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The eclipsing binary IT Librae is an unusual system of two B-type stars that is situated about 1 kpc above the Galactic plane. The binary was probably ejected from its birthplace in the disk, but the implied time of flight to its current location exceeds the evolutionary lifetime of the primary star. Here we present a study of new high-dispersion spectroscopy and an exquisite light curve from the Kepler K2 mission in order to determine the system properties and resolve the timescale discrepancy. We derive a revised spectroscopic orbit from radial-velocity measurements and determine the component effective temperatures through comparison of reconstructed and model spectra (T1 = 23.8 ± 1.8 kK, T2 = 13.7 ± 2.5 kK). We use the Eclipsing Light Curve code to model the K2 light curve, and from the inclination of the fit we derive the component masses (M1 = 9.6 ± 0.6 Me, M2 = 4.2 ± 0.2 Me) and mean radii (R1 = 6.06 ± 0.16 Re, R2 = 5.38 ± 0.14 Re). The secondary star is overluminous for its mass and appears to fill its Roche lobe. This indicates that IT Librae is a post-mass-transfer system in which the current secondary was the mass donor star. The current primary star was rejuvenated by mass accretion, and its evolutionary age corresponds to the time since the mass transfer stage. Consequently, the true age of the binary is larger than the ejection time of flight, thus resolving the timescale discrepancy. 

The eclipsing binary IT Librae is an unusual system of two B-type stars that is situated about 1 kpc above the Galactic plane. The binary was probably ejected from its birthplace in the disk, but the implied time of flight to its current location exceeds the evolutionary lifetime of the primary star. Here we present a study of new high-dispersion spectroscopy and an exquisite light curve from the Kepler K2 mission in order to determine the system properties and resolve the timescale discrepancy. We derive a revised spectroscopic orbit from radial-velocity measurements and determine the component effective temperatures through comparison of reconstructed and model spectra (T₁= 23.8 ± 1.8 kK,T₂= 13.7 ± 2.5 kK). We use the Eclipsing Light Curve code to model the K2 light curve, and from the inclination of the fit we derive the component masses (M₁= 9.6 ± 0.6M_⊙,M₂= 4.2 ± 0.2M_⊙) and mean radii (R₁= 6.06 ± 0.16R_⊙,R₂= 5.38 ± 0.14R_⊙). The secondary star is overluminous for its mass and appears to fill its Roche lobe. This indicates that IT Librae is a post-mass-transfer system in which the current secondary was the mass donor star. The current primary star was rejuvenated by mass accretion, and its evolutionary age corresponds to the time since the mass transfer stage. Consequently, the true age of the binary is larger than the ejection time of flight, thus resolving the timescale discrepancy.

 

HD 93521 is a massive, rapidly rotating star that is located about 1 kpc above the Galactic disk, and the evolutionary age for its estimated mass is much less than the time of flight if it was ejected from the disk. Here we present a reassessment of both the evolutionary and kinematical timescales for HD 93521. We calculate a time of flight of 39 ± 3 Myr based upon the distance and proper motions from Gaia EDR3 and a summary of radial velocity measurements. We then determine the stellar luminosity using a rotational model combined with the observed spectral energy distribution and distance. A comparison with evolutionary tracks for rotating stars from Brott et al. yields an evolutionary age of about 5 ± 2 Myr. We propose that the solution to the timescale discrepancy is that HD 93521 is a stellar merger product. It was probably ejected from the Galactic disk as a close binary system of lower-mass stars that eventually merged to create the rapidly rotating and single massive star we observe today.

 

Classical Be stars are possible products of close binary evolution, in which the mass donor becomes a hot, stripped O- or B-type subdwarf (sdO/sdB), and the mass gainer spins up and grows a disk to become a Be star. While several Be+sdO binaries have been identified, dynamical masses and other fundamental parameters are available only for a single Be+sdO system, limiting the confrontation with binary evolution models. In this work, we present direct interferometric detections of the sdO companions of three Be stars—28 Cyg, V2119 Cyg, and 60 Cyg—all of which were previously found in UV spectra. For two of the three Be+sdO systems, we present first orbits and preliminary dynamical masses of the components, revealing that one of them could be the first identified progenitor of a Be/X-ray binary with a neutron star companion. These results provide new sets of fundamental parameters that are crucially needed to establish the evolutionary status and origin of Be stars.