Radiative Transfer using the Monte Carlo Method

Details Radiative Transfer using the Monte Carlo Method Radiative Transfer using the Monte Carlo Method

Sep 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001phdt.......109c&link_type=abstract

PhD Thesis, Universidade de São Paulo

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Radiative Transfer, Monte Carlo

Scientific paper

The radiation of stellar objects surrounded by an envelope can undergo significant reprocessing by the circumstellar material. To investigate the nature of the central object, and also of the envelope, it is necessary a theoretical tool capable of modeling, in a satisfactory way, the radiative transfer. In this work, we present a Monte Carlo code which treats the radiative transfer of the polarized light in several media, which is able to simulate several kinds of observation: polarimetry, espectropolarimetry, imaging, photome-try and spectroscopy. The code was developed aiming at three different applications. The first was the solution of the radiative transfer in electronic clouds. The second one was the study of the radiative transfer of resonant lines in stellar winds. The third one was the solution of the radiative transfer coupled to the radiative equilibrium in dusty environments. In this work, we show the application of the code to two situations: resonant line formation in spherical stellar winds and the radiative transfer in dusty circumstellar envelopes. In the study of the resonant line formation we examine the effects of the wind parameters (velocity law, optical depth, thermal velocity, etc.) on the observables. Among the results shown, we highlight new results for the envelope brightness and polarization profiles and for line maps, which simulate spectropolarimetric observations of different parts of the wind. The main application of the code was the study of the radiative transfer and radiative equilibrium in dusty circumstellar envelopes. We studied the effects of the dust grain size on the energy spectral distribution of spherical envelopes. We introduced the concept of approximate scaling, which reveals important symmetries on the infrared emission of envelopes with different grain sizes and their consequences to the spectral energy distribution. We showed that, in some cases, the spectral energy distribution of models with differently sized grains can be practically indistinguishable, a result with important observational consequences. The concept of approximate scaling was very useful to systematize a large model grid. Finally, we studied the polarization of circumstellar axi-symmetric envelopes. In this study, we investigated the effects on the polarization of the grain optical properties, of the optical depth and of the envelope thermal emission. We developed a semi-analytical model that calculates, in a satisfactory way, the polarization for optically thin envelopes. We showed that the polarization, in contrast with the spectral energy distribution, can be useful to constrain the size of dust grains. Our study brings to evidence the importance of simultaneous observations of the polarization and spectral energy distribution on the study of circumstellar envelopes with dust.

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