Core-collapse Models Research Articles

Detection potential for the diffuse supernova neutrino background in the large liquid-scintillator detector LENA

The large-volume liquid-scintillator detector LENA (low energy neutrino astronomy) will provide high-grade background discrimination and enable the detection of diffuse supernova neutrinos (DSN) in an almost background-free energy window from {approx}10 to 25 MeV. Within ten years of exposure, it will be possible to derive significant constraints on both core-collapse supernova models and the supernova rate in the near universe up to redshifts z<2.

Constraining supernova models using the hot gas in clusters of galaxies

The hot Intra-Cluster Medium (ICM) in clusters of galaxies is a very large repository of metals produced by supernovae. We aim to accurately measure the abundances in the ICM of many clusters and compare these data with metal yields produced by supernovae. Using the data archive of the XMM-Newton X-ray observatory, we compile a sample of 22 clusters. We fit spectra extracted from the core regions and determine the abundances of silicon, sulfur, argon, alcium, iron, and nickel. The abundances from the spectral fits are subsequently fitted to supernova yields determined from several supernova type Ia and core-collapse supernova models. We find that the argon and calcium abundances cannot be fitted with currently favoured supernova type Ia models. We obtain a major improvement of the fit, when we use an empirically modified delayed-detonation model that is calibrated on the Tycho supernova remnant. The two modified parameters are the density where the sound wave in the supernova turns into a shock and the ratio of the specific internal energies of ions and electrons at the shock. Our fits also suggest that the core-collapse supernovae that contributed to the enrichment of the ICM had progenitors which were already enriched. The Ar/Ca ratio in clusters is a good touchstone for determining the quality of type Ia models. The core-collapse contribution, which is about 50% and not strongly dependent on the IMF or progenitor metallicity, does not have a significant impact on the Ar/Ca ratio. The number ratio between supernova type Ia and core-collapse supernovae suggests that binary systems in the appropriate mass range are very efficient (~ 5-16%) in eventually forming supernova type Ia explosions.

The Spin of the Near‐Extreme Kerr Black Hole GRS 1915+105

Based on a spectral analysis of the X-ray continuum that employs a fully relativistic accretion-disk model, we conclude that the compact primary of the binary X-ray source GRS 1915+105 is a rapidly-rotating Kerr black hole. We find a lower limit on the dimensionless spin parameter of a* greater than 0.98. Our result is robust in the sense that it is independent of the details of the data analysis and insensitive to the uncertainties in the mass and distance of the black hole. Furthermore, our accretion-disk model includes an advanced treatment of spectral hardening. Our data selection relies on a rigorous and quantitative definition of the thermal state of black hole binaries, which we used to screen all of the available RXTE and ASCA data for the thermal state of GRS 1915+105. In addition, we focus on those data for which the accretion disk luminosity is less than 30% of the Eddington luminosity. We argue that these low-luminosity data are most appropriate for the thin alpha-disk model that we employ. We assume that there is zero torque at the inner edge of the disk, as is likely when the disk is thin, although we show that the presence of a significant torque does not affect our results. Our model and the model of the relativistic jets observed for this source constrain the distance and black hole mass and could thus be tested by determining a VLBA parallax distance and improving the measurement of the mass function. Finally, we comment on the significance of our results for relativistic-jet and core-collapse models, and for the detection of gravitational waves.

Fragmentation of massive protostellar discs

We examine whether massive-star accretion discs are likely to fragment due to self-gravity. Rapid accretion and high angular momentum push these discs toward fragmentation, whereas viscous heating and the high protostellar luminosity stabilize them. We find that for a broad range of protostar masses and for reasonable accretion times, massive discs larger than ∼150 au are prone to fragmentation. We develop an analytical estimate for the angular momentum of accreted material, extending the analysis of Matzner & Levin to account for strongly turbulent initial conditions. In a core-collapse model, we predict that discs are marginally prone to fragmentation around stars of about 4–15 M⊙– even if we adopt conservative estimates of the discs' radii and tendency to fragment. More massive stars are progressively more likely to fragment, and there is a sharp drop in the stability of disc accretion at the very high accretion rates expected above 110 M⊙. Fragmentation may starve accretion in massive stars, especially above this limit, and is likely to create swarms of small, coplanar companions.

Principle of Universality of γ-Process Nucleosynthesis in Core-Collapse Supernova Explosions

Using core-collapse supernova explosion models, we investigate the principle of the universality of the γ-process, according to which the s/p abundance ratios produced by individual nucleosynthesis episode are almost constant over a wide range of atomic number. The universality originates from the shift of the γ-process layers, the independence of the s/p abundance ratios of the nuclear reactions, and the weak s-process in presupernovae. Our calculations further suggest an extended universality in which the s/p ratios in the γ-process layers are not only constant but also centered around a specific value of 3. To verify this, we propose an astronomical observation of indium isotopic abundance fractions.

