Stellar Abundances in the Early Galaxy and Two r-Process Components

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Abstract
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 I129 and Hf182 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 I129 and Hf182. 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 W182 or the low mass (A < or = 130) nuclei associated with I127 is the same for both the H and L events and follows the corresponding solar r-pattern in each mass region. 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 log epsilon (Eu) = -2.98 to -2.22 were formed from an ISM contaminated most likely by a single H event within the first 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 (about 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.
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