Session S8: Stellar Physics and Super Novae

Genesis light noble gas measurements.

Models of planetary evolution require knowledge of the base reservoir of the solar system, which is dominated by the sun. Currently, the only way to accurately determine the composition of trace elements in the sun is by examining the solar wind (SW). Previous solar wind measurements have come from lunar samples which varied by 5-10{\%} or spacecraft measurements which are not precise enough to differentiate from the planetary reservoirs. The Genesis mission directly collected the SW for about 852 days using ultra-pure collectors$^{1}$. From these samples, we have precisely measured the $^{20}$Ne/$^{22}$Ne and $^{36}$Ar/$^{38}$Ar ratio of the solar wind, obtaining values of 13.972 $\pm $ 0.025 and 5.501 $\pm $ 0.005 respectively$^{2}$. We are currently determining diffusion rates of light noble gases in the Genesis collectors to find out if any significant diffusive losses have occurred. $^{1}$Jurewicz A. et al$.$ \textit{Space Sci. Rev.} \textbf{105}, 535 (2003). $^{2}$Meshik A. et al. \textit{Science} \textbf{318}, 433 (2007).   

Imaging 2-20 MeV solar neutrons in the inner heliosphere with the SONNE detector

The Solar NeutroN Experiment (SONNE), a neutron detector with imaging and energy measurement capabilities sensitive to neutrons in the 2-20 MeV energy range, is specifically conceived as a candidate instrument for the Solar Sentinels program. Different design concepts have been explored to optimize the detection capabilities for solar-flare neutrons in the inner heliosphere. The detection principle is based on multiple elastic neutron-proton scatterings in organic scintillators. By measuring the scattering coordinates and determining the energy of recoil protons and time of flight of scattered neutrons, the energy spectrum and incident direction of primary neutrons can be reconstructed. We present the results of calibrations and further simulations that demonstrate that the instrument meets the requirements for unprecedented sensitive measurements of low-energy solar neutrons. We confirm that the instrument has an energy resolution of $\sim $20{\%} over a wide range of energies and that its angular resolution is of order 15 degrees allowing for heavy background suppression. Furthermore, the efficiency agrees with the Monte Carlo model allowing us to extrapolate to the full instrument that may be deployed on Solar Sentinels.   

The growth of magnetic turbulence and transport of cosmic rays in supernova remnant precursors.

The process of first order Fermi acceleration in supernova remnant shocks has long been the favoured mechanism for producing cosmic rays in our galaxy. One of the major difficulties with this picture however is that it must be pushed to its limits to even approach the so-called knee of the cosmic-ray spectrum, below which most particles are believed to be galactic in origin. The two greatest uncertainties in determining the maximum achievable energy in this picture are the magnetic field and the transport properties of the relativistic particles, although these are not independent of one another. Amplification of the magnetic field beyond the linear regime, via the non-resonant current driven instability is investigated as a possible method to resolve this problem. We report on recent numerical calculations of magnetic field growth and the transport properties of relativistic particles in the resulting amplified field. We speculate on the importance of these results in the context of particle acceleration.   

Surface Detonation Models of Type Ia Supernovae

Flame propagation and subsequent detonation in near-Chandrasekhar mass, carbon/oxygen white dwarf stars are studied using multi-dimensional, reactive hydrodynamic simulation. The single off-center bubble ignition models studied by Townsley et al. (2007) are extended through detonation and into the homologous expansion phase. In these models, detonation occurs in a collision region at the stellar surface. Careful attention is paid to accurately calculating the yield of material burned to nuclear statistical equilibrium (NSE) and then frozen out in the expansion following the detonation wave which sweeps over the white dwarf. A self regulating process comprised of neutronization and pre-expansion leads to $\sim$1.1M$_{\odot}$ of $^{56}$Ni synthesized in all of the single point ignition models studied. The yield of intermediate mass elements is $\sim$0.1-0.3$M_{\odot}$ and the explosion energies are $\sim$1.5$\times$10$^{51}$ ergs, comparable to observed luminous type Ia supernovae (Ia SNe). Multi-point ignition can lead to lower luminosity explosions by releasing more energy in the deflagration which goes into expanding the white dwarf prior to detonation. A suite of pre-expanded surface detonation models are presented which have explosion energies and $^{56}$Ni masses spanning those of observed Ia SNe. Synthetic spectra and light curves are being generated from the multi-dimensional models for more direct comparison to observed Ia SNe.   

