Issue |
A&A
Volume 369, Number 1, April I 2001
|
|
---|---|---|
Page(s) | 87 - 98 | |
Section | Galactic structure, stellar clusters, and populations | |
DOI | https://doi.org/10.1051/0004-6361:20010144 | |
Published online | 15 April 2001 |
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
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.
Key words: stars: abundances / stars: evolution / stars: population II / Galaxy: globular clusters
© ESO, 2001
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