Kr Isotopic Compositions in Stardust SiC grains and AGB Winds

Krypton (Kr) is a heavy noble gas that does not chemically react and hence does not condense into dust. However, it is found in trace amounts inside stardust silicon carbide (SiC) grains in meteorites, which are believed to have condensed in the C-rich envelopes of low-mass asymptotic giant branch (AGB) stars. The measured isotopic composition of Kr clearly reveals the signature of the s (slow neutron-capture) process. It is likely that Kr is ionised and implanted in stardust SiC grains via stellar winds in two different evolutionary phases: one during the AGB phase in small grains showing low ⁸⁶Kr/⁸²Kr, and another during the post-AGB phase in large grains showing high ⁸⁶Kr/⁸²Kr ratios. The low ⁸⁶Kr/⁸²Kr ratios observed in stardust SiC grains can be explained by model predictions of AGB winds. On the other hand, to explain the high ⁸⁶Kr/⁸²Kr ratios we need to look at the material in the winds of the post-AGB phase. We present Kr isotopic compositions predicted by s-process AGB-star models of different masses and metallicities, and compare them to data from stardust SiC grains. We find that to match the high ⁸⁶Kr/⁸²Kr ratios observed in the large grains, a proton ingestion during the thermal pulse (TP) may be required. We also find that the ⁸⁴Kr(n,γ)⁸⁵Kr neutron-capture cross section should to be lower than the current estimate in order for our models to match the pure s-process value.