Interpretation of Solar System Abundances Around the N = 50 Neutron Shell

Abstract

New measurements [l] of CI chondrites for Ni-Ru show a high degree of smoothness of the odd-A solar system abundance curve (SSAC) through the region of the N = 50 closed neutron shell. The resolved s- and r-process peaks at the N = 82 and 126 neutron shells [2] are not apparent for the N = 50 region. Our data confirm the necessity for a single element 89Y SSAC peak, presumably of s-process origin. If the total SSAC is smooth but made of contributions from more than one nucleosynthesis process, then at least the major contributing processes must also have smooth abundance curves. Within errors, a smooths-process abundance (N,) curve can be drawn using N, from Beer and co-workers. For A= 75-85 there are strong "non-s" contributions which could be flat or show a shallow maximum at mass 79-81. (N-N,; suppressed scale). This maximum would be analogous to the "r-process" peaks at A = 129 or 195. The reason that the two-peak structure for N = 50 is not apparent in the total abundance curve is that the lower mass peak is relatively broad, leading to unresolved sand non-speaks in the total SSAC. The rise in the N-N, curve below mass 75 is probably an error in the theoretical N,, so the "non-s" peak is better defined than at first glance. Below mass 69 it is hard to separate the contributions of n-capture nucleosynthesis from the high-mass tail of the iron group nuclei, the origin of which is not well understood.

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