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Stochastic chemical enrichment in metal-poor systems I.: Theory
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics, Theoretical Astrophysics.
2005 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 439, no 1, 93-106 p.Article in journal (Refereed) Published
Abstract [en]

A stochastic model of the chemical enrichment of metal-poor systems by core-collapse (Type II) supernovae is presented, allowing for large-scale mixing of the enriched material by turbulent motions and cloud collisions in the interstellar medium. Infall of pristine material is taken into account by following the evolution of the gas density in the medium. Analytical expressions were derived for the number of stars enriched by a given number of supernovae, as well as for the amount of mass with which the ejected material from a supernova is mixed before being locked up in a subsequently formed star. It is shown that for reasonable values of the gas density (~0.1 cm-3) and of the supernova rate (~0.25 kpc-3 Myr-1) of the Galactic halo, the resulting metallicity distributions of the extreme Population II stars show a distinct cut-off at [Fe/H] approximately -4. In fact, by assuming no low-mass Population III stars were able to form out of the primordial interstellar medium, the derived fraction of stars below [Fe/H]= -4 is in agreement with observations. Moreover, the probability is high that even the most metal-poor stars observed to date have been enriched by several contributing supernovae. This partly explains the relatively small star-to-star scatter in many chemical abundance ratios for stars down to [Fe/H]= -4, as recently found in several observational studies. Contribution from the thermonuclear (Type Ia) supernovae is found to be negligible over almost the entire extremely metal-poor regime. Although the fraction of contaminated stars may increase rapidly towards [Fe/H]= -2.5, the fraction of stars with iron primarily from Type Ia supernovae remains small. The stars that are heavily polluted by Type Ia supernovae are pushed towards higher metallicities, creating a hole/bump in the metallicity distribution. Such features could be used to reveal the possible presence of subpopulations of Galactic halo stars that have been enriched by Type Ia supernovae.

Place, publisher, year, edition, pages
2005. Vol. 439, no 1, 93-106 p.
National Category
Astronomy, Astrophysics and Cosmology
URN: urn:nbn:se:uu:diva-92466DOI: 10.1051/0004-6361:20041934OAI: oai:DiVA.org:uu-92466DiVA: diva2:165553
Available from: 2004-11-12 Created: 2004-11-12 Last updated: 2013-03-13Bibliographically approved
In thesis
1. Stochastic Chemical Evolution: A Study of Scatter in Relative Elemental Abundances in Extremely Metal-poor Stars
Open this publication in new window or tab >>Stochastic Chemical Evolution: A Study of Scatter in Relative Elemental Abundances in Extremely Metal-poor Stars
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Stokastisk grundämnestillväxt : En studie av spridningen i relativa grundämnesförekomster i extremt metallfattiga stjärnor
Abstract [en]

Chemical evolution addresses the problem of the formation of the chemical elements and their evolution throughout the history of the universe. This thesis discusses in particular the chemical evolution in the young universe and what we may learn from the observations of the oldest stars. The present day production of carbon in the Galaxy is also discussed. Interstellar media of young, metal-poor, star-forming systems are expected to show large chemical abundance inhomogeneities due to local supernova explosions. These inhomogeneities are reflected in the surface abundances of the population of longlived, low-mass stars. A stochastic model of the chemical evolution in such systems is presented and used to study the metallicity distribution and the scatter in chemical abundance ratios. The model takes into account mixing of the enriched material by turbulent motions and cloud collisions in the interstellar medium as well as infall of pristine matter. The predicted metallicity distribution shows, in accordance with observations of extreme Pop II strars in the Galactic halo, a distinct cut-off at [Fe/H]~-4. However, the fraction of stars below [Fe/H]=-4 agrees with observatrion only if a population of metal-free stars (Pop III) was never able to form. The predicted scatter in abundance ratios is demonstrated to be crucially dependent on the as yet uncertain supernova yields and the relatively small star-to-star scatter is tentatively explained by the averaging of a large number of contributing supernovae and by the selection effects favouring contributions from supernovae in a certain mass range for the most metal-poor stars. Furthermore, stars enriched by one single supernova are predicted to be found in very narrow sequences in the abundance ratio diagrams (so called A/A diagrams). Verification of the existence of such features, called single supernova sequences, is observationally challenging. Abundance analysis of carbon was performed in a large sample of solar-type stars in the Galactic disk using the forbidden [C I] line at 8727 Å. A comparison between the relation of [C/O] with metallicity for the Galactic stars and that of dwarf irregular galaxies suggests that large amounts of carbon are produced today by massive, so called Wolf-Rayet stars. Low-mass stars are less important. This was also demonstrated by modelling the chemical evolution of carbon.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2004. 44 p.
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 1051
Astronomy, nucleosynthesis, chemical evolution, Galactic evolution, supernovae, chemical abundances, Population II stars, Astronomi
National Category
Astronomy, Astrophysics and Cosmology
urn:nbn:se:uu:diva-4702 (URN)91-554-6117-4 (ISBN)
Public defence
2004-12-03, Polhemsalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 10:00
Available from: 2004-11-12 Created: 2004-11-12Bibliographically approved

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