The open cluster Pismis 11 and the very luminous blue supergiant HD 80077 ^*,**

I. Physical parameters of the cluster

Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal. Escuela Politécnica Superior, University of Alicante, Apdo. 99, 03080 Alicante, Spain e-mail: tobarra@dfists.ua.es

Received: 5 May 2008
Accepted: 11 October 2008

Abstract

Context. The very luminous blue supergiant HD 80077 has been claimed to be a member of the young open cluster Pismis 11, and hence a hypergiant. Membership of the cluster would mean that it is one of the brightest stars in the Galaxy, and one of the few evolved very massive stars whose distance can be accurately determined.

Aims. We carry out a comprehensive study of the open cluster Pismis 11, which allows us to derive with accuracy its distance and reddening.

Methods. We obtained UBVRI photometry of the cluster field and low-resolution spectroscopy of a number of putative members. We derive spectral types from the spectra and determine that the reddening in this direction is standard. We then carry out a careful photometric analysis that allows us to determine individual reddening values, deriving unreddened parameters that are used for the main sequence fit.

Results. We identify 43 likely members of Pismis 11 and determine individual reddenings. We study the variation of extinction across the face of the cluster and find some dispersion, with a trend to higher values in the immediate neighbourhood of HD 80077. We estimate a distance of 3.6 kpc for the cluster. If HD 80077 is a member, it has M_bol < -10.5 and it is one of the three visually brightest stars in the Galaxy. Several early type stars in the vicinity of Pismis 11 fit well the cluster sequence and are likely to represent an extended population at the same distance. About 18′ to the North of Pismis 11, we find a small concentration of stars, which form a clear sequence. We identify this group as a previously uncatalogued open cluster, which we provisionally call Alicante 5. The distance to Alicante 5 is also 3.6 kpc, suggesting that these two clusters and neighbouring early-type stars form a small association.

Conclusions. We have identified a small association around Pismis 11, located at a distance of 3.6 kpc. Based on its proper motion, HD 80077 is not a runaway star and may be a member of the cluster. If this is the case, it would be one of the brightest stars in the Galaxy.