Anna F. Pala

Doppler imaging of the planetary debris disc at the white dwarf SDSS J122859.93+104032.9

Debris discs which orbit white dwarfs are signatures of remnant planetary systems. We present 12 yr of optical spectroscopy of the metal-polluted white dwarf SDSS J1228+1040, which shows a steady variation in the morphology of the 8600 A Ca II triplet line profiles from the gaseous component of its debris disc. We identify additional emission lines of O I, Mg I, Mg II, Fe II and Ca II in the deep co-added spectra. These emission features (including Ca H & K) exhibit a wide range in strength and morphology with respect to each other and to the Ca II triplet, indicating different intensity distributions of these ionic species within the disc. Using Doppler tomography, we show that the evolution of the Ca II triplet profile can be interpreted as the precession of a fixed emission pattern with a period in the range 24–30 yr. The Ca II line profiles vary on time-scales that are broadly consistent with general relativistic precession of the debris disc.

A radio-pulsing white dwarf binary star.

White dwarfs are compact stars, similar in size to Earth but approximately 200,000 times more massive. Isolated white dwarfs emit most of their power from ultraviolet to near-infrared wavelengths, but when in close orbits with less dense stars, white dwarfs can strip material from their companions and the resulting mass transfer can generate atomic line and X-ray emission, as well as near- and mid-infrared radiation if the white dwarf is magnetic. However, even in binaries, white dwarfs are rarely detected at far-infrared or radio frequencies. Here we report the discovery of a white dwarf/cool star binary that emits from X-ray to radio wavelengths. The star, AR Scorpii (henceforth AR Sco), was classified in the early 1970s as a δ-Scuti star, a common variety of periodic variable star. Our observations reveal instead a 3.56-hour period close binary, pulsing in brightness on a period of 1.97 minutes. The pulses are so intense that AR Sco’s optical flux can increase by a factor of four within 30 seconds, and they are also detectable at radio frequencies. They reflect the spin of a magnetic white dwarf, which we find to be slowing down on a 107-year timescale. The spin-down power is an order of magnitude larger than that seen in electromagnetic radiation, which, together with an absence of obvious signs of accretion, suggests that AR Sco is primarily spin-powered. Although the pulsations are driven by the white dwarf’s spin, they mainly originate from the cool star. AR Sco’s broadband spectrum is characteristic of synchrotron radiation, requiring relativistic electrons. These must either originate from near the white dwarf or be generated in situ at the M star through direct interaction with the white dwarf’s magnetosphere.

Trace hydrogen in helium atmosphere white dwarfs as a possible signature of water accretion

A handful of white dwarfs with helium-dominated atmospheres contain exceptionally large masses of hydrogen in their convection zones, with the metal-polluted white dwarf GD 16 being one of the earliest recognized examples. We report the discovery of a similar star: the white dwarf coincidentally named GD 17. We obtained medium-resolution spectroscopy of both GD 16 and GD 17 and calculated abundances and accretion rates of photospheric H, Mg, Ca, Ti, Fe and Ni. The metal abundance ratios indicate that the two stars recently accreted debris, which is Mg-poor compared to the composition of bulk Earth. However, unlike the metal pollutants, H never diffuses out of the atmosphere of white dwarfs and we propose that the exceptionally high atmospheric H content of GD 16 and GD 17 (2.2 × 1024 and 2.9 × 1024 g, respectively) could result from previous accretion of water bearing planetesimals. Comparing the detection of trace H and metal pollution among 729 helium atmosphere white dwarfs, we find that the presence of H is nearly twice as common in metal-polluted white dwarfs compared to their metal-free counterparts. This highly significant correlation indicates that, over the cooling age of the white dwarfs, at least some fraction of the H detected in many He atmospheres (including GD 16 and GD 17) is accreted alongside metal pollutants, where the most plausible source is water. In this scenario, water must be common in systems with rocky planetesimals.

Effective temperatures of cataclysmic variable white dwarfs as a probe of their evolution

NASA [NAS 5-26555]; European Research Council [320964]; Fondecyt [1141269]; STFC [ST/L000733]; Robert Martin Ayers Sciences Fund; [GO-9357]; [GO9724]; [GO-12870]; [GO-13807]; [ASI-INAF I/037/12/0]

The age–metallicity relation in the solar neighbourhood from a pilot sample of white dwarf–main sequence binaries

The age–metallicity relation (AMR) is a fundamental observational constraint for understanding how the Galactic disc formed and evolved chemically in time. However, there is not yet an agreement on the observational properties of the AMR for the solar neighbourhood, primarily due to the difficulty in obtaining accurate stellar ages for individual field stars. We have started an observational campaign for providing the much needed observational input by using wide white-dwarf–main-sequence (WDMS) binaries. White dwarfs are ‘natural’ clocks and can be used to derive accurate ages. Metallicities can be obtained from the main-sequence companions. Since the progenitors of white dwarfs and the main-sequence stars were born at the same time, WDMS binaries provide a unique opportunity to observationally constrain in a robust way the properties of the AMR. In this work we present the AMR derived from analysing a pilot sample of 23 WDMS binaries and provide clear observational evidence for the lack of correlation between age and metallicity at young and intermediate ages (0–7 Gyr).

