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 e†ects of 6È13.6 eV far-ultraviolet (FUV) photons and Lyman continuum EUV photons incident upon disks or clumps idealized as spheres of radius and enclosed mass For sufficiently large values of the radiation r d M * . r d /M * , Ðeld evaporates the surface gas and dust. Analytical and numerical approximations to the resulting Ñows are presented ; the model depends on the Ñux of FUV and EUV photons, and the column r d , M * , density of neutral gas heated by FUV photons to high temperatures. Application of this model shows that the circumstellar disks cm) in the Orion Nebula (““ proplyds ÏÏ) are rapidly destroyed (r d D 1014È1015 by the external UV radiation Ðeld. Close cm) to h1 Ori C, the ionizing EUV photon Ñux controls the mass-loss rate, and the (d [ 1017 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 D1000 K. As the distance from h1 Ori C increases, the Lyman continuum Ñux declines, the PDR thickens, and the IF moves away from the disk surface. At d D 3] 1017 cm, the thickness of the PDR becomes comparable to the disk radius. Between 3] 1017 cm, spherical cm[ d [ 1018 divergence and the resultant pressure gradient in the 103 K PDR forms a mildly supersonic (D3È6 km s~1) but neutral Parker wind. This wind Ñows 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 stando† distance In this regime, the mass-loss rate is determined by the incident FUV r IF Z 2.5r d . photon Ñux and not the ionizing Ñux. However, at very large distances, cm, the FUV photon d Z 1018 Ñux drops to values that cannot maintain the disk surface temperature at D103 K. As the PDR temperature drops, the pressure of the FUV-powered Ñow declines with increasing distance from h1 Ori C, and again the EUV ionizing photons can penetrate close to the disk surface and dominate the evaporation rate. Radio, Ha, and [O III] observations of externally illuminated young stellar objects in the Trapezium region are used to determine and the projected distances, from h1 Ori C. The observed values of r IF d M , and are combined with the theory to estimate the disk sizes, mass-loss rates, surface densities, and r IF d M disk masses for the ensemble of extended sources in the Trapezium cluster. Observations of and r IF , d M , in HST 182[413 and a few other sources are used to calibrate parameters of the theory, especially r d the column of heated PDR gas. The disks have a range in sizes between mass14\ log [r d /(cm)]\ 15.2, loss rates of [7.7\ log surface densities at disk edge 0.7\ log [M0 /(M _ /yr)]\ [6.2, [&(r d )/(g cm~2)]\ 2.5 which imply disk surface densities at 1 AU from the central, embedded star of 2.8\ log cm~2)]\ 3.8 and disk masses of & and scale with the adopted ioniza[& 0 /(g 0.002\ M d /M _ \ 0.07. M d tion time, which we take to be 105 yr. The inferred for the ensemble of disks suggest that the t i , &(r d ) initial surface density power law of an individual disk, & P r~a, is bounded by 1[ a [ 1.5. Subject headings : accretion, accretion disks È circumstellar matter È H II regions È ISM: general È open clusters and associations : individual (Orion Nebula Cluster)