Observations of the circumstellar disk in the Bok globule CB 26 at 110, 230, and 270 GHz are presented together with the results of the simulations and estimates of the disk parameters. These observations were obtained using the SMA, IRAM Plateau de
Bure, and OVRO interferometers. The maps have relatively high angular resolutions (0.4-1), making it possible to study the spatial structure of the gas-dust disk. The disk parameters are reconstructed via a quantitative comparison of observational and theoretical intensity maps. The disk model used to construct the theoretical maps is based on the assumption of hydrostatic and radiative equilibrium in the vertical direction, while the radial surface density profile is described phenomenologically. The system of equations for the transfer of the infrared and ultraviolet radiation is solved in the vertical direction, in order to compute the thermal structure of the disk. The disk best-fit parameters are derived for each map and all the maps simultaneously, using a conjugate gradient method. The degrees of degeneracy of the parameters describing the thermal structure and density distribution of the disk are analyzed in detail. All three maps indicate the presence of an inner dust-free region with a radius of approximately 35 AU, in agreement with the conclusions of other studies. The inclination of the disk is 78 deg, which is smaller than the value adopted in our earlier study of rotating molecular outflows from CB 26. The model does not provide any evidence for the growth of dust particles above a_max=0.02 cm.
We present a self-consistent model of a protoplanetary disk: ANDES (AccretioN disk with Dust Evolution and Sedimentation). ANDES is based on a flexible and extendable modular structure that includes 1) a 1+1D frequency-dependent continuum radiative t
ransfer module, 2) a module to calculate the chemical evolution using an extended gas-grain network with UV/X-ray-driven processes surface reactions, 3) a module to calculate the gas thermal energy balance, and 4) a 1+1D module that simulates dust grain evolution. For the first time, grain evolution and time-dependent molecular chemistry are included in a protoplanetary disk model. We find that grain growth and sedimentation of large grains to the disk midplane lead to a dust-depleted atmosphere. Consequently, dust and gas temperatures become higher in the inner disk (R < 50 AU) and lower in the outer disk (R > 50 AU), in comparison with the disk model with pristine dust. The response of disk chemical structure to the dust growth and sedimentation is twofold. First, due to higher transparency a partly UV-shielded molecular layer is shifted closer to the dense midplane. Second, the presence of big grains in the disk midplane delays the freeze-out of volatile gas-phase species such as CO there, while in adjacent upper layers the depletion is still effective. Molecular concentrations and thus column densities of many species are enhanced in the disk model with dust evolution, e.g., CO2, NH2CN, HNO, H2O, HCOOH, HCN, CO. We also show that time-dependent chemistry is important for a proper description of gas thermal balance.
We study details of the UV radiation transfer in a protoplanetary disk, paying attention to the influence of dust growth and sedimentation on the disk density and temperature. Also, we show how the dust evolution affects photoreaction rates of key molecules, like CN and CS.
