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[33.01] Sandcastles in the Wind: Frustrated Small-Particle Growth in Circumstellar Disks?

H. B. Throop (Colorado / LASP), J. Bally (Colorado / CASA), L. W. Esposito (Colorado / LASP)

In the Orion nebula, thought to be a typical star-forming region for Sun-like stars, several dozen disks have been observed surrounding ~ 30% of 150 young stellar objects. While the circumstellar disks are common, they may not be long-lived. The region is dominated by O star \theta₁C, a strong UV source producing 10{⁸} photons cm^-2 s^-1 at a typical disc distance d = 10⁴ AU. The FUV flux has previously been shown to be sufficient to heat and remove the gas component of the circumstellar disks on timescales ~10⁶ yr (Johnstone, Hollenbach, Bally 1998, ApJ 499/758).

We here consider a different destruction effect: that of UV photosputtering into disk solid particles. The timescales for both a) grain destruction and b) grain coagulation for micron-sized particles are on the order 1-10 yr. The similarity of these timescales suggest that planetesimal formation may not be globally inevitable, but strongly dependent on the balance between creation and destruction processes.

We have computed 10^5-6 year numerical integrations of the evolution of small particles in the circumstellar disks. The initial size distribution is that of the ISM, and processes considered are a) particle collision and coagulation, b) photosputtering into and removal of ice surfaces, and c) photoevaporation and removal of the gas and entrained dust particles. Our model considers vertically-mixed size distributions over a range of disk sizes and initial conditions. For 100 AU disks, dust mass is lost by processes b) and c) each at ~10^-8 M_\hbox{sun} yr^-1, implying micron-sized particle survival times \lesssim 10⁴ yr. The formation of large icy planetesimals (and thus planets) may be possible only if coagulation occurs on significantly shorter timescales.

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The author(s) of this abstract have provided an email address for comments about the abstract: throop@broccoli.colorado.edu

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