Photoevaporation of protostellar disks. IV. Externally illuminated disks

By means of numerical simulations we investigate the photoevaporation of protostellar disks. Here we concentrate on the evolution of disks under the influence of an external Lyman continuum radiation field. The morphological evolution is illustrated in detail for the case of a relatively massive disk exposed to a modest ionizing flux. Cometary tails develop and break off into filaments which leave the immediate vicinity of the disk with the evaporating flow. It evolves into a relatively undisturbed disk completely enveloped by the ionization front. A low mass star-disk system is used to investigate the dependence of the evolution on the distance from the ionizing source. Distances and external Euv flux are adapted to the situation of the proplyds close to the Trapezium star theta (1) Ori C. The low-mass system becomes extremely deformed through the asymmetrical illumination. The total mass of the disk fragments which break off during the cometary phase ( ~ 2*E(4) yr) is of order 10% of the disk mass. With decreasing distance the densest parts of the disk remnant are more strongly disturbed. The dependence of the final photoevaporation rate on the distance dot {M}_ph~ d(-1.1) is consistent with analytical models. For comparison with observations we use a 3D ray-tracing procedure to calculate spectra, continuum maps and Hα line profiles. During the cometary phase the elongated tails of the low mass system are recognizable at almost all frequencies ranging from the radio region to the near-infrared. Furthermore, the disk remnants viewed face-on from the direction of the source appear as round heads in Hα resembling this class of proplyds quite well.