very small, indicating that this small mass cluster is chemically very homogenous. On the other hand, our results show clearly and for the first time that the O-Na anticorrelation (up to now seen only for stars on the red giant branches of globular clusters) is present among unevolved stars in the globular cluster NGC 6752, a more massive cluster than NGC 6397. A similar anticorrelation is present also for Mg and Al, and C and N. It is very difficult to explain the observed Na-O, and Mg-Al anticorrelation in NGC 6752 stars by a deep mixing scenario; we think it requires some non internal mechanism.