Neutron excess number and nucleosynthesis of heavy elements in a type Ia supernova explosion

Type Ia supernovae produce very powerful burst of light, which can be observed to high redshift. This fact is very attractive for cosmological applications. For supernova light curve modeling, it is very important to know the amount of Fe and Ni, formed during the explosion. In this paper, we explore both the chemical composition of the ejected supernova shells and the possibility of weak r-process under increased neutron excess number based on a set of trajectories of tracer particles, calculated in a hydrodynamic model of SNIa explosion. It is shown that no r-process elements are synthesized in the considered supernova model, even for an increased neutron excess number (Y_e ～ 0.4) because of the slow evolution of temperature and density along chosen trajectories. The results of explosive nucleosynthesis are discussed.     

Supernova constraints on neutrino mass and mixing

In this article I review the constraints on neutrino mass and mixing coming from type-II supernovae. The bounds obtained on these parameters from shock reheating, r-process nucleosynthesis and from SN1987A are discussed. Given the current constraints on neutrino mass and mixing the effect of oscillations of neutrinos from a nearby supernova explosion in future detectors will also be discussed.     

Magnetorotational Mechanism of the Explosion of Core-Collapse Supernovae

The idea of the magnetorotational explosion mechanism is that the energy of rotation of the neutron star formed in the course of a collapse is transformed into the energy of an expanding shock wave by means of a magnetic field. In the two-dimensional case, the time of this transformation depends weakly on the initial strength of the poloidal magnetic field because of the development of a magnetorotational instability. Differential rotation leads to the twisting and growth of the toroidal magnetic-field component, which becomes much stronger than the poloidal component. As a result, the development of the instability and an exponential growth of all field components occur. The explosion topology depends on the structure of the magnetic field. In the case where the initial configuration of the magnetic field is close to a dipole configuration, the ejection of matter has a jet character, whereas, in the case of a quadrupole configuration, there arises an equatorial ejection. In either case, the energy release is sufficient for explaining the observed average energy of supernova explosion. Neutrinos are emitted as the collapse and the formation of a rapidly rotating neutron star proceeds. In addition, neutrino radiation arises in the process of magnetorotational explosion owing to additional rotational-energy losses. If the mass of a newborn neutron star exceeds the mass limit for a nonrotating neutron star, then subsequent gradual energy losses may later lead to the formation of a black hole. In that case, the energy carried away by a repeated flash of neutrino radiation increases substantially. In order to explain an interval of 4.5 hours between the two observed neutrino signals from SN 1987A, it is necessary to assume a weakening of the magnetorotional instability and a small initial magnetic field (10⁹?10¹⁰ G) in the newly formed rotating neutron star. The existence of a black hole in the SN 1987A remnant could explain the absence of any visible pointlike source at the center of the explosion.     

233Pa(2nth, f ) cross-section determination using a fission track technique

The νp process is a primary nucleosynthesis process which occurs in core-collapse supernovae. An essential role in this process is being played by electron antineutrinos. They generate, by absorption on protons, a supply of neutrons which, by (n, p) reactions, allow to overcome waiting point nuclei with rather long beta-decay and proton-capture lifetimes. The synthesis of heavy elements by the νp process depends sensitively on the $\bar \nu _e$ luminosity and spectrum. As has been shown recently, the latter are affected by collective neutrino flavor oscillations which can swap the $\bar \nu _e$ and $\bar \nu _{\mu ,\tau }$ spectra above a certain split energy. Assuming such a swap scenario, we have studied the impact of collective neutrino flavor oscillations on the νp-process nucleosynthesis. Our results show that the production of light p-nuclei up to mass number A = 108 is very sensitive to collective neutrino oscillations.     

Production of transuranium elements in a binary model under conditions of pulse nucleosynthesis

A model for the production of transuranium isotopes under the conditions of pulse nucleosynthesis in a powerful neutron flux (∼10²⁴−10²⁵ neutron cm⁻²) is considered. The explosive nature of the process allows us to separate it into two parts: the process of multiple neutron captures (t < 10⁻⁶ s) and the process of the subsequent β-decays for neutron-rich nuclei. The model of multiple captures neutron includes a variation of the cross section of the (n, γ) reaction as a result of the adiabatic expansion of the explosive nucleosynthesis area. A binary mixture of ²³⁸U and ²³⁹Pu is used as the initial isotope composition.     

Neutrino-nucleus reaction in supernovae

Neutrino reactions play an important role at various stages of core-collapse supernova. During infall, neutrinos are produced by electron capture mainly on nuclei and contribute significantly to the cooling of the collapsing core. After core bounce the nascent neutron star cools by neutrino emission. It is a major goal to observe such neutrinos from a future supernova by earthbound detectors and to establish their spectra. Recently it has been shown that the spectrum of electron neutrinos from the early neutrino burst is significantly altered if inelastic neutrino-nucleus scattering is considered in supernova simulations. Finally spallation reactions induced by neutrinos when passing through the outer burning shells can produce certain nuclides in what is called neutrino nucleosynthesis.     