The first pre-supersoft X-ray binary

We report the discovery of an extremely close white dwarf plus F dwarf main-sequence star in a 12 h binary identified by combining data from the Radial Velocity Experiment survey and the Galaxy Evolution Explorer survey. A combination of spectral energy distribution fitting and optical and Hubble Space Telescope ultraviolet spectroscopy allowed us to place fairly precise constraints on the physical parameters of the binary. The system, TYC 6760-497-1, consists of a hot Teff ∼ 20 000 K, MWD∼0.6M⊙MWD∼0.6M⊙ white dwarf and an F8 star (MMS∼1.23M⊙MMS∼1.23M⊙, RMS∼1.3R⊙RMS∼1.3R⊙) seen at a low inclination (i ∼ 37°). The system is likely the descendant of a binary that contained the F star and an ∼2 M⊙ A-type star that filled its Roche lobe on the thermally pulsating asymptotic giant branch, initiating a common envelope phase. The F star is extremely close to Roche lobe filling and there is likely to be a short phase of thermal time-scale mass transfer on to the white dwarf during which stable hydrogen burning occurs. During this phase, it will grow in mass by up to 20 per cent, until the mass ratio reaches close to unity, at which point it will appear as a standard cataclysmic variable star. Therefore, TYC 6760-497-1 is the first known progenitor of a supersoft source system, but will not undergo a Type Ia supernova explosion. Once an accurate distance to the system is determined by Gaia, we will be able to place very tight constraints on the stellar and binary parameters.

Multiband photometry and spectroscopy of an all-sky sample of bright white dwarfs

The upcoming NASA Transiting Exoplanet Survey Satellite (TESS) will obtain space-based uninterrupted light curves for a large sample of bright white dwarfs distributed across the entire sky, providing a very rich resource for asteroseismological studies and the search for transits from planetary debris. We have compiled an all-sky catalogue of ultraviolet, optical and infrared photometry as well as proper motions, which we propose as an essential tool for the preliminary identification and characterization of potential targets. We present data for 1864 known white dwarfs and 305 high-probability white dwarf candidates brighter than 17 mag. We describe the spectroscopic follow-up of 135 stars, of which 82 are white dwarfs and 25 are hot subdwarfs. The new confirmed stars include six pulsating white dwarf candidates (ZZ Cetis), and nine white dwarf binaries with a cool main-sequence companion. We identify one star with a spectroscopic distance of only 25 pc from the Sun. Around the time TESS is launched, we foresee that all white dwarfs in this sample will have trigonometric parallaxes measured by the ESA Gaia mission next year.

A catalogue of white dwarf candidates in VST ATLAS.

The Sloan Digital Sky Survey (SDSS) has created a knowledge gap between the Northern and the Southern hemispheres, which is very marked for white dwarfs: Only ≃15 per cent of the known white dwarfs are south of the equator. Here, we make use of the VLT Survey Telescope (VST) ATLAS survey, one of the first surveys obtaining deep, optical, multiband photometry over a large area of the southern skies, to remedy this situation. Applying the colour and proper-motion selection developed in our previous work on SDSS to the most recent internal data release (2016 April 25) of VST ATLAS, we created a catalogue of ≃4200 moderately bright (g ≤ 19), high-confidence southern white dwarf candidates, which can be followed up individually with both the large array of southern telescopes or in bulk with ESOs forthcoming multi-object spectrograph 4MOST.

The cataclysmic variable QZ Lib: a period bouncer

While highly evolved cataclysmic variables (CVs) with brown dwarf donors, often called “period bouncers”, are predicted to make up ≃ 40 − 70% of the Galactic CV population, only a handful of such systems are currently known. The identification and characterization of additional period bouncers is therefore important to probe this poorly understood phase of CV evolution. We investigate the evolution of the CV QZ Lib following its 2004 super–outburst using multi–epoch spectroscopy. From time– resolved spectroscopic observations we measure the orbital period of the system, Porb = 0.06436(20) d, which, combined with the superhump period PSH = 0.064602(24) d, yields the system mass ratio, q = 0.040(9). From the analysis of the spectral energy distribution we determine the structure of the accretion disc and the white dwarf effective temperature, Teff = 10 500 ± 1500 K. We also derive an upper limit on the effective temperature of the secondary, Teff < 1700 K, corresponding to a brown dwarf of T spectral type. The low temperature of the white dwarf, the small mass ratio and the fact that the donor is not dominating the near–infrared emission are all clues of a post bounce system. Although it is possible that QZ Lib could have formed as a white dwarf plus a brown dwarf binary, binary population synthesis studies clearly suggest this scenario to be less likely than a period bouncer detection and we conclude that QZ Lib is a CV that has already evolved through the period minimum.

The Gaia/IPHAS and Gaia/KIS value-added catalogues

This article has been published in Monthly Notices of the Royal Astronomical Society: Letters.