The Multiwavelength Picture of Star Formation in the Very Young Open Cluster NGC 6383

We review the properties of the very young (∼ 2 Myr) open cluster NGC 6383. The cluster is dominated by the massive binary HD 159176 (O7 V + O7 V). The distance to NGC 6383 is consistently found to be 1.3 ± 0.1 kpc and the average reddening is determined to be E(B−V ) = 0.32 ± 0.02. Several pre-main sequence candidates have been identified using different criteria relying on the detection of emission lines, infrared excesses, photometric variability and X-ray emission. 1. NGC 6383 and its Surroundings NGC 6383 (α2000 = 173448, δ2000 = −3234.0; lII = 355.69, bII = +0.04) is a rather compact open cluster which could be part of the Sgr OB1 association together with NGC 6530 and NGC 6531. The cluster was originally discovered by John Herschel in 1834, and listed as h 3689 in Herschel’s Cape Catalogue published in 1847. Most probably due to a clerical error, it also appears in Dreyer’s New General Catalogue as NGC 6374, an identifier that is however not commonly used. Trumpler (1930) pointed out that NGC 6383 belongs to a category of clusters in which a few dozen faint stars are closely grouped around a bright hot central star that frequently happens to be a binary system. In the case of NGC 6383, the cluster is centered on the O-type binary HD 159176 (mV = 5.7, see Fig. 1) that dominates the emission from the cluster over a broad range of energies from the near-infrared to the X-ray domain. Apart from HD 159176, the cluster currently harbours no stars earlier than B1, although de Wit et al. (2005) suggested that the eclipsing binary candidate HD 158186 (O9.5 V, Marchenko et al. 1998) might have been ejected from NGC 6383 through dynamical interactions in the cluster core. NGC 6383 and more specifically HD 159176 are likely responsible for the ionization of the H II region RCW 132 (Rodgers, Campbell & Whiteoak 1960) also known as S 11 (Sharpless 1953)1 or Stromlo 67 (Gum 1955). Rodgers et al. (1960) described RCW 132 as a 110 arcmin × 80 arcmin medium brightness crescent-shaped region. Images of this emission nebula can be found for instance on plate 131 of Lyngå & Hansson (1972). During a survey of the galactic ridge at 1390 MHz, Westerhout (1958) detected a large (1.5 × 1.5) ring-like radio structure roughly centered on NGC 6383. However, Note that the S 11 identifier is not to be confused with the number of the nebula in the revised catalogue of H II regions published by Sharpless (1959). In the latter catalogue, the nebula corresponds to entry number 12 (in the SIMBAD database, this identifier is referred to as Sh 2-012).