SWS observations of IR emission features towards compact HII regions.
Heras, A. M.; Kessler, M. F.; de Graauw, T.; Barlow, M. J.; Caux, E.; van der Hucht, K. A.; Tielens, A. G. G. M.; Allamandola, L. J.; Cox, P.; Roelfsema, P. R.; Beintema, D.; Lim, T.; Cassinelli, J. P.; Clegg, P. E.; Churchwell, E.; Baluteau, J. -P.; Sandford, S. A.; Boxhoorn, D. R.; Huygen, R.; Hudgins, D. M.
Abstract
We present ISO Short Wavelength Spectrometer (SWS) grating spectra of six compact HII regions. In addition to strong emission lines from atomic species these spectra display infrared bands attributed to Polycyclic Aromatic Hydrocarbons (PAHs). The continuous spectral coverage of the present observations and the high spectral resolution allow to describe the detailed structure of the emission bands: the 7.7μm band is composed of two bands at 7.6 and 7.8μm, the 6.2 μm band has a long wavelength extension, there is a plateau of emission between 6 and 7μm and a new feature is reported at 11.0μm in addition to the well-known 11.2μm band. These observations also reveal large variations in the relative intensities of the dust bands, in particular between the 7.7 and 8.6μm bands. In one extreme case, the 8.6μm band is stronger than the 7.7μm band. These observations are compared to a mixed population of ionized PAHs, using new laboratory measurements.