Mysterious, Variable, and Extremely Hot: White Dwarfs Showing Ultra-High Excitation Lines I. Photometric Variability

Abstract

About 10% of all stars exhibit absorption lines of ultra-high excited (UHE) metals (e.g. OVIII) in their optical spectra when entering the white dwarf cooling sequence. The recent discovery of a both spectroscopic and photometric variable UHE white dwarf led to the speculation that the UHE lines might be created in a shock-heated circumstellar magnetosphere. We investigate (multi-band) light curves from several ground- and space-based surveys of all 16 currently known UHE white dwarfs (including one newly discovered) and eight white dwarfs that show only the HeII line problem, as both phenomena are believed to be connected. We find that $75_{-13}^{+8}$% of the UHE white dwarfs, and $75_{-19}^{+9}$% of the HeII line problem white dwarfs are significantly photometrically variable, with periods ranging from 0.22d to 2.93d and amplitudes from a few tenth to a few hundredth mag. The high variability rate is in stark contrast to the variability rate amongst normal hot white dwarfs (we find $9_{-2}^{+4}$%), marking UHE and HeII line problem white dwarfs as a new class of variable stars. The period distribution of our sample agrees with both the orbital period distribution of post-common envelope binaries and the rotational period distribution of magnetic white dwarfs if we assume that the objects in our sample will spin-up as a consequence of further contraction. The lack of increasing photometric amplitudes towards longer wavelengths, as well as the non-detection of optical emission lines arising from the highly irradiated face of a hypothetical secondary in the optical spectra of our stars, makes it seem unlikely that an irradiated late type companion is the origin of the photometric variability. Instead, we believe that spots on the surfaces of these stars and/or geometrical effects of circumstellar material might be responsible. (abridged)

24 pages, accepted for publication in A&A

References in corpus (42)

• The Gaia mission
• The generalised Lomb-Scargle periodogram. A new formalism for the floating-mean and Keplerian periodograms
• The K2 Mission: Characterization and Early results
• Achieving better than 1 minute accuracy in the Heliocentric and Barycentric Julian Dates
• A Technique for Extracting Highly Precise Photometry for the Two-Wheeled Kepler Mission
• The Catalina Surveys Periodic Variable Star Catalog
• The Zwicky Transient Facility: Surveys and Scheduler
• Magnetic White Dwarfs
• SDSS Standard Star Catalog for Stripe 82: the Dawn of Industrial 1% Optical Photometry
• New white dwarf stars in the Sloan Digital Sky Survey Data Release 10
• EVEREST: Pixel Level Decorrelation of K2 Light curves
• White Dwarf Rotation as a Function of Mass and a Dichotomy of Mode Linewidths: Kepler Observations of 27 Pulsating DA White Dwarfs Through K2 Campaign 8
• Binary Planetary Nebulae Nuclei towards the Galactic Bulge. I. Sample Discovery, Period Distribution and Binary Fraction
• Axions and the cooling of white dwarf stars
• Revisiting the axion bounds from the Galactic white dwarf luminosity function
• Accretion of a giant planet onto a white dwarf
• White dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 14
• The chemical diversity of exo-terrestrial planetary debris around white dwarfs
• Identifying close binary central stars of PN with Kepler
• A deep test of radial differential rotation in a helium-atmosphere white dwarf: I. Discovery of pulsations in PG 0112+104
• Spectral analyses of eighteen hot H-deficient (pre-) white dwarfs from the Sloan Digital Sky Survey Data Release 4
• Cold giant planets evaporated by hot white dwarfs
• Binary Central Stars of Planetary Nebulae Discovered Through Photometric Variability V: The Central Stars of HaTr 7 and ESO 330-9
• Limits on the neutrino magnetic dipole moment from the luminosity function of hot white dwarfs
• Pulsating hydrogen-deficient white dwarfs and pre-white dwarfs observed with TESS: I. Asteroseismology of the GW Vir stars RX J2117+3412, HS 2324+3944, NGC 6905, NGC 1501, NGC 2371, and K 1-16
• A quantitative in-depth analysis of the prototype sdB+BD system SDSS J08205+0008 revisited in the Gaia era
• Ultracool white dwarfs and the age of the Galactic disc
• Analysis of cool DO-type white dwarfs from the Sloan Digital Sky Survey Data Release 10
• Spectral analysis of the extremely hot DA white dwarf PG0948+534
• Stellar laboratories. IX. New Se V, Sr IV - VII, Te VI, and I VI oscillator strengths and the Se, Sr, Te, and I abundances in the hot white dwarfs G191-B2B and RE 0503-289
• The vertical Na-O relation in the bulge globular cluster NGC 6553
• Stellar laboratories. VIII. New Zr IV - VII, Xe IV - V, and Xe VII oscillator strengths and the Al, Zr, and Xe abundances in the hot white dwarfs G191-B2B and RE0503-289
• HS1857+5144: A hot and young pre-cataclysmic variable
• SDSSJ212531.92-010745.9 - the first definite PG1159 close binary system
• The new pre-cataclysmic binary PG 2200+085
• A Plague of Magnetic Spots Among the Hot Stars of Globular Clusters
• First discovery of trans-iron elements in a DAO-type white dwarf (BD$-22{}^{\circ }3467$)
• The nature of the photometric variability of HgMn stars: A test of simulated light curves of ϕ Phe against the TESS data
• Spectral analyses of DO white dwarfs and PG1159 stars from the Sloan Digital Sky Survey
• Complete spectral energy distribution of the hot, helium-rich white dwarf RX J0503.9-2854
• Distorted surfaces of magnetic helium-peculiar stars: An application to a Cen
• Understanding the rotational variability of K2 targets. HgMn star KIC 250152017 and blue horizontal branch star KIC 249660366

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