Influence of light nuclei on neutrino-driven supernova outflows

We study the composition of the outer layers of a protoneutron star and show that light nuclei are present in substantial amounts. The composition is dominated by nucleons, deuterons, tritons and alpha particles; ${}^3\mathrm{He}$ is present in smaller amounts. This composition can be studied in laboratory experiments with new neutron-rich radioactive beams that can reproduce similar densities and temperatures. After including the corresponding neutrino interactions, we demonstrate that light nuclei have a small impact on the average energy of the emitted electron neutrinos, but are significant for the average energy of antineutrinos. During the early post-explosion phase, the average energy of electron antineutrinos is slightly increased, while at later times during the protoneutron star cooling it is reduced by about 1 MeV. The consequences of these changes for nucleosynthesis in neutrino-driven supernova outflows are discussed.

Figure 1

Neutron star atmosphere profiles of density and temperature corresponding to model M15-l1-r1 of Ref. [7] for times $t=2,5,7$ and $10\hspace{1em}\mathrm{s}$ post bounce.

Figure 2

Mass fractions (defined as the mass density of species $i$ divided by the total mass density) for nuclei present around the surface of the protoneutron star using three different EOS: the one used in Ref. [7] (dashed lines), the NSE EOS (dotted lines), and the virial EOS [8, 21, 22, 23] (solid lines). Beta equilibrium was assumed in each case. The lines labeled “heavy” represent the mass fraction of nuclei with $A>4$, which are included in the NSE EOS but not in the others. From top to bottom the profiles correspond to $t=2,5,7$ and 10 s post bounce. The vertical lines mark the positions of the neutrinospheres of electron neutrinos (right line) and electron antineutrinos (left line).

Figure 3

Different contributions to the absorption opacity $\langle {\kappa }_{\mathrm{abs}}\rangle$ and effective opacity $\langle {\kappa }_{\mathrm{eff}}\rangle$ of electron antineutrinos as a function of radius for $t=2,5,7$ and $10\hspace{1em}\mathrm{s}$ post bounce (from top to bottom) and for two different EOS: the one used in Ref. [7] (dashed lines) and the virial EOS (solid lines), based on the target abundances displayed in Fig. 2. In addition, the total absorption (“abs”) and total effective opacity (“effective”) are shown. The vertical lines mark the positions of the neutrinospheres of electron antineutrinos for the two different EOS. In the second panel from the top the vertical dashed and solid lines coincide.

Figure 4

Profile of the electron fraction $Y_e$ in the region around the neutrinosphere at $t=2\hspace{1em}\mathrm{s}$ after core bounce. The dashed line corresponds to the EOS of Ref. [7] (case A), the dotted line to the NSE EOS (case C), and the solid line to the virial EOS (case D). All profiles are obtained assuming beta equilibrium for the corresponding EOS.