SIMBAD references

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 σ_ext) 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 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≳2r₀, have a shock between the disk surface and IF, and the mass-loss rates are determined by FUV photons. For σ_ext=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 (r_IF≲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, r_IF, depends mainly on r₀, G₀, and σ_ext inside the flow region. Using ten objects in Orion for which both r₀ and r_IF are directly observed, and for which G₀ can be estimated from the observed projected distance of the proplyd from θ¹C Ori, we find that σ_ext~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 (t_i/10⁵ yr) M_☉, where t_i is the illumination timescale. Comparison with millimeter observations of the disk masses (≲0.02 M_☉) indicate t_i~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.