48Ca Production in Matter Expanding from High Temperature and Density

Details 48Ca Production in Matter Expanding from High Temperature and Density 48Ca Production in Matter Expanding from High Temperature and Density

May 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...462..825m&link_type=abstract

Astrophysical Journal v.462, p.825

Astronomy and Astrophysics

Astronomy

38

Nuclear Reactions, Nucleosynthesis, Abundances, Stars: Supernovae: General

Scientific paper

We calculate with a large nuclear reaction network the nuclear dynamics associated with the expansions and cooling of initially hot and dense matter. We study matter with neutron excess near that of 48Ca, because one objective is to clarify the nucleosynthesis of that abundant neutron-rich nucleus, whose origin has been enigmatic. Expecting that supernovae provide the site of its origin, we take initial temperatures near T9 = 10 but survey a wide range of initial densities, corresponding to a wide range in initial entropies. The highest entropies are probably associated with winds from newborn neutron stars in Type II collapse events, whereas the smallest entropies are probably associated with very high density Type Ia cores. Our survey objective is the analysis of the dynamics of the nuclear assembly as it cools, and how the correct description of it depends on the specific entropy.
We show that resultant abundances of neutron-rich nuclei are very poorly represented by nuclear statistical equilibrium (NSE). The deviations from NSE are governed by the number of heavy nuclei assembled during the expansion, which differs significantly from the number demanded by NSE at both high and low entropy. High-entropy expansions are shown to contain too few nuclei, with the result that 48Ca cannot survive the expansion even though it would be expected to be abundant using NSE guidelines. Low-entropy expansions contain too many nuclei with respect to that guideline, with the result that 48Ca is more abundant than expected. In this case it is especially significant that the ratio of 48Ca to other major neutron-rich nuclei (e.g., 66Zn, 82Se) is substantially greater than NSE guidelines, which ameliorates overproduction limits from the latter. We show, furthermore, that the 48Ca nucleus itself plays a key role in the nuclear dynamics. In the low-entropy expansion, which is the one to which we must look for 48Ca origin, abundant 48Ca is a refractory post, a local abundance maximum, with quasi-equilibria attached to it in both the upward-mass direction (setting their abundances) and in the downward-mass direction (setting the disintegration rate of 48Ca, which governs the attempt to relax to NSE).
Although understanding the network is our key result, we discuss some immediate consequences of astrophysical importance. We argue that Type Ia cores must be the site of origin of 48Ca, and track one dense explosion of an oxygen-neon core to show that it adjusts its neutron richness during the burning to values similar to those in our survey (η = 0.17) and that the network dynamics are identical to those identified in our survey. We also discuss significant implications for the correlated endemic isotopic anomalies in neutron-rich isotopes discovered in calcium-aluminum-rich inclusions from meteorites.

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