ANÁLISE DE ESTRELAS B DO AGLOMERADO ABERTO NGC 3766

Abstract

The existence of multiple components of the main sequence has been verified in globular clusters and, surprisingly, in open clusters with ages between ~ 15 M yr and ~ 600 M yr (LI et al., 2019). This observational results challenge the old conceptions that the main sequence of a young cluster is formed by only one stellar population the same chemical composition. Recent studies have shown that open clusters may present splits in the main sequence (MS), with rapidly rotating stars forming a redder MS while slowly rotating stars are mainly on the blue component of the MS. This dissertation is based on a detailed spectroscopic analysis of B-type massive stars of the open cluster NGC 3766, which is known for presenting a high ratio of Be/B stars (MCSWAIN et al., 2009). We estimate the effective temperature (Teff ), surface gravity (log g), the projected rotational velocity (v seni), microturbulence velocity (ξ), and macroturbulence velocity(ζ), as well as abundances of silicon, oxygen, carbon, and nitrogen, adopting a semi-automatic method of non-ETL spectral synthesis. The observational data used in this analysis are high resolution spectra obtained with Flames/Giraffe coupled to the VLT UT2 telescope, Chile, in the context of the Gaia-Eso Survey (GES). The sample of B stars in NGC 3766 have effective temperatures between 18000 K and 23400 K, log g between 3.40 and 4.00, corresponding to stars with masses between 7 and 12 M⊙. The average abundances obtained for the open cluster NGC 3766 – A(Si) = 7.53±0.06, A(C) = 8.50±0.04, A(N) = 7.86±0.06, and A(O)=8.77±0.05 – are consistent with the solar abundances. We tested the hypothesis of nitrogen enrichment induced by stellar rotation: Star NGC 3766 5 has a discrepant N/O ratio relative to the full sample, but it has low v sini (less than 100km/s), which does not allow us to conclude if the high N abundance obtained may have been the result of mixture induced by rotation. The values of v seni spam between 5 km/s and 322 km/s, with an average value of 231±54 km/s for the Be stars or Be candidates, and 119±74 km/s for normal B stars. The distribution of v seni in the CMD of NGC 3766 suggests that the MS may present two components due to rotation.