Peter Vitello

Two-dimensional fluid model of an inductively coupled plasma with comparison to experimental spatial profiles

A key need for the development and testing of models suitable for chemically active, low pressure plasmas is detailed comparisons between model predictions and experimental measurements. In this paper, a two‐dimensional, axisymmetric fluid model of an inductively coupled plasma is described, and model predictions are compared to the experimental measurements of P. A. Miller, G. A. Hebner, K. E. Greenberg, P. D. Pochan, and B. P. Aragon [J. Res. Natl. Inst. Stand. Technol. 100, 427 (1995)] of electron density, electron temperature, and plasma potential. Comparisons between model predictions and experimental measurements were made in argon and chlorine discharges. Qualitative to semiquantitative agreement between the model predictions and experimental diagnostics was observed, suggesting that assumptions made in the model are reasonably accurate.

Winds from accretion disks - Ultraviolet line formation in cataclysmic variables

Winds from accretion disks in cataclysmic variable stars are ubiquitous. Observations by IUE reveal P Cygni-shaped profiles of high-ionization lines which are attributed to these winds. We have studied the formation of UV emission lines in cataclysmic variables by constructing kinematical models of biconical rotating outflows from disks around white dwarfs. The photoionization in the wind is calculated taking into account the radiation fields of the disk, the boundary layer, and the white dwarf. The 3D radiative transfer is solved in the Sobolev approximation. Effects on the line shapes of varying basic physical parameters of the wind are shown explicitly. We identify and map the resonant scattering regions in the wind which have strongly biconical character regardless of the assumed velocity and radiation fields. Rotation at the base of the wind introduces a radial shear which decreases the line optical depth and reduces the line core intensity. We find that it is possible to reproduce the observed P Cygni line shapes and make some predictions to be verified in high-resolution observations.

Ultraviolet line diagnostics of accretion disk winds in cataclysmic variables

The IUE data base is used to analyze the UV line shapes of the cataclysmic variables RW Sex, RW Tri, and V Sge. Observed lines are compared to synthetic line profiles computed using a model of rotating biconical winds from accretion disks. The wind model calculates the wind ionization structure self-consistently including photoionization from the disk and boundary layer and treats 3D line radiation transfer in the Sobolev approximation. It is found that winds from accretion disks provide a good fit for reasonable parameters to the observed UV lines which include the P Cygni profiles for low-inclination systems and pure emission at large inclination. Disk winds are preferable to spherical winds which originate on the white dwarf because they: (1) require a much lower ratio of mass-loss rate to accretion rate and are therefore more plausible energetically; (2) provide a natural source for a biconical distribution of mass outflow which produces strong scattering far above the disk leading to P Cygni profiles for low-inclination systems and pure line emission profiles at high inclination with the absence of eclipses in UV lines; and (3) produce rotation-broadened pure emission lines at high inclination.

Line-driven winds from accretion disks. I - Effects of the ionization structure

The basic characteristics of line-driven winds from UV accretion disks were investigated analytically and numerically. Stellar winds have been compared with the outflow from a disk surrounding a supermassive black hole. It is found that in the approximation of vertically streaming radiation close to the disk photosphere, the existence of steady disk winds depends crucially on their ionization structure, a basic difference when compared to the stellar winds. The necessary criteria for the line radiation force to support the disk wind are presented and discussed.

Ignition & Growth and JWL++ Detonation Models in Coarse Zones

“Ignition & Growth” (I&G) and JWL++ models are compared for a variety of problems. The detonation velocity becomes nearly constant with zoning at the edge of convergence, which for TATB, is 8 zones/mm for I&G and 4 for JWL++. The use of pressure in the rate for I&G makes the detonation velocity rapidly decrease as the zones are coarsened. Using pressure plus artificial viscosity to some power in the rate for JWL++ allows the correction for coarsening zones. In coarse zones, the pressure and the burn fraction turn on independently and this feature dominates model behavior. If pressure lags burn fraction, then the maximum pressure will be lower than expected. An unexpected phenomenon is saturation, i.e. the slowing down of the detonation velocity as a function of the fast rate constant. This slowing can be weak and produce a plateau, or it can be strong and cause the detonation velocity to approach an asymptote. The saturation effect comes from a combination of the 1−F term and declining pressures. Failure (critical diameter effect) occurs in reactive flow but optimizing for this undoes the settings for other results. In JWL++ , the fast reaction pressure exponent is near −1 for the best fit for the size (diameter) effect, 2 for the Pop plot and near −3 to fit failure. The Pop plot deflagration rate is derived, although it needs not to be the same as the detonation rate. The use of additive pressures is compared with the pressure equilibrator and no difference is found. Increased zoning by a factor of 5 and improved code structure will be needed for future improvement.

