Lawrence H. Auer

Short characteristic integration of radiative transfer problems: formal solution in two-dimensional slabs

Abstract A short characteristic method based on parabolic approximation of the source function is developed and applied to the solution of the two-dimensional radiative transfer problem on Cartesian meshes. The method is significantly faster for the evaluation of multidimensional radiation fields than those currently in use. Convergence as a functional of the grid resolution is discussed and linear and parabolic upwind interpolation are compared.

A rapidly convergent iterative solution of the non-LTE line radiation transfer problem

Abstract An iterative scheme has been developed for the solution of the non-LTE line radiation transfer problem. The method uses an approximate operator that is deliberately chosen to be local so that it can be easily extended to multidimensional geometry. The difference between the formal and approximate solutions is used as a driving term for the iterations. In one-dimensional, semiinfinite and free-standing slabs, the technique is found to be very fast, robust, and applicable to a large class of problems.

Diffusion, P1, and other approximate forms of radiation transport

Abstract Full transport solutions of time-dependent problems can be computationally very expensive. Therefore, considerable effort has been devoted to developing approximate solution techniques that are much faster computationally and yet are accurate enough for a particular application. Many of these approximate solutions have been used in isolated problems and have not been compared to each other. This paper presents two test problems that test and compare several approximate transport techniques. In addition to the diffusion and P1 approximations, we will test several different flux-limited diffusion theories and variable Eddington factor closures. For completeness, we will show some variations that have not yet appeared in the literature that have some interesting consequences. For example, we have found a trivial way to modify the P1 equations to get the correct propagation velocity of a radiation front in the optically thin limit without modifying the accuracy of the solution in the optically thick limit. Also, we will demonstrate nonphysical behavior in some published techniques.

On laboratory-frame radiation hydrodynamics☆

Abstract We derive and discuss the equations of radiation hydrodynamics with all radiation quantities expressed in the laboratory frame. Relativistically exact transformations from the comoving to laboratory frame are used. We obtain simple, fully covariant, expressions for the radiation energy and momentum source/sink terms using lab-frame radiation quantities. The mathematical simplicity of having lab-frame radiation quantities in conservative operators on the left-hand side is maintained. The resulting system can be solved efficiently using an Eddington tensor formulation for angular dependences, and an iterative frequency-splitting, in the spirit of the multifrequency-gray method. Use of lab-frame radiation quantities can present numerical difficulties in making an accurate connection to the diffusion limit for extremely optically thick media; but it may have no difficulty for neutron-transport problems where very large scattering-thicknesses are generally not encountered (for non-critical systems).

ACCELERATION OF CONVERGENCE

Acceleration of the convergence of approximate operator iteration schemes is discussed. Algorithms based on both residual minimization and use of conjugate vector spaces are presented. It is shown that both give dramatic improvement, at very low computational cost, in the iterative solution of radiative transfer problems in the presence of scattering.

Astronomical Refraction: Computational Method for All Zenith Angles

It is shown that the problem of computing astronomical refraction for any value of the zenith angle may be reduced to a simple, nonsingular, numerical quadrature when the proper choice is made for the independent variable of integration. The angle between the radius vector and the light ray is such a choice. The implementation of the quadrature method is discussed in its general form and illustrated by means of an application to a piecewise polytropic atmosphere. The flexibility, simplicity, and computational efficiency of the method are evident.

Wind Velocity Variations in the Luminous Blue Variable-Type Erupting Star of the Wolf-Rayet Binary HD 5980

We present the wind velocity and UV luminosity variations in the Wolf-Rayet system HD 5980 obtained over a time span during which one of the stars of the system was transformed into a luminous blue variable and underwent an eruption. We are able to separate the velocity components of the two stars in the system: a stable velocity component at -1700 km s-1 is associated with the nonerupting star, while the variable wind with velocities ranging from -500 to -3000 km s-1 corresponds to the eruptor. The development of a fast wind following the slow wind eruptive phase is observed. Under the assumption of radiatively driven winds, these changing velocities indicate that the radius of the photosphere gradually increased during at least 12 years prior to the 1994 eruption, decreasing rapidly thereafter. An estimate of the stellar parameters indicates that the erupting star is massive (M > 40 M☉) and very luminous (L > 106 L☉), and that during the eruption its radius extended beyond the binary orbit (R* > 100 R☉).

Modeling radiative transfer in molecular clouds. 1: HCO(+) in the star-forming region W49A North

A new general multilevel, non-Local Thermodynamic Equilibrium (LTE) radiative transfer code, valid for any velocity field, is applied to HCO(+) observations of W49A North. Three classes of collapse models are considered: free-fall collapse (v proportional to 1/sq. root of r), rho proportional to r(exp -3/2) throughout the molecular cloud, successfully reproduces the features of the observations and gives the best fit to the J = 1-0 and J = 3-2 profiles both toward the prominent H II component G of W49A North and off the center. In addition to a slow radial fall-off of density, the theoretical modeling implies the following for the molecular cloud: the large line widths result from motions occurring within the inner 1 pc, and there are probably one or more fragments with peculiar velocities within this same region.

Influence of Solar Heating and Precipitation Scavenging on the Simulated Lifetime of Post—Nuclear War Smoke

The behavior of smoke injected into the atmosphere by massive fires that might follow a nuclear war was simulated. Studies with a three-dimensional global atmospheric circulation model showed that heating of the smoke by sunlight would be important and might produce several effects that would decrease the efficiency with which precipitation removes smoke from the atmosphere. The heating gives rise to vertical motions that carry smoke well above the original injection height. Heating of the smoke also causes the tropopause, which is initially above the smoke, to reform below the heated smoke layer. Smoke above the tropopause is physically isolated from precipitation below. Consequently, the atmospheric residence time of the remaining smoke is greatly increased over the prescribed residence times used in previous models of nuclear winter.

The Wind-Wind Collision Region of the Wolf-Rayet Binary V444 Cygni: How Much Optical Line Emission Does It Produce?

We model the emission-line profile variations that are expected to be produced by physical and wind eclipses in the Wolf-Rayet (W-R + O) binary system V444 Cyg. A comparison of the theoretical profiles with the He II 4686 A line observed in V444 Cyg allows us to isolate the effects that are likely to be due to the wind-wind collision region in this particular line. We estimate that the wind-wind collision region contributes no more than ~12% of the equivalent width of the emission line, with smaller values during elongations, when part of the shock cone is being eclipsed by the O star. The upper limit implies a maximum contribution from the wind-wind collision region of ~1 × 1035 ergs s-1 to the total luminosity of He II 4686 A line. Using the analytical solution of Canto et al., we find that the bulk of this emission arises along the shock cone walls where the flow velocity is ~800 km s-1, at a distance of ~8 R☉ from the O stars surface, and at θ = 65°-75° from the line joining the centers of the two stars, with origin in the O star. The derived surface density of this region is σ = 0.22 g cm-2, which, together with the He II 4686 A luminosity, indicates that the thickness of the shock lies in the range 2-10 × 1010 cm and the total density is 1-6 × 1012 cm-3.