"Evaporating Grains in Reflection Nebulae"
ISOCAM spectral data of Ced 201 indicate that there is a component underlying the PAH bands that Cesarsky et al. (2000) have attributed to emission from very small carbonaceous grains (VSGs). These grains would not be the classical small silicate grains observed in the interstellar medium and HII regions, but amorphous carbon particles PAH clusters that could comprise the smallest of the interstellar grains. Very small carbonaceous grains with some aromaticity have been suggested as contributors to the interstellar extinction curve, but have never been directly confirmed. Cesarsky et al. also suggest that close to the exciting star of the nebula, PAH molecules are either liberated from the VSGs, or the VSGs are aromaticized by the UV from the central star. Their observations show that the 7.7 band appears to get broader towards the edges of the nebula, as expected if the PAHs are in clusters, but the ISOCAM data do not have sufficient spectral resolution (about 40) to determine if the apparent broadening is due to multiple components or true broadening of the feature. We propose to study the emission as a function of position in Ced 201 and two other morphologically similar reflection nebulae at the higher spectral resolution of the Spitzer IRS and at longer wavelengths than was possible with ISOCAM to determine:
1. whether the apparent broadening of the 7.7 micron band is due to intrinsic broadening or to separate components that vary in relative strength as a function of distance from the star.
2. whether the other PAH emission bands, particularly the 11.3 micron band, broaden as well, as expected if the PAHs exist as clusters or VSGs.
3. whether the temperature of the continuum decreases with distance from the star (as for classical VSGs), maintains a constant value as for grains small enough to see just single photon events, or whether it is a combination of the two.
4. if VSGs are present, the size of the VSGs required to produce the observed continuum.