3-D Radiation Transfer in Line-Blanketed Circumstellar Media

Details 3-D Radiation Transfer in Line-Blanketed Circumstellar Media 3-D Radiation Transfer in Line-Blanketed Circumstellar Media

Dec 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995aas...187.1909b&link_type=abstract

American Astronomical Society, 187th AAS Meeting, #19.09; Bulletin of the American Astronomical Society, Vol. 27, p.1313

Mathematics

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Scientific paper

We present a fully 3-D Monte Carlo radiation transfer (MCRT) technique for calculating metal line blanketing by rotating, expanding (or contracting) circumstellar envelopes around early-type stars. Current MCRT algorithms follow individual monochromatic photon packets, moving through the envelope. The scattering opacity determines the scattering probability within a given volume element, while the absorptive opacity determines the destruction probability. A difficulty with MCRT is that when large absorptive opacities (i.e., spectral lines) are present, most photons are destroyed before they exit the envelope. Consequently, a very large number of stellar photons must be generated to obtain enough emergent photons to measure the flux. Furthermore, to incorporate the large number of blanketing spectral lines, one requires an extremely fine frequency sampling. Here, we present a technique (for electron scattering atmospheres) that may circumvent these problems. Since electron scattering is wavelength-independent, we extend the monochromatic photon packet to include all frequencies simultaneously (i.e., a spectrum) when calculating the photon scattering locations and subsequent directions. Instead of destroying photons upon absorption, we attenuate the spectrum (after it emerges) by the absorptive optical depth accumulated along the entire path. Finally, owing to Doppler shifts within the expanding atmosphere, we only require the cumulative frequency integral of the line opacity when calculating the attenuation. This allows us to include the large number of lines required for line blanketing. Furthermore, the frequency sampling of our spectra is determined by the expansion velocity rather than the line thermal widths. In this paper, we present a feasibility study of this method applied to the axisymmetric circumstellar disk around the Be star zeta Tau. By matching the amount of excess Fe line absorption within the flux spectrum, as well as the amount of UV depolarization observed by the Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE), we hope to determine the temperature, column density, and ionization structure of the circumstellar disk.

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