Two-dimensional Radiation Hydrodynamic Simulations Research Articles

The Spin Periods and Rotational Profiles of Neutron Stars at Birth

We present results from an extensive set of one- and two-dimensional radiation-hydrodynamic simulations of the supernova core collapse, bounce, and postbounce phases, and focus on the protoneutron star (PNS) spin periods and rotational profiles as a function of initial iron core angular velocity, degree of differential rotation, and progenitor mass. For the models considered, we find a roughly linear mapping between initial iron core rotation rate and PNS spin. The results indicate that the magnitude of the precollapse iron core angular velocities is the single most important factor in determining the PNS spin. Differences in progenitor mass and degree of differential rotation lead only to small variations in the PNS rotational period and profile. Based on our calculated PNS spins, at ~ 200-300 milliseconds after bounce, and assuming angular momentum conservation, we estimate final neutron star rotation periods. We find periods of one millisecond and shorter for initial central iron core periods of below ~ 10 s. This is appreciably shorter than what previous studies have predicted and is in disagreement with current observational data from pulsar astronomy. After considering possible spindown mechanisms that could lead to longer periods we conclude that there is no mechanism that can robustly spin down a neutron star from ~ 1 ms periods to the "injection" periods of tens to hundreds of milliseconds observed for young pulsars. Our results indicate that, given current knowledge of the limitations of neutron star spindown mechanisms, precollapse iron cores must rotate with periods around 50-100 seconds to form neutron stars with periods generically near those inferred for the radio pulsar population.

Two‐dimensional Radiation‐Hydrodynamic Model for Limit‐Cycle Oscillations of Luminous Accretion Disks

We investigate the time evolution of luminous accretion disks around black holes by conducting two-dimensional radiation-hydrodynamic simulations. We adopt the α prescription for the viscosity. The radial-azimuthal component of the viscous stress tensor is assumed to be proportional to the total pressure in the optically thick region and the gas pressure in the optically thin regime. The viscosity parameter, α, is taken to be 0.1. We find the limit-cycle variation in luminosity between high and low states. When we set the mass input rate from the outer disk boundary to be 100LE/c2, the luminosity suddenly rises from 0.3LE to 2LE, where LE is the Eddington luminosity. It decays after retaining the high value for about 40 s. Our numerical results can explain the variability amplitude and duration of the recurrent outbursts observed in microquasar GRS 1915+105. We show that multidimensional effects play an important role in the high-luminosity state. In this state, the outflow is driven by the strong radiation force, and some part of the radiation energy dissipated inside the disk is swallowed by the black hole due to the photon-trapping effects. This trapped luminosity is comparable to the disk luminosity. We also calculate two more cases: one with a much larger accretion rate than the critical value for the instability and the other with the viscous stress tensor being proportional to the gas pressure only, even when the radiation pressure is dominant. We find no quasi-periodic light variations in these cases. This confirms that the limit-cycle behavior found in the simulations is caused by the disk instability.

Supercritical Accretion Flows around Black Holes: Two‐dimensional, Radiation Pressure–dominated Disks with Photon Trapping

The quasi-steady structure of supercritical accretion flows around a black hole is studied based on two-dimensional radiation-hydrodynamic (2D-RHD) simulations. The supercritical flow is composed of two parts: the disk region and the outflow regions above and below the disk. Within the disk region the circular motion and the patchy density structure are observed, which is caused by Kelvin-Helmholtz instability and probably by convection. The mass accretion rate decreases inward, roughly in proportion to the radius, and the remaining part of the disk material leaves the disk to form the outflow because of the strong radiation pressure force. We confirm that photon trapping plays an important role within the disk. Thus, matter can fall onto the black hole at a rate exceeding the Eddington rate. The emission is highly anisotropic and moderately collimated so that the apparent luminosity can exceed the Eddington luminosity by a factor of a few in the face-on view. The mass accretion rate onto the black hole increases with the absorption opacity (metallicity) of the accreting matter. This implies that the black hole tends to grow faster in metal-rich regions, such as in starburst galaxies or star-forming regions.

Indirectly driven target design for fast ignition with proton beams

The heavy ion beam (HIB) indirectly driven compression of a small amount of DT fuel, and its fast ignition by a beam of laser accelerated protons (PB) is considered (Roth M. et al 2001 Phys. Rev. Lett. 86 436). First, a representative working point (0.8 mg of DT at 400 g cm−3) has been derived analytically from the energy balance of an hypothetical power plant. Second, through self-consistent two-dimensional radiation hydrodynamic simulations with the MULTI code (Ramis R. et al 1988 Comput. Phys. Commun. 49 475) we define and optimize a HIB-driven hohlraum target to produce the above conditions; also, with ignition simulations, we determine the thermonuclear yield (≃83 MJ) and the minimum PB energy (≃33 kJ) needed to ignite the asymmetrically compressed fuel. The target is found to be a factor 5 less sensitive to implosion asymmetries than a conventional central spot one. This allows us to reduce the hohlraum size (≃6 mm), improve the driver coupling, reduce the compression energy to the 0.8–1.2 MJ range, and have enough clearance for the installation of the PB converter foil at distances (3–4 mm) appropriate for an efficient ignition.

