Abstract
Radiative transfer calculations are an essential part of modelling in stellar atmospheres. While the standard integral methods used today have seen great success over the last decades, they do have their limitations. In particular, they tend to be slow when applied to 3D non-LTE problems. Meanwhile, Monte Carlo radiative transfer is used many other places in astrophysics, where it is the preferred choice in 3D, scatteringdominated regimes; the areas where traditional methods fall short.
In this thesis, we test Monte Carlo radiative transfer simulations on a solar atmosphere model. One of the drawbacks of the method, is its inefficiency in high optical depth regions. To overcome this, we introduce a boundary condition that excludes thermal regions in the lower atmosphere. With some adjustment, we are able to efficiently calculate the mean radiation field for all the wavelengths covering the bound-free and bound-bound transitions of a 2-level plus continuum hydrogen model atom.
We then let this be part of an iterative radiative transfer calculation, where we use the statistical equilibrium equations to update the atom populations, and the Monte Carlo simulation to update the radiation field until the populations converge. Doing this, we achieve convergence in 9 iterations. The resulting populations and radiation field are comparable to results from an integral method, but differ from it in ways that can be traced back to the shortcomings of the method.
Veileder: Førsteamanuensis Tiago Pereira, Institutt for teoretisk astrofysikk, UiO
Intern sensor: Professor Boris V. Gudviksen, Institutt for teoretisk astrofysikk, UiO
Ekstern sensor: Researcher Jaime de la Cruz Rodriguez, Department of Astronomy, Stockholm University