Improving Monte Carlo radiative transfer in the regime of high optical depths: The minimum scattering order

Abstract

Radiative transfer (RT) simulations are a powerful tool that enables the calculation of synthetic images of a wide range of astrophysical objects. These simulations are often based on the Monte Carlo (MC) method, as it provides the needed versatility that allows the consideration of the diverse and often complex conditions found in those objects. However, this method faces fundamental problems in the regime of high optical depths which may result in noisy images and underestimated flux values. In this study, we propose an advanced MCRT method, i.e., an enforced minimum scattering order that is aimed at providing a minimum quality of determined flux estimates. For that purpose, we extended our investigations of the scattering order problem and derived an analytic expression for the minimum number of interactions that depends on the albedo and optical depth of the system, which needs to be considered to achieve a certain coverage of the scattering order distribution. The method is based on the utilization of this estimated minimum scattering order and enforces the consideration of a sufficient number of interactions during a simulation. Moreover, we identified two notably distinct cases that shape the kind of complexity that arises in MCRT simulations: the albedo-dominated and the optical depth-dominated case. Based on that, we analyzed implications regarding the best usage of a stretching method as a means to alleviate the scattering order problem. We find that its most suitable application requires taking into account the albedo and the optical depth. Then, we argue that the derived minimum scattering order can be used to assess the performance of a stretching method with regard to the scattering orders its usage promotes. Finally, we stress the need for developing advanced pathfinding techniques to fully solve the problem of MCRT simulations in the regime of high optical depths.