AAS 207th Meeting, 8-12 January 2006
Session 10 Circumstellar Disks II
Poster, Monday, 9:20am-7:00pm, January 9, 2006, Exhibit Hall

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[10.03] Silicate Emission Profiles for Selected Orion Proplyds: Evidence for Grain Growth and Thermal Processing in Externally Illuminated Circumstellar Disks.

M. Kassis (Keck Observatory), R.Y. Shuping (USRA-SOFIA, NASA ARC), M. Morris (UCLA), N. Smith, J. Bally (Univ. of Colorado)

In the Orion Nebula, the intense ultraviolet (UV) radiation field of the high-mass Trapezium externally illuminates protoplanetary disks (proplyds) surrounding young (< 106 year old) low-mass stars in the nebula cluster. The UV radiation heats the disk surfaces, drives mass-loss, and produces bright ionization fronts. The photoevaporation of these disks places strong temporal constraints on planet formation mechanisms in irradiated environments, which is important since a significant fraction of stars form in clusters with high-mass stars.

As part of our ongoing studies to understand the grain composition and size distribution in the proplyd disks, we have obtained 8-13 \mum~low resolution spectra (R~100) of 10 targets in Orion using both MIRSI at the NASA IR Telescope Facility and LWS at the W. M. Keck Observatory. Most of the sources in our sample show prominent silicate emission profiles that are qualitatively similar to those observed for T Tauri and HAeBe stars. The peak flux of the silicate band appears to correlate roughly with projected distance from \theta1~Ori C, suggesting that grain heating by the incident UV radiation field is important. For most sources, the silicate profile is broad and flat between 9 and 12 \mum. Ratios of the 11.3 to 9.8 \mum~flux from circumstellar disks have been used as a strong indicator of grain growth. Most of the sources in our sample have F(11.3)/F(9.8) ratios in the 0.8 to 0.9 range, indicating that the grain size is relatively large. Some of the proplyds also show a very slight emission bump near 11.3 \mum~which we attribute to crystalline forsterite. The results of our 8-13 \mum~spectral analysis indicate that the grains in proplyd disks have undergone significant growth and perhaps some thermal annealing despite ongoing photoablation by the external UV radiation from \theta1~Ori C.

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