ITAL]HST[/ITAL]/WFPC2 and VLT/ISAAC Observations of Proplyds in the Giant H [CSC]ii[/CSC] Region NGC 3603

We report the discovery of three proplyd-like structures in the giant H II region NGC 3603. The emission nebulae are clearly resolved in narrowband and broadband HST/WFPC2 observations in the optical and broadband VLT/ISAAC observations in the near-infrared. All three nebulae are tadpole shaped, with the bright ionization front at the head facing the central cluster and a fainter ionization front around the tail pointing away from the cluster. Typical sizes are 6000 AU × 20,000 AU The nebulae share the overall morphology of the proplyds (PROto PLanetarY DiskS) in Orion, but are 20 to 30 times larger in size. Additional faint filaments located between the nebulae and the central ionizing cluster can be interpreted as bow shocks resulting from the interaction of the fast winds from the high-mass stars in the cluster with the evaporation flow from the proplyds. Low-resolution spectra of the brightest nebula, which is at a projected separation of 1.3 pc from the cluster, reveal that it has the spectral excitation characteristics of an ultra compact H II region with electron densities well in excess of 104 cm-3. The near-infrared data reveal a point source superposed on the ionization front. The striking similarity of the tadpole-shaped emission nebulae in NGC 3603 to the proplyds in Orion suggests that the physical structure of both types of objects might be the same. We present two-dimensional radiation hydrodynamical simulations of an externally illuminated star-disk-envelope system, which was still in its main accretion phase when first exposed to ionizing radiation from the central cluster. The simulations reproduce the overall morphology of the proplyds in NGC 3603 very well, but also indicate that mass-loss rates of up to 10-5 M☉ yr-1 are required in order to explain the size of the proplyds. We report the discovery of three proplyd-like structures in the giant H II region NGC 3603. The emission nebulae are clearly resolved in narrowband and broadband HST/WFPC2 observations in the optical and broadband VLT/ISAAC observations in the near-infrared. All three nebulae are tadpole shaped, with the bright ionization front at the head facing the central cluster and a fainter ionization front around the tail pointing away from the cluster. Typical sizes are 6000 AU × 20,000 AU. The nebulae share the overall morphology of the proplyds (PROto PLanetarY DiskS) in Orion, but are 20 to 30 times larger in size. Additional faint filaments located between the nebulae and the central ionizing cluster can be interpreted as bow shocks resulting from the interaction of the fast winds from the high-mass stars in the cluster with the evaporation flow from the proplyds. Low-resolution spectra of the brightest nebula, which is at a projected separation of 1.3 pc from the cluster, reveal that it has the spectral excitation characteristics of an ultra compact H II region with electron densities well in excess of 104 cm−3. The near-infrared data reveal a point source superposed on the ionization front. The striking similarity of the tadpole-shaped emission nebulae in NGC 3603 to the proplyds in Orion suggests that the physical structure of both types of objects might be the same. We present two-dimensional radiation hydrodynamical simulations of an externally illuminated star-disk-envelope system, which was still in its main accretion phase when first exposed to ionizing radiation from the central cluster. The simulations reproduce the overall morphology of the proplyds in NGC 3603 very well, but also indicate that mass-loss rates of up to 10−5 M☉ yr−1 are required in order to explain the size of the proplyds. Due to these high mass-loss rates, the proplyds in NGC 3603 should only survive ≈105 yr. Despite this short survival time, we detect three proplyds. This indicates that circumstellar disks must be common around young stars in NGC 3603 and that these particular proplyds have only recently been exposed to their present harsh UV environment.

Detailed characterization of laser plasmas for benchmarking of radiation-hydrodynamic modeling

We have applied Thomson scattering for accurate measurements of densities, temperatures, and ionization levels in dense laser-produced gold plasmas. These experiments give a unique data set that has allowed us to make critical comparisons with two-dimensional radiation-hydrodynamic simulations showing that dielectronic recombination is an important process in cooling laser plasmas. Consistent with this result, additional X-ray spectroscopic measurements of the emission from the gold plasma also indicate a rapidly recombining plasma. These findings are important for the development of kinetics models relevant to high- Z laser-irradiated plasmas and to test our calculational capability for future high-density plasma experiments.

The physics issues that determine inertial confinement fusion target gain and driver requirements: A tutorial

This paper presents a simplified, tutorial approach to determining the gains of inertial confinement fusion (ICF) targets, via a basic, zero-dimensional (“0-D”), energy “bookkeeping” of input (parametrized by ICF drivers’ coupling efficiencies to the target, and subsequent hydrodynamic efficiencies of implosion) versus output (thermonuclear burn efficiency and target fuel mass). Physics issues/constraints such as hydrodynamic instabilities, symmetry and implosion velocity requirements will be discussed for both the direct drive (driver impinging directly on the target) and indirect drive (x-ray implosion within a driver heated hohlraum) approaches to ICF. Supplementing the 0-D model with simple models for hohlraum wall energy loss (to predict coupling efficiencies) and a simple one-dimensional (1-D) model of the implosion as a spherical rocket (to predict hydrodynamic implosion efficiencies) allows gains to be predicted that compare well with the results of complex two-dimensional (2-D) radiation hydrodynamic simulations.

Numerical Simulation of an ICF Target Explosion in a Stratified Gas Atmosphere

Two-dimensional radiation hydrodynamic simulations of a light ion fusion target generated microfireball in a stratified gas atmosphere have been performed. The target location in a two region cavity was varied with the intent to reduce the overpressure on the diodes at the walls of a target chamber with a single cavity gas. Helium and nitrogen at 15 torr were used as the cavity gases; target explosions of 200 and 800 MJ were investigated. It was found that placing the target in a helium region surrounded by nitrogen could reduce the overpressure by a factor of 2 when compared with a single gas cavity of nitrogen. The surface heat flux was also reduced from a pure helium gas cavity.