Silicate Emission Profiles from Low-Mass Protostellar Disks in the Orion Nebula: Evidence for Growth and Thermal Processing of Grains

摘要

We present 8--13 micron low resolution spectra (R~100) of 8 low-massprotostellar objects ("proplyds") in the Orion Nebula using the Long WavelengthSpectrometer (LWS) at the W. M. Keck Observatory. All but one of the sources inour sample show strong circumstellar silicate emission, with profiles that arequalitatively similar to those seen in some T Tauri and Herbig Ae/Be stars. Thesilicate profile in all cases is significantly flattened compared to theprofile for typical interstellar dust, suggesting that the dominant emittinggrains are significantly larger than those found in the interstellar medium.The 11.3-to-9.8 micron flux ratio--often used as an indicator of graingrowth--is in the 0.8 to 1.0 range for all of our targets, indicating that thetypical grain size is around a few microns in the surface layers of theattendant circumstellar disk for each object. Furthermore, the silicateprofiles show some evidence of crystalline features, as seen in other youngstellar objects. The results of our analysis show that the grains in thephotoevaporating protostellar disks of Orion have undergone significant growthand perhaps some annealing, suggesting that grain evolution for these objectsis not qualitatively different from other young stellar objects.