2020A&A...642A.228W

The subgiant UCAC2 46706450 is a late-type star with an ultraviolet (UV) excess. It was considered as a candidate to establish a sample of stars of spectral type F, G, and K with white dwarf (WD) companions that could be used to test binary evolution models. To verify the WD nature of the companion, UV spectroscopy has previously been performed by other authors. Via a detailed model-atmosphere analysis, we show that the UV source is an extremely hot WD with an effective temperature of T_eff=105000±5000K, mass of M/M_☉=0.54±0.02, radius of R/R_☉=0.040_–0.004^+0.005, and luminosity of L/L_☉=176_–49⁺⁵⁵, meaning that the compact object is just about to enter the WD cooling sequence. Investigating spectra of the cool star (T_eff=4945±250K), we found that it is a K-type subgiant with M/M_☉=0.8-2.4, R/R_☉=5.9_–0.5^+0.7, and L/L_☉=19_–5⁺⁵ that is rapidly rotating with vsin(i)=81km/s. Optical light curves reveal a period of two days and an o-band peak-to-peak amplitude of 0.06mag. We suggest that it is caused by stellar rotation in connection with star spots. With the radius, we infer an extremely high rotational velocity of v_rot=151_–13⁺¹⁸km/s, thus marking the star as one of the most rapidly rotating subgiants known. This explains chromospheric activity observed by H α emission and emission-line cores in CaII H and K as well as NUV flux excess. From equal and constant radial velocities of the WD and the K subgiant as well as from a fit to the spectral energy distribution, we infer that they form a physical, wide (though unresolved) binary system. Both components exhibit similar metal abundances and show iron-group elements with slightly oversolar (up to 0.6dex) abundance, meaning that atomic diffusion in the WD atmosphere is not yet active due to a residual, weak radiation-driven wind. Kinematically and from its height above the Galactic plane, the system belongs to the Galactic thick disk, indicating that it is an old system and that the initial masses of both stars were close to 1M_☉.