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

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

Apr 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...515..669s&link_type=abstract

The Astrophysical Journal, Volume 515, Issue 2, pp. 669-684.

Astronomy and Astrophysics

Astronomy

115

Stars: Circumstellar Matter, Ism: Clouds, Ism: Individual (Orion Nebula), Line: Formation, Stars: Formation

Scientific paper

We have modeled the neutral flows emerging from circumstellar disks or small clumps of size r_0 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 theta^1C 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 (G_0, r_0, and sigma_ext) in which far-ultraviolet (FUV)-dominated, as opposed to extreme-ultraviolet (EUV)-dominated, flows exist. G_0 is the FUV (6 eV~2r_0, have a shock between the disk surface and IF, and the mass-loss rates are determined by FUV photons. For sigma_ext=8x10^-22 cm^2 and a UV source similar to theta^1 C Ori, the FUV-dominated region extends from G_0~5x10^4 to G_0~2x10^7 (or distances from theta^1 C Ori of 0.3-0.01 pc), for disk or clump size of r_0~10^14-10^15 cm. Outside this parameter space, hydrogen-ionizing EUV photons dominate the photoevaporation, and the IF is close to the disk surface (r_IF<~2r_0). 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 r_0, G_0, and sigma_ext inside the flow region. Using ten objects in Orion for which both r_0 and r_IF are directly observed, and for which G_0 can be estimated from the observed projected distance of the proplyd from theta^1C Ori, we find that sigma_ext~8x10^-22 cm^2 best fits the observations. In these models, the disk mass-loss rates are roughly 10^-7 M_solar yr^-1. We have determined the disk masses for circular and radial proplyd orbits. For circular orbits around theta^1C Ori, the disk masses range between 0.005 and 0.04 (t_i/10^5 yr) M_solar, where t_i is the illumination timescale. Comparison with millimeter observations of the disk masses (<~0.02 M_solar) indicate t_i~10^5 yr, suggesting that theta^1C Ori is a young (<~10^5 yr old) O star in this scenario. The timescale for the disks to significantly lose mass and shrink is ~10^5 yr. If the disks cross the Trapezium cluster on radial orbits, the proplyd masses range between 0.002 and 0.01 M_solar. For radial orbits, the lifetime of the proplyds can be as large as the age of the Orion Cluster (~1 Myr), and theta^1C Ori can be significantly older than 10^5 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 H_2 1-0 S(1) and the H_2 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 H_2 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.

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