The birth-cluster of the galactic luminous blue variable WRA 751

¹ Institute of Astronomy, ETH Hönggerberg, HPF, 8093 Zürich, Switzerland
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: pasquali@mpia.de
³ ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany e-mail: fcomeron@eso.org
⁴ STScI, 3700 San Martin Drive, MD28201 Baltimore, USA e-mail: nota@stsci.edu

Received: 3 August 2005
Accepted: 8 November 2005

Abstract

We present the results of NTT/VLT UBV imaging of a 260 arcmin² region containing the Galactic Luminous Blue Variable WRA 751, in search for its birth-cluster, i.e. a cluster of young and massive stars spatially and physically associated with it. On the basis of the classical reddening-free parameter Q, we have identified a sample of 24 early-type stars with colours typical of spectral types earlier than B3. Interestingly, these stars are clustered within a radius of 1' from WRA 751, corresponding to about 1% of the imaged field. These stars tightly distribute around , which in turn defines a mean extinction  mag. The 5 brighter () and bluer () stars of the sample have been subsequently observed with FORS1 and classified as 3 late O- and 2 early B-stars. The absence of stars earlier than O8 indicates an age of the cluster older than 4 Myr, although it could be due to an incomplete sampling of the upper end of the main sequence. Nevertheless, the detection of OB stars of class I certainly indicates an age of a few million years. At an assumed distance of 6 kpc, we estimate a cluster radius of 3.4 pc and a total mass of ³ . Our discovery is only the second known instance of a Galactic Luminous Blue Variable associated with its birth-cluster.