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H. B. Throop, J. Bally, L. W. Esposito (Univ. Colorado)
In the Orion nebula, several dozen dark disks have been observed surrounding ~ 30% of 150 young stellar objects. The circumstellar disks are common and short-lived, due to the presence of O star \theta1C, at a typical disc distance d = 104-5 AU, causing disk mass loss rates ~10-6 M\hbox{sun} yr-1 (Johnstone, Hollenbach, Bally 1998, ApJ 499/758). We consider the effect of \theta1C on planetesimal formation in the region.
We here consider the competing effect of particle growth and particle loss on the abundance of solid material and the formation of planets in the disks. The processes included are a) growth by grain coagulation, and loss by b) photosputtering of ices and c) photoevaporation of the gas disk and small particles entrained within. We have computed 105-6 year numerical integrations of the evolution of small particles in the circumstellar disks. The initial size distribution is that of the ISM, and we consider vertically-mixed size distributions over a range of disk sizes and initial conditions.
We find for a set of nominal disk parameters, particles inward of R ~ 100 AU coagulate quickly and are large enough to survive against loss processes. However, at distances outward of 100 AU, particles remain small enough, r \lesssim 100\ \mum, to be entrained by the photevaporating gas, and nearly all particle mass is lost from the disk within 105 yr. The disk extinction profile is sharply terminated at 100 AU. The formation of large bodies outward of this edge is possible only if the disks are given significant head-start times before the onset of photoevaporation.
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