Mid-infrared imaging spectroscopy in Ophiuchus

The dust emission spectrum between 5 and 16 mu m has been measured at the northern edge of the dense cloud in Ophiuchus at a position where the radiation field intensity is estimated to be 10-20 times the mean Solar neighborhood value. The spectrum shows the well known emission bands at 6.2, 7.7, 8.6, 11.3 and 12.7 mu m. ISO allows to detect the emission features for a radiation field intensity two orders of magnitude smaller than in the objects previously observed. Since all of these bands are considered to be characteristic of C-C or C-H bonds in aromatic hydrocarbons this observation strongly supports the existence of large aromatic molecules in the general interstellar medium. An important result of the observation is the presence of a significant continuum below the features at all wavelengths, and in particular, a featureless continuum between the 8.6 and 11.3 mu m features and beyond that at 12.7 mu m. The Ophiuchus spectrum is remarkably similar to that measured in NGC 7023. We argue that both the bands and the continuum emission must come from molecules or small aggregates with less than a few hundred carbon atoms.