PARAMETRIC STUDY OF HEAVY ELEMENT ISOTOPIC ABUNDANCES IN SIC GRAINS

This paper presents a new interpretation of the non-solar isotopic com position of a series of elements from Si to Sm found in small-sized (m um) SiC grains of presumably circumstellar origin. It relies on a para metrized model that aims at investigating in as broad a way as possibl e the neutron exposures or range of such exposures that could replicat e the series of measured isotopic compositions without relying on any specific (and sometimes highly uncertain) astrophysical scenario. It i s found that the general trends of the isotopic data of Si, Ca, Ti, Sr , Kr, Xe, Ba, Nd and Sm can be reproduced with a much weaker neutron e xposure than contemplated in previous studies. Such a neutron irradiat ion history does not necessitate the classical assumption of an admixt ure of a s-process component to normal solar abundances. Instead, the linear correlations observed in three-isotope plots are interpreted in terms of a mixture of components having experienced slightly differen t neutron exposures. In this framework, predictions are provided for a ll the elements from Si to Bi, and implications for possible astrophys ical sites are discussed.