Effects of Grain Size on the Spectral Energy Distribution of Dusty Circumstellar Envelopes

We study the effects of dust grain size on the spectral energy distribution (SED) of spherical circumstellar envelopes. Based on the self-similarity relations of dusty SEDs derived by Ivezic & Elitzur (1997), we expect an approximate invariance of the IR SED for models with different grain sizes. Approximate invariance follows from the fact that differently sized grains have similar optical properties at long wavelengths where the dust reprocesses the starlight. In this paper, we discuss what are the physical requirements on the model parameters to maintain the approximate invariance of the IR SED. Single grain size models are studied for a wide range of grain sizes in three optical depth regimes. In this study, we find limits for the cases where the IR SED is and is not capable of conveying information about grain sizes, and to what extent it does so. We find that approximate invariance occurs for a much larger range of grain sizes than previously believed, and, when approximate invariance holds, the SED is controlled mainly by one parameter, the reprocessing optical depth, a quantity that measures the fraction of starlight that is absorbed by the dust grains. Models with a grain size distribution are studied as well. For these models, we find that, in many instances, the concept of approximate invariance may be extended from the IR SED to all wavelengths. This means that, for a wide range of optical depths, models with different grain size distributions will produce very similar SEDs and, hence, the reprocessing optical depth is the only quantity that can be unambiguously obtained from the SED. The observational consequences of this result are discussed in detail

Towards Understanding The B[e] Phenomenon: IV. Modeling of IRAS 00470+6429

FS CMa type stars are a recently described group of objects with the B[e] phenomenon that exhibit strong emission-line spectra and strong IR excesses. In this paper we report the first attempt for a detailed modeling of IRAS 00470+6429, for which we have the best set of observations. Our modeling is based on two key assumptions: the star has a main-sequence luminosity for its spectral type (B2) and the circumstellar envelope is bimodal, composed of a slowly outflowing disk-like wind and a fast polar wind. Both outflows are assumed to be purely radial. We adopt a novel approach to describe the dust formation site in the wind that employs timescale arguments for grain condensation and a self-consistent solution for the dust destruction surface. With the above assumptions we were able to reproduce satisfactorily many observational properties of IRAS 00470+6429, including the H line profiles and the overall shape of the spectral energy distribution. Our adopted recipe for dust formation proved successful in reproducing the correct amount of dust formed in the circumstellar envelope. Possible shortcomings of our model, as well as suggestions for future improvements, are discussed.Comment: 11 pages, 7 figures, accepted for publication in The Astrophysical Journa