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

Abstract

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.