MadSci Network: Astronomy

Re: reaction speed of nucleosynthesis, theory vs. experiment, why differ?

Date: Tue Jan 7 20:21:01 2014
Posted By: Vladimir Escalante-Ramirez, Faculty
Area of science: Astronomy
ID: 1386769094.As

Calculations of reaction rates of heavy nuclei are very uncertain. There are no sure ways to calculate them, and differences with experimental results must be expected. The question here is how sensitive are calculations of supernova explosions to changes in the nuclear reaction rates. Calculations of supernova explosions use a complex network of thousands of nuclear reactions. Current models start with a Chandrasekar–mass dwarf composed of about equal parts of 12C and 16O. The main reactions are 12C +12C, which produces mainly 16O, 20Ne and 24Mg, and 16O +16O, which produces mainly 28Si, 32S, 36Ar, and 40Ca.

Bravo and Martínez–Pinedo published a a detailed study of sensitivity of nucleosynthesis of type Ia supernovae to variations of nuclear reaction rates in Physical Review C, Vol. 85, 055805 (2012) . Surprisingly they found that variations of individual reaction rates when they are multiplied by factors of 10 or 0.1 were quite modest in the results in the energy yield of the explosion. As expected the largest changes occur with modifications of the rates of the 12C +12C and 16O +16O, but even in those cases the energy of the explosion changes by less than 4%. Changes in the yields of products of the reactions with a mass fraction larger than 0.02 vary by less than a factor of 2.

The explanation for these small variations is that there are thousands of reactions that occur simultaneously during the explosion. Bravo and Martínez–Pinedo used a network of 3138 nuclear reactions. Unless there is a simultaneous variation of certain rates in phase, a large variation of a few of them will not cause large variations in the luminosity of the explosion of the nucleosynthesis because other rates producing the same element remain constant. On the other hand large random variations of several reactions will tend compensate one another, and the result is again little change in the energy and nucleosynthesis yields. A similar conclusion was reached by Parikh, Jose, Seitenzahl, and Roepke, who used a network of 2305 reactions. . Their work has been accepted for publication in Astronomy and Astrophysics. Therefore my guess is that variations in the nuclear reaction rates will not modify predictions by current supernova models.

Vladimir Escalante Ramírez
Center for Radio Astronomy and Astrophysics
Morelia, Michoacán, México

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