Photoevaporation of disks around massive stars and application to ultracompact H II regions
Por:
Hollenbach D., Johnstone D., Lizano S., Shu F.
Publicada:
1 ene 1994
Resumen:
Young massive stars produce sufficient Lyman continuum photon luminosity ?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 < rg = 1015M1 cm, where M* = 10 M? M1 is the stellar mass. For r ? 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 Mw of the fast, ? 1000 km s-1, stellar wind from the massive star. A critical mass-loss rate 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, Mw, < Mcr, the diffuse photons from the atmosphere above rg produce a photoevaporative mass-loss rate from the disk at r ? rg of order 1 × 10-5?49 1/2M1 1/2 M? yr-1, where ?i, = 1049?49 s-1. The resulting slow (10-50 km s-1) ionized outflow, which persists for ? 105 yr for disk masses Md ? 0.3M*, may explain the observational characteristics of unresolved, ultracompact H II regions. In the strong stellar wind solution, Mw, > Mcr, the ram pressure of the stellar wind blows down the atmosphere for r < 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>rg; however, it is now dominated by the flow at rw (>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 × 10-5 Mw-6 ?w8?49 -1/2 M? yr-1, where Mw-6 = Mw/10-6 M? yr-1 and ?w8 = ?w8=?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 (?i,M*, Mw) and are independent of disk parameters as long as an extended (r ? rg), neutral disk exists. We compare both weak-wind and strong-wind model results to the observed ra
Filiaciones:
Hollenbach D.:
MS 245-3, NASA Ames Research Center, Moffett Field, CA 94035, United States
Johnstone D.:
Astronomy Department, University of California, Berkeley, Berkeley, CA 94720, United States
Lizano S.:
Institute de Astronomia, UNAM, Ciadad Universitaria, Apdo 70-264, DF 04510, Mexico City, Mexico
Shu F.:
Astronomy Department, University of California, Berkeley, Berkeley, CA 94720, United States
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