arXiv subjects

R. Buras

Publications and source records attributed to R. Buras.

At least 19 recordsLinked to original sources

arXiv2019

ALIS: An efficient method to compute high spectral resolution polarized solar radiances using the Monte Carlo approach

An efficient method to compute accurate polarized solar radiance spectra using the (three-dimensional) Monte Carlo model MYSTIC has been developed. Such high resolution spectra are measured by various satellite instruments for remote sensing of atmospheric trace gases. ALIS (Absorption Lines Importance Sampling) allows the calculation of spectra by tracing photons at only one wavelength: In order to take into account the spectral dependence of the absorption coefficient a spectral absorption weight is calculated for each photon path. At each scattering event the local estimate method is combined with an importance sampling method to take into account the spectral dependence of the scattering coefficient. Since each wavelength grid point is computed based on the same set of random photon paths, the statistical error is the almost same for all wavelengths and hence the simulated spectrum is not noisy. The statistical error mainly results in a small relative deviation which is independent of wavelength and can be neglected for those remote sensing applications where differential absorption features are of interest. Two example applications are presented: The simulation of shortwave-infrared polarized spectra as measured by GOSAT from which CO$_2$ is retrieved, and the simulation of the differential optical thickness in the visible spectral range which is derived from SCIAMACHY measurements to retrieve NO$_2$. The computational speed of ALIS (for one- or three-dimensional atmospheres) is of the order of or even faster than that of one-dimensional discrete ordinate methods, in particular when polarization is considered.

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arXiv2008

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

It has been recently proposed that the shocked surface layers 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 significant factors. This suggests that, either the formation of r-process elements works differently than suggested by Ning et al. (2007, NQM07), or that some essential core properties with influence on the explosion dynamics might be different from those predicted by Nomoto's progenitor model.

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arXiv2008

Nucleosynthesis in Early Neutrino Driven Winds

Nucleosynthesis in early neutrino winds is investigated. Presented is a brief overview of two recent problems of supernova nucleosynthesis. In the first part we investigate the effect of nuclear parameters on the synthesis of Mo92 and Mo94. Based on recent experimental results, we find that the proton rich winds of the model investigated here can not be the source of solar Mo92 and Mo94. In the second part we investigate the nucleosynthesis from neutron rich bubbles and show that they do not contribute to the overall nucleosynthesis.

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arXiv2006

Nucleosynthesis in neutrino heated matter: The vp-process and the r-process

This manuscript reviews recent progress in our understanding of the nucleosynthesis of medium and heavy elements in supernovae. Recent hydrodynamical models of core-collapse supernovae show that a large amount of proton rich matter is ejected under strong neutrino fluxes. This matter constitutes the site of the vp-process where antineutrino absorption reactions catalyze the nucleosynthesis of nuclei with A > 64. Supernovae are also associated with the r-process responsible for the synthesis of the heaviest elements in nature. Fission during the r-process can play a major role in determining the final abundance patter and in explaining the almost universal features seen in metal-poor r-process-rich stars.

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arXiv2006

Two-Dimensional Hydrodynamic Core-Collapse Supernova Simulations with Spectral Neutrino Transport II. Models for Different Progenitor Stars

1D and 2D supernova simulations for stars between 11 and 25 solar masses are presented, making use of the Prometheus/Vertex neutrino-hydrodynamics code, which employs a full spectral treatment of the neutrino transport. Multi-dimensional transport aspects are treated by the ``ray-by-ray plus'' approximation described in Paper I. Our set of models includes a 2D calculation for a 15 solar mass star whose iron core is assumed to rotate rigidly with an angular frequency of 0.5 rad/s before collapse. No important differences were found depending on whether random seed perturbations for triggering convection are included already during core collapse, or whether they are imposed on a 1D collapse model shortly after bounce. Convection below the neutrinosphere sets in about 40 ms p.b. at a density above 10**12 g/cm^3 in all 2D models, and encompasses a layer of growing mass as time goes on. It leads to a more extended proto-neutron star structure with accelerated lepton number and energy loss and significantly higher muon and tau neutrino luminosities, but reduced mean energies of the radiated neutrinos, at times later than ~100 ms p.b. In case of an 11.2 solar mass star we find that low (l = 1,2) convective modes cause a probably rather weak explosion by the convectively supported neutrino-heating mechanism after ~150 ms p.b. when the 2D simulation is performed with a full 180 degree grid, whereas the same simulation with 90 degree wedge fails to explode like all other models. This sensitivity demonstrates the proximity of our 2D models to the borderline between success and failure, and stresses the need of simulations in 3D, ultimately without the axis singularity of a polar grid. (abridged)

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arXiv2005

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 electron neutrino and positron captures on neutrons are faster than those for the inverse captures on protons. Following a suggestion by Frohlich et al. 2005, we also include the possibility that, in addition to the protons, alpha-particles, and heavy seed, a small flux of neutrons is maintained by the reaction p(bar(nu_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) and (n,gamma) reactions. Using the unmodified ejecta histories from a recent two-dimensional supernova model by Janka et al. 2003, we find synthesis of p-rich nuclei up to 102Pd. However, if the entropy of these ejecta is increased by a factor of two, the synthesis extends to 120Te. Still larger increases in entropy, that might reflect the role of magnetic fields or vibrational energy input neglected in the hydrodynamical model, result in the production of numerous r-, s-, and p-process nuclei up to A approximately 170, even in winds that are proton-rich.

