We present quantitative predictions for the abundances of r-process elements in stars formed very early in the Galactic history using a phenomenological two-component r-process model based on the 129I and 182Hf inventory in the early solar system. This model assumes that a standard mass of the ISM dilutes the debris from an individual supernova. High-frequency supernova H events and low-frequency supernova L events are proposed in the model with characteristics determined by the meteoritic data on 129I and 182Hf. The yields in an H or L event are obtained from these characteristics and the solar r-process abundances under the assumption that the yield template for the high-mass (A>130) nuclei associated with 182W or the low-mass (A≤130) nuclei associated with 127I is the same for both the H and L events and follows the corresponding solar r-pattern in each mass region. This choice of the yield templates is justified by the regular solar-like r-process abundance pattern for Ba and higher atomic numbers observed in very metal-poor stars. The abundance of Eu, not Fe, is proposed as a key guide to the age of very metal-poor stars. We predict that stars with logε(Eu)=-2.98 to -2.22 were formed from an ISM contaminated most likely by a single H event within the first ~107yr of the Galactic history and should have an Ag/Eu abundance ratio less than the corresponding solar r-process value by a factor of at least 10. Many of the very metal-poor stars observed so far are considered here to have been formed from an ISM contaminated by many (~10) r-process events. Stars formed from an ISM contaminated only by a pure L event would have an Ag/Eu ratio higher than the corresponding solar r-process value but would be difficult to find due to the low-frequency of the L events. However, variations in the relative abundances of the low- and high-mass regions should be detectable in very metal-poor stars.
Bibliographical noteFunding Information:
We greatly appreciate the support by John Cowan and Christopher Sneden in freely providing us information on their work and in maintaining a continued level of interest in testing alternative models, however speculative. Discussions with Andrew McWilliam on abundances in very metal-poor stars were of considerable aid. We thank Petr Vogel for comments on an earlier draft of the paper. This work was supported in part by the US Department of Energy under contract W-7405-ENG-36 and grant DE-FG03-88ER-13851, and by NASA under grant NAG 5-4076, Caltech Division Contribution No. 8641(1032). Y.-Z.Q. was supported by the J. Robert Oppenheimer Fellowship at Los Alamos National Laboratory.
- Abundances in metal-poor stars
- Galactic chemical evolution
- r-process nucleosynthesis