Photoevaporation of protostellar disks - IV. Externally illuminated disks

By means of numerical simulations we investigate the photoevaporation of pr otostellar disks. Here we concentrate on the evolution of disks under the i nfluence of an external Lyman continuum radiation field. The morphological evolution is illustrated in detail for the case of a relatively massive dis k exposed to a modest ionizing flux. Cometary tails develop and break off i nto filaments which leave the immediate vicinity of the disk with the evapo rating flow. It evolves into a relatively undisturbed disk completely envel oped by the ionization front. A low mass star-disk system is used to invest igate the dependence of the evolution on the distance from the ionizing sou rce. Distances and external EUV flux are adapted to the situation of the pr oplyds close to the Trapezium star theta(1) Ori C. The low-mass system beco mes extremely deformed through the asymmetrical illumination. The total mas s of the disk fragments which break off during the cometary phase (similar to 2 x 10(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 dep endence of the final photoevaporation rate on the distance (M) over dot ph proportional to d(-1.1) is consistent with analytical models. For comparison with observations we use a 3D ray-tracing procedure to calcu late spectra, continuum maps and H alpha line profiles. During the cometary phase the elongated tails of the low mass system are recognizable at almos t all frequencies ranging from the radio region to the near-infrared. Furth ermore, the disk remnants viewed face-on from the direction of the source a ppear as round heads in H alpha resembling this class of proplyds quite wel l.