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Astron. Astrophys. 362, 310-324 (2000)

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1. Introduction

N 66 (Henize et al. 1956) is the largest and most luminous H II  region in the Small Magellanic Cloud (SMC, m-M=18.94 Laney & Stobie 1994). It is also known as DEM S103 (Davies et al. 1976) or NGC 346 referring to the main exciting star cluster.

Radio continuum observations (Taisheng Ye et al. 1991) and low-resolution H[FORMULA] observations (Le Coarer et al. 1993, Fig. 1) show that the brightest emission region, N 66, is along and to the SW of an oblique (SE-NW) "bar". A more compact H II  region is located at [FORMULA](J2000)=00h 59m 16s, [FORMULA](J2000)=-72o 10´ is N 66A. A supernova remnant is located to the East of the region.

A dense cluster of massive young stars is located in N 66, but there are also young stars outside, in particular the ionizing stars of N 66A. Massey et al. (1989) have performed an extensive study of the stellar content of the region, which contains at least 33 O stars, including 11 of type O6.5 or earlier. 22 of these O stars are contained in the central star cluster, and the others are isolated or in small groups. The hottest star, W 3 is classified O3 III(f*) (Walborn & Blades 1986). The most massive star W 1 of the central cluster, classified O4 III(n)(f) by Walborn & Blades (1986), is in fact multiple and the mass of the brightest component is at most 85 [FORMULA] (Heydari-Malayeri & Hutsemékers 1991). The brightest star in the whole region is HD 5980, a OB?+WN eclipsing binary with V [FORMULA] 11.5 (see for a recent study Heydari-Malayeri et al. 1997); it is located outside the dense cluster. The region was mapped in the CO(2-1) line with the SEST telescope and the results are shown here for the first time. These data show that N66 does not contain much molecular gas, except for a small cloud to the NE of the bar. The whole region has been recently reobserved in the CO(2-1) line with the SEST at higher sensitivity. These new observations show that there is also weak molecular emission in the N66 bar, connected with the H II  region but not associated with the cloud detected previously both spatially and in the velocity space (Rubio et al. 2000). The region is also deficient in H I  (Staveley-Smith et al. 1997) and the weak 21-cm line emission detected in this area shows no correlation with the components of N 66 and may be unrelated. Probably most of the gas is ionized outside the molecular clouds. A broad area including the region we studied has been mapped at 68" resolution in the [C II ] 157 µm line by Israïl & Maloney (1993); the intensity peaks on the bar.

In this paper we present mid-IR spectrophotometric observations of N 66 obtained with the 32[FORMULA]32 pixel ISOCAM camera on board the Infrared Space Observatory (ISO) of the European Space Agency. These observations belong to a wider program aimed to study the interplay between the interstellar medium (ISM) and star formation in our and in external nearby galaxies. The Magellanic Clouds represented obvious sources to include in this project because of their proximity but also because they offer the opportunity to study how metallicity influences this process. Thus, many HII complexes in both the Large and the Small Magellanic Clouds were observed with ISOCAM in spectro-imaging mode between 5 and 18 µm and N66 is one of these sources. A 6´[FORMULA]6´ field centered on N66 was mapped in 7 broad-band filters and the central 3´[FORMULA]3´ have been observed with the Circular Variable Filters (CVFs) as dispersive elements. These observations provide a wealth of data on warm dust, fine-structure line and Aromatic Infrared Bands (AIBs).

The present paper describes the observations, their reduction (Sect. 2) and the analysis of the whole region. We will investigate the distribution of the fine-structure line emission in Sect. 3, of the dust emission of both discrete peaks and diffuse regions in Sects. 4 and 5. Sect. 6 gives the conclusions. Appendix A describes how we built the ISRF map at 1600 [FORMULA] and Appendix B contains a short discussion of the stars seen in our observations.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 19100