Open clusters: III. Fundamental parameters of B stars in NGC 6087, NGC 6250, NGC 6383 and NGC 6530. B type stars with circumstellar envelopes.

Abstract

Context. Stellar physical properties of star clusters are poorly known and the cluster parameters are often quite uncertain.Aims. Our goals are to perform a spectrophotometric study of the B star population in open clusters to derive accurate stellar parameters, search for the presence of circumstellar envelopes and discuss the characteristics of these stars.Methods. The BCD spectrophotometric system is a powerful method to obtain stellar fundamental parameters from direct measurements of the Balmer discontinuity. To this end, we wrote the interactive code MIDE3700. The BCD parameters can also be used to infer the main properties of open clusters: distance modulus, color excess and age. Furthermore, we inspect the aspect of the Balmer discontinuity to put in evidence the presence of circumstellar disks and identify Be-star candidates. An additional set of high resolution spectra, in the Hα region, is used to confirm the Be nature of these stars.Results. In this work we provide Teff , log g, Mv , Mbol and spectral types for a sample of 68 stars in the field of the open clustersNGC 6087, NGC 6250, NGC 6383, and NGC 6530, as well as the cluster distances, ages and reddening. Then, based on a sample of 230 B stars in the direction of the eleven open clusters studied along these three series of papers, we report new six Be stars, four blue straggler candidates, and fifteen B type stars (called Bdd) with a double Balmer discontinuity which indicates the presence of circumstellar envelopes. We discuss the distribution of the fraction of B, Be and Bdd star cluster members per spectral subtype. The majority of the Be stars are dwarfs and present a maximum at the spectral type B2-B4 in young and intermediate-age open clusters (< 40 Myr). Another maximum of Be stars is observed at the spectral type B6-B8 in open clusters older than 40 Myr, where thepopulation of Bdd stars also becomes relevant. The Bdd stars seem to be in a ?passive? emission phase.Conclusions. Our results support previous statements that the Be phenomenon is present along the whole main sequence band and occurs in very different evolutionary states. We find clear evidence of an enhance of stars with circumstellar envelopes with cluster age. The Be phenomenon reaches it maximum in clusters of intermediate-age (10 − 40 Myr) and the number of B stars with circumstellar envelopes (Be plus Bdd stars) keeps also high for the older clusters (40 − 100 Myr).