Hot white dwarfs in detached binaries from the Rosat WFC All Sky Survey

White dwarfs in unresolved pairs with normal stars (spectral type K or earlier) are invisible at optical wavelengths, due to the close proximity of the much more luminous main sequence companion. ROSAT has provided evidence for the existence of a growing sample of these hidden white dwarfs through the detection of EUV and soft X-ray emission. For companions of spectral type ~A5 or earlier, the white dwarf can be spectroscopically identified at far-ultraviolet wavelengths by IUE. Eleven such systems had previously been found in this way from ROSAT, EUVE and IUE observations. A search for fainter, less obvious samples of these binaries is presented, and five new systems have been discovered.;Three new close, pre-CV WD+dM binaries have also been found in the ROSAT WFC survey. Intriguingly, all three degenerates are rare mixed hydrogen/helium atmosphere DAO white dwarfs. The EUVE spectrum of one of these new systems, RE J0720-318, is analysed in detail. In particular, it is found that, while the optical spectrum can only be reproduced with a homogeneously mixed atmosphere, the EUVE spectrum can only be matched by a layered model, implying that the underlying structure of the white dwarf is stratified. The hydrogen layer mass of 3x10-14 M is the lowest measured for any white dwarf from EUVE spectra. In addition, an unprecedented HeI/HI ratio of ~1 is detected for the absorbing column along the line of sight, implying a hydrogen ionisation fraction of >90%, if all of this material resides in local interstellar medium. It is suggested that most of the helium lies in the vicinity of the star, possible in the form of a circumbinary disk left over from the common envelope phase. These results have important implications for our understanding of the evolutionary status of DAO white dwarfs in particular, and for post-common envelope systems in general.;A catalogue of all the detected white dwarf binaries found in the ROSAT survey is presented, with an analysis of the white dwarf mass distribution. Compared with optically selected samples, a significant excess of hot, massive objects is detected. This excess probably arises from the slower cooling rates of massive (>0.9M) white dwarfs in comparison to normal mass (0.6M) stars

Imaging planets around nearby white dwarfs

We suggest that Jovian planets will survive the late stages of stellar evolution, and that white dwarfs will retain planetary systems in wide orbits (≳5 au). Utilizing evolutionary models for Jovian planets, we show that infrared imaging with 8-m class telescopes of suitable nearby white dwarfs should allow us to resolve and detect companions ≳3MJUP. Detection of massive planetary companions to nearby white dwarfs would prove that such objects can survive the final stages of stellar evolution, place constraints on the frequency of main-sequence stars with planetary systems dynamically similar to our own and allow direct spectroscopic investigation of their composition and structure

SDSS J000555.90-100213.5: a hot, magnetic carbon-dominated atmosphere WD rotating with a 2.1 d period

A surprisingly large fraction (70 per cent) of hot, carbon-dominated atmosphere (DQ) white dwarfs are magnetic and/or photometrically variable on short time-scales up to ∼1000 s. However, here, we show that the hot DQ magnetic white dwarf SDSS J000555.90−100213.5 is photometrically variable by 11 per cent on a longer time-scale, with a period of 2.110 ± 0.045 d. We find no evidence of the target fluctuating on short time-scales at an amplitude of ≲±0.5 per cent. Short period hot DQ white dwarfs have been interpreted as non-radial pulsators, but in the case of SDSS J0005−1002, it is more likely that the variability is due to the rotation of the magnetic hot DQ white dwarf. We suggest that some hot DQ white dwarfs, varying on short time-scales, should be more carefully examined to ascertain whether the variability is due to rotation rather than pulsation. All hot DQs should be monitored for long-period modulations as an indicator of rotation and magnetism

Transit detection limits for sub-stellar and terrestrial companions to white dwarfs

The SuperWASP project is a ground-based ultra wide angle search for extra-solar planetary transits that has successfully detected 15 previously unknown planets in the last two years. We have used SuperWASP photometric data to investigate the transit characteristics of and detection limits for brown dwarfs, gas giants and terrestrial companions in orbit around white dwarfs. The relatively small size of a white dwarf host star (approximately 1 Earth radius), implies that any sub-stellar or gas giant companion will completely eclipse it, while terrestrial bodies smaller than the Moon will produce relatively large (> 1%) transits, detectable in good S/N light-curves. We performed extensive simulations using SuperWASP photometric data and we found that for Gaussian random noise we are sensitive to companions as small as the Moon. Our sensitivity drops in the presence of co-variant noise structure, nevertheless Earth-size bodies remain readily detectable in relatively low S/N data. We searched for eclipses and transit signals in a sample of 174 WASP targets, resulting from a cross-correlation of the McCook & Sion catalogue and the SuperWASP data archive. This study found no evidence for sub-stellar or planetary companions in close orbits around our sample of white dwarfs

Exoplanet-induced Radio Emission from M Dwarfs

We consider the magnetic interaction of exoplanets orbiting M dwarfs, calculating the expected Poynting flux carried upstream along Alfvén wings to the central star. A region of emission analogous to the Io footprint observed in Jupiter's aurora is produced, and we calculate the radio flux density generated near the surface of the star via the electron-cyclotron maser instability. We apply the model to produce individual case studies for the TRAPPIST-1, Proxima Centauri, and dwarf NGTS-1 systems. We predict steady-state flux densities of up to ~10 μJy and sporadic bursts of emission of up to ~1 mJy from each case study, suggesting these systems may be detectable with the Very Large Array and the Giant Metrewave Radio Telescope, and perhaps the Square Kilometre Array in the future. Finally, we present a survey of 85 exoplanets orbiting M dwarfs, identifying 11 such objects capable of generating radio emission above 10 μJy

Where are all the Sirius-like binary systems?

