Photoevaporation of Disks and Clumps by Nearby Massive Stars: Application to Disk Destruction in the Orion Nebula

We present a model for the photoevaporation of circumstellar disks or dense clumps of gas by an external source of ultraviolet radiation. Our model includes the thermal and dynamic effects of 6-13.6 eV far-ultraviolet (FUV) photons and Lyman continuum EUV photons incident upon disks or clumps idealized as spheres of radius r d and enclosed mass M * . For sufficiently large values of r d / M * , the radiation field evaporates the surface gas and dust. Analytical and numerical approximations to the resulting flows are presented; the model depends on r d , M * , the flux of FUV and EUV photons, and the column density of neutral gas heated by FUV photons to high temperatures. Application of this model shows that the circumstellar disks ( r d ~ 10 14 -10 15 cm) in the Orion Nebula ("proplyds") are rapidly destroyed by the external UV radiation field. Close ( d ≲ 10 17 cm) to θ 1 Ori C, the ionizing EUV photon flux controls the mass-loss rate, and the ionization front (IF) is approximately coincident with the disk surface. Gas evaporated from the cold disk moves subsonically through a relatively thin photodissociation region (PDR) dominated by FUV photons and heated to ~1000 K. As the distance from θ 1 Ori C increases, the Lyman continuum flux declines, the PDR thickens, and the IF moves away from the disk surface. At d ~ 3 × 10 17 cm, the thickness of the PDR becomes comparable to the disk radius. Between 3 × 10 17 cm ≲ d ≲ 10 18 cm, spherical divergence and the resultant pressure gradient in the 10 3 K PDR forms a mildly supersonic (~3-6 km s -1 ) but neutral Parker wind. This wind flows outward until it passes through a shock, beyond which gas moves subsonically through a stationary D-type IF. The IF is moved away from the disk surface to a standoff distance r IF ≳ 2.5 r d . In this regime, the mass-loss rate is determined by the incident FUV photon flux and not the ionizing flux. However, at very large distances, d ≳ 10 18 cm, the FUV photon flux drops to values that cannot maintain the disk surface temperature at ~10 3 K. As the PDR temperature drops, the pressure of the FUV-powered flow declines with increasing distance from θ 1 Ori C, and again the EUV ionizing photons can penetrate close to the disk surface and dominate the evaporation rate. Radio, Hα, and [O III] observations of externally illuminated young stellar objects in the Trapezium region are used to determine r IF and the projected distances, d ⊥ , from θ 1 Ori C. The observed values of r IF and d ⊥ are combined with the theory to estimate the disk sizes, mass-loss rates, surface densities, and disk masses for the ensemble of extended sources in the Trapezium cluster. Observations of r IF , d ⊥ , and r d in HST 182-413 and a few other sources are used to calibrate parameters of the theory, especially the column of heated PDR gas. The disks have a range in sizes between 14 < log [ r d /(cm)] < 15.2, mass-loss rates of -7.7 < log [ /( M /yr)]<-6.2, surface densities at disk edge 0.7 < log [Σ( r d )/(g cm -2 )] < 2.5 which imply disk surface densities at 1 AU from the central, embedded star of 2.8 < log [Σ 0 /(g cm -2 )] < 3.8 and disk masses of 0.002 < M d / M ☉ < 0.07. Σ and M d scale with the adopted ionization time, t i , which we take to be 10 5 yr. The inferred Σ( r d ) for the ensemble of disks suggest that the initial surface density power law of an individual disk, Σ ∝ r -α , is bounded by 1 ≲ α ≲ 1.5.