The r-Process in Black Hole Winds

All the current r-process scenarios relevant to core-collapse supernovae are facing severe difficulties. In particular, recent core-collapse simulations with neutrino transport show no sign of a neutron-rich wind from the proto-neutron star. In this paper, we discuss nucleosynthesis of the r-process in an alternative astrophysical site, "black hole winds", which are the neutrino-driven outflow from the accretion torus around a black hole. This condition is assumed to be realized in double neutron star mergers, neutron star - black hole mergers, or hypernovae.Comment: 6 pages, 4 figures, invited talk at OMEG10, March 2010, to be published in the proceedings of OMEG10 (AIP

Physical conditions for the r-process I. radioactive energy sources of kilonovae

Radioactive energies from unstable nuclei made in the ejecta of neutron star mergers play principal roles in powering kilonovae. In previous studies power-law-type heating rates (e.g., ~ t^-1.3) have frequently been used, which may be inadequate if the ejecta are dominated by nuclei other than the A ~ 130 region. We consider, therefore, two reference abundance distributions that match the r-process residuals to the solar abundances for A >= 69 (light trans-iron plus r-process elements) and A >= 90 (r-process elements). Nucleosynthetic abundances are obtained by using free-expansion models with three parameters: expansion velocity, entropy, and electron fraction. Radioactive energies are calculated as an ensemble of weighted free-expansion models that reproduce the reference abundance patterns. The results are compared with the bolometric luminosity (> a few days since merger) of the kilonova associated with GW170817. We find that the former case (fitted for A >= 69) with an ejecta mass 0.06 M_sun reproduces the light curve remarkably well including its steepening at > 7 days, in which the mass of r-process elements is ~ 0.01 M_sun. Two beta-decay chains are identified: 66Ni -> 66Cu -> 66Zn and 72Zn -> 72Ga -> 72Ge with similar halflives of parent isotopes (~ 2 days), which leads to an exponential-like evolution of heating rates during 1-15 days. The light curve at late times (> 40 days) is consistent with additional contributions from the spontaneous fission of 254Cf and a few Fm isotopes. If this is the case, the event GW170817 is best explained by the production of both light trans-iron and r-process elements that originate from dynamical ejecta and subsequent disk outflows from the neutron star merger.Comment: 15 pages, 5 figures, accepted for publication in Ap

Cold r-Process in Neutrino-Driven Winds

The r-process in a low temperature environment is explored, in which the neutron emission by photodisintegration does not play a role (cold r-process). A semi-analytic neutrino-driven wind model is utilized for this purpose. The temperature in a supersonically expanding outflow can quickly drop to a few 10^8 K, where the (n, gamma)-(gamma, n) equilibrium is never achieved during the heavy r-nuclei synthesis. In addition, the neutron capture competes with the beta-decay owing to the low matter density. Despite such non-standard physical conditions for the cold r-process, a solar-like r-process abundance curve can be reproduced. The cold r-process predicts, however, the low lead production compared to that expected in the traditional r-process conditions, which can be a possible explanation for the low lead abundances found in a couple of r-process-rich Galactic halo stars.Comment: 5 pages, 3 figures, accepted for publication in ApJ

Neutron Star Mergers as the Origin of r-Process Elements in the Galactic Halo Based on the Sub-halo Clustering Scenario

Binary mergers (NSMs) of double neutron star (and black hole-neutron star) systems are suggested to be major sites of r-process elements in the Galaxy by recent hydrodynamical and nucleosynthesis studies. It has been pointed out, however, that the estimated long lifetimes of neutron star binaries are in conflict with the presence of r-process-enhanced halo stars at metallicities as low as [Fe/H] ~ -3. To resolve this problem, we examine the role of NSMs in the early Galactic chemical evolution on the assumption that the Galactic halo was formed from merging sub-halos. We present simple models for the chemical evolution of sub-halos with total final stellar masses between 10^4 M_solar and 2 x 10^8 M_solar. Typical lifetimes of compact binaries are assumed to be 100 Myr (for 95% of their population) and 1 Myr (for 5%), according to recent binary population synthesis studies. The resulting metallcities of sub-halos and their ensemble are consistent with the observed mass-metallicity relation of dwarf galaxies in the Local Group, and the metallicity distribution of the Galactic halo, respectively. We find that the r-process abundance ratios [r/Fe] start increasing at [Fe/H] <= -3 if the star formation efficiencies are smaller for less massive sub-halos. In addition, the sub-solar [r/Fe] values (observed as [Ba/Fe] ~ -1.5 for [Fe/H] < -3) are explained by the contribution from the short-lived (~1 Myr) binaries. Our results indicate that NSMs may have a substantial contribution to the r-process element abundances throughout the Galactic history.Comment: 5 pages, 2 figures, accepted for publication in ApJ

Electron-capture supernovae as sources of 60Fe

We investigate the nucleosynthesis of the radionuclide 60Fe in electron-capture supernovae (ECSNe). The nucleosynthetic results are based on a self-consistent, two-dimensional simulation of an ECSN as well as models in which the densities are systematically increased by some factors (low-entropy models). 60Fe is found to be appreciably made in neutron-rich ejecta during the nuclear quasi-equilibrium phase with greater amounts being produced in the lower-entropy models. Our results, combining them with the yields of core-collapse supernovae (CCSNe) in the literature, suggest that ECSNe account for at least 4-30% of live 60Fe in the Milky Way. ECSNe co-produce neutron-rich isotopes, 48Ca, 50Ti, 54Cr, some light trans-iron elements, and possibly weak r-process elements including some radionuclides such as 93Zr, 99Tc, and 107Pd, whose association with 60Fe might have been imprinted in primitive meteorites or in the deep ocean crust on the Earth.Comment: 6 pages, 2 figures, accepted for publication in ApJ

