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Short-lived radionuclides and early solar system chronology

According to astronomical observations, numerical calculations and theoretical models, astronomers are able to model the different stages of evolution of low-mass stars, such as our sun. Recent results from Spitzer Space Telescope suggest that low-mass stars in a pre-main sequence stage evolve rapidly, where dust in a disk dissipates in about few My. Chandra X-ray Observatory data also indicate that powerful energetic flares are ubiquitous for an extended period of time. Based on these observations, one may infer that our solar system must have evolved through these various stages, from gravitational collapse of a molecular cloud to formation of planetary-sized bodies.

 

Short-lived radionuclides, which are opposite to long-lived radionuclides such as 238U, 232Th, 40K and so on, provide solutions for this purpose. Since the timescale we are interested in here is much shorter than the age of our solar system, long-lived radioisotopes are not capable of providing enough resolution at this temporal scale due to their long half-lives. In this proposal, the nuclides considered being “short-lived” are those whose half-lives are on the order of 100 Myr, including 41Ca (0.1 Myr), 26Al (0.7 Myr), 10Be (1.5 Myr), 60Fe (1.5 Myr) 53Mn (3.7 Myr), 107Pd (6.5 Myr), 182Hf (9 Myr), 129I (15.7 Myr), 92Nb (15.7 Myr), 244Pu (82 Myr) and 146Sm (103 Myr). The mean-lives of these radionuclides are extremely short compared with the age of the solar system thus imply that these nuclides are now “extinct”. On the other hand, they are long-lived enough to survive over a sufficient amount of time in the solar nebula to be incorporated into the oldest rocks formed during that time. Due to these properties, extinct nuclides play an important role in the early solar system chronology: their short half-lives allow us to constrain the timing of early solar system events, including the timescale of molecular cloud collapse, early chemical fractionation between parent and daughter isotopes, the formation of CAIs and chondrules, duration of high temperature events, the onset of planetary differentiation, and so on, with high temporal resolution in accordance with their abundances preserved in primitive meteorites.

 

Moreover, the abundances of these short-lived nuclides also constrain early solar system processes, such as possible formation mechanisms of the solar system (a triggered origin by shockwaves from a supernova vs. self-gravitational collapse), the origin of the short-lived radionuclides (delivery by shockwaves or stellar winds from stellar source(s) vs. local production), the formation environment of the solar system (in an isolated molecular cloud vs. in an OB association) as well as possible nuclear reactions in the solar nebula.  Those are important issues to examine the possibilities of different numerical and theoretical models for the origin of the solar system and short-lived radionuclides. Currently I choose to focus on 41Ca, 26Al and 10Be to study nebular chronologies and processes in the early solar system as these are the most highly refractory short-lived isotopes.

 

Implication of stable isotope anomalies for chemical evolution of the solar nebula and stellar  nucleosynthesis

A number of distinct isotopic signatures in terms of enrichments or depletions relative to terrestrial values, including O, Mg, Ca, Ti, Cr, Sr, Zr, Mo, Ba and Sm, have been recognized in meteorites. All these suggest that under some conditions, products of stellar nucleosynthesis were able to dodge the homogenization in the early solar nebula as well as in the meteorite parent bodies, and that there might exist multiple isotopic reservoirs to give rise to both positive and negative anomalies of the same isotopes. Isotopic anomalies also provide constraints on the nucleosynthetic mechanisms that are responsible for producing corresponding exotic components that result in anomalous isotopic compositions in primitive meteorites. In other words, these anomalies serve one of the good proxies to probe how elements are formed in stellar interiors, which also invokes the knowledge of stellar evolutions. Special attentions will be paid to 48Ca, 50Ti and oxygen isotopes in this study as they are the most prominent anomalies ever found. By comparison with the abundances of short-lived radionuclides (e.g. 41Ca, 26Al and 10Be), we will seek for possible temporal constraints for the evolution of isotopic reservoirs in the solar nebula.

 

 

Committee members: Kevin D. McKeegan (Chair), Edward D. Young, John T. Wasson and Roger K. Ulrich (outside member)

Collaborators: Andrew M. Davis (U. of Chicago) and Trevor R. Ireland (Australian National University)