Photodissociation Region Models of Photoevaporating Circumstellar Disks and Application to the Proplyds in Orion

Abstract

We have modeled the neutral flows emerging from circumstellar disks or small clumps of size r0 illuminated 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 θ1C Ori. Our models improve upon the simpler models of Johnstone, Hollenbach, & Ballyby including the results of both equilibrium and nonequilibrium 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 (G0, r0, and σext) in which far-ultraviolet (FUV)-dominated, as opposed to extreme-ultraviolet (EUV)-dominated, flows exist. G0 is the FUV (6 eV <13.6 eV) flux (in units of the local average interstellar flux) incident on the neutral flow at the ionization front (IF), and σext is the dust FUV extinction cross section per H nucleus in the flow region. FUV-dominated flows are extended with sizes of the IF rIF>~2r0, have a shock between the disk surface and IF, and the mass-loss rates are determined by FUV photons. For σext=8×10-22 cm2 and a UV source similar to θ1 C Ori, the FUV-dominated region extends from G0~5×104 to G0~2×107 (or distances from θ1 C Ori of 0.3-0.01 pc), for disk or clump size of r0~1014-1015 cm. Outside this parameter space, hydrogen-ionizing EUV photons dominate the photoevaporation, and the IF is close to the disk surface (rIF