C-enhanced metal poor stars

Details C-enhanced metal poor stars C-enhanced metal poor stars

Feb 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004phdt........14l&link_type=abstract

PhD Thesis, Universita di Padova

Computer Science

Sound

3

Stars: Chemically Peculiar, Stars: Agb And Post-Agb, Stars: Binaries

Scientific paper

The dissertation presents the results relative to the analysis of a sample of 13 C-enhanced metal poor stars. Such topic has a crucial importance for the study of the early phases of the Galaxy, given that the surveys looking for extremely metal poor stars find that as many as 25% of stars with [Fe/H] ≤-2.5 are C-enhanced. The analyses of these objects published insofar are not very numerous nor include a systematic coupling of the chemical composition to the dynamical information (i.e. radial velocity variability).
For the purpose of this investigation we used data collected with UVES at VLT and HIRES at Keck telescope, selecting targets from the largest metal poor star surveys so far, the HK and Hamburg/ESO surveys.
We measured the chemical composition of these objects through abundance analysis via line equivalent widths measurements and via spectral synthesis , finding that, with a single exception, they all showed evidence of enrichment from the slow(s) neutron-capture process. We note that no selection effect toward s-process enrichment was applied, and that this observed fraction likely reflects that present among metal poor, C-enhanced stars.
Given the evolutionary state of the objects in our sample (main sequence stars, subgiants and stars in their early subgiant phase), such an abundance pattern cannot originate from the stars themselves. This result suggests that these objects are members of binary systems with companions which once were Intermediate Mass Stars (IMS); when the latter underwent the third dredge-up during their Asymptotic Giant Branch phases, they dumped the shell processed enriched material on the stars we are now observing. This scenario is supported from another of our findings, i.e. that the fraction of detected binaries among s-process rich, C-enhanced, extremely metal poor stars (CEMP-s) indicate that all of them are in fact in binary system. These results show that these objects are the metal poor counterparts to classical CH-stars and Ba stars.
Our results have a strong implication on the Initial Mass Function (IMF)
in the early Galaxy. In fact, as already mentioned, the fraction of C-enhanced stars at low metallicity is very large. Given our conclusion about the origin of such objects, the immediate implication is that CEMP-s stars bear the mark of the past existence of a population of IMS which already evolved and is now impossible to observe directly. Therefore, the fraction of CEMP-s is a crucial indicator to the fraction of IMS that were shining in the early Galaxy, hence it is possible to directly count them.
On the basis of the observational data and a few reasonable and physically sound assumptions we obtain that IMS stars in the early Galaxy accounted for as much as 36% of stars (by number). This value is incompatible with the "classical" IMF's, such as the Salpeter and Miller & Scalo ones.
This simple consideration on the basis of the data provides the first direct observational evidence of a different IMF in the early Galaxy, characterized by a larger typical mass than presently forming stars.

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