The Roles of Nuclear Physics during Stellar Core Collapse

Nuclear electron capture and the nuclear equation of state play important roles during the collapse of a massive star and the subsequent supernova. The nuclear equation of state controls the nature of the bounce which initially forms the supernova shock while electron capture determines the location where the shock forms. Advances in nuclear structure theory have allowed a more realistic treatment of electron capture in supernovae to be developed. With this improvement, we have shown that electron capture on nuclei with masses larger than 50 dominates electron capture on free protons, producing significant changes in the hydrodynamics of core collapse and bounce. We will present explorations of the impact of weak interactions with heavy nuclei in supernovae, focusing on the consequences across the range of supernova progenitors. Examination of the sensitivity of these effects to variations in the electron capture rates will also be presented. Additionally, we will present simulations showing the impact of a variety of nuclear equations of state on supernova shock propagation and the interplay between electron capture and the equation of state.   

Evolutionary Tracks for Betelgeuse

We have constructed a series of quasi-hydrostatic evolutionary models for the M2 Iab supergiant Betelgeuse ($\alpha~Orionis$). Our models are constrained by the observed temperature, luminosity, surface composition and mass loss for this star, along with recent parallax measurements and high resolution imagery which directly determine its radius. The surface convective zone obtained in our model naturally accounts for observed variations in surface luminosity and the size of detected surface bright spots. In our models these result from upflowing convective material from regions of high temperature in a surface convective zone. We also account for the observed periodic variability as the result of the effective equation of state in a simple linear pulsation model. Based upon a comparison between the accumulated mass loss in the observed circumstellar shell, and the lower limit on luminosity we suggest that this star most likely has a mass of either $\approx 16 \pm 2$ M$_\odot$ if a Reimers lass loss rate applies or $20 \pm 2$ for the de Jager mass loss rate. For any mass loss rate the star must be close to the tip of the first ascent up the giant branch.   

Non-MHD gravity-driven Hamiltonian dynamo for driving astrophysical jets

Conservation of canonical angular momentum $P_{\phi }=m_{\sigma }r^{2}\dot{\phi}+\ (2\pi )^{-1}q_{\sigma } \psi (r,z,t)$ shows that charged particles are typically constrained to stay within a poloidal Larmor radius of a poloidal magnetic flux surface $\psi (r,z,t) $. However, more detailed consideration shows that particles with a critical charge to mass ratio can have zero canonical angular momentum and so be both immune from centrifugal force and not constrained to stay in the vicinity of a specific flux surface. Suitably charged dust grains can have zero canonical angular momentum and in the presence of a gravitational field will spiral inwards across poloidal magnetic surfaces toward the central object and accumulate. This accumulation results in a gravitationally-driven dynamo [1], i.e., a mechanism for converting gravitational potential energy into a battery-like electric power source. \newline \newline [1] P. M. Bellan, Phys. Plasmas 14, Art. No. 122901, 2007   

Nucleosynthesis Yields from the Explosion of Massive Stars

The large number of recent abundance observations in metal-poor stars and the progressing field of galactic evolution pose a need for improved predictions of nucleosynthesis yields from core collapse supernovae. The innermost ejecta and especially the Fe-group nuclei are directly affected by the explosion mechanism. Induced explosion models employing a piston or thermal bomb fail to predict the observed yields because the effects of neutrino interactions are not included. However, comprehensive core collapse supernova simulations are a complex and long standing problem. Despite continuous improvement they still bear important uncertainties. We will present detailed nucleosynthesis yields based on a model for the supernova ejecta featuring accurate Boltzmann neutrino transport and detailed neutrino-matter interaction in the nuclear network. The results will be confronted with recent observations of metal-poor stars and their impact on Galactic chemical evolution will be addressed.   

The lowest-mass stellar black holes: catastrophic death of neutron stars in gamma-ray bursts

Mergers of double neutron stars are considered the most likely progenitors for short gamma-ray bursts. Indeed such a merger can produce a black hole with a transient accreting torus of nuclear matter and the conversion of the torus mass-energy to radiation can power a gamma-ray burst. Using available binary pulsar observations supported by our extensive evolutionary calculations of double neutron star formation, we demonstrate that the fraction of mergers that can form a black hole -- torus system depends very sensitively on the (largely unknown) maximum neutron star mass. We show that the available observations and models put a very stringent constraint on this maximum mass under the assumption that a majority of short gamma-ray bursts originate in double neutron star mergers. Specifically, we find that the maximum neutron star mass must be within 2--2.5 Msun. Moreover, a single unambiguous measurement of a neutron star mass above 2.5 Msun would exclude double neutron star mergers as short gamma-ray burst progenitors.