Neutrino-induced nucleosynthesis of A>64 nuclei: the nu p process

We present a new nucleosynthesis process that we denote as the nu p process, which occurs in supernovae (and possibly gamma-ray bursts) when strong neutrino fluxes create proton-rich ejecta. In this process, antineutrino absorptions in the proton-rich environment produce neutrons that are immediately captured by neutron-deficient nuclei. This allows for the nucleosynthesis of nuclei with mass numbers A>64, , making this process a possible candidate to explain the origin of the solar abundances of (92,94)Mo and (96,98)Ru. This process also offers a natural explanation for the large abundance of Sr seen in a hyper-metal-poor star.     

Supernova-Explosion Mechanism Involving Neutrinos

A major part of the energy released upon the gravitational collapse of massive-star cores is carried away by neutrinos. Neutrinos play a crucial role in collapsing supernovae (SNe). At the present time, mathematical models of core-collapse SNe are based on multidimensional gas dynamics and thermonuclear reactions, whereas the neutrino transport is frequently treated in simplified way. An accurate analysis of neutrinos in a spherically symmetric gravitational collapse is performed on the basis of Boltzmann kinetic equations including all weak-interaction reactions with exact quantum-mechanical matrix elements. The role of multidimensional effects is studied bymeans of multidimensional gas dynamics allowing for the neutrino transport via diffusion treated by employing flux limiters. The possibility of largescale convection, which is of interest both from the point of view of explaining a type II supernova (SN) and from the point of view of implementing an experiment aimed at detecting possible energetic (?10 MeV) neutrinos from an SN, is discussed. Thermonuclear burning leads to the explosion of a type I SN. A hot central region and the subsequent large-scale convection may also play an important role in the SN mechanism. If neutrinos and convection play a key role for a type II SN, then, in order to explain gamma radiation from product radioactive elements, convection is of importance in the case of SNe belonging to both types. In addition, convection may be important for bright type I SNe. Original methods are presented for multidimensional gas dynamics involving thermonuclear burning and for multitemperature gas dynamics involving radiative transfer.     

Selected topics in nuclear astrophysics

In this lectures after a brief introduction to stellar reaction rates and its implementation in nuclear networks I discuss the nuclear aspects of the collapse of the inner core of massive stars once it has run out of its nuclear energy source and of the star's explosion as a type II supernova and the explosive nucleosynthesis occurring during this explosion which leads to the production of heavy elements by the rapid neutron capture process and potentially also by the recently discovered νp process.     

High energy neutrinos from gamma-ray bursts with precursor supernovae

The high energy neutrino signature from proton-proton and photo-meson interactions in a supernova remnant shell ejected prior to a gamma-ray burst provides a test for the precursor supernova, or supranova, model of gamma-ray bursts. Protons in the supernova remnant shell and photons entrapped from a supernova explosion or a pulsar wind from a fast-rotating neutron star remnant provide ample targets for protons escaping the internal shocks of the gamma-ray burst to interact and produce high energy neutrinos. We calculate the expected neutrino fluxes, which can be detected by current and future experiments.     

Calculation of transuranium element synthesis in intensive neutron fluxes under adiabatic conditions

The creation of transuranium isotopes based on intense pulsed nucleosynthesis is considered. The model of multiple neutron captures takes into account the variation of the (n, γ)-cross section resulting from adiabatic expansion of the explosive nucleosynthesis area. The calculated yields of transuranium isotopes obtained under conditions close to a “Par” nuclear explosion, enable us to improve the agreement between the model results and the experimental data within the wide range of atomic mass number A = 248–257, provided the adiabatic conditions are taken into account.     

Resonance neutrino bremsstrahlung ν → νγ in a strong magnetic field

High energy neutrino bremsstrahlung ν → ν + γ in a strong magnetic field (B B_s) is studied in the framework of the Standard Model (SM). A resonance probability and a four-vector of the neutrino energy and momentum loss are presented. A possible manifestation of the neutrino bremsstrahlung in astrophysical cataclysm of type of a supernova explosion or a merger of neutron stars, as an origin of cosmological γ-burst is briefly discussed.     

Ignition of thermonuclear reactions by the neutrino radiation and supernova explosions

It is pointed out that in collapsing carbon-oxygen stars (M ? 2M_⊙) the neutrino heating initiates carbon and oxygen explosion in the stellar mantle leading to its ejection, i.e., to Supernova explosion. The main heating mechanism is the neutrino-electron scattering.     

Dirac-neutrino magnetic moment and the dynamics of a supernova explosion

The double conversion of neutrino chirality ν_L → ν_R → ν_L has been analyzed for supernova conditions, where the first stage is due to the interaction of the neutrino magnetic moment with plasma electrons and protons in the supernova core, and the second stage, due to the resonance spin flip of the neutrino in the magnetic field of the supernova envelope. It is shown that, in the presence of the neutrino magnetic moment in the range 10^?13 μ_B < μ_ν < 10^?12 μ_B and a magnetic field of ～10¹³ G between the neutrinosphere and the shock-stagnation region, an additional energy of about 10⁵¹ erg, which is sufficient for a supernova explosion, can be injected into this region during a typical shock-stagnation time.     

