Carla Fröhlich,1 Raphael Hirschi,1 Matthias Liebendörfer,1 Friedrich-Karl Thielemann,1 Gabriel Martínez Pinedo2 & Eduardo Bravo3
1Department of Physics and Astronomy, Universität Basel, Switzerland 2 Gesellschaft für Schwerionenforschung, Darmstadt, Germany 3Departament de Física i Enginyeria Nuclear, Universitat Politecnica de Catalunya,
Galactic chemical evolution witnesses the enrichment of the interstellar medium with elements heavier than H, He, and Li that originate from the Big Bang. These heavier elements can be traced via the surface compositions of low-mass stars of various ages, which have remained unaltered since their formation and therefore measure the composition in the interstellar medium at the time of their birth. Thus, the metallicity [Fe/H] is a measure of the enrichment with nucleosynthesis products and indirectly of the ongoing duration of galactic evolution. For very early times, when the interstellar medium was essentially pristine, this interpretation might be wrong and perhaps we see the ejecta of individual supernovae where the amount of H with which these ejecta mix is dependent on the energy of the explosion and the mass of the stellar progenitor. Certain effects are qualitatively well understood, i.e. the early ratios of alpha elements (O, Ne, Mg, Si, S, Ar, Ca, Ti) to Fe, which represent typical values from Type-II supernova explosions that originate from rapidly evolving massive stars. On the other hand, Type-Ia supernovae, which are responsible for the majority of Fe-group elements and are the products of binary evolution of lower-mass stars, later emit their ejecta and reduce the alpha/Fe ratio. In addition to being a measure of time, the metallicity [Fe/H] also enters stellar nucleosynthesis in two other ways. (i) Some nucleosynthesis processes are of secondary nature, e.g. the s-process, requiring initial Fe in stellar He-burning. (ii) Other processes are of primary nature, e.g. the production of Fe-group elements in both types of supernovae. These explosive nucleosynthesis yields originate in both cases from initial H, which is burned during stellar evolution and in the final explosion, but the question is whether the initial metallicity affects the way of explosive processing (e.g. by changing the neutron-richness of matter measured by ye) or influences the stellar evolution and consequently the final nucleosynthesis products. In the present paper we will first outline the general
Increasing explosion energy
SN explosion and black hole?
Direct blackhole formation
20 25 30 35 Progenitor Mass (Ms)
Figure 37.1. Left panel: C-O core masses from stellar-evolution calculations for various metallicities as a function of the initial stellar mass, with Vinit = 300 km s-1 (Hirschi et al. 2005). Right panel: explosion energy as a function of progenitor mass for core-collapse supernovae (of Solar metallicity) due to neutrino absorption (Liebendorfer et al. 2003). The explosion energy peaks around ~25M0, increasing with increasing progenitor mass at lower masses and decreasing fast for higher masses. Above ~40M0 no explosions are observed.
questions addressed here and then analyze the abundance trend of individual elements.
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