Martinez Pinedo, G.

Möller, H.

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Impact of nuclear reactions on the fate of intermediate-mass stars∗H. Mo¨ller1 and G. Martı´nez-Pinedo1,21TU Darmstadt, Darmstadt, Germany; 2GSI, Darmstadt, GermanyThe evolution of intermediate-mass stars (8 - 12 M⊙)represents one of the most challenging subjects in nuclearastrophysics (see e.g. [1,2]). Their final fate is still uncer-tain and strongly model dependent. They can become whitedwarfs, undergo electron-capture or core-collapse super-novae or even proceed towards explosive oxygen-burningand a subsequent thermonuclear explosion. An accuratedescription of the involved nuclear reactions is crucial forthe determination of the pre-supernova structure of thesestars. We argue that due to the possible development of anoxygen-deflagration, a hydrodynamic description coupledto a nuclear reaction network has to be used. For selectednuclear species, we include a set of updated reaction rates,for which we discuss their role for the evolution of the stel-lar core, at the example of SAGB-star models from [2].In this report, we want to point out the recent advancesthat we made in understanding the key nuclear reactionsthat are relevant for the proper modeling of the late stagesof these stars. Compared to massive stars, nuclear reac-tions will operate at much higher densities (109 comparedto 106 g cm−3), where electron-capture (EC) reactions canalready operate and influence the evolution of the star. Aspointed out in [2], the O-Ne-core of such stars may first un-dergo a phase of URCA-cooling due to EC on odd-A nuclei∗ supported by SFB 634, HIC for FAIR, HGS-HIRe and NAVI-15-10-5 0 5 10 8  8.5  9  9.5  10rate: log10(NA <σv> [cm3 s-1  mol-1])temperature: log10(T [K])a)-20-15-10-5 0 0  0.2  0.4  0.6  0.8  1  1.2  1.4flux: log 10(f [g mol-1  s-1 ])enclosed baryon mass [Msun]b)he4+ne20->mg24he4+ne20->mg23+nhe4+ne20->na23+phe4+o20->ne24he4+o20->ne23+nhe4+o20->f23+p-30-25-20-15-10-5 0 5 8  8.5  9  9.5  10rate: log10(NA <σv> [cm3 s-1  mol-1])temperature: log10(T [K])c)O16 + O16 → S32O16 + O20 → S36O20 + O20 → S40-40-35-30-25-20-15-10-5 0 0  0.2  0.4  0.6  0.8  1  1.2  1.4flux: log 10(f [g mol-1  s-1 ])enclosed baryon mass [Msun]d)Figure 1: Rates and fluxes for the Ne- and O-burning phaseand then a phase of rapid energy release due to double-ECon even-A nuclei, rising the temperature in the stellar core.Prior to the Ne-burning phase, 24Mg is converted into24Ne and later 20Ne into 20O by EC. Hence, the center ofthe core consists mainly of 16O and 20Ne but also up to4% of 20O. This opens new reaction channels that have sofar not been considered. They modify the Ne-burning thatnow proceeds by the reactions 20Ne (γ, α) 16O, followedby 20O (α, γ) 24Ne. The JINA reaclib rates [3] are dis-played in panel a) of Figure 1. Here, we compare the ratesof the different reaction channels for the α-capture on 20Neand 20O. The reaction 20O (α, n) 23Ne is dominating andwe argue that the α-capture on 20O is a competitive processthat should be considered in future calculations. This canbe seen even better in panel b) where we look at the reac-tion fluxes for conditions of stellar models from [2] priorto Ne-burning. For a binary rate, the flux is defined as:fAB→X = ρNA 〈σv〉AB→X YAYB, with density ρ, projectileabundances YA and YB and reaction rate NA 〈σv〉AB→X.Once the core reaches temperatures & 1.7 GK, the fu-sion reactions of neutron-rich oxygen isotopes, 16,20O +20O → 36,40S∗ may become important (besides 16O +16O → 32S∗). Due to the lack of experimental data, we relyon a model by [4] for the calculation of the astrophysical S-factors and calculate the reaction rate, taking into accountthe different branching ratios. In panels c) and d) of Fig-ure 1, it can be seen that the reaction rates are actually verycomparable. In panel d) are shown again the reaction fluxesfor the same conditions as in panel b). We find that thecontribution of the 16O + 20O-channel reaches up to 15%of the 16O + 16O-channel, while the 20O + 20O-channel ismuch less important. Considering the uncertainties presentin all of these reactions, it is important to perform a carefulevaluation of the corresponding S-factors as they can sub-stantially affect oxygen burning in intermediate-mass stars.In addition to that, the fusion involving 20O has Q-valuesof ∼ 30 MeV. This does not only increase the rate of en-ergy release during the fusion phase of oxygen by up 30%,but it also allows for exotic decay channels including theemission of up to 5 neutrons. See [5] for more details.References[1] K. Nomoto, ApJ 322 (1987), 206–214.[2] S. Jones, et al., ApJ 772 (2013), 150.[3] R. H. Cyburt, et al., ApJS 189 (2010), 240–252.[4] D. G. Yakovlev, et al., Phys. Rev. C 82 (2010), 044609.[5] H. Mo¨ller, et al., PoS(NIC XIII)125 (2014).GSI SCIENTIFIC REPORT 2014 MU-THEORY-03DOI:10.15120/GR-2015-1-MU-THEORY-03 339

Impact of nuclear reactions on the fate of intermediate-mass stars

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Impact of nuclear reactions on the fate of intermediate-mass stars

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