Implications of low-energy fusion hindrance on stellar burning and nucleosynthesis

Abstract

We investigate the consequences of a new phenomenological model prediction of strongly reduced low-energy astrophysical S-factors for carbon and oxygen fusion reactions on stellar burning and nucleosynthesis. The new model drastically reduces the reaction rates in stellar matter at temperatures T < ∼ (3-10) × 10 8 K, especially at densities ρ > ∼ 10 9 g cm −3 , in a strongly screened or even pycnonuclear burning regime. We show that these modifications change the abundance of many isotopes in massive late-type stars and in particular strongly enhance the abundances of long-lived radioactive isotopes such as 26 Al and 60 Fe. The reduced reaction rates also significantly complicate carbon ignition (shift carbon ignition to higher temperatures and densities) in massive accreting white dwarfs exploding as type Ia supernovae and in accreting neutron stars producing superbursts. This would require much higher ignition densities for white dwarf supernovae and would widen the gulf between theoretical and inferred ignition depths for superbursts.

Thermonuclear fusion in dense stars

“…In modern literature (Brown & Bildsten 1998;Cumming & Bildsten 2001;Gasques et al 2007), a simplified "one-zone approximation" is used for the cooling rate in Eq. (24):…”

“…The account of the heat diffusion is taken by A&A 538, A115 (2012) detailed calculation of the thermal structure of neutron star envelopes and corresponding heat fluxes. The result is compared to the simplified "one-zone approximation" (Brown & Bildsten 1998;Cumming & Bildsten 2001;Gasques et al 2007). Finally, we consider the effects of strong magnetic fields on the ignition curves in neutron star envelopes.…”

Uncertainties and robustness of the ignition process in type Ia supernovae

“…Some of them (Cussons et al 2002;Itoh et al 2003) state that resonant screening effects could have a dramatic effect (either suppression or enhancement, respectively, for the two cited papers) on the reaction rate at low energy. Jiang et al (2007) and Gasques et al (2007) predict that the fusion cross section has a low-energy hindrance in a range of temperatures and densities which include the conditions in the WD core. Gasques et al (2007) further compute the SN Ia ignition curve in this case, finding that with the reduced burning rate the ignition criteria will be fulfilled at a larger temperature.…”

“…Jiang et al (2007) and Gasques et al (2007) predict that the fusion cross section has a low-energy hindrance in a range of temperatures and densities which include the conditions in the WD core. Gasques et al (2007) further compute the SN Ia ignition curve in this case, finding that with the reduced burning rate the ignition criteria will be fulfilled at a larger temperature. All these theoretical expectations will not be addressed explicitly in the present work.…”

Investigating energy dissipation through nucleon transfer reactions

“…An abundance of recent measurements has shown this hindrance effect to be present in a wide variety of systems [5][6][7][8], such that it appears to be a general feature of heavy ion fusion. Importantly, this may affect fusion reactions of astrophysical importance [9,10], suggesting important consequences for stellar evolution. The physical origin of this phenomenon has been extensively sought [11][12][13], but no convincing explanation capable of consistently describing fusion above and below the Coulomb barrier has been forthcoming.…”