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arXiv2005

Two-dimensional hydrodynamic core-collapse supernova simulations with spectral neutrino transport. I. Numerical method and results for a 15 M_sun star

Supernova models with a full spectral treatment of the neutrino transport are presented, employing the Prometheus/Vertex neutrino-hydrodynamics code with a ``ray-by-ray plus'' approximation for treating two- (or three-) dimensional problems. The method is described in detail and critically assessed with respect to its capabilities, limitations, and inaccuracies in the context of supernova simulations. In this first paper of a series, 1D and 2D core-collapse calculations for a (nonrotating) 15 M_sun star are discussed, uncertainties in the treatment of the equation of state -- numerical and physical -- are tested, Newtonian results are compared with simulations using a general relativistic potential, bremsstrahlung and interactions of neutrinos of different flavors are investigated, and the standard approximation in neutrino-nucleon interactions with zero energy transfer is replaced by rates that include corrections due to nucleon recoil, thermal motions, weak magnetism, and nucleon correlations. Models with the full implementation of the ``ray-by-ray plus'' spectral transport were found not to explode, neither in spherical symmetry nor in 2D with a 90 degree lateral wedge. The success of previous 2D simulations with grey, flux-limited neutrino diffusion can therefore not be confirmed. Omitting the radial velocity terms in the neutrino momentum equation leads to ``artificial'' explosions by increasing the neutrino energy density in the convective gain layer by about 20--30% and thus the integral neutrino energy deposition in this region by about a factor of two. (abbreviated)

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arXiv2005

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

We investigate the possibility to approximate 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 the effective relativistic potential introduced by Rampp & Janka (2002) 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 be 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.

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arXiv2005

On ion-ion correlation effects during stellar core collapse

The role of ion-ion correlations in suppressing neutrino-nucleus elastic scattering during stellar core collapse is reinvestigated, using two different equations of state. We test the improved description by Itoh et al. against the treatment suggested by Horowitz and find that the stronger cross section reduction for small momentum transfer in the former case does not lead to noticeable changes of the core deleptonization and entropy increase during collapse, because the improvements are relevant below neutrino trapping conditions only for very low neutrino energies, corresponding to a very small phase space volume. Treating screening effects for ionic mixtures by the linear mixing rule, applied to the collection of representative heavy nucleus, alpha particles, and free nucleons, which is assumed to characterize the composition in nuclear statistical equilibrium, we cannot determine mentionable differences during stellar collapse, because alpha particles are not sufficiently abundant and their coherent scattering opacity is too small.

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arXiv2004

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 $\sim 25$ milliseconds after bounce when the SN emits with very high luminosity mainly $\nu_e$ neutrinos. We examine the dependence of this burst on variations in the input of current SN models and find that recent improvements of the electron capture rates as well as uncertainties in the nuclear equation of state or a variation of the progenitor mass have only little effect on the signature of the neutronization peak in a megaton water Cherenkov detector for different neutrino mixing schemes. We show that exploiting the time-structure of the neutronization peak allows one to identify the case of a normal mass hierarchy and large 13-mixing angle $\theta_{13}$, where the peak is absent. The robustness of the predicted total event number in the neutronization burst makes a measurement of the distance to the SN feasible with a precision of about 5%, even in the likely case that the SN is optically obscured.

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arXiv2004

Neutrino-Driven Supernovae: an Accretion Instability in a Nuclear Physics Controlled Environment

New simulations demonstrate that low-mode, nonradial hydrodynamic instabilities of the accretion shock help starting hot-bubble convection in supernovae and thus support explosions by the neutrino-heating mechanism. The prevailing conditions depend on the high-density equation of state which governs stellar core collapse, core bounce, and neutron star formation. Tests of this sensitivity to nuclear physics variations are shown for spherically symmetric models. Implications of current explosion models for r-process nucleosynthesis are addressed.

