Yutaka Komiya¹, Shimako Yamada², Takuma Suda¹, and Masayuki Y. Fujimoto^3,4

¹National Astronomical Observatory of Japan, Osawa, Mitaka, Tokyo 181-8588, Japan
²Department of Cosmoscience, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan
³Nuclear reaction data center, Graduate School of Science, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan
⁴Visiting researcher, Faculty of Engineering, Hokkai-gakuen University, Sapporo, Hokkaido 062-8605, Japan

We investigate the chemical enrichment of r-process elements in the early evolutionary stages of the Milky Way halo within the framework of hierarchical galaxy formation using a semi-analytic merger tree. In this paper, we focus on heavy r-process elements, Ba and Eu, of extremely metal-poor (EMP) stars and give constraints on their astronomical sites. Our models take into account changes of the surface abundances of EMP stars by the accretion of interstellar medium (ISM). We also consider metal-enrichment of intergalactic medium by galactic winds and the resultant pre-enrichment of proto-galaxies. The trend and scatter of the observed r-process abundances are well reproduced by our hierarchical model with ~10% of core-collapse supernovae in low-mass end (~10 M_☉) as a dominant r-process source and the star formation efficiency of ~10^-10 yr–1. For neutron star mergers as an r-process source, their coalescence timescale has to be ~10⁷ yr, and the event rates ~100 times larger than currently observed in the Galaxy. We find that the accretion of ISM is a dominant source of r-process elements for stars with [Ba/H] < –3.5. In this model, a majority of stars at [Fe/H] < –3 are formed without r-process elements, but their surfaces are polluted by the ISM accretion. The pre-enrichment affects ~4% of proto-galaxies, and yet, is surpassed by the ISM accretion in the surface of EMP stars.

Reference
Komiya Y, Yamada S, Suda T and Fujimoto MY (2014) The New Model of Chemical Evolution of r-process Elements Based on the Hierarchical Galaxy Formation. I. Ba and Eu.  The Astrophysical Journal 783:132.
[doi:10.1088/0004-637X/783/2/132]

Link to Article