Physics – Nuclear Physics
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.548m&link_type=abstract
Meteoritics, vol. 30, no. 5, page 548
Physics
Nuclear Physics
Anomalies, Isotopic, Calcium-Aluminum-Rich Inclusions, Nucleosynthesis, Supernova
Scientific paper
Calculations of neutron-rich nuclear statistical equilibrium (NSE) have long been used as guides to the synthesis of 48Ca, e.g. [1]. To the astrophysicist, the success of these calculations has suggested the solar system's supply of this isotope was produced in neutron-rich matter expanding from high temperature and density, presumably near the core of a Type II (core-collapse of a massive star) supernova. To the meteoriticist, neutron-rich NSE calculations have provided insight into the correlated anomalies among 48Ca, 50Ti, and 54Cr observed in CAIs, e.g. [2]. So that we do not delude ourselves with conclusions drawn from NSE, however, we should question how accurately neutron-rich NSE predicts the yield of 48Ca produced in expansions from high temperature and density. In order to study this question, we drop the assumption of NSE and employ a nuclear network code to follow 48Ca synthesis in expanding matter. A description of this code may be found in [3,4,5]. We assumed the material began at T = 10^(10)K and expanded until T ~ 10^7K, by which point the nuclear reactions had ceased. We characterized all calculations by phi, the number of photons per nucleon. phi scales monotonically with the entropy per nucleon. We performed calculations for constant phi=5, a high-entropy expansion, and for constant phi=1, a low entropy expansion. For each calculation, the neutron richness was eta=0.2 and the matter expanded on a 0.2 second timescale. We compared these results with neutron-rich NSE calculations. The results are shown in table 1. Shown for both calculations are the final mass fractions of 48Ca. For comparison are shown the 48Ca mass fractions computed in NSE at temperatures T9 = T/10^9K = 3, T9=3.5, T9=4, and T9=5, reasonable expectations for the freezeout temperatures. Not surprisingly, the 48Ca yield from the high-entropy expansion (phi=5) differs greatly from that in neutron-rich NSE. What is surprising is that this is not a freezeout effect, but rather the result of a shifting quasi-static equilibrium. The mass fractions of 48Ca in the high-entropy expansion and in neutron-rich NSE differ for temperatures as high as 6.5 x 10^9K. NSE is not a good guide at all to 48Ca synthesis in this case. NSE is also not a good guide to 48Ca synthesis in the low-entropy expansion (phi=1). This is due to the fact that material produces more, but lighter, nuclei (including 48Ca) than NSE demands. This result has dramatic consequences for the ratio of 66Zn/48Ca in these freezeouts [6]. In summary, one should be extremely cautious in applying the results of neutron-rich NSE calculations to study of correlated anomalies in CAIs. References: [1] Hartmann D. H. et al. (1985) Astrophys. J., 297, 837-845. [2] Lee T. (1988) in Meteorites and the Early Solar System (J. F. Kerridge and M. S. Matthews, eds.), 1021-1062, Univ. of Arizona, Tucson. [3] Meyer B. S. and Walsh J. H. (1993) in Nuclear Physics in the Universe (M. W. Guidry and M. R. Strayer, eds.), 9-30, IOP, Bristol. [4] Fuller G. M. and Meyer B. S. (1995) Astrophys. J., in press. [5] Meyer B. S. (1995) Astrophys. J. Lett., 449, in press. [6] Meyer B. S. (1995) this volume.
Clayton Donald D.
Krishnan T.
Meyer Bradley S.
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