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Astron. Astrophys. 322, 554-564 (1997)

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3. High-resolution images

There are several indications that massive stars form in groups (see, e.g. Lada et al. 1991 , Larson 1992 , Hodapp 1994 ). This is in line with recent high-resolution ground-based results, namely the decomposition of the so-called Magellanic supermassive stars into tight clusters of massive components (see Heydari-Malayeri 1996 for a review), as well as HST observations (e.g. Walborn et al. 1995 ). HD 5980 is a binary and a third component was suspected to lie at [FORMULA] from the main star (Massey et al. 1989 ). It is therefore necessary to see to what extent we can resolve this star and to check the presence of possible line-of-sight components that may contribute to the light of HD 5980.

A sub-arcsecond image of HD 5980 in the R band taken with SUSI at NTT is presented in Fig. 1. A resolution of 0 [FORMULA].65 FWHM makes it, to our knowledge, the best quality optical image ever published of the HD 5980 field. The closest star to HD 5980 visible on this image, lying at [FORMULA] 4 [FORMULA].1 SE, has a V mag of [FORMULA] 19.1. Also, the closest fainter star is a V [FORMULA] 20.5 mag situated at [FORMULA] 4 [FORMULA].4 SE. We did not try to resolve HD 5980 on these SUSI observations because of the presence of saturated pixels. An attempt to resolve HD 5980 was made using our adaptive optics images which allow us to put constraints on the presence of stellar components and extended structure around HD 5980. There are no stellar components down to 6.7 mag fainter than HD 5980 in K at a separation of 1 [FORMULA].0 and S/N = 3. For a separation of 0 [FORMULA].3 this upper bound is 4.1 mag fainter than HD 5980. These are in line with the results of HST observations reported by Koenigsberger et al. (1995 ). Furthermore, the observations of January 1996 would have detected a structure 3.9 mag arcsec-2 fainter than HD 5980 at 1 [FORMULA].0 and the 3 [FORMULA] level.

[FIGURE] Fig. 1. An R image of HD 5980 obtained using NTT+SUSI. Resolution 0 [FORMULA].65 FWHM without applying any restoration technique. Field [FORMULA] 50 [FORMULA] [FORMULA] 50 [FORMULA]. Exposure time 10 s. North is at the top and east to the left. The arrow indicates the closest star to HD 5980, visible on our images, at [FORMULA] 4 [FORMULA].1 SE and V [FORMULA] 19.1 mag.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998