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)

arXiv comment24 pages, accepted for publication in A&A

References in corpus (97)

• Measuring Reddening with SDSS Stellar Spectra and Recalibrating SFD
• The Gaia mission
• The Seventh Data Release of the Sloan Digital Sky Survey
• The generalised Lomb-Scargle periodogram. A new formalism for the floating-mean and Keplerian periodograms
• The K2 Mission: Characterization and Early results
• First Results from the Catalina Real-time Transient Survey
• ATLAS: A High-Cadence All-Sky Survey System
• Gaia Early Data Release 3: Parallax bias versus magnitude, colour, and position
• 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
• A $Gaia$ Data Release 2 catalogue of white dwarfs and a comparison with SDSS
• Binarity in Brown Dwarfs: T Dwarf Binaries Discovered with the Hubble Space Telescope WPFC2
• The Zwicky Transient Facility: Surveys and Scheduler
• Magnetic White Dwarfs
• A First Catalog of Variable Stars Measured by the Asteroid Terrestrial-impact Last Alert System (ATLAS)
• A Spectroscopic Survey & Analysis of Bright, Hydrogen-Rich White Dwarfs
• SDSS Standard Star Catalog for Stripe 82: the Dawn of Industrial 1% Optical Photometry
• The Element Abundances in Bare Planetary Nebula Central Stars and the Shell Burning in AGB Stars
• Planetary nebulae seen with TESS: Discovery of new binary central star candidates from Cycle 1
• New white dwarf stars in the Sloan Digital Sky Survey Data Release 10
• New cooling sequences for old white dwarfs
• EVEREST: Pixel Level Decorrelation of K2 Light curves
• Know Your Neighborhood: A Detailed Model Atmosphere Analysis of Nearby White Dwarfs
• 3D maps of interstellar dust in the Local Arm: using $Gaia$, 2MASS and APOGEE-DR14
• Pulsating white dwarfs: new insights
• 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
• The True Incidence of Magnetism among Field White Dwarfs
• Revisiting the axion bounds from the Galactic white dwarf luminosity function
• Magnetic White Dwarf Stars in the Sloan Digital Sky Survey
• Accretion of a giant planet onto a white dwarf
• Binary stars as the key to understanding planetary nebulae
• DA white dwarfs in SDSS DR7 and a search for infrared excess emission
• Physical properties of IP Pegasi: an eclipsing dwarf nova with an unusually cool white dwarf
• Precise mass and radius values for the white dwarf and low mass M dwarf in the pre-cataclysmic binary NN Serpentis
• Post-common envelope binaries from SDSS - VII: A catalogue of white dwarf-main sequence binaries
• White dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 14
• Post common envelope binaries from SDSS. XII: The orbital period distribution
• An update to the EVEREST K2 pipeline: Short cadence, saturated stars, and Kepler-like photometry down to Kp = 15
• Detection of a period decrease in NN Ser with ULTRACAM: evidence for strong magnetic braking or an unseen companion?
• The chemical diversity of exo-terrestrial planetary debris around white dwarfs
• The population of hot subdwarf stars studied with Gaia II. The Gaia DR2 catalogue of hot subluminous stars
• Fundamental parameter accuracy of DA and DB white dwarfs in Gaia Data Release 2
• The Age of the Milky Way Inner Halo
• Binary central stars of PN discovered through photometric variability. I. What we know and what we would like to find out
• Population of hot subdwarf stars studied with Gaia III. Catalogue of known hot subdwarf stars: Data Release 2
• New evolutionary sequences for hot H-deficient white dwarfs on the basis of a full account of progenitor evolution
• Limits on the dependence of the fine-structure constant on gravitational potential from white-dwarf spectra
• Identifying close binary central stars of PN with Kepler
• The White Dwarfs within 25 Parsecs of the Sun: Kinematics and Spectroscopic Subtypes
• The Age of the Galactic Stellar Halo from Gaia White Dwarfs
• A study of high proper-motion white dwarfs I. Spectropolarimetry of a cool hydrogen-rich sample
