Stellar abundances in the early galaxy and two r-process components

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 I-129 and Hf-182 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 I-129 and Hf-182. 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 W-182 Or the low-mass (A less than or equal to 130) nuclei associated with I-127 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 loge(Eu) = -2.98 to -2.22 were formed from an ISM contaminated most likely by a single H event within the first similar to 10(7) yr 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 (similar to 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. (C) 2000 Elsevier Science B.V. All rights reserved.