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[127.03] The ``Aerogel'' Model for the Origin of the Short-Lived Radionuclides in the Early Solar System

S.J. Desch, N. Ouellette, J.J. Hester, L.A. Leshin (Arizona State University)

Isotopic analyses of meteorites have revealed that our Solar System contained a number of live short-lived radionuclides at its birth. These include {}⁴¹{\rm Ca} (t_1/2 = 0.10 Myr), {}³⁶{\rm Cl} (0.30 Myr), {}²⁶{\rm Al} (0.71 Myr), {}¹⁰{\rm Be} (1.5 Myr), {}⁶⁰{\rm Fe} (1.5 Myr), {}⁵³{\rm Mn} (3.7 Myr), {}¹⁰⁷{\rm Pd} (6.5 Myr), {}¹²⁹{\rm I} (15.7 Myr), and {}¹⁸²{\rm Hf} (9 Myr). The radionuclide {}¹⁰{\rm Be}, which must be created by spallation reactions, is known to be decoupled in meteorites from the other radionuclides, and must have a separate origin that predates the Solar System. Its origin has been attributed to trapping of {}¹⁰{\rm Be} Galactic cosmic rays in the Sun's molecular cloud core (Desch et al.\ 2004; ApJ 602, 528). The most plausible explanation for the other radionuclides is a nearby supernova. Most models of injection of supernova radioactivities into the early Solar System hypothesize that the supernova triggered the collapse of the Sun's molecular cloud core. Chevalier (2000; ApJ 538, L151) has suggested instead that the supernova occurred after the Sun's protoplanetary disk had formed, and at a distance of < 1 pc, in analogy to the proplyds observed in the Orion Nebula only a few tenths of a parsec from \theta¹ Ori C. We use meteoritical and astrophysical evidence to argue that this is by far the most plausible scenario for how the Solar System acquired its short-lived radionuclides. We hypothesize that radionuclides in the supernova ejecta condensed into grains which were then injected into our protoplanetary disk; there they were stopped like dust grains lodged in aerogel. Because of the proximity of the disk to the supernova, a key prediction of this ``aerogel'' model is the presence of very short-lived radionuclides in the early Solar System (< 10⁴ yr). We discuss the recent, tentative evidence for live {}⁶³{\rm Ni} (t_1/2 = 101 yr) in the early Solar System (Luck et al.\ 2003; GCA 67, 143) in this context, and discuss the effect of the injected radioactivities on the ionization state of the solar nebula.

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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.