• A deep test of radial differential rotation in a helium-atmosphere white dwarf: I. Discovery of pulsations in PG 0112+104
• Periodic Variability of Low-Mass Stars in SDSS Stripe 82
• 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
• Two-colour photometry of the binary planetary nebula nuclei UU Sagitte and V477 Lyrae: oversized secondaries in post-common-envelope binaries
• 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
• Stellar laboratories III. New Ba V, Ba VI, and Ba VII oscillator strengths and the barium abundance in the hot white dwarfs G191-B2B and RE0503-289
• Stellar laboratories IV. New Ga IV, Ga V, and Ga VI oscillator strengths and the gallium abundance in the hot white dwarfs G191-B2B and RE0503-289
• SALT HRS discovery of the binary nucleus of the Etched Hourglass Nebula MyCn 18
• 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
• Stellar laboratories. VI. New Mo IV - VII oscillator strengths and the molybdenum abundance in the hot white dwarfs G191-B2B and RE0503-289
• Spectral analysis of the extremely hot DA white dwarf PG0948+534
• Probing the Gravitational Dependence of the Fine-Structure Constant from Observations of White Dwarf Stars
• Stellar laboratories: new Ge V and Ge VI oscillator strengths and their validation in the hot white dwarf RE 0503-289
• 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
• Modelling of variability of the chemically peculiar star phi Draconis
• 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
• Metal abundances in hot white dwarfs with signatures of a superionized wind
• The vertical Na-O relation in the bulge globular cluster NGC 6553
• SALT HRS discovery of a long period double-degenerate binary in the planetary nebula NGC 1360
• HS1857+5144: A hot and young pre-cataclysmic variable
• Stellar laboratories II. New Zn IV and Zn V oscillator strengths and their validation in the hot white dwarfs G191-B2B and RE0503-289
• Unravelling the baffling mystery of the ultrahot wind phenomenon in white dwarfs
• SDSSJ212531.92-010745.9 - the first definite PG1159 close binary system
• The new pre-cataclysmic binary PG 2200+085
• Three-dimensional abundance distributions in ApBp star atmospheres: non-axisymmetric magnetic geometry
• 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$-22{}^{\circ }3467$)
• Stellar laboratories. V. The Xe VI ultraviolet spectrum and the xenon abundance in the hot DO-type white dwarf RE0503-289
• On the post-common-envelope central star of the planetary nebula NGC 2346
• Identification of a DO White Dwarf and a PG1159 Star in the ESO SN Ia Progenitor Survey (SPY)
• Search for trans-iron elements in hot, helium-rich white dwarfs with the HST Cosmic Origins Spectrograph
• 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
• The hot white dwarf in the peculiar binary nucleus of the planetary nebula EGB6
• 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
• Discovery of two bright DO-type white dwarfs
• Understanding the rotational variability of K2 targets. HgMn star KIC 250152017 and blue horizontal branch star KIC 249660366

Cited by in corpus (9)

• The bright blue side of the night sky: Spectroscopic survey of bright and hot (pre-) white dwarfs
• The Rapid Rotation of the Strongly Magnetic Ultramassive White Dwarf EGGR 156
• Measuring White Dwarf Variability from Sparsely Sampled Gaia DR3 Multi-Epoch Photometry
• The unusual planetary nebula nucleus in the Galactic open cluster M37 and six further hot white dwarf candidates
• Pulsating hydrogen-deficient white dwarfs and pre-white dwarfs observed with TESS -- IV. Discovery of two new GW Vir stars: TIC0403800675 and TIC1989122424
• Interacting planetary nebulae III: Verification and galactic population based on the measurements of Gaia EDR3
• How an overweight and rapidly rotating PG 1159 star in the Galactic halo challenges evolutionary models
• Ancient 'ghost' planetary nebulae discovered with amateur telescopes
• Hot Degenerates in the MCT Survey. III. A Sample of White Dwarf Stars in the Southern Hemisphere