Photoevaporation of disks around massive stars and application to ultracompact H II regions

Young massive stars produce sufficient Lyman continuum photon luminosity Phii to significantly affect the structure and evolution of the accretion disks surrounding them. A nearly static, ionized, isothermal 10⁴ K atmosphere forms above the neutral disk for disk radii r less than rg = 10¹⁵ M₁ cm, where M* = 10 solar mass M₁ is the stellar mass. For r approximately greater than rg the diffuse field created by hydrogen recombinations to the ground state in the photoionized gas above the disk produces a steady evaporation at the surface of the disk, and this H II gas flows freely out to the ISM (the 'disk wind'). The detailed structure depends on the mass-loss rate dot-Mw of the fast, approximately greater than 1000 km/sec, stellar wind from the massive star. A critical mass-loss rate dot-Mcr is defined such that the ram pressure of the stellar wind equals the thermal pressure of the H II atmosphere at rg. In the weak stellar wind solution, dot-Mw less than dot-Mcr, the diffuse photons from the atmosphere above rg produce a photoevaporative mass-loss rate from the disk at r approximately greater than rg of order 1 x 10⁻⁵(Phi₄₉)1/2(M₁)1/2 solar mass/year, where Phii = 10⁴⁹ Phi₄₉/sec. The resulting slow (10 to 50 km/sec) ionized outflow, which persists for approximately greater than 10⁵ year for disk masses Md approximately 0.3 M*, may explain the observational characteri stics of unresolved, ultracompact H II regions. In the strong stellar wind solution, dot-Mw greater than dot-Mcr, the ram pressure of the stellar wind blows down the atmosphere for r less than rg and allows the stellar photons to penetrate to greater radii and smaller heights. A slow, ionized outflow produced mainly by diffuse photons is again created for r less than rg; however, it is now dominated by the flow at rw(greater than rg), the radius at which the stellar wind ram pressure equals the thermal pressure in the evaporating flow. The mass-loss rate from the disk is of order 6 x 10⁻⁵dot-Mw-6 vw8(Phi ₄₉)-1/2 solar mass/year, where dot-Mw-6 = Mw/10⁻⁶ solar mass/year and vw8 = vw/1000 km/sec is the stellar wind velocity. The resulting outflow, which also persists for approximately greater than 10⁵ year may explain many of the more extended (r approximately greater than 10¹⁶ cm) ultracompact H II regions. Both the weak-wind and the strong-wind models depend entirely on stellar parameters Phii, M*, dot-Mw) and are independent of disk parameters as long as an extended r much greater than (rg), neutral disk exists. We compare both weak-wind and strong-wind model results to the observed radio free-free spectra and luminosities of ultracompact H II regions and to the interesting source MWC 349.