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Stochastic chemical enrichment in metal-poor systems II.: Abundance ratios and scatter
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics, Theoretical Astrophysics.
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. 436, no 6, 879-894 p.Article in journal (Refereed) Published
Abstract [en]

A stochastic model of the chemical enrichment of metal-poor systems by core collapse supernovae is used to study the scatter in stellar abundance ratios. Large-scale mixing of the enriched material by turbulent motions and cloud collisions in the interstellar medium, and infall of pristine matter are taken into account. The resulting scatter in abundance ratios, e.g. as functions of the overall metallicity, is demonstrated to be crucially dependent on the as yet uncertain supernovae yields. The observed abundance ratios and their scatters therefore have diagnostic power as regards the yields. The relatively small star-to-star scatter observed in many chemical abundance ratios, e.g. by Cayrel et al. (2004) for stars down to [Fe/H]= -4, is tentatively explained by the averaging of a large number of contributing supernovae and by the cosmic selection effects favoring contributions from supernovae in a certain mass range for the most metal-poor stars. The scatter in observed abundances of -elements is understood in terms of observational errors only, while additional spread in yields or sites of nucleosynthesis may affect the odd-even elements Na and Al. For the iron-group elements we find some systematic deviations from observations in abundance ratios, such as systematically too high predicted Cr/Fe and Cr/Mg ratios, as well as differences between the different sets of yields, both in terms of predicted abundance ratios and scatter. The semi-empirical yields recently suggested by Francois et al. (2004) are found to lead to scatter in abundance ratios significantly greater than observed, when applied in the inhomogeneous models. "Spurs", very narrow sequences in abundance-ratio diagrams, may disclose a single-supernova origin of the elements of the stars on the sequence. Verification of the existence of such features, called single supernova sequences (SSSs), is challenging. This will require samples of several hundred stars with abundance ratios observed to accuracies of 0.05 dex or better.

Place, publisher, year, edition, pages
2005. Vol. 436, no 6, 879-894 p.
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:uu:diva-92467DOI: 10.1051/0004-6361:20042168OAI: oai:DiVA.org:uu-92467DiVA: diva2:165554
Available from: 2004-11-12 Created: 2004-11-12 Last updated: 2017-12-14Bibliographically 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.
Series
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 1051
Keyword
Astronomy, nucleosynthesis, chemical evolution, Galactic evolution, supernovae, chemical abundances, Population II stars, Astronomi
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
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
Opponent
Supervisors
Available from: 2004-11-12 Created: 2004-11-12Bibliographically approved

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