John M. Blondin

Colliding winds from early-type stars in binary systems

The dynamics of the wind and shock structure formed by the wind collision in early-type binary systems is examined by means of a 2D hydrodynamics code, which self-consistently accounts for radiative cooling, and represents a significant improvement over previous attempts to model these systems. The X-ray luminosity and spectra of the shock-heated region, accounting for wind attenuation and the influence of different abundances on the resultant level and spectra of X-ray emission are calculated. A variety of dynamical instabilities that are found to dominate the intershock region is examined. These instabilities are found to be particularly important when postshock material is able to cool. These instabilities disrupt the postshock flow and add a time variability of order 10 percent to the X-ray luminosity. The X-ray spectrum of these systems is found to vary with the nuclear abundances of winds. These theoretical models are used to study several massive binary systems, in particular V444 Cyg and HD 193793.

Stability of Standing Accretion Shocks, with an Eye toward Core-Collapse Supernovae

We examine the stability of standing, spherical accretion shocks. Accretion shocks arise in core-collapse supernovae (the focus of this paper), star formation, and accreting white dwarfs and neutron stars. We present a simple analytic model and use time-dependent hydrodynamics simulations to show that this solution is stable to radial perturbations. In two dimensions we show that small perturbations to a spherical shock front can lead to rapid growth of turbulence behind the shock, driven by the injection of vorticity from the now nonspherical shock. We discuss the ramifications this instability may have for the supernova mechanism.

PULSAR WIND NEBULAE IN EVOLVED SUPERNOVA REMNANTS

For pulsars similar to the one in the Crab Nebula, most of the energy input to the surrounding wind nebula occurs on a timescale 103 yr; during this time, the nebula expands into freely expanding supernova ejecta. On a timescale ~104 yr, the interaction of the supernova with the surrounding medium drives a reverse shock front toward the center of the remnant, where it crushes the pulsar wind nebula (PWN). We have carried out one- and two-dimensional, two-fluid simulations of the crushing and reexpansion phases of a PWN. We show that (1) these phases are subject to Rayleigh-Taylor instabilities that result in the mixing of thermal and nonthermal fluids, and (2) asymmetries in the surrounding interstellar medium give rise to asymmetries in the position of the PWN relative to the pulsar and explosion site. These effects are expected to be observable in the radio emission from evolved PWN because of the long lifetimes of radio-emitting electrons. The scenario can explain the chaotic and asymmetric appearance of the Vela X PWN relative to the Vela pulsar without recourse to a directed flow from the vicinity of the pulsar. The displacement of the radio nebulae in G327.1-1.1, MSH 15-56 (G326.3-1.8), G0.9+0.1, and W44 relative to the X-ray nebulae may be due to this mechanism. On timescales much greater than the nebular crushing time, the initial PWN may be mixed with thermal gas and become unobservable, so that even the radio emission is dominated by recently injected particles.

Transition to the Radiative Phase in Supernova Remnants

The evolution of a supernova remnant through the transition from an adiabatic Sedov-Taylor blast wave to a radiative pressure-driven snowplow phase is studied using one- and two-dimensional hydrodynamic simulations. This transition is marked by a catastrophic collapse of the postshock gas, forming a thin, dense shell behind the forward shock. After the transition, the shock front is characterized by a deceleration parameter, Vt/R ≈ 0.33, which is considerably higher than the analytic estimate of for a pressure-driven snowplow. In two dimensions, the catastrophic collapse is accompanied by violent dynamical instabilities of the thin, cool shell. The violence of the collapse and the subsequent instability of the shell increase with increasing ambient density. Preshock density perturbations as small as 1% in an ambient medium with density of 100 cm-3 can lead to distortions of the shock front larger than 10% of the radius of the remnant.

