R. Buras

Improved models of stellar core collapse and still no explosions: what is missing?

Two-dimensional hydrodynamic simulations of stellar core collapse are presented which for the first time were performed by solving the Boltzmann equation for the neutrino transport including a state-of-the-art description of neutrino interactions. Stellar rotation is also taken into account. Although convection develops below the neutrinosphere and in the neutrino-heated region behind the supernova shock, the models do not explode. This suggests missing physics, possibly with respect to the nuclear equation of state and weak interactions in the subnuclear regime. However, it might also indicate a fundamental problem with the neutrino-driven explosion mechanism.

Nucleosynthesis in Early Supernova Winds II: The Role of Neutrinos

One of the outstanding unsolved riddles of nuclear astrophysics is the origin of the so-called p-process nuclei from A = 92 to 126. Both the lighter and heavier p-process nuclei are adequately produced in the neon and oxygen shells of ordinary Type II supernovae, but the origin of these intermediate isotopes, especially 92,94Mo and 96,98Ru, has long been mysterious. Here we explore the production of these nuclei in the neutrino-driven wind from a young neutron star. We consider such early times that the wind still contains a proton excess because the rates for νe and positron captures on neutrons are faster than those for the inverse captures on protons. Following a suggestion by Frohlich and coworkers, we also include the possibility that—in addition to the protons, α-particles, and heavy seed—a small flux of neutrons is maintained by the reaction p(e, e+)n. This flux of neutrons is critical in bridging the long waiting points along the path of the rp-process by (n, p) reactions. Using the unmodified ejecta histories from a recent two-dimensional supernova model by Janka and coworkers, we find synthesis of p-rich nuclei up to 102Pd, although our calculations do not show efficient production of 92Mo. If the entropy of these ejecta is increased by a factor of 2, the synthesis extends to 120Te. Still larger increases in entropy, which might reflect the role of magnetic fields or vibrational energy input neglected in the hydrodynamical model, result in the production of nuclei up to A ≈ 170. Elements synthesized in these more extreme outflows include numerous s- and p-process nuclei, and even some r-process nuclei can be synthesized in these proton-rich conditions.

Toward gravitational wave signals from realistic core-collapse supernova models

We have computed the gravitational wave signal from supernova core collapse by using the most realistic input physics available at present. We start from state-of-the-art progenitor models of rotating and nonrotating massive stars and simulate the dynamics of their core collapse by integrating the equations of axisymmetric hydrodynamics, together with the Boltzmann equation for the neutrino transport, including an elaborate description of neutrino interactions, and a realistic equation of state. Using the Einstein quadrupole formula we compute the quadrupole wave amplitudes, the Fourier wave spectra, the amount of energy radiated in the form of gravitational waves, and the signal-to-noise ratios for the Laser Interferometer Gravitational-Wave Observatory (LIGO) I and the tuned Advanced LIGO (LIGO II) interferometers resulting from both nonradial mass motion and anisotropic neutrino emission. The simulations demonstrate that the dominant contribution to the gravitational wave signal is produced by neutrino-driven convection behind the supernova shock. For stellar cores rotating at the extreme of current stellar evolution predictions, the core bounce signal is detectable (S/N 7) with LIGO II for a supernova up to a distance of ~5 kpc, whereas the signal from postshock convection is observable (S/N 7) with LIGO II up to a distance of ~100 kpc and with LIGO I to a distance of ~5 kpc. If the core is nonrotating, its gravitational wave emission can be measured with LIGO II up to a distance of ~15 kpc (S/N 8), while the signal from the Ledoux convection in the deleptonizing nascent neutron star can be detected up to a distance of ~10 kpc (S/N 8). Both kinds of signals are generically produced by convection in any core-collapse supernova.

Exploring the relativistic regime with Newtonian hydrodynamics: an improved effective gravitational potential for supernova simulations

We investigate the possibility approximating relativistic effects in hydrodynamical simulations of stellar core collapse and post-bounce evolution by using a modified gravitational potential in an otherwise standard Newtonian hydrodynamic code. Different modifications of a previously introduced effective relativistic potential are discussed. Corresponding hydrostatic solutions are compared with solutions of the TOV equations, and hydrodynamic simulations with two different codes are compared with fully relativistic results. One code is applied for one- and two-dimensional calculations with a simple equation of state and employs either the modified effective relativistic potential in a Newtonian framework or solves the general relativistic field equations under the assumption of the conformal flatness condition (CFC) for the three-metric. The second code allows for full-scale supernova runs including a microphysical equation of state and neutrino transport based on the solution of the Boltzmann equation and its moments equations. We present prescriptions for the effective relativistic potential for self-gravitating fluids to he used in Newtonian codes, which produce excellent agreement with fully relativistic solutions in spherical symmetry, leading to significant improvements compared to previously published approximations. Moreover, they also approximate qualitatively well relativistic solutions for models with rotation.

ELECTRON NEUTRINO PAIR ANNIHILATION: A NEW SOURCE FOR MUON AND TAU NEUTRINOS IN SUPERNOVAE

We show that in a supernova core the annihilation process νee → νμ,τμ,τ is always more important than the traditional reaction e+e- → νμ,τμ,τ as a source for muon and tau neutrino pairs. We study the impact of the new process by means of a Monte Carlo transport code with a static stellar background model and by means of a self-consistent hydrodynamic simulation with Boltzmann neutrino transport. Nucleon bremsstrahlung NN → NNνμ,τμ,τ is also included as another important source term. Taking into account νee → νμ,τμ,τ increases the νμ and ντ luminosities by as much as 20%, while the spectra remain almost unaffected. In our hydrodynamic simulation, the shock was somewhat weakened. Elastic νμ,τνe and νμ,τe scattering is not negligible but less important than νμ,τe± scattering. Its influence on the νμ,τ fluxes and spectra is small after all other processes have been included.

Dynamics of shock propagation and nucleosynthesis conditions in O-Ne-Mg core supernovae

It has been recently proposed that the shocked surface layer s of exploding O-Ne-Mg cores provide the conditions for r-process nucleosynthesis, because their rapid expansion and high entropies enable heavy r-process isotopes to form even in an environment with very low initial neutron excess of the matter. We show here that the most sophisticated available hydrodynamic simulations (in spherical and axial symmetry) do not support this new r-process scenario because they fail to provide the necessary conditions of temperature, entropy, and expansion timescale by significa nt factors. This suggests that, either the formation of r-pr ocess elements works differently than suggested by Ning et al. (2007, NQM07), or that some essential core properties with influence on the explosio n dynamics might be different from those predicted by Nomoto’s progenitor model.

Exploiting the neutronization burst of a galactic supernova

One of the robust features found in simulations of core-collapse supernovae (SNe) is the prompt neutronization burst, i.e. the first

Supernova neutrinos: flavor-dependent fluxes and spectra

\sim 25

The mechanism of core-collapse supernovae and the ejection of heavy elements

milliseconds after bounce when the SN emits with very high luminosity mainly

A lower bound on sin2β from minimal flavour violation

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