2014A&A...570A...3S

Recent detection of a yellow supergiant star as a possible progenitor of a supernova has posed serious questions about our understanding of the evolution of massive stars. The spectroscopic binary star HD50975 with an unseen hot secondary was studied in detail with the main goal of estimating fundamental parameters of both components and the binary system. A comprehensive analysis and modeling of collected long-term radial velocity measurements, photometric data, and spectra was performed to calculate orbital elements, atmospheric parameters, abundances, and luminosities. The spectrum in an ultraviolet region was studied to clarify the nature of an unseen companion star. The orbital period was found to be 190.22±0.01-days. The primary star (hereafter HD50975A) is a yellow supergiant with an effective temperature T_eff=5900 ±150K and a surface gravity of log(g)=1.4±0.3(cgs). The atmosphere of HD50975 A is slightly metal deficient relative to solar, [Fe/H]=-0.26±0.06dex. Abundances of Si and Ca are close to the scaled solar composition. The r-process element europium is enhanced, [Eu/H]=+0.61±0.07. The bolometric magnitude of the primary was estimated to be M_bol=-5.5±0.3mag and its mass to be 10.7±2.0M_☉. The secondary (hereafter HD50975B) is a hot star of spectral type ∼B2 near ZAMS with an effective temperature of T_eff≃21000K and a mass M≃8.6M_☉. The distance between HD50975A and B is about 370R_☉. The binary star is near a semi-detached configuration with a radius, R_A≃107R_☉, and a radius of Roche lobe of about 120R_☉ for the primary star.