Radiative transfer models of non-spherical prestellar cores

Abstract

We present 2D Monte Carlo radiative transfer simulations of prestellar cores. We consider two types of asymmetry: disk-like asymmetry, in which the core is denser towards the equatorial plane than towards the poles; and axial asymmetry, in which the core is denser towards the south pole than the north pole. In both cases the degree of asymmetry is characterized by the ratio e between the maximum optical depth from the centre of the core to its surface and the minimum optical depth from the centre of the core to its surface. We limit our treatment here to mild asymmetries with e = 1.5 and 2.5. We consider both cores which are exposed directly to the interstellar radiation field and cores which are embedded inside molecular clouds. The SED of a core is essentially independent of the viewing angle, as long as the core is optically thin. However, the isophotal maps depend strongly on the viewing angle. Maps at wavelengths longer than the peak of the SED (e.g. 850 μm) essentially trace the column-density. This is because at long wavelengths the emissivity is only weakly dependent on temperature, and the range of temperature in a core is small (typically Tmax/Tmin ≲ 2). Thus, for instance, cores with disk-like asymmetry appear elongated when mapped at 850 μm from close to the equatorial plane. However, at wavelengths near the peak of the SED (e.g. 200 μm), the emissivity is more strongly dependent on the temperature, and therefore, at particular viewing angles, there are characteristic features which reflect a more complicated convolution of the density and temperature fields within the core. These characteristic features are on scales 1/5 to 1/3 of the overall core size, and so high resolution observations are needed to observe them. They are also weaker if the core is embedded in a molecular cloud (because the range of temperature within the core is then smaller), and so high sensitivity is needed to detect them. Herschel, to be launched in 2007, will in principle provide the necessary resolution and sensitivity at 170 to 250 μm.