Toward three-dimensional simulations of stellar core collapse with magnetic fields

AbstractIn spherical symmetry, very reliable models of stellar core collapse, bounce, and the postbounce phase can be constructed based on general relativistic Boltzmann neutrino transport. However, even if the time-integrated neutrino luminosity in the region between the surface of the protoneutron star and the stalled accretion shock is one or two orders of magnitude larger than the energy of a supernova explosion, it is generally accepted that the net energy transfer is not efficient enough to drive an explosion, unless the fluid instabilities in this regime are taken into account. Complementary to other groups, who are elaborating an extension of the accurate neutrino physics to axisymmetric simulations, we construct efficient parameterizations of the neutrino physics that enable three-dimensional magneto-hydrodynamics simulations that do not constrain the fluid instabilities by artificially imposed symmetries. We evaluate our approximations with respect to spherically symmetric Boltzmann neutrino transport, present preliminary MHD simulations with a resolution of 600 zones cubed, and illustrate the questions that can be addressed by this approach.

A magnetorotational core-collapse model with jets

We present the results of the 2D simulation of a magnetorotational (MR) core-collapse model accompanied by jet formation in the core-collapse model explosion. The initial magnetic field used in the simulations has a dipole-like symmetry. Contrary to the simulations of the MR core-collapse model with an initial quadrupole-like magnetic field, where the matter was ejected mainly near the equatorial plane, in the presence of the dipole-like initial magnetic field the core-collapse model explosion is developing preferably along a rotational axis and leads to the formation of a protojet. We expect that protojet propagation through the envelope of the star will be accompanied by its collimation. The magnetorotational instability (MRI) was found in the simulations, similar to the earlier considered case of the quadrupole-like initial magnetic field. Our estimations show that the characteristic time for the reconnection of the magnetic field is much larger than the MRI development time. The supernova explosion energy for the dipole-like field is about 0.61 x 10 51 erg, and about 0.13 M ⊙ of mass was ejected during the explosion.

Evidence for p-process nucleosynthesis recorded at the Solar System abundances

Analyzing the Solar System abundance, we have found two universal scaling laws concerning the p- and s-nuclei with the same atomic number. They are evidence of the p-nucleus origin that has been discussed for long years. The first scaling is the correlation of the isotope abundances between a p-nucleus and an s-nucleus that is two neutron heavier than the p-nucleus. The abundance ratios of the s-nucleus to the p-nucleus are almost constant in a wide range of atomic numbers. This scaling indicates that the p-nucleus is dominantly produced from the s-nucleus by (γ, n) reactions in huge photon bath in supernova explosions. The second scaling indicates that the isotope abundance ratio between two p-nuclei with the same atomic number is almost unity. These two scalings are a new useful method to verify nucleosynthesis models for the p-process. We have calculated these ratios by a p-process nucleosynthesis based on a typical core-collapse supernova explosion model. The results are consistent with the scaling in the solar abundances. In addition, the scalings provide new concepts: a universality of the p-process, a rate meter for the s-process and a new nuclear cosmochronometer.

On Rapidly Rotating Magnetic Core‐Collapse Supernovae

We have analyzed the magnetic effects that may occur in rapidly rotating core collapse supernovae. We consider effects from both magnetic turbulence and the formation of magnetic bubbles. For magnetic turbulence we have made a perturbative analysis for our spherically symmetric, core-collapse supernova model that incorporates the buildup of magnetic field energy in the matter accreting onto the proto-neutron star shortly after collapse and bounce. This significantly modifies the pressure profile and increases the heating of the material above the proto-neutron star resulting in an explosion even in rotating stars that would not explode otherwise. Regarding magnetic bubbles, we show that a model with an initial uniform magnetic field ~108 G and uniform angular velocity of ~0.1 rad s-1 can form magnetic bubbles due to the very nonhomologous nature of the collapse. It is estimated that the buoyancy of the bubbles causes matter in the proto-neutron star to rise, carrying neutrino-rich material to the neutron star surface. This increases the neutrino luminosity sufficiently at early times to achieve a successful neutrino-driven explosion. Both magnetic mechanisms thus provide new means for initiating a Type II core-collapse supernova.

Scaling relations for the supernova γ-process and the 176Lu nuclear cosmochronometer

Analyzing the solar system abundances, we find two universal scaling relations concerning the p- and s-nuclei. These scalings indicate that the γ -process nucleosynthesis in supernova (SN) explosions is the most probable origin of the p-nuclei. In addition the scalings lead to new concepts: a universality of the γ -process and a new nuclear cosmochronometer. Calculations of the γ -process are carried out for typical core-collapse SN explosion models. These confirm the observed scalings.

