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Abstract:
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Multidimensional radiative transfer is an essential ingredient of modern ap- proach to modeling of astrophysical objects. Realistic modeling calls for the as- sumption of non-local thermodynamic equilibrium (NLTE), which, in turn requires self-consistent solution of coupled equations of radiative transfer statistical equilib- rium. This approach allows us to compute emergent spectrum from a given model of the object, which is, in principle, a necessary step in interpretation of observational results. Thanks to the high-resolution and high signal to noise observations, it is often possible to measure not only intensity of the light but also its state of po- larization. For interpretation of such observations it is necessary to solve radiative transfer problem for polarized radiation.
This thesis deals with non-LTE transfer of (generally polarized) radiation in two- dimensional media. Thesis can be divided in two parts. In the first part, we present a numerical method for the formal solution of the radiative transfer equation in 2D Cartesian coordinate system. This method allows us to explicitly account for the contribution of non-local source functions to the local specific intensity, and, hence, to the local scattering integral. The knowledge of these contributions is necessary for an iterative solution of coupled equations of radiative transfer and statistical equilibrium. Based on this formal solution we introduce two novel schemes for multidimensional NLTE radiative transfer which have so far been used only in 1D geometry: symmetric Gauss-Seidel iteration and “Sweep-by-sweep” implicit lambda iteration, latter one being based on “Forth-and-back” implicit lambda iteration. Both methods utilize implicit use of the local source function and the source func- tion corrections each sweep of the computational grid (four times per iteration). “Sweep-by-sweep” implicit lambda iteration also uses the idea of iteration factors and achieves acceleration of about factor of seven with respect to the referent Ja-
cobi method. Both new methods also significantly surpass both Jacobi iteration and Gauss-Seidel iteration on problems with periodic boundary conditions. Also, it turns out that “Sweep-by-sweep” implicit lambda iteration scales with grid resolu- tion better than the Jacobi iteration.
The second part of the thesis deals with numerical polarized radiative transfer on 2D cylindrical grids. The method is based on the second order short characteristics for the formal solution and uses standard Jacobi iteration with Ng acceleration to solve the polarized non-LTE problem (Generalization to other iterative schemes is given in appendix A). This method allows for the self-consistent solution of coupled equations of radiative transfer and statistical equilibrium equation for a two level atom model for polarized radiation. The method employs reduced intensity basis where intensity and source function are written as six-vectors and source function does not depend on direction which allows for significant saving in memory and computing time. It is applicable for modeling of axisymmetric objects such as as- trophysical disks. The method has been tested on simple models of circumstellar and self-emitting disks. The most important conclusion of these computations is that the presence of the disk in the system introduces a significant degree of linear polarization due to the scattering processes and that one is able to model it in detail using our approach. Also, it is shown that the presence of rotation in self-emitting disks dramatically changes not only intensity, but also polarized spectral lines pro- files. Interplay of non-LTE, multidimensional effects and rotation results in very complicated line profiles which are non-trivial for interpretation. However, the main effect is that the rotation decreases the amount of Stokes Q component and, de- pending on the rotation velocity causes appearance of double-lobed U polarization profile. If these effects can be observed, this kind of modeling provides a useful tool for interpretation of the spectropolarimetric observations. |