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Problems Related to Interplanetary Matter (1961)

Chapter: COMMENTS ON THE TIME DEPENDENCE OF NUCLEO-SYNTEHSIS

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Suggested Citation:"COMMENTS ON THE TIME DEPENDENCE OF NUCLEO-SYNTEHSIS." National Research Council. 1961. Problems Related to Interplanetary Matter. Washington, DC: The National Academies Press. doi: 10.17226/18683.
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Suggested Citation:"COMMENTS ON THE TIME DEPENDENCE OF NUCLEO-SYNTEHSIS." National Research Council. 1961. Problems Related to Interplanetary Matter. Washington, DC: The National Academies Press. doi: 10.17226/18683.
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Page 8

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COMMENTS ON THE TIME DEPENDENCE OF NUCLEOSYNTHESIS Truman P. Kohman Department of Chemistry Carnegie Institute of Technology The discovery by Reynolds of excess Xe ' in meteorites, about which we have just heard, has opened extremely important new avenues to the study of the early events of the solar system. Earlier today we heard from Cameron of a remarkably complete and detailed account of nucleosynthesis and planetary system formation which would account for Reynold's observations and many other facts. Another noteworthy feat of deductive inference has been accomplished by Anders and his collaborators (Goles, Fish and Anders, 1960; Fish, Goles and Anders, 1960), who give convincing arguments for a relatively intense but short-lived source of heat in the meteorite parent bodies. Calculations of the speaker (Kohman 1960) have shown, however, that the continuous nucleosynthesis rapid- mixing model cannot supply it at the right time (Kohman 1956). One way out of this difficulty is Cameron's suggestion of medium-lived radioactiv- ity induced by a high flux of high-energy particles in the solar nebula, but such a radical postulate is not really necessary. Calculations have been made for a "mixed nucleosynthesis" model, in which the bulk of the stable and long-lived nuclides are made more-or- less continuously over a long period of time, and shortly prior to the for- mation of the solar system a "spike" of freshly synthesized material from a near-by supernova, to the extent of 1 - 10 percent of the total, is added. Then both the I1*' and the heating can be accounted for very nicely. Under this model, about 100 million years more than Cameron assumed [~200 m.y. in all] is available for the processes of condensation of the solar system to the point where the meteoritic parent bodies had cooled sufficiently to retain Xe. Cameron: These models depend very sensitively on the mixing time assumed. If mixing requires two or three half-lives of 1^9, then it should be possible to get local enrichment of this nuclide. The prob- lem is, in a sense, a philosophical one: should we try to construct a general model, assuming complete mixing, or should we work with the special case of the arrival of just what is needed at just the right time? 7

Kohman: In order to avoid invoking a rare coincidence to account for the solar system, would it not be possible to assume that the "hot breeze" from the local supernova had some necessary correlation, causal or otherwise, with the initiation of condensation? Cameron: The trouble with that suggestion is that it appears to be neces- sary, from Jeans' criterion, for the galactic cloud to remain undis- turbed for a long period while condensing into protostars. Any large- scale disturbance of the cloud would probably result in its dispersal. Anders: On the other hand, the more massive stars in such a condensing cloud would evolve very quickly, within about 10 million years, and would then explode and inject freshly-synthesized material into the cloud. Cameron: I would expect that such events would result in the disruption of the cluster by sweeping out the nebular gas and dust and thereby re- ducing the total mass so that the remaining stars would no longer be gravitationally bound. REFERENCES Fish, R. A., Goles. G. G.. and Anders, E. (1960) Astrophys. J. 132, 243. Goles, G. G., Fish, R. A., and Anders, E. (1960) Geochimica et Cosmo- chimica Acta, 19, 177. Kohman, T. P. (1960) Paper presented at Cleveland Meeting of American Chemical Society, April 1960; to be published in J. Chem. Ed. Kohman, T. P. (1956) Ann. N. Y. Acad. Sci. 62, 503.

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