PHOTOEVAPORATION OF PROTOSTELLAR DISKS .2. THE IMPORTANCE OF UV DUST PROPERTIES AND IONIZING FLUX

We study the hydrodynamic evolution of protostellar disks under the in fluence of a central source of ionizing radiation and its stellar wind . Here we examine the effects of an important contribution to the diff use radiation field-the scattering of hydrogen-ionizing photons on dus t grains (dust scattering). We present and discuss the resulting chang es in the evolution of the system under a variety of conditions both w ith and without dust scattering. An important consequence is the signi ficant increase in the photoevaporation rate. Depending on the scatter ing coefficient assumed, the presence of dust within the ionized regio n can increase the density and flow of ionized material and correspond ingly shorten the disk's lifetime by a factor of two or more. In addit ion, the temperature of the ionized outflowing gas is slightly higher and remains more nearly constant over the extent of the ionized region , even in the regions shadowed from direct stellar radiation. We also investigate the influence of other major parameters of the problem, wi nd velocity, wind mass loss rate, and stellar ionizing flux, by system atically varying these parameters. Over a large range of values of ste llar ionizing flux S-star we find a power law dependence of the disk m ass loss rate due to photoionization (M) over dot(ph) proportional to S-star(0.58) which is comparable to analytic estimates. Deviations fro m this power law occurred for moderate values S-star greater than or s imilar to 10(47) s(-1) due to our finite disk size and for low values S-star less than or similar to 10(45) s(-1) due to the resulting non-s teady flow pattern. Because we have assumed a warm (T approximate to 1 0(4) K) wind and have included heating, cooling, ionization, and recom bination processes in the stellar wind, we find that the disk's photoe vaporation rate depends on the assumed wind parameters in a manner whi ch can be explained by the loss of UV ionizing photons very close to t he central star.