The anomalous 3.43 and 3.53 micron emission features toward HD 97048 and Elias 1: C-C vibrational modes of polycyclic aromatic hydrocarbons?

Abstract

We have obtained 5-8 microns spectra toward the two protostellar sources HD 97048 and Elias 1. Besides the well-known family of IR emission bands at 3.3, 6.2 "7.7," 8.7, and 11.3 microns, these objects show strong anomalous emission features at 3.43 and 3.53 microns. No related anomalous bands were found in the new spectra. Combining our results with earlier data, it is shown that, while the anomalous bands are emitted from within 0".05 (approximately 10 AU) of HD 97048, the emission in the general IR features is extended on at least a 20" scale. Some possible assignments of the anomalous emission features are discussed, namely C-H stretching modes in -CHO or -CH2-/-CH3 groups (either in dust grains or as sidegroups on polycyclic aromatic hydrocarbon molecules [PAHs]), and vibrational modes of PAHs without sidegroups. The absence of related anomalous emissions in the 5-8 microns region as well as the high-excitation conditions in the emission zone of the anomalous features make an origin in molecular (side-)groups in grains or on PAHs unlikely. Given the high energy density in the emission zone, as well as the apparent correspondence of the anomalous 3.43 and 3.53 microns features with weak emission shoulders associated with the general family of IR emission bands, it is concluded that an explanation in terms of C-C overtones and combination bands of highly excited, large PAHs or PACs (polycyclic aromatic carbons, i.e., dehydrogenated PAHs; > 500-1000 C atoms) is at the moment most attractive.