PHOTODISSOCIATION REGION MODELS OF PHOTOEVAPORATING CIRCUMSTELLAR DISKS AND APPLICATION TO THE PROPLYDS IN ORION

摘要

We have modeled the neutral flows emerging from circumstellar disks or small clumps of size r_o illu- minated by an external source of ultraviolet radiation. The models are applied to the disks (proplyds) in the Orion Nebula, most of which are illuminated by θ ~1C Ori. Our models improve upon the simpler models of Johnstone, Hollenbach, & Ballyby including the results of both equilibrium and nonequilib- rium photodissociation region (PDR) codes, and by treating the flow speed off the disk surface in a more consistent manner. We present a study that delineates the parameter space (G_o, r_o, and σ_ext) in which far-ultraviolet (FUV)—dominated, as opposed to extreme-ultraviolet (EUV)—dominated, flows exist. G_o is the FUV (6 eV ＜ hν ＜ 13.6 eV) flux (in units of the local average interstellar flux) incident on the neutral flow at the ionization front (IF), and σ_ext is the dust FUV extinction cross section per H nucleus in the flow region. FUV-dominated flows are extended with sizes of the IF r_IF ≈＞2r_o, have a shock between the disk surface and IF, and the mass-loss rates are determined by FUV photons. For σ _ext = 8×10~(-22) cm~2 and a UV source similar to θ~1 C Ori, the FUV-dominated region extends from G_o≈5×10~4 to G_o≈2 x 10~7 (or distances from θ~1 C Ori of 0.3-0.01 pc), for disk or clump size of r_o ≈ 10~14 - 10~15 cm. Outside this parameter space, hydrogen-ionizing EUV photons dominate the photoevaporation, and the iF is close to the disk surface (r_IF ≈＜ 2r_o). We show that FUV-dominated flows can explain the observed sizes of the ionization fronts