(2020) The Origin of S-Process Isotope Heterogeneity in the Solar Protoplanetary Disk
Ek M, Hunt AC, Lugaro M & Schönbächler M
01g: Plenary Hall, Thursday 25th June 22:18 - 22:21
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In your last slide, discussion based on your data show S-process enrichment then towards Sun, which do not require isotope dichotomy separated by Jupiter. What makes difference between Mo isotope and other heavy elements?
The dichotomy is essentially a constant offset between the CC and NC meteorites and requires a separate process than that which created the progressive enrichment of s-process elements towards the sun. This dichotomy resolved by looking at the p-process isotopes. The problem with p-process isotopes is that they usually have low abundances and always have isobaric interferences. This makes high precision isotope measurements on these isotopes particularly difficult. Mo is unusual in that it its p-process isotopes have an unusually high abundance ( ~10%) making high precision isotope measurements much easier than for other elements. It might very well be that the dichotomy seen on Mo is also present for other elements but that we have so far been unable to resolve it. This is the case for Pd where the precision on the p-process isotope is not high enough to confidently confirm, or rule out, the presence of such a dichotomy.
This is a question for you and Quinn. You both suggest that the incomplete condensation of Yb and Pd in SiC accounts for the smaller than expected s-process variations, so could you expect anomalies in Yb and Pd to be correlated in leachates for example? Would this fall on an s-process mixing line? Also, Pd shows heterogeneity at the bulk meteorite scale while Yb doesn't, so does this tell us more about the carriers of these elements, or the metallicity of the AGB stars that contributed material to our solar system?
I wouldn’t expect Pd and Yb to correlate in leachates but I wouldn’t necessarily be surprised if they did either. The stardust carriers for Pd and Yb are both currently unknown so it could be that they are hosted in the same phase but my initial suspicion would be that they don’t have the same carrier. That being said their respective carriers could have similar chemical resistance so that they are both destroyed in the same leech step. So there are a lot of unknowns. There are to many unknowns to be able to explain why Pd shows nucleosynthetic offsets on the bulk rock scale but Yb doesn’t. Perhaps the biggest is that we do not know what the condensation temperatures of these elements actually are around AGB stars. Another big unknown is the carrier phase for each of these elements and how resistant those carriers might be to weathering in the ISM and protoplanetary disk. I think the fact that the nucleosynthetic signature for the refractory REE are much smaller than Pd’s refractory neighbours lends credence to the idea that a large fraction of stardust originates from high metallicity AGB stars. But there is certainly more work to be done to properly test this idea.
Any idea what process is responsible for the depletion of elements with low condensation temperatures in stardust? Also, what phase(s) are the expected carrier for Pd?
There is no nucleosynthetic reason why AGB stars would produce less of the elements with a lower condensation temperature. The process must therefore be related to fractional condensation of elements around AGB stars. There are two main possibilities that we discuss in our paper: 1) First is that the more volatile elements simply didn’t condense into dust around AGB stars but were mainly concentrated in the gas phase when mixed with the ISM. 2) The second possibility is that the more volatile elements condense into stardust that is much more readily destroyed in the ISM than the carrier of the refractory elements. It is difficult to distinguish between these two possibilities especially since the carrier for Pd and many other more volatile elements is unknown. The carrier phase for Pd is a mystery. It is probably not SiC as there has been at least one study which ought to have identified Pd in SiC had it been present.
Is there observational evidence regarding the modelled element yields for AGB stars?
The trend that the poduction of heavy element decreases as the metallicity increases is a feature of the 13C neutron source in AGB stars as can be seen in the last supplementary figure of our paper. This has been observed in Ba stars, which are companion stars to AGB stars in binary system, where the Ce/Y ratio decreases with metallicity as predicted by models. My co-author Maria Lugaro has just had a paper accepted on this which is avaliable as a prepint here https://arxiv.org/abs/2006.11101 for anyone interested.
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