Determination of the s-PROCESS Neutron Capture Time

Abstract

The relative abundance that an isotope or element attains during the s-process depends critically, in many cases, upon the mean neutron capture time, T . The latter is very poorly known, however, with estimates ranging over a factor of 1O for a particular isotope. We show that it may be possible to determine T for a given isotope and star frotn observations. We have calculated the relative abundances of all the isotopes that participate in the s-process as a function of T under steady- state conditions. The assumption of steady state is not critical as long as we confine our attention to ratios of abundances of elements which are adjacent in the periodic table. We have also included in our calculations processes which may alter the effective fl-decay lifetime, such as the photo-beta process, and find that the effects on elemental abundance ratios are negligible. The calculations can be applied in a variety of ways. For example, in some highly evolved stars, material processed in the interior is apparently transported to the surface in such quantities as to completely dominate the atmospheric abundances of the heavy elements. In principle, one should be able to determine r,' for a given isotope for such a star by comparing observations with these calculations. With few exceptions, this does not appear possible as yet since the spectra of such stars are notoriously difficult to interpret. For R Cyg, however, Zr isotopic abundance observations rule out both very small and very large values of r,,. For solar-system material, examination of the abundance of isotopes produced only by the s-process indicates that r was probably on the order of 10 years. Subject headings: abundances, solar - abundances, stellar - Ba ii stars - interiors, stellar - nuclear reactions - S-type stars