Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.559o&link_type=abstract
Meteoritics, vol. 30, no. 5, page 559
Other
Anomalies, Isotopic, Diamond, Interstellar, Dust, Gases, Noble, Supernova
Scientific paper
Xenon-HL carried by interstellar diamonds in primitive meteorites [1] resembles xenon produced in the p- and r-processes of nucleosynthesis (thought to occur in supernovae) in that it is enriched in the light (hence 'L'), p-only, isotopes 124,126Xe and the heavy (hence 'H'), r-only, isotopes 134,136Xe. Detailed comparison reveals different levels of enhancement in Xe-H, however, of 134Xe and 136Xe. As a result, ad-hoc scenarios have been suggested, like a mini-r-process, intermediate between s-and r-process, for producing Xe-H [2]. However, inventing a special process in order to account for a significant fraction of one element (several percent of the Xe in primitive meteorites is Xenon-HL) without evidence for such a process to have contributed significantly to anything else, is not a very attractive solution. Here we propose to consider instead that Xe-H is basically 'normal' r-process Xe that a chemical fractionation effect has turned into Xe-H. In doing so we compare r-process Xe with 'pure 'Xe-HL (i.e. Xe-HL extrapolated to 130Xe degrees 0, where 'real' HL-Xe is the product of mixing with ~ normal Xe). The r-process acts on a rapid (~1 sec) timescale (e.g. [3]), producing neutron-rich nuclides far from stability, that subsequently decay via a series of beta-decays into stable endproducts. As the precursors of the different Xe isotopes have different lifetimes, the isotopic composition of the Xe in supernova ejecta will be time-dependent on that time-scale. From the figure, where the development of the ratio 134Xe/136Xe is shown, it is obvious that after approx. 2 hours this ratio is equal to that of 'pure' Xe-H. If, at that time, a chemical separation can be achieved between the Te and I precursors on one hand and Xe on the other, we are left with Xe for which this ratio is identical to that in Xe-H. At this time little 129,131,132Xe will have been produced, because their precursors have much longer half-lives. In order to account for the small, but non-zero abundances in pure Xe-H of these isotopes, on the order of 5% of fully developed r-process Xe may have to be admixed, an observation supported by the fact that in 'pure' Xe-H these isotopes occur in ratios relative to each other that are consistent with r-process Xe proper. Also, because in the p-process 126Xe is in part originally produced as 126Ba with a half-life of 97 min., a separation of Ba from Xe on a similar time-scale may account for the fact that (126Xe/124Xe)L < (126Xe/124Xe)p. Condensation comes to mind as an obvious means to achieve a separation between xenon and the other elements, but timescales usually associated with the formation of supernova condensates are years rather than hours. We note, however, that for certain assumptions about the cooling process of supernovae, the first condensates may form in the ejecta after about 10^3-10^4sec. already [4]. Taken at face value, the existence of Xe-HL may serve to support such a fast cooling scenario. References: [1] Lewis R. S. et al. (1987) Nature, 326, 160-162. [2] Clayton D. D. (1989) Astrophys. J., 340, 613-619. [3] Kratz K.-L. et al. (1993) Astrophys. J., 403, 216-238. [4] Lattimer J. M. et al. (1978) Astrophys. J., 219, 230-249.
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