Cosmic Explosions: Rapporteur Summary of the 10th Maryland Astrophysics Conference

This meeting covered the range of cosmic explosions from solar flares to gamma-ray bursts. A common theme is the role of rotation and magnetic fields. A rigorous examination is underway to characterize systematic effects that might alter the Type Ia supernova results suggesting an accelerating Universe. The discovery of the central point of X-ray emission in Cas A by CXO should give new insight into the core collapse problem in general and the nature of the still undetected compact remnant in SN 1987A in particular. Jets were described from protostars to microquasars to blazars to gamma-ray bursts. Polarization studies of core-collapse supernovae lead to the conclusion that core collapse is not merely asymmetric, but strongly bi-polar. To account for normal core-collapse supernovae, the explosion must be jet-like in routine circumstances, that is, in the formation of neutron stars, not only for black holes. Given the observed asymmetries, estimates of explosion energies based on spherically-symmetric models must be regarded with caution. The strong possibility that at least some gamma-ray bursts arise from massive stars means that it is no longer possible to decouple models of the gamma-ray burst and afterglow from considerations of the "machine." The implied correlation of gamma-ray bursts with star formation and massive stars and evidence for jets does not distinguish a black hole collapsar model from models based on the birth of a magnetar. Calorimetry of at least one afterglow suggests that gamma-ray bursts cannot involve highly inefficient internal shock models. Essentally all gamma-ray burst models involve the "Blandford Anxiety," the origin of nearly equipartition magnetic fields in the associated relativistic shocks.Comment: 22 pages LaTeX, one eps figure, to be published in the Proceedings of the 10th Maryland Conference on Astrophysics, eds, S. Holt and W. Zhang, AI

The Role of the Magnetorotational Instability in the Sun

We calculate growth rates for nonaxisymmetric instabilities including the magnetorotational instability (MRI) throughout the Sun. We first derive a dispersion relation for nonaxisymmetric instability including the effects of shear, convective buoyancy, and three diffusivities (thermal conductivity, resistivity, and viscosity). We then use a solar model evolved with the stellar evolution code MESA and angular velocity profiles determined by Global Oscillations Network Group (GONG) helioseismology to determine the unstable modes present at each location in the Sun and the associated growth rates. The overall instability has unstable modes throughout the convection zone and also slightly below it at middle and high latitudes. It contains three classes of modes: large-scale hydrodynamic convective modes, large-scale hydrodynamic shear modes, and small-scale magnetohydrodynamic (MHD) shear modes, which may be properly called MRI modes. While large-scale convective modes are the most rapidly growing modes in most of the convective zone, MRI modes are important in both stably stratified and convectively unstable locations near the tachocline at colatitudes theta less than 53 degrees. Nonaxisymmetric MRI modes grow faster than the corresponding axisymmetric modes; for some poloidal magnetic fields, the nonaxisymmetric MRI growth rates are similar to the angular rotation frequency Omega, while axisymmetric modes are stabilized. We briefly discuss the saturation of the field produced by MRI modes, finding that the implied field at the base of the convective zone in the Sun is comparable to that derived based on dynamos active in the tachocline and that the saturation of field resulting from the MRI may be of importance even in the upper convection zone.Comment: 20 pages, 11 figure

Magnetic Fields in Core Collapse Supernovae: Possibilities and Gaps

Spectropolarimetry of core collapse supernovae has shown that they are asymmetric and often, but not universally, bi-polar. The Type IIb SN1993J and similar events showed large scatter in the Stokes parameter plane. Observational programs clearly have much more to teach us about the complexity of asymmetric supernovae and the physics involved in the asymmetry. Jet-induced supernova models give a typical jet/torus structure that is reminiscent of some objects like the Crab nebula, SN1987A and perhaps Cas A. Jets, in turn, may arise from the intrinsic rotation and magnetic fields that are expected to accompany core collapse. We summarize the potential importance of the magneto-rotational instability for the core collapse problem and sketch some of the effects that large magnetic fields, ~10^{15} G, may have on the physics of the supernova explosion. Open issues in the problem of multi-dimensional magnetic core collapse are summarized and a critique is given of some recent MHD collapse calculations.Comment: 20 pages, 2 figures, to appear in the proceedings of the INT workshop "Open Issues in Understanding Core Collapse Supernovae," Seattle, 2004, ed. T. Mezzacappa (World Scientific

The Non-Monotonic Dependence of Supernova and 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.Comment: 20 pages, 6 figures, accepted for publication in The Astrophysical Journal. Few typos are correcte

Hydrogen-Poor Circumstellar Shells from Pulsational Pair-Instability Supernovae with Rapidly Rotating Progenitors

In certain mass ranges, massive stars can undergo a violent pulsation triggered by the electron/positron pair instability that ejects matter, but does not totally disrupt the star. After one or more of these pulsations, such stars are expected to undergo core-collapse to trigger a supernova explosion. The mass range susceptible to this pulsational phenomena may be as low as 50-70 Msun if the progenitor is of very low metallicity and rotating sufficiently rapidly to undergo nearly homogeneous evolution. The mass, dynamics, and composition of the matter ejected in the pulsation are important aspects to determine the subsequent observational characteristics of the explosion. We examine the dynamics of a sample of stellar models and rotation rates and discuss the implications for the first stars, for LBV-like phenomena, and for superluminous supernovae. We find that the shells ejected by pulsational pair-instability events with rapidly rotating progenitors (>30% the critical value) are hydrogen-poor and helium and oxygen-rich.Comment: 14 pages, 2 figure

The Supernova-Gamma Ray Burst Connection

Supernovae 1998bw and its corresponding relativistically expanding radio source are coincident with the \grb source GRB 980425. We show that of six recent SN Ib/c for which an outburst epoch can be estimated with some reliability, four have radio outbursts and all are correlated in time and space with BATSE \grbs. The joint probability of all six correlations is 1.5$\times10^{-5}$. No such correlation exists for SN Ia and SN II. The \gr\ energy associated with the SN/GRB events is $\sim10^{46} - 10^{48}$ ergs if emitted isotropically. Economy of hypotheses leads us to propose that all \grbs are associated with supernovae and that the \grb events have a quasi-isotropic component that cannot be observed at cosmological distances and a strongly collimated and Doppler-boosted component that can only be seen if looking nearly along the collimation axis. Such collimation requires a high rate of occurrence perhaps consistent with a supernova rate. The collimated flow may be generated by core collapse to produce rotating, magnetized neutron stars. All core collapse events may produce such jets, but only the ones that occur in supernovae with small or missing hydrogen envelopes, Type Ib or Ic, can propagate into the interstellar medium and yield a visible \grb. We suggest that asymmetries in line profiles and spectropolarimetry of SN II and SN Ib/c, pulsar runaway velocities, soft \gr repeaters and \grbs are associated phenomena.Comment: Submitted to ApJL on May 19, 1998. Revised on Jun 15, 199

Observations and Theory of Supernovae

This Resource Letter provides a guide to the literature on the observations of supernovae and the theory of their explosion mechanisms. Journal articles and books are cited for the following topics: observations of the spectra, spectropolarimetry, and light curves of supernovae of various types, theory of thermonuclear explosions, core collapse, and radioactive decay, applications to cosmology, and possible connections to gamma-ray bursts.Comment: AAPT/AJP Resource Letter, American Journal of Physics, in press, 19 pages, 208 reference