A circumbinary debris disk in a polluted white dwarf system

Planetary systems commonly survive the evolution of single stars, as evidenced by terrestrial-like planetesimal debris observed orbiting and polluting the surfaces of white dwarfs. This letter reports the identification of a circumbinary dust disk surrounding a white dwarf with a substellar companion in a 2.27 hr orbit. The system bears the dual hallmarks of atmospheric metal pollution and infrared excess, however the standard (flat and opaque) disk configuration is dynamically precluded by the binary. Instead, the detected reservoir of debris must lie well beyond the Roche limit in an optically thin configuration, where erosion by stellar irradiation is relatively rapid. This finding demonstrates that rocky planetesimal formation is robust around close binaries, even those with low mass ratios.Comment: accepted to Nature Astronomy, this is the authors' versio

Most EL CVn systems are inner binaries of hierarchical triples

In spite of their importance for modern astronomy, we do not fully understand how close binary stars containing at least one white dwarf form from main sequence binary stars. The discovery of EL CVn binaries, close pre-white dwarfs with A/F main sequence star companions, offers now the unique possibility to test models of close compact binary star formation. Binary evolution theories predict that these EL CVn stars descend from very close main sequence binaries with orbital periods shorter than 3 days. If this is correct, nearly all EL CVn stars should be inner binaries of hierarchical triples because more than 95 per cent of very close main sequence binaries (the alleged progenitor systems) are found to be hierarchical triples. We here present SPHERE/IRDIS observations of five EL CVn binaries, finding in all of them tertiary objects, as predicted. We conclude that EL CVn systems are inner binaries of hierarchical triples and indeed descend from very close main sequence binaries that experience stable mass transfer

The population of white dwarf binaries with hot subdwarf companions

Hot subdwarfs (sdBs) are core helium-burning stars, which lost almost their entire hydrogen envelope in the red-giant phase. Since a high fraction of those stars are in close binary systems, common envelope ejection is an important formation channel. We identified a total population of 51 close sdB+WD binaries based on time-resolved spectroscopy and multi-band photometry, derive the WD mass distribution and constrain the future evolution of these systems. Most WDs in those binaries have masses significantly below the average mass of single WDs and a high fraction of them might therefore have helium cores. We found 12 systems that will merge in less than a Hubble time and evolve to become either massive C/O WDs, AM\,CVn systems, RCrB stars or even explode as supernovae type Ia.Comment: 5 pages, 2 figures, to appear in the proceedings of the 19th European White Dwarf Workshop, ASP Conf. Se

Magnetic dynamos in white dwarfs – II. Relating magnetism and pollution

We investigate whether the recently suggested rotation and crystallization driven dynamo can explain the apparent increase of magnetism in old metal polluted white dwarfs. We find that the effective temperature distribution of polluted magnetic white dwarfs is in agreement with most/all of them having a crystallizing core and increased rotational velocities are expected due to accretion of planetary material which is evidenced by the metal absorption lines. We conclude that a rotation and crystallization driven dynamo offers not only an explanation for the different occurrence rates of strongly magnetic white dwarfs in close binaries, but also for the high incidence of weaker magnetic fields in old metal polluted white dwarfs

Orbital periods and component masses of three double white dwarfs

The merger of close double white dwarfs (CDWDs) is one of the favourite evolutionary channels for producing Type Ia supernovae (SN Ia). Unfortunately, current theories of the evolution and formation of CDWDs are still poorly constrained and have several serious uncertainties, which affect the predicted SN Ia rates. Moreover, current observational constraints on this evolutionary pathway for SN Ia mainly rely on only 18 double-lined and/or eclipsing CDWDs with measured orbital and stellar parameters for both white dwarfs. In this paper we present the orbital periods and the individual masses of three new double-lined CDWDs, derived using a new method. This method employs mass ratios, the Hα core ratios and spectral model-fitting to constrain the masses of the components of the pair. The three CDWDs are WD0028–474 (Porb=9.350 ± 0.007 hours, M1 = 0.60 ± 0.06 M⊙, M2 = 0.45 ± 0.04 M⊙), HE0410– 1137 (Porb= 12.208 ± 0.008 hours, M1 = 0.51 ± 0.04M⊙, M2 = 0.39 ± 0.03 M⊙) and SDSSJ031813.25–010711.7 (Porb= 45.908 ± 0.006 hours, among the longest period systems, M1 = 0.40 ± 0.05M⊙, M2 = 0.49 ± 0.05M⊙). While the three systems studied here will merge in timescales longer than the Hubble time and are expected to become single massive (& 0.9 M⊙) white dwarfs rather than exploding as SN Ia, increasing the small sample of CDWDs with determined stellar parameters is crucial for a better overall understanding of their evolution

