The Birth-Cluster of the Galactic Luminous Blue Variable WRA751

TL;DR

This study identifies a young star cluster associated with the Galactic Luminous Blue Variable WRA751, providing insights into its age, mass, and environment, and marking only the second such known association.

Contribution

The paper reports the discovery and characterization of the birth-cluster of WRA751, a rare association of a Galactic Luminous Blue Variable with its natal cluster.

Findings

Cluster contains 24 early-type stars within 1 arcmin of WRA751.

Estimated cluster age is older than 4 million years.

Cluster distance is approximately 6 kpc with a total mass of 2200 solar masses.

Abstract

We present the results of NTT/VLT UBV imaging of a 260 square arcmin region containing the Galactic Luminous Blue Variable WRA751, 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 arcmin from WRA751, corresponding to about 1% of the imaged field. These stars tightly distribute around (B-V) = 1.67, which in turn defines a mean extinction A(V) = 6.1 mag. The 5 brighter (V > 16.2) and bluer (Q < -0.9) 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 2200 solar masses. Our discovery is only the second known instance of a Galactic Luminous Blue Variable associated with its birth-cluster.