The Distribution of Warm Dust in the Star-forming Region Cepheus A: Infrared Constraints

Abstract

In order to determine the distribution of dust in the cloud associated with the star-forming region Cepheus A, we have obtained new, high angular resolution far-infrared (FIR) maps (at 50 and 100 μm) of this extended infrared source and polarimetric images (1.65 and 2.2 μm) of the reflection nebulosity, IRS 6, associated with this young stellar object. From our FIR maps we calculate the dust temperature and optical depth at 100 μm. Cepheus A has moderate optical depths (τ<SUB>100</SUB> μm ≤ 0.6), and its dust temperature ranges from 30 to 55 K. The two-dimensional map of the 100 microns optical depth indicates that there is a region of lower dust column density near the peak of the FIR emission. A radiative transfer code was used to model the available photometry and the FIR data of Cepheus A. A spherical dust cloud with a central young star was assumed, and the input parameters in this model were varied in order to reproduce the spectral energy distribution and the high angular resolution profiles at FIR wavelengths. The model that gives the best fit to the observations requires a dust cloud of the following characteristics: R<SUB>outer</SUB> = 0.5 pc, R<SUB>inner</SUB> = 0.07 pc, τ<SUB>100</SUB> = 0.15, α = 1.5, where R<SUB>outer</SUB>, R<SUB>inner</SUB>, τ<SUB>100</SUB> and α are the outer radius, inner radius, optical depth at 100 μm, and the exponent of the power law in the emitting-dust density gradient: n<SUB>d</SUB>(r) = n<SUB>0</SUB>(r/r<SUB>0</SUB>)<SUP>-α</SUP>. The inner radius used in this model (R<SUB>inner</SUB> = 0.07 pc) is similar in size to the "cavity" derived from the two-dimensional map of the dust optical depth at 100 μm. For small distances (r &lt; 0.15 pc) from the infrared peak, a second density gradient is derived from the distribution of the near-infrared (NIR) polarization. In this inner region of the dust cloud the NIR polarization distribution indicates that the density of the scattering dust should remain constant or increase slightly with distance. Our results are consistent with current star formation theories: a young stellar object surrounded by an infalling envelope with a characteristic density distribution of: n<SUB>d</SUB>(r) ∝r<SUP>-1.5</SUP>, a circumstellar disk, and a cavity (R<SUB>inner</SUB> ∼ 0.07 pc) in which n<SUB>d</SUB> is constant, created by the dispersal of the initial dust cloud by a strong stellar wind.