4. Tentative scenario
According to the results presented above, we can distinguish between two separate stellar populations. Roughly 50% of the stars in our sample show a range of moderate overabundances of the -elements and a slowly varying abundance of the s -process elements relative to the iron peak. The other 50% of the stars in the sample show a constant (and maximum) overabundance of the -elements relative to the iron-peak elements, and varying s -process abundances. This behaviour must be related to nucleosynthesis processes.
The first interpretation which comes to mind is to relate one of these populations to the most metal-rich part of the halo and the other to the most metal-poor part of the disk. This interpretation is somewhat similar to what has been recently proposed by Nissen and Schuster (1997). However, upon examination of the kinematical data for our sample, there is no clear distinction between these populations on this basis alone, both populations containing high velocity stars typical of halo kinematics.
We therefore propose an alternative interpretation in which the halo stars can be divided into two sub-classes of Pop II stars, namely Pop IIa and Pop IIb, forming the two branches in Fig. 3. The stars belonging to the disk do not exhibit such correlations in their element abundances, unless they are very metal-poor. We will discuss these points in more details in a subsequent paper. In the following, we propose a scenario explaining the origin of the two sub-classes of halo stars.
4.1. General picture
First we assume a burst of star formation with at least some massive stars. As these massive stars evolve and end their lives in supernova (SN) explosions, -elements and r -process elements are ejected in the surrounding interstellar matter (ISM). A second generation of stars will form out of this continuously enriched ISM. These stars will form the Pop IIa stars, with values of [ /Fe] and [r /Fe] increasing with time. The slope in [Y/Fe] versus [Ti/Fe] for Pop IIa stars indicates an overproduction of Y relative to Fe in massive stars. Our results show the same tendency for Sr and Zr.
Assume now that after this burst phase no more massive stars are formed. The lower mass stars are either still reaching the main sequence or in a more evolved phase, maybe already processing s -elements. These elements will be ejected through stellar wind or superwind events and will contaminate the surrounding ISM. After the SN phase, the ISM was already enriched in and r -process elements, showing a unique [ /Fe] and [r /Fe]. The interstellar matter will continue to condense in new stars, now with a constant value of [ /Fe] and increasing values of [s /Fe]. These stars will form the Pop IIb stars.
Note that [Eu/Fe] shows a perfect correlation with [ /Fe] in Pop IIa stars, as expected. The points representative of Pop IIb stars are clumped at the maximum value of [ /Fe] and [r /Fe], i.e. at the values reached at the end of the massive stars outburst. This shows that, if produced by lower mass stars, it must be in the same proportions as Fe.
4.2. Globular clusters and EASE scenario
We now suggest that the formation of the field halo stars takes place in the globular clusters (GCs). This requires two reasonable assumptions. The first one is that the evaporation of low mass stars from GCs happens since the early phases of the evolution of the cluster and accounts for the field Pop II stars. The second one is that the matter ejected by SNe and stellar winds, although generally assumed to be mostly expelled from the cluster, nevertheless contributes to the enrichment of the lower mass stars, first by mixing with the ISM and then by accretion at the surface of already formed stars. The possibility of self-enrichment by SNe has been discussed by Smith (1986, 1987) and Morgan and Lake (1989).
In the early phase of the GC evolution, massive stars will form SNe until all stars more massive than about have completed their evolution. This fixes the end of the and r elements synthesis and the maximum value of [ /Fe] observed in Fig. 3. The second phase will lead to a relative enrichment of s elements only.
Our two phases scenario nicely explains the features observed in Fig. 3. Pop IIa stars are evaporated during the massive stars outburst, [ /Fe] increasing with time, and the Pop IIb stars escape later in the evolution of the cluster, after the end of the SN phase. The stars located at the top of the vertical branch are those which have escaped the cluster in the most advanced phases of its evolution. A schematical illustration is given in Fig. 5.
The range of metallicity at a given location in Fig. 3 corresponds
to stars evaporated from clusters of various global
enrichments (due to different initial mass functions) and, thus, different present day metallicity. As the evolution time for a star of a given mass is about the same in all GCs, it is no surprise that the abundance ratios, contrary to the metallicity, do not depend on the cluster from which the halo stars have evaporated.
The similar numbers of Pop IIa and Pop IIb stars suggest that either the evaporation was much more efficient in the early phases of the GC evolution or that a large fraction of Pop IIa stars originate from GCs which have been disrupted during the massive stars outburst. This is in agreement with the view that GCs with a flat mass function are weakly bound (Meylan and Heggie, 1997). This is also in agreement with the recent work of Brown et al. (1995), where they develop a model for the early dynamical evolution and self-enrichment of GCs.
The EASE (Evaporation/Accretion/Self-Enrichment) scenario also nicely explains the larger metallicity range covered by the field halo stars, extending to much lower metallicities than the GCs. The very metal-poor stars would be evaporated from the GCs at a very early stage of the outburst phase, when the self-enrichment of the cluster was still very low.
© European Southern Observatory (ESO) 1998
Online publication: January 16, 1998