Pulsar spins from an instability in the accretion shock of supernovae

Rotation-powered radio pulsars are born with inferred initial rotation periods of order 300 ms (some as short as 20 ms) in core-collapse supernovae. In the traditional picture, this fast rotation is the result of conservation of angular momentum during the collapse of a rotating stellar core. This leads to the inevitable conclusion that pulsar spin is directly correlated with the rotation of the progenitor star. So far, however, stellar theory has not been able to explain the distribution of pulsar spins, suggesting that the birth rotation is either too slow or too fast. Here we report a robust instability of the stalled accretion shock in core-collapse supernovae that is able to generate a strong rotational flow in the vicinity of the accreting proto-neutron star. Sufficient angular momentum is deposited on the proto-neutron star to generate a final spin period consistent with observations, even beginning with spherically symmetrical initial conditions. This provides a new mechanism for the generation of neutron star spin and weakens, if not breaks, the assumed correlation between the rotational periods of supernova progenitor cores and pulsar spin.

A MILLION-SECOND CHANDRA VIEW OF CASSIOPEIA A

We introduce a million second observation of the supernova remnant Cassiopeia A with the Chandra X-Ray Observatory. The bipolar structure of the Si-rich ejecta (northeast jet and southwest counterpart) is clearly evident in the new images, and their chemical similarity is confirmed by their spectra. These are most likely due to jets of ejecta as opposed to cavities in the circumstellar medium, since we can reject simple models for the latter. The properties of these jets and the Fe-rich ejecta will provide clues to the explosion of Cas A.

Hydrodynamic instabilities in supernova remnants : self-similar driven waves

The initial interaction of a supernova with its surroundings involves the uniformly expanding, roughly power-law density profile of the outer parts. If the supernova gas has a steep power-law density profile and the surrounding density can be described by ρ ∞ r -s , with s = 0 for a uniform interstellar medium and s = 2 for a stellar wind, the interaction region is given by a self-similar solution. The profiles of the physical quantities in the shocked region show that the flow is subject to convective or Rayleigh-Taylor instability; the convective growth rate is largest at the contact discontinuity between the shocked supernova gas and the shocked surroundings

Hydrodynamic simulations of stellar wind disruption by a compact X-ray source

This paper presents two-dimensional numerical simulations of the gas flow in the orbital plane of a massive X-ray binary system, in which the mass accretion is fueled by a radiation-driven wind from an early-type companion star. These simulations are used to examine the role of the compact object (either a neutron star or a black hole) in disturbing the radiatively accelerating wind of the OB companion, with an emphasis on understanding the origin of the observed soft X-ray photoelectric absorption seen at late orbital phases in these systems. On the basis of these simulations, it is suggested that the phase-dependent photoelectric absorption seen in several of these systems can be explained by dense filaments of compressend gas formed in the nonsteady accreation bow shock and wake of the compact object. 61 refs.

The Spherical Accretion Shock Instability in the Linear Regime

We use time-dependent, axisymmetric, hydrodynamic simulations to study the linear stability of the stalled, spherical accretion shock that arises in the postbounce phase of core-collapse supernovae. We show that this accretion shock is stable to radial modes, with decay rates and oscillation frequencies in close agreement with the linear stability analysis of Houck and Chevalier. For nonspherical perturbations we find that the l = 1 mode is always unstable for parameters appropriate to core-collapse supernovae. We also find that the l = 2 mode is unstable, but typically has a growth rate smaller than that for l = 1. Furthermore, the l = 1 mode is the only mode found to transition into a nonlinear stage in our simulations. This result provides a possible explanation for the dominance of an l = 1 sloshing mode seen in many two-dimensional simulations of core-collapse supernovae.

The structure and evolution of radiatively cooling jets

The two-dimensional simulations presently used to characterize the structure and evolution of radiatively cooling supersonic jets reveal that cooling jet morphologies resemble those of adiabatic outflows, but with the fundamental difference that a dense, cold shell will condense out of the shocked gas at the head of the jet when the cooling distance behind either of the two principal shocks is smaller than the jet radius. For very high cooling rates, the material that accumulates at the head of the jet forms an extended plug of cold gas resembling the nose cone observed in numerical simulations of strongly magnetized adiabatic jets. An investigation is made of the dependence of jet properties on the density ratio between the beam and the ambient medium, as well as on the strength of radiative cooling. 52 refs.