Galactic4He and heavy elements enrichment by stellar nucleosynthesis |
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| Authors: | M Busso R Gallino A Masani |
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| Institution: | (1) Istituto di Fisica Generale Dell'Università, Torino, Italia;(2) Laboratorio di Cosmogeofisica del CNR, Torino, Italia |
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| Abstract: | The contribution to the galactic abundance of He and heavy elements by stellar nucleosynthesis is calculated as a function of time, keeping account of present knowledge about stellar and galactic evolution. A model is used which distinguishes the phase of the contracting halo from the subsequent history of the disc. Various uncertainties involved both in stellar and in galactic evolutionary theory are discussed. The amount of4He produced by stars of different masses and ejected in interstellar medium is fairly well known from stellar theory, while we have assumed its primordial abundance as a free parameter, ranging from 0 up to 0.4. We find that stellar activity provides a significant contribution to the cosmic4He, though not sufficient to explain the observed abundance. The best agreement with observational data (Y
  0.26 andY
now0.28) is obtained starting with a primordial abundanceY
 =(0.20–0.23), which is consisten with the  Big-Bang theory predictions and with recent observational estimates. The contribution to the abundance of heavy elements depends on the last stellar stages and on the final explosion mechanism, which are only now beginning to be understood. Nevertheless, in the framework of present theories, we individuate a stellar evolutionary scheme reproducing the observedZ abundances for Populationi and Populationii stars, with the correctly estimated  Y/Z value. In this scheme, only stars belonging to two narrow mass ranges (10 m/m
15 andm/m
 80) are allowed to eject metal-enriched matter, possibly with the solar (C+O)/(Si+Fe) ratio. |
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