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

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 Teff = 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 (Teff = 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 vrot = 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⊙.

Towards a volumetric census of close white dwarf binaries – I. Reference samples

“…The identification of WD+AFGK binary candidates has been pioneered by Parsons et al (2016) and Rebassa-Mansergas et al (2017) who cross-matched AFGK-type stars spectroscopically confirmed by the RAVE and LAMOST surveys respectively, with the GALEX ultraviolet photometry. Parsons et al (2016) obtained HST spectroscopy of nine of these potential WD+AFGK binaries, all of which contained white dwarfs (see also Werner et al 2020). 4 Follow-up spectroscopy of TGAS-selected WD+AFGK candidates led to the confirmation of 23 PCEBs (Ren et al 2020, hereafter the "Ren sample").…”

Looking into the cradle of the grave: J22564–5910, a potential young post-merger hot subdwarf

“…Magnetically active (sub-)giants, for example, show chromospheric emission lines in Hα, but at the same time also in the cores of the Ca ii H and K lines, as well as sometimes other lines in the optical or ultraviolet (Wilson 1963(Wilson , 1968Gray & Corbally 2009). For some of these stars, time-variability in the chromospheric Hα emission, which is not correlated to the rotation period, has also been reported, though, the exact origin of the variability is not yet understood (e.g., Dorren et al 1984;Vida et al 2015;Kővári et al 2019;Werner et al 2020).…”

The Spatial and Emission Properties of the Large [O iii] Emission Nebula Near M31

“…Moreover, none of the central stars of the sample of large old PNe derived above reaches this luminosity-temperature domain in the Hertzprung-Russell diagram. We conclude that such a luminous white dwarf, even if masked by a main-sequence star in a binary system (e.g., UCAC2 46706450; Werner et al 2020), would not be missed at this low distance and the complete coverage of the region by GALEX.…”