THE DISTRIBUTION OF WARM DUST IN THE STAR-FORMING REGION CEPHEUS-A - INFRARED CONSTRAINTS

In order to determine the distribution of dust in the cloud associated with the star-forming region Cepheus A, we have obtained new, high an gular resolution far-infrared (FIR) maps (at 50 and 100 mu m) of this extended infrared source and polarimetric images (1.65 and 2.2 mu m) o f the reflection nebulosity, IRS 6, associated with this young stellar object. From our FIR maps we calculate the dust temperature and optic al depth at 100 pm. Cepheus A has moderate optical depths (tau(100) mu m less than or equal to 0.6), and its dust temperature ranges from 30 to 55 K. The two-dimensional map of the 100 mu m optical depth indica tes 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 a vailable photometry and the FIR data of Cepheus A. A spherical dust cl oud with a central young star was assumed, and the. input parameters i n this model were varied in order to reproduce the spectral energy dis tribution 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(outer) = 0.5 pc, R(inner) = 0.07 pc, tau(100) = 0.15, alpha = 1.5, where R(outer), R(inner), tau 100, and alpha are the outer radius, inner radius, optical depth at 10 0 mu m, and the exponent of the power law in the emitting-dust density gradient: n(d)(r)= n(0)(r/r(0))(-alpha). The inner radius used in thi s model (R(inner) = 0.07 pc) is similar in size to the ''cavity'' deri ved from the two-dimensional map of the dust optical depth at 100 mu m . For small distances (r less than or equal to 0.15 pc) from the infra red peak, a second density gradient is derived from the distribution o f the near-infrared (NIR) polarization. In this inner region of the du st cloud the NIR polarization distribution indicates that the density of the scattering dust should remain constant or increase slightly wit h distance. Our results are consistent with current star formation the ories: a young stellar object surrounded by an infalling envelope with a characteristic density distribution of: n(d)(r) proportional to r(- 1.5) a circumstellar disk, and a cavity (R(inner) similar to- 0.07 pc) in which nd is constant, created by the dispersal of the initial dust cloud by a strong stellar wind.