Astron. Astrophys. 361, 877-887 (2000)

1. Introduction

The giant H II region N 11 (Henize 1956) or DEM 34 (Davies et al. 1976) is the second most important H emission complex in the Large Magellanic Cloud (LMC) after the famous 30 Dor (see e.g. Rosado et al. 1996and references therein). Interestingly, this region has been suggested to be reminiscent of an evolved, some 2 10⁶ years older version of the 30 Dor starburst (Walborn & Parker 1992). The H II region N11 C (NGC 1769), lying at the eastern periphery of the bubble created by the central association LH 9 (Walborn & Parker 1992), is one of the brightest and youngest nebular components of the N 11 complex. N11 C was studied by Heydari-Malayeri et al. (1987, hereafter Paper I) regarding its physical properties (gas density, excitation, chemical abundances, extinction, etc.) as well as its stellar content. They determined the spectral types for 9 stars and gave B and V photometry for 57 stars in the region. These observations also revealed the presence of an anonymous compact cluster (hereafter labelled HNT) south-west of N 11C (Fig. 1). Moreover, they identified Wo 599, an O3-O4 V star, as the main ionizing source of N11 C, instead of the central object Sk-66^o41 (HDE 268743) considered previously to be one of the most massive stars with a mass well over 120 (Humphreys 1983).

Fig. 1. The LMC H II region N 11C as seen through the Strömgren b filter. The image is based on co-adding 7 dithered individual exposures. The main objects are labelled. The field size is 116" 127", corresponding to 29 32 pc. North is up and east is left.

Subsequently, on the basis of sharp images obtained in good seeing conditions and using high-resolution CCDs assisted by advanced image restoration methods, Heydari-Malayeri et al. (1988) showed that Sk-66^o41 is actually a star cluster made up of at least six components, the main star having a ZAMS mass of 90. More recently, adaptive optics observations at the ESO 3.6 m telescope resolved Sk-66^o41 into a tight cluster of at least 12 components, the brightest component corresponding to a ZAMS mass of 50 (Heydari-Malayeri & Beuzit 1994, hereafter Paper II).

The present paper is devoted to the stellar content of N11 C, which constitutes the central part of the OB association LH 13 (Lucke & Hodge 1970). We particularly focus on the two tight star clusters Sk-66^o41 and HNT. Although adaptive optics observations have resolved Sk-66^o41 (Paper II), we need colors, especially in the visible, for studying the properties of the individual components. With regard to HNT, no studies have previously been devoted to this cluster.

© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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