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arXiv2004

Nucleosynthesis in the Hot Convective Bubble in Core-Collapse Supernovae

As an explosion develops in the collapsed core of a massive star, neutrino emission drives convection in a hot bubble of radiation, nucleons, and pairs just outside a proto-neutron star. Shortly thereafter, neutrinos drive a wind-like outflow from the neutron star. In both the convective bubble and the early wind, weak interactions temporarily cause a proton excess (Ye > 0.5) to develop in the ejected matter. This situation lasts for at least the first second, and the approximately 0.05 - 0.1 solar masses that is ejected has an unusual composition that may be important for nucleosynthesis. Using tracer particles to follow the conditions in a two-dimensional model of a successful supernova explosion calculated by Janka, Buras & Rampp (2003), we determine the composition of this material. Most of it is helium and 56Ni. The rest is relatively rare species produced by the decay of proton-rich isotopes unstable to positron emission. In the absence of pronounced charged-current neutrino capture, nuclear flow will be held up by long-lived waiting point nuclei in the vicinity of 64Ge. The resulting abundance pattern can be modestly rich in a few interesting rare isotopes like 45Sc, 49Ti, and 64Zn. The present calculations imply yields that, when compared with the production of major species in the rest of the supernova, are about those needed to account for the solar abundance of 45Sc and 49Ti. Since the synthesis will be nearly the same in stars of high and low metallicity, the primary production of these species may have discernible signatures in the abundances of low metallicity stars. We also discuss uncertainties in the nuclear physics and early supernova evolution to which abundances of interesting nuclei are sensitive.

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arXiv2004

Core-Collapse Supernovae: Modeling between Pragmatism and Perfectionism

We briefly summarize recent efforts in Garching for modeling stellar core collapse and post-bounce evolution in one and two dimensions. The transport of neutrinos of all flavors is treated by iteratively solving the coupled system of frequency-dependent moment equations together with a model Boltzmann equation which provides the closure. A variety of progenitor stars, different nuclear equations of state, stellar rotation, and global asymmetries due to large-mode hydrodynamic instabilities have been investigated to ascertain the road to finally successful, convectively supported neutrino-driven explosions.

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arXiv2004

Core-Collapse Supernovae at the Threshold

Recent progress in modeling core-collapse supernovae is summarized and set in perspective. Two-dimensional simulations with state-of-the-art treatment of neutrino transport still fail to produce powerful explosions, but evidence is presented that they are very close to success.

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arXiv2003

Supernova neutrinos: Flavor-dependent fluxes and spectra

Transporting nu_mu and nu_tau in a supernova (SN) core involves several processes that have been neglected in traditional simulations. Based on a Monte Carlo study we find that the flavor-dependent spectral differences are much smaller than is often stated in the literature. A full-scale SN simulation using a Boltzmann solver and including all relevant neutrino reactions confirms these results. The flavor-dependent flux differences are largest during the initial accretion phase.

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arXiv2003

Improved Models of Stellar Core Collapse and Still no Explosions: What is Missing?

Two-dimensional hydrodynamic simulations of stellar core-collapse with and without rotation 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. 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 of the neutrino-driven explosion mechanism.

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arXiv2002

Explosion Mechanisms of Massive Stars

One of the central problems in supernova theory is the question how massive stars explode. Understanding the physical processes that drive the explosion is crucial for linking the stellar progenitors to the final remnants and for predicting observable properties like explosion energies, neutron star and black hole masses, nucleosynthetic yields, explosion anisotropies, and pulsar kicks. Here we review different suggestions for the explosion mechanism and discuss the constraints that can or cannot be deduced from observations. The most refined and detailed hydrodynamical models are available for neutrino-driven supernova explosions, although the viability of this mechanism has yet to be demonstrated convincingly. Since spherical models do not explode, the hope rests on the helpful effects of convection inside the nascent neutron star and in the neutrino-heated region behind the stalled shock. We present the first two-dimensional simulations of these processes which have been performed with a Boltzmann solver for the neutrino transport and a state-of-the-art description of neutrino-matter interactions. Our most complete models fail, but convection brings them encouragingly close to a success. An explosion could be obtained by just a minor modification of the neutrino transport, in which case the exploding model fulfills important requirements from observations (abridged).

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arXiv2002

Core Collapse and Then? The Route to Massive Star Explosions

The rapidly growing base of observational data for supernova explosions of massive stars demands theoretical explanations. Central of these is a self-consistent model for the physical mechanism that provides the energy to start and drive the disruption of the star. We give arguments why the delayed neutrino-heating mechanism should still be regarded as the standard paradigm to explain most explosions of massive stars and show how large-scale and even global asymmetries can result as a natural consequence of convective overturn in the neutrino-heating region behind the supernova shock. Since the explosion is a threshold phenomenon and depends sensitively on the efficiency of the energy transfer by neutrinos, even relatively minor differences in numerical simulations can matter on the secular timescale of the delayed mechanism. To enhance this point, we present some results of recent one- and two-dimensional computations, which we have performed with a Boltzmann solver for the neutrino transport and a state-of-the-art description of neutrino-matter interactions. Although our most complete models fail to explode, the simulations demonstrate that one is encouragingly close to the critical threshold because a modest variation of the neutrino transport in combination with postshock convection leads to a weak neutrino-driven explosion with properties that fulfill important requirements from observations.

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