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H. B. Throop (Colorado / LASP), J. Bally (Colorado / CASA), L. W. Esposito (Colorado / LASP), M. J. McCaughrean (Astrophys. Inst. Potsdam)
Approximately 10 young (\tau ~ Myr) circumstellar discs have been observed by HST (McCaughrean & O'Dell 1996, {\sl AJ} {\bf 111}, 1977). These discs are seen in silhouette, back-illuminated by nebulosity in the Orion region.
We have measured and analyzed the radial profiles of several of these discs for wavelengths between 0.2 -- 0.67 \mum. We find that both the size and shape of the radial profiles are independent of wavelength. The discs' lack of measured color places strong constraints on particle sizes in the discs' outer edges. Using multiple scattering calculations, we find that the discs are consistent with silicate-ice particles of r \gtrsim 1.5 \mum. Smaller particles would cause the discs to appear smaller with increasing wavelength. Our observations show that the discs are strongly inconsistent with `primordial' r = 0.2 \mum particles measured for regions; such small particles have been assumed in earlier disc models (e.g., McCaughrean & O'Dell 1996). Because we find the particles to be much larger, the discs may be `hiding' several orders of magnitude more mass than early lower limits indicated.
Existing models of grain coagulation in the early solar nebula typically predict grain growth to mm-sized particles in 103-4 years; e.g., Ruden & Pollack 1996 ({\sl ApJ} {\bf 375}, 740). Our observational results are the first confirmation of this aspect of the theoretical models, and the first direct confirmation of grain accretion in young circumstellar discs.
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The author(s) of this abstract have provided an email address for comments about the abstract: throop@colorado.edu