Models of highly extended dust shells around R Coronae Borealis

Abstract

Radiation transport models are constructed for the dust shells of the hydrogen deficient supergiant star R Coronae Borealis (R CrB). IRAS observations of R CrB are used as constraints in selecting the model parameters. Based on suggestions from earlier work, a double-shell model is employed as a standard configuration for R CrB. The first shell is a hot inner shell of radius 5 arcsec surrounding the central star. The second shell is a cold remnant shell which is highly extended, with a radius of 10 arcmin (derived assuming a source distance of 1.6 kpc). The two shells can be spatially separated, and can have completely different mechanisms for the heating of their dust grains. A detailed parametric study of the system is undertaken using simple power law density and opacity for single sized grains. The models of R CrB clearly point to the configuration of a large well-separated cold dust shell surrounding a tiny hot dust shell, which supports the existing view that this extended shell is a fossil shell. However, the separation between the two shells cannot be determined purely from the radiative transfer modelling. This can still be treated as a free parameter of the system for modelling purposes. The interstellar radiation field (ISRF) incident on the outer boundary of the system plays a considerable role in explaining the 60- and 100-μm surface brightness of R CrB measured by IRAS. The observed constant temperature of the fossil shell, however, poses serious theoretical problems. We propose models in which the density varies as r^-gamma (gamma~=1.0-1.5). The source of dust heating in our models is a combination of central star radiation and ISRF. The amorphous carbon grains seem to give better fits to the IRAS data than the crystalline grains do. A comparison of both the analytic models and the radiative transfer models is made, to clarify their usefulness in modelling the density and the surface brightness data of R CrB. The models are schematic in nature, and are not intended as best fitting models for the R CrB fossil shell.