Catálogo de publicaciones - libros

Unevolved or scarcely evolved stars of globular clusters do not have either high enough temperatures in their cores or large enough convective envelopes to dredge up to the surface the products of high temperature proton-capture reactions. Hence, these stars represent an ideal diagnostic to study the pattern of the chemical anomalies observed in the C, N, O, Na, Mg, Al elements. I will review the status of researches on this presently debated subject, presenting also the latest results from high resolution UVES spectra on dwarf and subgiant stars in NGC 6397, NGC 6752 and 47 Tuc.

The nucleosynthetic histories of the outer halo globular clusters, as inferred by their chemical abundance patterns, are crucial tracers of the early formation of the Galactic halo and subsequent evolution of the Galaxy. Some outer halo clusters have been purported to be associated with galactic merger events such as the present-day merger with the Sgr dwarf galaxy. Reported here are preliminary abundance results for a sub-sample of the data acquired for this investigation.

It is now clear that abundance variations from star-to-star among the light elements, particularly C, N, O, Na and Al, are ubiquitous within galactic globular clusters; they appear seen whenever data of high quality is obtained for a sufficiently large sample of stars within such a cluster. The correlations and anti-correlations among these elements and the range of variation of each element appear to be independent of stellar evolutionary state, with the exception that enhanced depletion of C and of O is sometimes seen just at the RGB tip. While the latter behavior is almost certainly due to internal production and mixing, the internal mixing hypothesis can now be ruled out for producing the bulk of the variations seen. We focus on the implications of our new data for any explanation invoking primordial variations in the proto-cluster or accretion of polluted material from a neighboring AGB star.

I present preliminary results for a sample of ~700 red giants in ω Cen, observed during the Ital-FLAMES Consortium GTO time in May 2003, for the Bologna Project on ω Cen. Preliminary Fe and Ca abundances confirm previous results: while the metal-poor and intermediate populations show a normal halo α-enhancement of [α/Fe]≃+0.3, the most metal-rich stars show a signi.cantly lower [α/Fe]≃+0.1. If the metal-rich stars have evolved within the cluster in a process of self-enrichment, the only way to lower their α-enhancement would be SNe type Ia intervention.

Abundance analysis of Carbon and Nitrogen has been performed in a sample of 32 late F and early G type dwarf metal poor stars in the metallicity range −3.3 < [Fe/H] < 0 using molecular lines of CH and NH in the near-UV. We find that [C/Fe] decreases slowly with increasing [Fe/H] while [N/Fe] remains flat. Furthermore we derived uniform and accurate C/O and N/O ratios using oxygen abundances from near-UV OH lines employed in our previous studies. We confirm the metallicity dependence of C/O ratio known from previous studies and caused by the metallicity dependence of the C yields from massive stars with mass loss. [C/O] does not remain constant below [O/H]=−0.5 but increases again with a large scatter. We find that a primary component is required in order explain the observations of N/O and that the N production history is similar in our Galaxy and DLAs.

Most of the work reported here has been conducted within the ESO Large Programme 165.N-0276 “Galaxy Formation, Early Nucleosynthesis, and the First Stars”, which has covered 4 periods 65–68, from April 2000 to November 2001, with a total of 38 nights in visitor mode. The team had R. Cayrel as PI, and 13 CoIs:

Theoretical models for nucleosynthesis in asymptotic giant branch stars predict a large contribution to the cosmic nitrogen abundance from intermediate-mass stars [1]. In particular, hot-bottom-burning in stars above a certain mass produces [C/N]≲ −1 [2]. However, observations of C and N abundances in C-rich, metal-poor stars, usually using the CH and CN bands, show [C/N] values that vary between −0.5 and 1.5. (Fig. 1). If any of these stars have been polluted by intermediate mass AGB stars, then they should have lower [C/N] ratios. However, most of the CH stars with detailed abundances have [C/Fe] > 1.0, and it is more likely than stars mildly enhanced in C have been polluted by N-rich stars.

In the framework of the VLT Large program “First Stars” (165.N-0276(A)), we have measured the abundance of 13 heavy elements using high quality UVES spectra. In this paper, we report on the abundance of Sr and Ba in this sample of stars. In 1995, McWilliam et al. [4] showed that the [Sr/Fe] and [Ba/Fe] ratios exhibited a large dispersion in metal-poor stars. If these 2 elements are produced by the same nucleosynthetic process, then the variation of [Sr/Ba] as a function of metallicity should be constant . However, it is known (see Arlandini et al. 1999 [1] for example) that a significant part of Sr is built by s-process in massive stars. As this process is a secondary process, it is unlikely that this process is fully in operation at the early stages of the chemical evolution. On figure 1a, the [Sr/Ba] vs [Fe/H] are plotted together with some data found in the literature.

The s process in AGB stars is mainly driven by the C(α,n)O reaction. During a third dredge up episode, penetration of a small amount of protons from the envelope into the top layers of the C-rich and He-rich zone gives rise to the formation of a so-called C pocket [3]. At any given metallicity, a large range of C-pocket efficiencies is required for the interpretation of the s-process distributions observed in s-enhanced stars in the Galactic disk [4]. Stellar models predict wide ranges of [hs/Fe], [ls/Fe], [hs/ls], where ls=ls(Y, Zr) represents the first s-peak at neutron magic N = 50 and hs=hs(Ba, La, Nd, Sm) the second s-peak at neutron magic N = 82.

We present here the results of the measurement of the sulphur abundance in very metal-poor stars. Our sample covers the [-4;-2] range of metallicity, and thus allows us to constraint the chemical evolution models and also to put some key constraints on the supernovae models.