Photoevaporation of Disks and Clumps by Nearby Massive Stars: Application to Disk Destruction in the Orion Nebula
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Abstract
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 613.6 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 * , eld evaporates the surface gas and dust.Analytical and numerical approximations to the resulting ows are presented ; the model depends on the ux 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 ) are rapidly destroyed (r d D 10141015 by the external UV radiation eld.Close cm) to h1 Ori C, the ionizing EUV photon ux 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 ux 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 (D36 km s~1) but neutral Parker wind.This wind ows 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 distanceIn this regime, the mass-loss rate is determined by the incident FUV r IF Z 2.5r d .photon ux and not the ionizing ux.However, at very large distances, cm, the FUV photon d Z 1018 ux drops to values that cannot maintain the disk surface temperature at D103 K.As the PDR temperature drops, the pressure of the FUV-powered ow 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 mass-14 \ log [r d /(cm)] \ 15.2, loss rates of [7.7 \ log surface densities at disk edge 0.7 \ log [M 0 /(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.