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Astron. Astrophys. 359, 1139-1146 (2000)

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1. Introduction

N 66 (Henize 1956) is the largest and most luminous H II  region in the Small Magellanic Cloud (SMC). It is also known as DEM S103 (Davies et al. 1976) or NGC 346; the latter designates indifferently the H II  region and its ionizing cluster. A bright emission region (that we will call N 66 in what follows) is along and to the SW of an oblique (SE-NW) "bar" (Fig. 1). The H II  region is limited on the SW side by a well-defined arc. A more compact H II  region 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 excites N 66, but there are also young stars outside, as 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. Fig. 1 is a 60 sec OIII image obtained with EMMI/NTT on December 1995, and extracted from the ESO NTT archive through the ESO Science Archive Facility. The image was reduced using reference files obtained during the same observing run using a Tek 2048[FORMULA]2048 CCD, covering a field of view 9.2´ [FORMULA] 8.6´ with a pixel size of 0.27 ".

[FIGURE] Fig. 1. An archival CCD image of N 66 taken with the New Technology Telescope of ESO through an interference filter centered on the [O III ][FORMULA] 5007 line. This image displays clearly the structure of the H II  region and of its ionizing star cluster, NGC 346. The bright main H II  region is limited by a straight boundary on the NE side, probably due to a lack of gas. On the SW side, it is limited by a curved front which is clearly a photodissociation region (PDR) eating into a neutral cloud. Notice the narrow absorption features along this front, that continue to the south forming a S shape. The small H II  region N 66A is NE of the main H II  region (N 66 proper). The bright star to the East of N 66A is HD 5980, an OB?+WN binary. Other stellar objects in the field are either red supergiants or young OB stars or small clusters. They are identified in Massey et al. (1989) and in Fig. 5 of Paper I. The curved filaments on the top left quarter of the image delineate the supernova remnant SNR 0057-724, which is also a X-ray source (Kahabka et al. 1999).

N 66 has been observed at many wavelengths. In particular Contursi et al. (2000, hereafter Paper I) have presented and discussed 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. A 6´[FORMULA]6´ field 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 provided a wealth of data on warm dust, fine-structure lines and Aromatic Infrared Bands (AIBs). Mid-IR emission peaks coincide with the main features of the ionized gas map. Fig. 2 shows the contour map in the ISOCAM LW2 filter (5.0-8.0 µm) which is dominated by the 6.2 and 7.7 µm AIB emission, superimposed on the [O III ] image of Fig. 1. CO(2-1) line observations made with the Schottky receiver of the SEST telescope at La Silla were also presented. These relatively low-sensitivity observations showed that N 66 does not contain much molecular gas, except for a small cloud to the NE of the bar.

[FIGURE] Fig. 2. Contour map in the ISOCAM LW2 filter (5.0-8.0 µm) which is dominated by the 6.2 and 7.7 µm AIB emission, superimposed on the [O III ] image. For contour levels and designation of the emission peaks, see Fig. 9. The emission of a number of red supergiants is visible, as well as emission by material close to blue stars or groups of stars (see Paper I). There is mid-IR emission associated with the different H II  regions, and with the absorption marks below the main H II  region. The mid-IR "spur" to the NE is due to emission by the surface of a molecular cloud (see Fig. 3) visible as an obscuration on the [O III ] image.

The present paper describes and discusses new observations of the region of N 66. The CVF observations of the central peak made with ISO are discussed in another paper (Contursi et al. 2000a), with emphasis on the silicate emission. Sect. 2 presents high-sensitivity CO(2-1) line observations made with the SIS receiver of the SEST, which reveal the presence of molecular gas associated with the H II  region. In Sect. 3, we present near-IR maps made in the v=(1-0) S(1) line of H2 and in the adjacent continuum at 2.14 µm. Sect. 4 contains a discussion and Sect. 5 the conclusions.

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© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000