The Nonmonotonic Dependence of Supernova and Compact Remnant Formation on Progenitor Rotation

Traditional models of core collapse suggest the issue of successful versus failed supernova explosions and neutron star versus black hole formation depends monotonically on the mass (and metallicity) of the progenitor star. Here we argue that the issue of success or failure of the explosion or other possible outcomes may depend non--monotonically on the rotation of the progenitor star even at fixed progenitor mass and composition. We have computed "shellular" models of core collapse for a star of 15 M_solar with initial central angular velocity, Omega_0, in the range 0.1 -- 8 rad/s until a few hundred ms after bounce to explore qualitative trends. The non--monotonic behavior will be manifested in the rotation of the proto--neutron star and hence in the strength of the associated magnetic field that will be generated by shear in that rotating environment. We estimate that our maximally rotating and shearing models generate toroidal fields approaching or exceeding 10^17G, strengths nearing dynamical significance.

Evidence for Nucleosynthesis in the SupernovaγProcess: Universal Scaling forpNuclei

Analyzing the solar system abundance, we find two universal scaling laws concerning the p and s nuclei. They indicate that the gamma process in supernova (SN) explosions is the most probable origin of the p nuclei that has been discussed with many possible nuclear reactions and sites in about 50 years. In addition, the scalings lead to new concepts: a universality of the gamma process and a new nuclear cosmochronometer. We carry out gamma-process nucleosynthesis calculations for typical core-collapse SN explosion models, and the results satisfy the observed scalings.

The gravitational wave background from neutron star birth throughout the cosmos

We present calculations of the stochastic gravitational wave (GW) background result- ing from neutron star birth throughout the Universe. We update previous calculations by employing three GW waveforms from Dimmelmeier, Font & Muller (2002), based on three models incorporating general relativistic eects for the axisymmetric core col- lapse of rotating massive stars. Source-rate evolution is accounted for by using a sim- ulated star formation rate model based on a 'flat-⁄' cosmology (Hernquist & Springle 2003). We show that the GW background is only weakly dependent on the source-rate evolution model. Prominent features in the single-source GW spectral strain can be related to the calculated background spectra, even though the features are broadened and redshifted because of contributions from high-redshift sources. The background spectral closure density is reduced by 1-2 orders of magnitude compared to previous calculations, a consequence of using relativistic single-source GW waveforms and a more slowly evolving source-rate density. Duty cycle estimates for this background imply a non-continuous signal, assuming it consists predominantly of 'regular' and 'rapid' core-collapse models. We note that the background may be considerably en- hanced by the inclusion of dynamic post-collapse instabilities (bar-modes), which are not included in this work, so our estimates for this background may be lower limits.

Core-collapse model effects on nucleosynthesis

Although simulations of the core-collapse supernova mechanism have not yet converged on a final answer, much of the intuition gained from these simulations can change our current picture of supernova nucleosynthesis. Here we review the basic supernova explosion mechanism with an update on the current status of this mechanism. We then discuss, how this picture leads to changes in the current calculations of nucleosynthetic yields from these outbursts. The behavior of the electron fraction, mixing and explosion energy (as a function of mass) are quite different, than the assumptions used in many nucleosynthesis calculations, and we will focus on these issues.

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.

Ther‐Process in Neutrino‐driven Winds from Nascent, “Compact” Neutron Stars of Core‐Collapse Supernovae

We present calculations of r-process nucleosynthesis in neutrino-driven winds from the nascent neutron stars of core-collapse supernovae. A full dynamical reaction network for both the α-rich freezeout and the subsequent r-process is employed. The physical properties of the neutrino-heated ejecta are deduced from a general relativistic model in which spherical symmetry and steady flow are assumed. Our results suggest that proto-neutron stars with a large compaction ratio provide the most robust physical conditions for the r-process. The third peak of the r-process is well reproduced in the winds from these compact proto-neutron stars even for a moderate entropy, ~ 100NAk-200NAk, and a neutrino luminosity as high as ~1052 ergs s-1. This is due to the short dynamical timescale of material in the wind. As a result, the overproduction of nuclei with A 120 is diminished (although some overproduction of nuclei with A ≈ 90 is still evident). The abundances of the r-process elements per event is significantly higher than in previous studies. The total integrated nucleosynthesis yields are in good agreement with the solar r-process abundance pattern. Our results have confirmed that the neutrino-driven wind scenario is still a promising site in which to form the solar r-process abundances. However, our best results seem to imply both a rather soft neutron-star equation of state and a massive proto-neutron star that is difficult to achieve with standard core-collapse models. We propose that the most favorable conditions perhaps require that a massive supernova progenitor forms a massive proto-neutron star by accretion after a failed initial neutrino burst.

