Astron. Astrophys. 341, 539-546 (1999)

4. Discussion

The main results for the two stars (III-3 and II-85) that we have analysed can be summarized as follows:

1. Iron is a factor subsolar.

2. The -elements Mg, Si, Ca, Ti are enhanced relative to iron by dex.

3. The odd-Z elements Na and Al that are built during carbon burning are enhanced.

4. The s-process elements Y, Zr, Ba, La do not show a systematic trend, some appear to be enhanced, some depleted compared to iron.

5. The r-process element Eu is solar compared to iron.

This general pattern is consistent with the one exhibited by inner halo globular clusters of comparable metallicity, see e.g. Table 1 in Wheeler, Sneden, Truran (1989) for [Fe/H] clusters. An overabundance of -elements relative to iron is now generally interpreted as evidence of fast chemical enrichment. Under such circumstances stars form out of an ISM that was enriched predominantly by short lived massive stars exploding as SNs of Type II, before the explosion of most SNs of Type Ia belonging to the same generation(s). This interpretation applies to the well known -element overabundance in the galactic Halo, as well as to a possible overabundance in elliptical galaxies (e.g. Davies et al. 1993).

Models of bulge formation in which much of the stellar build up is completed in a time shorter than the assumed time for the bulk of SNIa products to be released naturally produce an -element enhancement (e.g. Matteucci & Brocato 1990). Besides depending on the theoretical SN yields, the detailed abundance pattern predicted by these models is mainly controlled by the ratio of the two relevant timescales: the star formation timescale and the timescale for the bulk (e.g. 50%) of the SNIa products to be released. Fitting to an observed set of elemental abundances allows to recover this ratio, i.e. the star formation timescale is derived modulo the SNIa timescale. While this latter timescale is generally considered to be of the order of 1 Gyr, it is worth recalling that no firm theoretical or observational limit exists on this quantity (e.g. Greggio 1996).

The situation is more confused as far as the s-process elements are concerned. First of all, none of the elements for which we have obtained the abundance is a pure s-process element, but some r-process contribution is also present. Second, some s-processing takes place in massive stars (Raiteri et al. 1993), as well as in relatively short lived and long lived intermediate mass stars (Gallino et al. 1998). In any event, when referring to -elements the s-process elements are underabundant with respect to the sun, i.e. [s/, which also argues for the bulge material out of which NGC 6553 formed having failed to experience enrichment by long lived stars.

All in all, we can notice that the elemental abundances in the two program stars favor a scenario in which the galactic bulge underwent a rapid chemical enrichment, as indeed indicated also by the old age of NGC 6553, and by the small age dispersion of bulge stars as inferred from their luminosity function (Ortolani et al. 1995).

© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998
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