2000A&A...362..310C

We present observations with the mid-infrared camera ISOCAM on board the Infrared Space Observatory of the major star-forming region N66 in the Small Magellanic Cloud (SMC) and of its surroundings. These observations were performed with broad filters and Circular Variable Filters giving a spectral resolution of about 40. In addition, CO(2-1) data are presented, allowing us to identify and study how hot dust relates with the different phases of the Interstellar Medium (ISM) present in N66. The spectra are dominated by the strong emission of fine-structure line. Monochromatic maps have been made in the [NeIII]15.6µm and [SIV]10.5µm line. There are significant differences between their distributions, due to the effects of density and of shocks. Aromatic Infrared Bands (AIBs) are seen at various places in the field but they are generally faint. They exhibit a variety of shapes and relative intensities, suggesting that a diversity of carbonaceous materials are present. Silicate emission is also clearly visible in the central condensation and in a few others and emission from hot small grains (Very Small Grains, VSGs) longward of 10µm is present in the whole region. All these dust components are heated by the very strong far-UV radiation of the many young, massive stars contained in the region. The interstellar radiation field (ISRF) at 1600Å is ≥10⁵ times the ISRF of the solar neighborhood in the peaks of mid-infrared emission. The relative contributions of these components (AIB carriers, VSGs and silicate grains) to the mid-infrared spectra seem to depend on the intensity and the hardness of the far-UV field. In general the 15/6.75µm intensity ratio is higher than in relatively quiescent galactic regions (Cesarsky et al., 1996A&A...315L.305C, Abergel et al., 1996A&A...315L.329A) but it is not as high as expected for a linear increase with ISRF. We interpret this behavior as due to the destruction of both AIBs carriers and VSGs in a very high ISRF. Finally several stars have been detected at 6.75µm. Two are red supergiants; the other stars are blue and the IR emission is due to circumstellar matter or to interstellar matter heated by the star.