Astronomy and Astrophysics Letters dedicated to ISO

SWS observations of IR emission features towards compact HII regions

P.R. Roelfsema, P. Cox, A.G.G.M. Tielens, L.J. Allamandola, J.-P. Baluteau, M.J. Barlow, D. Beintema, D.R. Boxhoorn, J.P. Cassinelli, E. Caux, E. Churchwell, P.E. Clegg, Th. de Graauw, A.M. Heras, R. Huygen, K.A. van der Hucht, D.M. Hudgins, M.F. Kessler, T. Lim, S.A. Sandford


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 7.7 micron band is composed of two bands at 7.6 and 7.8 microns  , the 6.2 micron   band has a long wavelength extension, there is a plateau of emission between 6 and 7 microns   and a new feature is reported at 11.0 microns   in addition to the well-known 11.2 micron   band. These observations also reveal large variations in the relative intensities of the dust bands, in particular between the 7.7 and 8.6  bands. In one extreme case, the 8.6 micron   band is stronger than the 7.7 micron   band. These observations are compared to a mixed population of ionized PAHs, using new laboratory measurements.

ISM : dust -- ISM : HII regions -- ISM : molecules -- Infrared : ISM : lines and bands

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