3. Initial conditions
For the calculations two different star-disk models are used. They are the results of collapse simulations of rotating molecular clouds. Model I is a relatively massive disk already used in Paper I and II. Model II originates from a collapse simulation of a cloud. For this calculation angular momentum transport was considered via a -prescription (Shakura & Sunyaev 1973) in the manner described by Yorke & Bodenheimer (1998). was adjusted according to the evolution of the disk's Toomre parameter Q, in order to simulate the effects of angular momentum transport due to tidal effects. The -viscosity module is also active in our code when starting with model II, but we used a relatively small value . The full set of hydrodynamical equations including angular momentum transport can be found for example in Kley et al. (1993).
The star and disk masses and other characteristic parameters of the models are listed in Table 1. The radius of the disk is defined by the sudden decrease in density of about two orders of magnitude. The luminosity of the central star of model I is the luminosity of a main sequence star. of model II was computed by the collapse code and is composed of an intrinsic core luminosity and an accretion luminosity. We need to determine the dust temperature with the help of the continuum radiation transfer module. This module was adopted from the collapse code and also employs the grey flux-limited diffusion approximation. The maximum dust temperature close to the central star is also given in Table 1. The remaining parameters describe the grid setup. Note the larger extent of the computational domain and the coarser resolution of model II.
Table 1. Star-disk models. For each model the mass and luminosity of the central star, the central dust temperature , the mass and radius of the disk, the number of nested grids and grid cells , the extension of the coarsest grid , and the resolution on the finest grid are given.
The start parameters for the simulations are summarized in Table 2. For each case the star-disk model, the distance d of the ionizing source, the stellar EUV photon rate and the resulting EUV photon flux at the center of the disk are given. For case A model I is illuminated. Cases B, C and D begin with model II. They are exposed to the same EUV photon rate with decreasing distance from the source. In all cases a modest stellar wind originating from the star at the center of the disk is turned on in order to avoid mass accretion onto the star. The wind velocity and the mass loss rate are also shown in Table 2.
Table 2. Parameters for the cases calculated: star-disk model (c.f. Table 1), distance d and EUV photon rate of the ionizing star, EUV photon flux [in ] at the disk's center, wind velocity and stellar mass loss rate of the stellar wind originating from the star in the center of the disk. Also given are mass , radius and evaporation rate of the disk t years after the start of the external illumination.
© European Southern Observatory (ESO) 1998
Online publication: November 9, 1998