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.

http://iopscience.iop.org/article/10.1086/305708/pdf

Transition to the Radiative Phase in Supernova Remnants

We discuss molecular cloud formation by large-scale supersonic compressions in the diffuse warm neutral medium (WNM). Initially, a shocked layer forms, and within it, a thin cold layer. An analytical model and high-resolution one-dimensional simulations predict the thermodynamic conditions in the cold layer. After ~1 Myr of evolution, the layer has column density ~2.5 × 1019 cm-2, thickness ~0.03 pc, temperature ~25 K, and pressure ~6650 K cm-3. These conditions are strongly reminiscent of those recently reported by Heiles and coworkers for cold neutral medium sheets. In the one-dimensional simulations, the inflows into the sheets produce line profiles with a central line of width ~0.5 km s-1 and broad wings of width ~1 km s-1. Three-dimensional numerical simulations show that the cold layer develops turbulent motions and increases its thickness until it becomes a fully three-dimensional turbulent cloud. Fully developed turbulence arises on times ranging from ~7.5 Myr for inflow Mach number M1,r = 2.4 to >80 Myr for M1,r = 1.03. These numbers should be considered upper limits. The highest density turbulent gas (HDG, n > 100 cm-3) is always overpressured with respect to the mean WNM pressure by factors of 1.5-4, even though we do not include self-gravity. The intermediate-density gas (IDG, 10 < n/cm-3 < 100) has a significant pressure scatter that increases with M1,r, so that at M1,r = 2.4 a significant fraction of the IDG is at a higher pressure than the HDG. Our results suggest that the turbulence and at least part of the excess pressure in molecular clouds can be generated by the compressive process that forms the clouds themselves and that thin CNM sheets may be formed transiently by this mechanism, when the compressions are only weakly supersonic.

/pdf/molecular-cloud-evolution-i-molecular-cloud-and-thin-cold-1rs4ezivw5.pdf

Molecular cloud evolution. i. molecular cloud and thin cold neutral medium sheet formation

The X-ray flux of the bright galactic bulge source GX5 - 1 shows intensity-dependent quasi-periodic oscillations between 20 and 40 Hz, appearing as a broad peak in the power spectrum whose centroid frequency, width, and integrated excess power strongly depend on the source intensity. The strength and steepness of low-frequency noise present in the power spectra below 15 Hz also depend on the source intensity. No evidence is found for coherent X-ray pulsations between 0.5 and 2000 Hz. Possible mechanisms to explain these new phenomena are discussed.

/pdf/intensity-dependent-quasi-periodic-oscillations-in-the-x-ray-3uufh20b3w.pdf

Intensity-dependent quasi-periodic oscillations in the X-ray flux of GX5 - 1

https://arxiv.org/pdf/astro-ph/9605015.pdf

Accretion processes on a black hole

Context. Plasma accreting onto classical T Tauri stars (CTTS) is believed to impact the stellar surface at free-fall velocities, generating a shock. Current time-dependent models describing accretion shocks in CTTSs are one-dimensional, assuming that the plasma moves and transports energy only along magnetic field lines (β � 1). Aims. We investigate the stability and dynamics of accretion shocks in CTTSs, considering the case of β > 1 in the post-shock region. In these cases the 1D approximation is not valid and a multi-dimensional MHD approach is necessary. Methods. We model an accretion stream propagating through the atmosphere of a CTTS and impacting onto its chromosphere by performing 2D axisymmetric MHD simulations. The model takes into account the stellar magnetic field, the gravity, the radiative cooling, and the thermal conduction (including the effects of heat flux saturation). Results. The dynamics and stability of the accretion shock strongly depend on the plasma β. In the case of shocks with β> 10, violent outflows of shock-heated material (and possibly MHD waves) are generated at the base of the accretion column and intensely perturb the surrounding stellar atmosphere and the accretion column itself (therefore modifying the dynamics of the shock). In shocks with β ≈ 1, the post-shock region is efficiently confined by the magnetic field. The shock oscillations induced by cooling instability are strongly influenced by β :f or β> 10, the oscillations may be rapidly dumped by the magnetic field, approaching a quasi-stationary state, or may be chaotic with no obvious periodicity due to perturbation of the stream induced by the post-shock plasma itself; for β ≈ 1 the oscillations are quasi-periodic, although their amplitude is smaller and the frequency higher than those predicted by 1D models.

/pdf/x-ray-emitting-mhd-accretion-shocks-in-classical-t-tauri-2lb0d5jeo4.pdf

X-ray emitting MHD accretion shocks in classical T Tauri stars - Case for moderate to high plasma-β values

We report the discovery of a new kind of thermal instability in accretion flows onto stellar atmospheres. The instability results in periodic variations in the height of the standoff shock which forms above the stellar surface. The hard X-ray temperature and luminosity vary with this period, as does any other radiation produced in the hot gas between the shock and the surface. Conditions for the appearance of this instability are most favorable in accreting white dwarf stars, such as cataclysmic variables and AM Her stars. We suggest that a search for such variability be carried out in AM Her stars in both the hard X-ray and UN-optical bands. The resulting period, which is of order 1--100 s for AM Her stars, permits the determination of the accretion rate per unit area at the stellar surface.

Thermal instability in accretion flows onto degenerate stars

Time-dependent accretion onto magnetized white dwarfs.

Previous accretion models have shown that if the cooling is due to optically thin bremsstrahlung, the flow is unstable. The shock height oscillates with a period approx.1 s, which corresponds to the cooling time for parameters characteristic of the AM Her binaries. Here we show that the effects of cyclotron cooling stabilize the flow if the magnetic field is large enough. For a 0.5 M/sub sun/ white dwarf accreting 1 g cm S s , the flow is stable if B is more than or approximately equal to 2 x 10X gauss. We also show that the bremsstrahlung X-ray emission and the optical cyclotron emission are 180 out of phase when oscillations are present. The relevance of these calculations to the observed optical oscillations, with periods of approx.1 s, in two AM Her stars is discussed.

G. Chanmugam

Papers

Thermal instability in accretion flows onto degenerate stars

Time-dependent accretion onto magnetized white dwarfs.

Time-dependent accretion onto magnetic white dwarfs: Effects of cyclotron emission