Rotating Winds from Accretion Disks in Cataclysmic Variables: Eclipse Modeling of V347 Puppis

We study the eclipsing nova-like variable V347 Pup by matching its UV emission line profiles in and out of eclipse to synthetic lines using a 3D kinematic and radiation transfer model. Our results support the accretion disk origin of winds in non-magnetic CVs as opposite to the WD origin. Our main point concerns the importance of rotation for the UV emission line shapes in such systems. In particular, we show that the narrowing of the UV emission lines in V347 Pup during eclipse can be easily explained by the eclipse of the innermost part of the wind by the secondary and the resulting reduction in the contribution of rotational broadening to the width of the lines. During the eclipse, the residual line flux is very sensitive to the maximal temperature of disk radiation. Good fits for reasonable mass-loss rates have been obtained for maximum disk temperatures of 50,000 degrees. This constraint was imposed either by leveling off the inner disk temperature profiles, in agreement with recent observations of some nova-like objects, or by assuming that the accretion disk does not extend to the surface of the white dwarf, in which case V347 up would be an intermediate polar. In anticipation of high-speed spectrophotometry of CVs by the HST, we provide numerical model of a time-resolved eclipse of V347 Pup or similar such system to be verified by future observations.

DYNAMICS OF LINE-DRIVEN WINDS FROM DISKS IN CATACLYSMIC VARIABLES. II. MASS-LOSS RATES AND VELOCITY LAWS

We analyze the dynamics of two-dimensional stationary, line-driven winds from accretion disks in cataclysmic variable (CV) stars by generalizing the formalism of Castor, Abbott, and Klein (CAK) for O stars. In Paper I, we solved the wind Euler equation, derived its two eigenvalues, and addressed the solution topology and wind geometry. Here, we focus on mass-loss rates and velocity laws of the wind. We find that disk winds, even in luminous nova-like variables, have low optical depth, even in the strongest driving lines. This suggests that thick-to-thin transitions in these lines occur in the wind. For disks with a realistic radial temperature law, the mass loss is dominated by gas emanating from the inner decade in radius. The total mass-loss rate associated with the wind from a disk of luminosity 10 L☉ is ~10-12 M☉ yr-1, or 10-4 of the mass accretion rate. This is 1 order of magnitude below the lower limit obtained from fitting P Cygni line profiles using kinematical wind models when the Lyman continuum is suppressed. The difficulties associated with such small mass-loss rates for line-driven winds from disks in CVs are principal and confirm our previous work on this subject. We conjecture that this issue may be resolved by detailed non-LTE calculations of the CAK line force within the context of CV disk winds and/or by better accounting for the disk energy distribution and wind ionization structure. We find that the wind velocity profile is well approximated by the empirical law used in kinematical modeling. The acceleration length scale is given by the footpoint radius of the wind streamline in the disk. This suggests an upper limit of ~10rwd to the acceleration scale, which is smaller by factor of a few as compared with values derived from line fitting.

Hot-spot contributions in shocked high explosives from mesoscale ignition models

High explosive performance and sensitivity is strongly related to the mesoscale defect densities. Bracketing the population of mesoscale hot spots that are active in the shocked ignition of explosives is important for the development of predictive reactive flow models. By coupling a multiphysics-capable hydrodynamics code (ale3d) with a chemical kinetics solver (cheetah), we can parametrically analyze different pore sizes undergoing collapse in high pressure shock conditions with evolving physical parameter fields. Implementing first-principles based decomposition kinetics, burning hot spots are monitored, and the regimes of pore sizes that contribute significantly to burnt mass faction and those that survive thermal conduction on the time scales of ignition are elucidated. Comparisons are drawn between the thermal explosion theory and the multiphysics models for the determination of nominal pore sizes that burn significantly during ignition for the explosive 1,3,5-triamino-2,4,6-trinitrobenzene.

Reactive Flow and the Size Effect

The detonation reaction rate in μs−1 is derived from Size Effect data using the relation – DUs(∂Us/∂y)−1, where y =1/Ro, where Us is the detonation velocity for a ratestick of radius Ro and D is the infinite-radius detonation velocity. These rates are generally not constant with radius and have pressure exponents ranging from  5. JWL++, a simple Reactive Flow code, is run with one rate constant on many samples to compare its rates. JWL++s pressure exponents vary from about 0.5 to 2.5, and failure occurs outside this range. There are three classes of explosives: (1) those for which the pressure exponent is between 1 and 2 and the rate is nearly constant (e.g. porous urea nitrate); (2) higher pressure explosives with a concave-down shape and large positive pressure exponents (dense TNT); and (3) explosives with negative pressure exponents and concave-up shapes (porous PETN). JWL++ fits only the first class well and has the most trouble with class 3. The pressure exponent in JWL++ is shown to be set by the shape of the Size Effect curve – a condition that arises in order to keep a constant reaction rate for all radii. Some explosives have too much bend to be modeled with one rate constant, e.g. Comp. B near failure. A study with creamed TNT shows that the rate constant need not be changed to account for containment. These results may well be pertinent to a larger consideration of the behavior of Reactive Flow models.

A theory of gamma-ray bursts based on resonant Compton scattering

Photon spectra produced from the resonant Compton scattering of blackbody photons by a beam of energetic electrons and positrons is calculated. The beam is assumed to be directed radially outward along the magnetic axis of a highly magnetized neutron star. A dipole magnetic field geometry is assumed and the effects of Compton-scattering energy losses on the particles in the beam during transport are included. A range of surface temperatures near 1 keV and magnetic field strengths near 10 12 G are studied for monoenergetic and power-law electron and positron source functions. Photon spectra similar to observed gamma-ray burst spectra are obtained for power-law beam injection functions with low-energy cutoffs, provided that (1) a large fraction of the neutron star is radiating and that (2) the particles in the beam lose most of their energy close to the neutron-star surface