Bipolar Supernova Explosions

We discuss the optical spectropolarimetry of several core-collapse supernovae, SN 1996cb (Type IIB), SN 1997X (Type Ic), and SN 1998S (Type IIn). The data show polarization evolution of several spectral features at levels from 0.5% to above 4%. The observed line polarization is intrinsic to the supernovae and not of interstellar origin. These data suggest that the the distribution of ejected matter is highly aspherical. In the case of SN 1998S, the minimum major to minor axis ratio must be larger than 2.5 to 1 if the polarization is 3% from an oblate spheroidal ejecta seen edge on. A well-defined symmetry axis can be deduced from spectropolarimetry for the peculiar Type IIn supernova SN 1998S but the Type IIB events SN 1993J and SN 1996cb seem to possess much more complicated geometries with polarization position angles showing larger irregular variations across spectral features; the latter may be associated with large scale clumpiness of the ejecta. The observed degree of polarization of the Type Ic SN 1997X is above 5%. The data reveal a trend that the degree of polarization increases with decreasing envelope mass and with the depth within the ejecta. We speculate that Type IIB, Type Ib, and Type Ic may be very similar events viewed from different aspect angles. The high axial ratio of the ejecta is difficult to explain in terms of the conventional neutrino driven core-collapse models for Type II explosions. Highly asymmetric explosion mechanisms such as the formation of bipolar jets during core-collapse may be a necessary ingredient for models of all core-collapse supernovae.

XPS measurements for detection of collapsed-core atoms

A considerable heat capacity anomaly of metals with unfilled d states at high temperature is explained using the Core Collapse Model (CCM). The XPS measurement of Pt atoms embedded in Cu matrix has been performed at different concentrations. The analogous measurement of Au atoms has been used as a reference. A difference in Pt levels' behavior and their dramatic narrowing with diluting have been interpreted in favor of the CCM.

The Progenitor of the New COMPTEL/[ITAL]ROSAT[/ITAL] Supernova Remnant in Vela

We show that (1) the newly discovered supernova remnant (SNR) GROJ0852-4642/RXJ0852.0-4622 was created by a core-collapse supernova of a massive star and (2) the same supernova event that produced the Ti-44 detected by COMPTEL from this source is probably also responsible for a large fraction of the observed Al-26 emission in the Vela region detected by the same instrument. The first conclusion is based on the fact that the remnant is currently expanding too slowly given its young age for it to be caused by a Type la supernova. If the current SNR shell expansion speed is greater than 3000 km/s, a 15 solar mass. Type II supernova with a moderate kinetic energy exploding at about 150 pc away is favored. If the SNR expansion speed is lower than 2000 km/s, as derived naively from X-ray data, a much more energetic supernova is required to have occurred at approximately 250 pc away in a dense environment at the edge of the Gum Nebula. This progenitor has a preferred ejecta mass of less than or equal to 10(Solar Mass), and therefore it is probably a Type Ib or Type Ic supernova. However, the required high ambient density of n(sub H) greater than or equal to 100 cu cm in this scenario is difficult to reconcile with the regional CO data. A combination of our estimates of the age/energetics of the new SNR and the almost perfect positional coincidence of the new SNR with the centroid of the COMPTEL Al-26 emission feature of the Vela region strongly favors a causal connection. If confirmed, this will be the first case in which both Ti-44 and Al-26 are detected from the same young SNR, and together they can be used to select preferred theoretical core-collapse supernova models.

A Survey for Inward Motions in the Dense Gas around Young Stellar Clusters

We report the results of a spectroscopic search for inward motions of the dense gas around 19 young stellar clusters. Single point observations of CS(2-1) and N2H+(1-0) were made using the Haystack 37 m telescope at 21'' resolution, and CS(2-1) and C34S(2-1) maps were made using the FCRAO 14 m telescope at 50'' resolution. Strong, spatially extended CS self-absorption is seen in Mon OB1-D, IRAS 20050+2720, and Cepheus A, which indicates that these sources are good candidates for follow-up observations of kinematic structure. In Cepheus A, in particular, we identify a region that shows inward motions over a region 0.2 pc in extent, which suggests that models of core collapse should take into account the turbulent motions in these sources. We use the normalized velocity difference, δv=[v(thick)-v(thin)]/Δv(thin), between the optically thick (CS) and thin (N2H+ or C34S) lines to quantify the relative motions between the forward and central layers of dense gas. The average value of this velocity difference is not significantly different from zero for either data set, and there is no correlation with the source bolometric temperature as determined from IRAS flux measurements. Thus this cluster sample has no significant excess of sources with inward motions, unlike the samples of individual YSOs recently reported by Gregerson et al. and Mardones et al. The lower incidence of high absolute values of δv in our sample can be attributed to a lower optical depth in the CS line due to the greater distance, and therefore poorer linear resolution, of the sources here, and to the higher kinetic and excitation temperatures in these more massive, cluster forming environments.