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1999ApJ...515..669S - Astrophys. J., 515, 669-684 (1999/April-3)

Photodissociation region models of photoevaporating circumstellar disks and application to the proplyds in Orion.


Abstract (from CDS):

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<hν<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=8x10–22 cm2 and a UV source similar to θ1 C Ori, the FUV-dominated region extends from G0~5x104 to G0~2x107 (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≲2r0).

We show that FUV-dominated flows can explain the observed sizes of the ionization fronts around many of the photoevaporating disks in Orion. The size of the neutral flow region, rIF, depends mainly on r0, G0, and σext inside the flow region. Using ten objects in Orion for which both r0 and rIF are directly observed, and for which G0 can be estimated from the observed projected distance of the proplyd from θ1C Ori, we find that σext~8x10–22 cm2 best fits the observations. In these models, the disk mass-loss rates are roughly 10–7 M.yr–1. We have determined the disk masses for circular and radial proplyd orbits. For circular orbits around θ1C Ori, the disk masses range between 0.005 and 0.04 (ti/105 yr) M, where ti is the illumination timescale. Comparison with millimeter observations of the disk masses (≲0.02 M) indicate ti~105 yr, suggesting that θ1C Ori is a young (≲105 yr old) O star in this scenario. The timescale for the disks to significantly lose mass and shrink is ∼105 yr. If the disks cross the Trapezium cluster on radial orbits, the proplyd masses range between 0.002 and 0.01 M. For radial orbits, the lifetime of the proplyds can be as large as the age of the Orion Cluster (∼1 Myr), and θ1C Ori can be significantly older than 105 yr.

We have calculated the thermal and chemical structure of the flow region in the observationally best studied object HST 182-413 (HST 10) and the representative object HST 155-338. A region of atomic hydrogen extends from the IF toward the disk surface, but close to the surface hydrogen becomes molecular. The temperatures inside the atomic layer are several thousand K. We have calculated the H2 1-0 S(1) and the H2 2-1 S(1) vibrational line intensities, the [C II] 158 µm and [O I] 63 µm fine-structure line intensities, and the [O I] 6300 Å line intensity. We find good agreement between the observed H2 1-0 S(1) line intensity and the theoretically predicted one. The models can also reproduce the [O I] 6300 Å line emission observed close to the disk surface in HST 182-413, HST 155-338, and the other proplyds where the disks can be resolved in the [O I] line. The other lines are not yet observed; we present them here as predictions for future observations.

Abstract Copyright:

Journal keyword(s): Stars: Circumstellar Matter - ISM: Clouds - ISM: individual (Orion Nebula) - Line: Formation - Stars: Formation

Simbad objects: 14

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