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H. B. Throop (Univ. Colorado), J. Bally (Univ. Colorado), L. W. Esposito (Univ. Colorado), M. J. McCaughrean (Astr. Inst. Potsdam)
Planet formation is generally believed to start with the coagulation of sub-micron-sized interstellar dust into planetesimals and eventually planets. The earliest stages of grain growth from interstellar dust into cm-sized pebbles has been described theoretically but not previously observed: a direct observation would complete a causal link between young circumstellar disks and evolved planetary systems. Here we present three lines of evidence for grain growth in young proto-planetary disks in the Orion nebula: 1) We find large, translucent regions of the disks to be achromatic as observed by the the Hubble Space Telescope implying grains larger than 4\ \mu\rm m; 2) We propose that the non-detection of the disks in the 1.3~mm continuum (Bally et al. 1998, AJ 116/854) can be most easily explained by grains larger than 1~cm; and 3) New numerical evolutionary modeling of grain growth within the photo-evaporative environment of the Orion nebula indicates that grains can grow to cm-sizes or larger in 105\rm\ yr.
Observations indicate that most stars form in high-mass clusters where photo-evaporation rapidly destroys young circumstellar disks and may frustrate planetary formation. While terrestrial planet formation may be largely unaffected by external illumination, Jovian planets and icy bodies must form quickly to capture sufficient mass before the volatiles in the proto-planetary nebula are lost to photo-evaporation.
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