The SDSS spectroscopic catalogue of white dwarf-main-sequence binaries: new identifications from DR 9–12

We present an updated version of the spectroscopic catalogue of white dwarf-main-sequence (WDMS) binaries from the Sloan Digital Sky Survey (SDSS). We identify 938 WDMS binaries within the data releases (DR) 9–12 of SDSS plus 40 objects from DR 1–8 that we missed in our previous works, 646 of which are new. The total number of spectroscopic SDSS WDMS binaries increases to 3294. This is by far the largest and most homogeneous sample of compact binaries currently available. We use a decomposition/fitting routine to derive the stellar parameters of all systems identified here (white dwarf effective temperatures, surface gravities and masses, and secondary star spectral types). The analysis of the corresponding stellar parameter distributions shows that the SDSS WDMS binary population is seriously affected by selection effects. We also measure the Na I λλ 8183.27, 8194.81 absorption doublet and H α emission radial velocities (RV) from all SDSS WDMS binary spectra identified in this work. 98 objects are found to display RV variations, 62 of which are new. The RV data are sufficient enough to estimate the orbital periods of three close binaries

Magnetic dynamos in white dwarfs – III: explaining the occurrence of strong magnetic fields in close double white dwarfs

The origin of strong (⁠∼>1MG⁠) magnetic fields in white dwarfs has been a puzzle for decades. Recently, a dynamo mechanism operating in rapidly rotating and crystallizing white dwarfs has been suggested to explain the occurrence rates of strong magnetic fields in white dwarfs with close low-mass main-sequence star companions. Here, we investigate whether the same mechanism may produce strong magnetic fields in close double white dwarfs. The only known strongly magnetic white dwarf that is part of a close double white dwarf system, the magnetic component of NLTT 12758, is rapidly rotating and likely crystallizing and therefore the proposed dynamo mechanism represents an excellent scenario for the origin of its magnetic field. Presenting a revised formation scenario for NLTT 12758, we find a natural explanation for the rapid rotation of the magnetic component. We furthermore show that it is not surprising that strong magnetic fields have not been detected in all other known double white dwarfs. We therefore conclude that the incidence of magnetic fields in close double white dwarfs supports the idea that a rotation- and crystallization-driven dynamo plays a major role in the generation of strong magnetic fields in white dwarfs

Circular polarimetry of suspect wind-accreting magnetic pre-polars

We present results from a circular polarimetric survey of candidate detached magnetic white dwarf – M dwarf binaries obtained using the Nordic Optical Telescope, La Palma. We obtained phase resolved spectropolarimetry and imaging polarimetry of seven systems, five of which show clearly variable circular polarisation. The data indicate that these targets have white dwarfs with magnetic field strengths >80 MG. Our study reveals that cyclotron emission can dominate the optical luminosity at wavelengths corresponding to the cyclotron emission harmonics, even in systems where the white dwarfs are only wind-accreting. This implies that a very significant fraction of the the stellar wind of the companion star is captured by the magnetic white dwarf reducing the magnetic braking in pre-CVs. Furthermore, the polarimetric confirmation of several detached, wind-accreting magnetic systems provides observational constraints on the models of magnetic CV evolution and white dwarf magnetic field generation. We also find that the white dwarf magnetic field configuration in at least two of these systems appears to be very complex