Open clusters

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 T eff , log g, M v , M bol and spectral types for a sample of 68 stars in the field of the open clusters NGC 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 the population 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).

Centrifugally driven mass-loss and outbursts of massive stars

“…Our work suggests that some Be stars can be produced as a result of core contraction and surface rotational acceleration due to efficient AM transport. This single star scenario, proposed by Maeder & Meynet (2001), also explains the higher abundance of Be stars in slightly aged populations (older than 10 Myr) and stars near the TAMS (Keller, Bessell & Da Costa 2000;Aidelman et al 2018), because more envelope spin-up occurs as stars age and evolve toward the TAMS. Additionally, the larger abundance of Be stars in low-metallicity environments (Peters et al 2019 and references therein) is naturally explained by the smaller mass/AM loss through winds (rather than having to appeal to larger rotation rates at birth), allowing more stars to reach their breakup rotation rate.…”

Be and Bn stars: Balmer discontinuity and stellar-class relationship

“…The rapid rotation that characterizes Bn stars, the occasional appearance of some emission in the Balmer lines (at least in Hα), and the presence of signatures for some sBD are motivate us to think that Bn and Be probably belong to the same class of objects. Some attempts to show a possible correlation between fastrotating late B-type stars and Be stars based on the presence of a sBD were discussed by Aidelman et al (2018) in open clusters.…”

“…Contrarily, in the diagrams where the parameters were corrected for rotational effects, we see that Bn stars can have age ratios 0.2 (t/t MS ) Bn 0.8 if M/M 4, whereas they have 0.0 (t/t MS ) Bn 0.2 for masses M/M 5. According to Aidelman et al (2018), the Be phenomenon is observed along the whole MS, and appears on average at an earlier age in massive stars than in the less massive stars. Thus, the possibility that the most massive Bn stars display the Be phenomenon at any time should not be excluded.…”

Astrometric and photometric study of Dias 4, Dias 6, and other five open clusters using ground-based and Gaia DR2 data

“…There is only one study based on CCD photometry with UBVRI data performed by Sagar & Cannon (1997). We refer the reader to the very recent study of Aidelman et al (2018) for a complete compilation of the parameters of the cluster determined by different authors, using different data and techniques.…”