PHOTOEVAPORATION OF DISKS AROUND MASSIVE STARS AND APPLICATION TO ULTRACOMPACT H II REGIONS

Young massive stars produce sufficient Lyman continuum photon luminosity Φ_i, 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 < r_g = 10¹⁵M₁ cm, where M_* = 10 M_⊙ M₁ is the stellar mass. For r ≳ r_g 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 Ṁ_w of the fast, ≳ 1000 km s^-1, stellar wind from the massive star. A critical mass-loss rate Ṁ_cr is defined such that the ram pressure of the stellar wind equals the thermal pressure of the H II atmosphere at r_g. In the weak stellar wind solution, Ṁ_w < Ṁ_cr, the diffuse photons from the atmosphere above r_g produce a photoevaporative mass-loss rate from the disk at r ≳ r_g of order 1 × 10^-5Φ₄₉^1/2M₁^1/2 M_⊙ yr^-1, where Φ_i, = 10⁴⁹Φ₄₉ s^-1. The resulting slow (10-50 km s^-1) ionized outflow, which persists for ≳ 10⁵ yr for disk masses M_d ∼ 0.3M_*, may explain the observational characteristics of unresolved, ultracompact H II regions. 
In the strong stellar wind solution, Ṁ_w, > Ṁ_cr, the ram pressure of the stellar wind blows down the atmosphere for r < r_g 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>r_g; however, it is now dominated by the flow at r_w (>r_g), 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 × 10^-5 Ṁ_w-6 ν_w8Φ₄₉^-1/2 M⊙ yr^-1, where Ṁ_w-6 = Ṁ_w/10^-6 M⊙ yr^-1 and ν_w8 = ν_w/1000 km s^-1 is the stellar wind velocity. The resulting outflow, which also persists for ≳ 10⁵ yr may explain many of the more extended (r ≳ 10¹⁶ cm) ultracompact H II regions. Both the weak-wind and the strong-wind models depend entirely on stellar parameters (Φ_i,M_*, Ṁ_w) and are independent of disk parameters as long as an extended (r ≫ r_g), 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.