Evaluating the yield of transuranium nuclides with masses of up to A = 270 via pulsed nucleosynthesis

A model of the creation of transuranium isotopes of up to A = 270 under conditions of pulse nucleosynthesis in a neutron flux with densities of up to ??10²⁵ neutron cm^?2 is considered. The pulse process allows us to divide it in time into two stages: the process of multiple neutron captures (t < 10^?6 s) and the subsequent ??-decay of neutron-excess nuclei. The modeling of the transuranium yields takes into account the adiabatic character of the process, the probability of delayed fission, and the emission of delayed neutrons. A target with a binary composition of ²³⁸U and ²³⁹Pu, ²⁴⁸Cm, and ²⁵¹Cf isotopes is used to predict the yields of heavy and superheavy isotopes.     

The beta strength function and the astrophysical site of ther-process

The beta strength function has been calculated for ～6000 nuclei between the line of beta stability and the neutron drip line. The calculations — performed by using a schematical Brown-Bolsterli model — yield more reliable beta decay half lives,P _n values andβ-delayed fission rates than the strongly oversimplified assumptions on S_β used up to now in astrophysical calculations. The calculated beta rates are shown in this paper to be decisive in the discussion of the problem of the astrophysical site of ther-process, which is responsible for the production of the heavy elements in the universe. In particular we concentrate on the neutron capture processes during the explosive burning of He in massive stars, initiated by the outgoing shock wave from a supernova explosion. It is shown that as consequence of the revisedβ-decay rates explosive He-burning represents a very convincing alternative scenario to the classicalr-process, which avoids the presently existing problems of the latter:

1. The computed relative abundances ofr-nuclei resemble on the average their solar system counterparts.
2. The calculated abundance peaks essentially coincide in position with the observed ones.
3. The new site of ther-process avoids the problems of overproduction of heavy elements and of the mass-cut.

Our results are based on realistic stellar models and hydrodynamical explosion calculations which for the first time are applied here to the problem of heavy element nucleosynthesis. The results turn out to be rather insensitive to the details of those models. The shorterβ-decay half lives obtained are of importance also in the investigation of further astrophysical sites producing heavy elements such as then-processes in explosive C or Ne burning.     

New measurement of the neutron lifetime with a large gravitational trap

The lifetime of the neutron is one of the key physical quantities used to determine the weak interaction parameters and to test predictions of the theory of primary nucleosynthesis. The lifetime of the neutron has been measured in the reported experiment by the method of storing neutrons in a material trap with a gravitational valve. Fomblin grease UT-18 hydrogen-free fluorine polymer has been used as coating. The resistance of the coating to repeated cooling down to 80 K combined with heating up to 300 K has been studied. The probability of losses in the trap is as small as 1.5% of the neutron decay probability. The lifetime of the neutron τ_n = (881.5 ± 0.7_stat ± 0.6_syst)s obtained at the new step is in good agreement with a commonly accepted value of (880.2 ± 1.0) s presented by the Particle Data Group.     

Impact of massive tau-neutrinos on primordial nucleosynthesis. Exact calculations

The influence of a massive Majorana v_τ on primordial nucleosynthesis is rigorously calculated. The system of three integro-differential kinetic equations is solved numerically for m_vτ in the interval from 0 to 20 MeV It is found that the usual assumption of kinetic equilibrium is strongly violated and non-equilibrium corrections considerably amplify the effect. Even a very weak restriction from nucleosynthesis, allowing for one extra massless neutrino species, permits to conclude that m_vτ < 1 MeV For a stricter bound, e.g. for ΔN_v < 0.3, the limit is tm_vτ < 0.35MeV.     

Methods for neutral-current neutrino detection in the Sudbury neutrino observatory

The Sudbury Neutrino Observatory will permit a comparison of the rate of neutral-current neutrino interactions to charged-current interactions, which is a test of neutrino mixing and mass. The neutral-current process can be measured by: a) comparison of the rate for electron elastic scattering to the pure charged-current rate on deuterium, b) neutron capture on Cl ions added to the heavy water, and, c) neutron capture in an array of ³He-filled proportional counters. The design and construction of suitable proportional counters are described.     

Measurement of the ratio of total and differential cross sections on neutrons and protons for charged-current neutrino events

Charged-current neutrino interactions have been analysed in a sample of pictures from BEBC equipped with a TST. Using a method independent of both the neutrino flux and nuclear interaction corrections, the ratio R= σ_n/σ_p has been measured. The result is R=1.98±0.19 for the ratio of total cross sections. Bjorken x distributions for proton and neutron targets and for u and d quarks are compared.