HD 50975: a yellow supergiant in a spectroscopic binary system^⋆

¹ Vilnius University Observatory, Ciurlionio 29, Vilnius 03100, Lithuania
e-mail: zacs@latnet.lv
² Laser Center, University of Latvia, Raiņa bulvāris 19, 1586 Rīga, Latvia
³ Institute of Astronomy of the Russian AS, 48 Pyatnitskaya st., 119017 Moscow, Russia
⁴ Terskol Branch of Institute of Astronomy of the Russian AS, 361605 Peak Terskol, Kabardino-Balkaria, Russia
⁵ Special Astrophysical Observatory of the Russian AS, 369167 Nizhnij Arkhyz, Russia

Received: 21 October 2013
Accepted: 4 August 2014

Abstract

Context. 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.

Aims. The spectroscopic binary star HD 50975 with an unseen hot secondary was studied in detail with the main goal of estimating fundamental parameters of both components and the binary system.

Methods. 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.

Results. The orbital period was found to be 190.22 ± 0.01 days. The primary star (hereafter HD 50975A) is a yellow supergiant with an effective temperature T_eff = 5900 ± 150 K and a surface gravity of log (g) = 1.4 ± 0.3 (cgs). The atmosphere of HD 50975 A is slightly metal deficient relative to solar, [Fe/H] = −0.26 ± 0.06 dex. 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 ± 0.3 mag and its mass to be 10.7 ± 2.0 M_⊙. The secondary (hereafter HD 50975B) is a hot star of spectral type ~B2 near ZAMS with an effective temperature of T_eff ≃ 21 000 K and a mass M ≃ 8.6 M_⊙. The distance between HD 50975A and B is about 370 R_⊙. The binary star is near a semi-detached configuration with a radius, R_A ≃ 107 R_⊙, and a radius of Roche lobe of about 120 R_⊙ for the primary star.