Photoevaporation of protostellar disks. I. The evolution of disks around early B stars.

The evolution of protostellar disks around hot, newly formed early B stars is calculated numerically. The interaction of the star with the disk is assumed to occur via stellar hydrogen-ionizing photons and a stellar wind. It is shown that for modest Lyc fluxes (S~10^46^ to 10^47^s^-1^) circumstellar disks are difficult to destroy by these processes alone. Photoevaporation time scales exceed several 10^6^yr. Observational consequences are also discussed. Circumstellar disks in the process of photoevaporation provide a continuous source of high density ionized material and would be interpreted as an ultracompact HII region (UCHII). The UCHII phase in the evolution of massive stars will occur while the disk and central source are still embedded in their parent molecular clump. The relative longevity of this phase is consistent with the observed ratio of optically obscured UCHIIs to optically visible OB-stars (about 15%). Due to the interaction of the circumstellar material with an isotropic stellar wind, multiple shock fronts and contact discontinuities appear in the flow. The outflow can be hydrodynamically focused along the rotation axis, producing a bipolar outflow over a wide range of stellar wind parameters and photoionization rates. Rayleigh-Taylor instabilities and Helmholtz "water wave" instabilities could lead to disk destruction on a shorter time scale than that expected for photoevaporation alone. Angular momentum transport in the disks -- not considered here -- should act on time scales of the order of or shorter than the photoevaporation time scale. Although external sources of radiation or winds are not considered here, photoionization of disks will occur whenever sufficient hydrogen-ionizing UV photons are present. Thus, many of the basic processes considered here are also relevant for "proplyds" which are associated with remnants of circumstellar material around newly formed low-mass stars in the vicinity of luminous O-stars.