Approximately 70 per cent of the nearby white dwarfs appear to be single stars, with the remainder being members of binary or multiple star systems. The most numerous and most easily identifiable systems are those in which the main-sequence companion is an M star, since even if the systems are unresolved the white dwarf either dominates or is at least competitive with the luminosity of the companion at optical wavelengths. Harder to identify are systems where the non-degenerate component has a spectral type earlier than M0 and the white dwarf becomes the less luminous component. Taking Sirius as the prototype, these latter systems are referred to here as ‘Sirius like’. There are currently 98 known Sirius-like systems. Studies of the local white dwarf population within 20 pc indicate that approximately 8 per cent of all white dwarfs are members of Sirius-like systems, yet beyond 20 pc the frequency of known Sirius-like systems declines to between 1 and 2 per cent, indicating that many more of these systems remain to be found. Estimates are provided for the local space density of Sirius-like systems and their relative frequency among both the local white dwarf population and the local population of A to K main-sequence stars. The great majority of currently unidentified Sirius-like systems will likely turn out to be closely separated and unresolved binaries. Ways to observationally detect and study these systems are discussed

The origin of hot white dwarf circumstellar features

We have analysed a sample of 23 hot DAs to better understand the source of the circumstellar features reported in previous work. Unambiguous detections of circumstellar material are again made at eight stars. The velocities of the circumstellar material at three of the white dwarfs are coincident with the radial velocities of interstellar medium (ISM) along the sight-line to the stars, suggesting that the objects may be ionizing the ISM in their locality. In three further cases, the circumstellar velocities are close to the ISM velocities, indicating that these objects are ionizing either the ISM or evaporated planetesimals/material in a circumstellar disc. The circumstellar velocity at WD 1614-084 lies far from the ISM velocities, indicating the ionization of either an undetected ISM component or circumstellar material. The material seen at WD0232+035 can be attributed to the photoionization of material lost from its M dwarf companion. The measured column densities of the circumstellar material lie within the ionized ISM column density ranges predicted to exist in hot DA Strömgren spheres.

NGTS-1b: A hot Jupiter transiting an M-dwarf

We present the discovery of NGTS-1b, a hot-Jupiter transiting an early M-dwarf host (Teff,∗=3916 +71 −63 K) in a P = 2.647 d orbit discovered as part of the Next Generation Transit Survey (NGTS). The planet has a mass of 0.812 +0.066 −0.075 MJ making it the most massive planet ever discovered transiting an M-dwarf. The radius of the planet is 1.33 +0.61 −0.33 RJ . Since the transit is grazing, we determine this radius by modelling the data and placing a prior on the density from the population of known gas giant planets. NGTS-1b is the third transiting giant planet found around an M-dwarf, reinforcing the notion that close-in gas giants can form and migrate similar to the known population of hot Jupiters around solar type stars. The host star shows no signs of activity, and the kinematics hint at the star being from the thick disk population. With a deep (2.5%) transit around a K = 11.9 host, NGTS-1b will be a strong candidate to probe giant planet composition around M-dwarfs via JWST transmission spectroscop

Orbital and evolutionary constraints on the planet hosting binary GJ 86 from the Hubble Space Telescope

This paper presents new observations of the planet-hosting, visual binary GJ 86 (HR 637) using the Hubble Space Telescope. Ultraviolet and optical imaging with WFC3 confirms the stellar companion is a degenerate star and indicates the binary semimajor axis is larger than previous estimates, with a ≳ 28 au. Optical STIS spectroscopy of the secondary reveals a helium-rich white dwarf with C2 absorption bands and Teff = 8180 K, thus making the binary system rather similar to Procyon. Based on the 10.8 pc distance, the companion has 0.59 M⊙ and descended from a main-sequence A star of 1.9 M⊙ with an original orbital separation a ≳ 14 au. If the giant planet is coplanar with the binary, the mass of GJ 86Ab is between 4.4 and 4.7 MJup. The similarity of GJ 86 and Procyon prompted a re-analysis of the white dwarf in the latter system, with the tentative conclusion that Procyon hosts a planetesimal population. The periastron distance in Procyon is 20 per cent smaller than in α Cen AB, but the metal-enriched atmosphere of Procyon B indicates that the planet formation process minimally attained 25 km bodies, if not small planets as in α Cen

New Praesepe white dwarfs and the initial mass-final mass relation

We report the spectroscopic confirmation of four further white dwarf members of Praesepe. This brings the total number of confirmed white dwarf members to eleven making this the second largest collection of these objects in an open cluster identified to date. This number is consistent with the high mass end of the initial mass function of Praesepe being Salpeter in form. Furthermore, it suggests that the bulk of Praesepe white dwarfs did not gain a substantial recoil kick velocity from possible asymmetries in their loss of mass during the asymptotic giant branch phase of evolution. By comparing our estimates of the effective temperatures and the surface gravities of WD0833+194, WD0840+190, WD0840+205 and WD0843+184 to modern theoretical evolutionary tracks we have derived their masses to be in the range 0.72−0.76M⊙ and their cooling ages ~300Myrs. For an assumed cluster age of 625±50Myrs the infered progenitor masses are between 3.3−3.5M⊙. Examining these new data in the context of the initial mass-final mass relation we find that it can be adequately represented by a linear function (a0=0.289±0.051, a1=0.133±0.015) over the initial mass range 2.7M⊙ to 6M⊙. Assuming an extrapolation of this relation to larger initialmasses is valid and adopting a maximum white dwarf mass of 1.3M⊙, our results support a minimum mass for core-collapse supernovae progenitors in the range ~6.8-8.6M⊙