Electron-capture supernovae as origin of 48Ca

We report that electron-capture supernovae (ECSNe), arising from collapsing oxygen-neon-magnesium cores, are a possible source of 48Ca, whose origin has remained a long-standing puzzle. Our two-dimensional, self-consistent explosion model of an ECSN predicts ejection of neutron-rich matter with electron fractions Ye = 0.40-0.42 and relatively low entropies, s = 13-15 kB per nucleon (kB is the Boltzmann constant). Post-processing nucleosynthesis calculations result in appreciable production of 48Ca in such neutron-rich and low-entropy matter during the quasi-nuclear equilibrium and subsequent freezeout phases. The amount of ejected 48Ca can account for that in the solar inventory when we consider possible uncertainties in the entropies or ejecta-mass distribution. ECSNe could thus be a site of 48Ca production in addition to a hypothetical, rare class of high-density Type Ia supernovae.Comment: 6 pages, 5 figures, accepted for publication in ApJ

The rp-Process in Neutrino-driven Winds

Recent hydrodynamic simulations of core-collapse supernovae with accurate neutrino transport suggest that the bulk of the early neutrino-heated ejecta is proton rich, in which the production of some interesting proton-rich nuclei is expected. As suggested in recent nucleosynthesis studies, the rapid proton-capture (rp) process takes place in such proton-rich environments by bypassing the waiting point nuclei with the beta-lives of a few minutes via the faster capture of neutrons continuously supplied from the neutrino absorption by protons. In this study, the nucleosynthesis calculations are performed with the wide ranges of the neutrino luminosities and the electron fractions (Ye), using the semi-analytic models of proto-neutron star winds. The masses of proto-neutron stars are taken to be 1.4 Msolar and 2.0 Msolar, where the latter is regarded as the test for somewhat high entropy winds (about a factor of two). For Ye > 0.52, the neutrino-induced rp-process takes place in many wind trajectories, and the p-nuclei up to A ~ 130 are synthesized with interesting amounts. However, 92Mo is somewhat underproduced compared to those with similar mass numbers. For 0.46 < Ye < 0.49, on the other hand, 92Mo is significantly enhanced by the nuclear flows in the vicinity of the abundant 90Zr that originates from the alpha-process at higher temperature. The nucleosynthetic yields are averaged over the ejected masses of winds, and further the Ye distribution predicted by the recent hydrodynamic simulation of a core-collapse supernova. Comparison of the mass-Ye-averaged yields to the solar compositions implies that the neutrino-driven winds can be potentially the origin of light p-nuclei up to A ~ 110, including 92,94Mo and 96,98Ru that cannot be explained by other astrophysical sites.Comment: 29 pages, 18 figures, accepted for publication in Ap

The r-Process in the Proto-Neutron-Star Winds with Anisotropic Neutrino Emission

The astrophysical origin of the r-process nuclei is still unknown. Even the most promising scenario, the neutrino-driven winds from a nascent neutron star, encounters severe difficulties in obtaining requisite entropy and short dynamic timescale for the r-process. In this study, the effect of anisotropy in neutrino emission from a proto-neutron star surface is examined with semi-analytic neutrino-driven wind models. The increase of neutrino number density in the wind owing to the anisotropy is modeled schematically by enhancing the effective neutrino luminosity. It is shown that the neutrino heating rate from neutrino-antineutrino pair annihilation into electron-positron pairs can significantly increase owing to the anisotropy and play a dominant role for the heating of wind material. A factor of five increase in the effective neutrino luminosity results in 50% higher entropy and a factor of ten shorter dynamic timescale owing to this enhanced neutrino heating. The nucleosynthesis calculations show that this change is enough for the robust r-process, producing the third abundance peak A = 195 and beyond. Future multi-dimensional studies with accurate neutrino transport will be needed if such anisotropy relevant for the current scenario (more than a factor of a few) is realized during the wind phase (~1-10 s).Comment: 8 pages, 3 figures, accepted for publication in ApJ Letter

Uncertainties in the nu p-process: supernova dynamics versus nuclear physics

We examine how the uncertainties involved in supernova dynamics as well as in nuclear data inputs affect the nup-process in the neutrino-driven winds. For the supernova dynamics, we find that the wind termination by the preceding dense ejecta shell, as well as the electron fraction (Y_{e, 3}; at 3 10^9 K) play a crucial role. A wind termination within the temperature range of (1.5-3) 10^9 K greatly enhances the efficiency of the nu p-process. This implies that the early wind phase, when the innermost layer of the preceding supernova ejecta is still 200-1000 km from the center, is most relevant to the nup-process. The outflows with Y_{e, 3} = 0.52-0.60 result in the production of the p-nuclei up to A=108 with interesting amounts. Furthermore, the p-nuclei up to A=152 can be produced if Y_{e, 3} = 0.65 is achieved. For the nuclear data inputs, we test the sensitivity to the rates relevant to the breakout from the pp-chain region (A < 12), to the (n, p) rates on heavy nuclei, and to the nuclear masses along the nup-process pathway. We find that a small variation of the rates of triple-alpha and of the (n, p) reaction on 56Ni leads to a substantial change in the p-nuclei production. We also find that 96Pd (N=50) on the nup-process path plays a role as a second seed nucleus for the production of heavier p-nuclei. The uncertainty in the nuclear mass of 82Zr can lead to a factor of two reduction in the abundance of the p-isotope 84Sr.Comment: 20 pages, 22 figures, accepted for publication in Ap