An extremely hot white dwarf with a rapidly rotating K-type subgiant companion: UCAC2 46706450

¹ Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Eberhard Karls Universität, Sand 1, 72076 Tübingen, Germany
e-mail: werner@astro.uni-tuebingen.de
² Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Straße 24/25, Germany
³ Departament de Física, Universitat Politècnica de Catalunya, c/Esteve Terrades 5, 08860 Castelldefels, Spain
⁴ Institut d’Estudis Espacials de Catalunya, Ed. Nexus-201, c/Gran Capità 2-4, 08034 Barcelona, Spain
⁵ National Astronomical Research Institute of Thailand, Sirindhorn AstroPark, Donkaew, Mae Rim, Chiang Mai 50180, Thailand
⁶ Key Laboratory of Space Astronomy and Technology, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China

Received: 3 June 2020
Accepted: 5 September 2020

Abstract

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 = 105 000 ± 5000 K, 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 ± 250 K), 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) = 81 km s⁻¹. Optical light curves reveal a period of two days and an o-band peak-to-peak amplitude of 0.06 mag. 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⁺¹⁸ km s⁻¹, 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 Ca II 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 0.6 dex) 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 1 M_⊙.