I will report on a programme of double white dwarf discovery from targets selected from Gaia DR2 from their location in the H-R diagram. Targets are chosen from their location above the main white dwarf cooling sequence in terms of luminosity. This favours double white dwarfs with equal contributions from the two stars and a number of new systems have been discovered. These are of particular...
The characterization of the local double white dwarf (DWD) population is crucial to our understanding of multiple questions, from stellar evolution, through the progenitors of Type-Ia supernovae (SNe Ia), to gravitational wave sources. From a spectroscopic sample of 439 WDs from the SPY survey, we measure the maximal changes in radial-velocity (DRVmax) between epochs, and model the observed...
We present the completed ELM Survey, a targeted survey of extremely low mass white dwarfs. The final sample contains 106 He+CO white dwarf binaries; their median orbital period is 6 hr. Combining our spectroscopy with Gaia astrometry reveals that half of the binaries orbit in the disk, half in the halo. We compare the disk and halo samples, and discuss the merger rate of He+CO white dwarf...
Common envelope evolution (CEE) is presently a poorly understood, yet critical, process in binary stellar evolution. I present results from a suite of high resolution global 3D hydrodynamical adaptive mesh refinement simulations of CEE involving a red giant branch or asymptotic giant branch primary and a white dwarf or main sequence secondary. The simulations are analyzed to understand how...
Gaia-DR2 has provided an unprecedented wealth of information about the white dwarf population of our Galaxy. In particular, our estimates show that the sample up to 100 pc from the Sun can be considered as practically complete. This fact permits to obtain accurate statistics about the percentages of the different subpopulations, such as the number of resolved/unresolved double degenerate...
Extremely low-mass white dwarfs (ELM WDs) are helium WDs with a mass less than $\sim 0.30\;M_\odot$. Most ELM WDs are found in double degenerates (DDs) in the ELM Survey led by Brown and Kilic. These systems are supposed to be significant gravitational-wave (GW) sources in the mHz frequency. In this talk, I first systematically investigated the formation of ELM WDs in DDs by a combination of...
With the data release of Gaia DR2 we were able to derive, for the first time, realistic strain, SNR predictions for the known LISA verification binaries. In this talk I will discuss our approach and result. I will also present an overview and the survey strategy of the ZTF high-cadence Galactic Plane survey which aims to find short period variables at low Galactic latitudes.
The Zwicky Transient Facility (ZTF) is in the process of obtaining well sampled lightcurves of all stars in the Northern hemisphere. In this sample of billions of lightcurves, we can expect to find on the order of a hundred eclipsing double white dwarfs. We are in the process of systematically searching the ZTF lightcurve for these systems, both by doing targeted searches as well a general...
Here we give new updates on an eclipsing double white dwarf binary with an orbital period of only 6.9 minutes. Additionally, we highlight several other candidate LISA DWD sources discovered using the optical time domain and discuss the future of this class of sources in the era of surveys such as LSST.
I will present a refined measurement of orbital decay from gravitational radiation in the second-most compact detached stellar binary known, the eclipsing 12.75-min double white dwarf system J0651+2844. Based on more than 490 hr of ground-based photometry over a nearly 8-year baseline, we measure a shift in mid-eclipse times caused by an orbital decay that can constrain the orbital decay to...
The tidal interactions of white dwarf binaries are crucial for understanding the fates of these systems when they merge, connecting them to different classes of astrophysical explosions and other binary systems. New discoveries of compact white dwarf binaries are putting important constraints on these tidal interactions. In the future, we will hopefully have the opportunity to combine...
AM CVn stars are ultracompact, accreting binaries in which both stars are either degenerate or semi-degenerate. Several formation channels have been suggested for AM CVn binaries, including a favoured model in which they descend directly from double white dwarf binaries. The double white dwarf channel has uncertainties around the fraction of double degenerate binaries which will reach a state...
Compact white dwarf (WD) binaries are expected to be the most abundant source detected by the upcoming gravitational-wave observatory, LISA. Based on both observational indications and theoretical studies, a fraction of these close WD binaries are expected to be undergoing mass transfer. The nature of this mass transfer will determine the long-term survival of the systems and will have...
The upcoming LISA mission is the only experiment that offers the opportunity to map the Milky Way through gravitational wave radiation, exploiting signals from double white dwarf (DWD) binaries. I will show that the large number of DWD detections will allow us to use these systems as tracers of the Milky Way's shape and to measure scale parameters of the bulge, disc and bar. Furthermore, in...
The double-degenerate scenario is, nowadays, the preferred model to
explain the majority of Type Ia supernovae. In order to explore it, a
vast number of studies have been performed during the last decades using
a large variety of methods. From the theoretical point of view,
numerical simulations using the Smoothed Particle Hydrodynamics (SPH)
method have played an essential role understanding...
We study the physical properties of the interacting processes between the components of two double white dwarf binary stars, V803 Cen and CR Boo, depending on the evolutionary stages for each of the two targets, separately. Both objects are semi-detached binaries, influenced by the mass transfer mechanism. The initial conditions suggest disturbances in the flow parameters, followed by...
Close double white dwarfs(CDWDs) are good tests for theories of binary evolution, potential progenitors of Type Ia supernovae, and important contributors of gravitational wave signal at low frequencies. We used a binary population synthesis code of a population of 1 million binaries to study the characteristics of CDWDs, the mass transfer stability criterion comes from adiabatic mass loss...
White dwarf-neutron star binaries are among the main progenitors for ultra-compact X-ray binaries. They spiral in to contact by emitting gravitational waves and depending on the mass of the white dwarf turn into stably transferring ultra-compact X-ray binaries or produce supernova-like events following a tidal disruption of the white dwarf. Nearly all the stably transferring systems evolve...
The white dwarf luminosity function is an essential tool to understand the nature and the history of the different components of our Galaxy. However, observational white dwarf samples from which the luminosity functions are built are contaminated by unresolved double-degenerates. These systems are considered as 'single' white dwarfs from an observational point of view and therefore contribute...
About 10% of white dwarfs in the solar neighborhood are strongly magnetic, with field strengths of up to 1000 MG. Curiously, high field magnetic white dwarfs are never found in wide binary systems with late-type stellar companions. This suggests that merging binaries within a common envelope may be required to explain the incidence of magnetism in these objects. However, there are also several...
There are several indications that stellar mergers are common outcomes of binary evolution. Mergers involving white dwarfs are linked to several classes of transients, such as supernova type, and may leave imprints in other observables as well, e.g. the WD mass distribution. The alleged excess of WDs with masses around 0.8solar masses has been linked to double white dwarf mergers. Here we...
Recently, Gaia DR2 has revealed an enhancement of high-mass WDs on the H-R diagram, called the Q branch. The distribution of photometric ages and velocities of WDs around the branch suggest an extra cooling delay beyond current WD cooling models. To explore the properties of this delay, we statistically compare two age indicators -- the dynamical age reflected by transverse velocity and the...
The collisional-triple SN Ia progenitor model posits that SNe Ia result from head-on collisions of binary white dwarfs (WDs), driven by dynamical perturbations by the tertiary stars in mild-hierarchical triple systems. To reproduce the Galactic SN Ia rate, some 30-55 per cent of all WDs would need to be in triple systems of a specific architecture. We test this scenario by searching the Gaia...
Despite the importance today of Type Ia supernova explosions (SNe Ia) for a number of branches of astrophysics and cosmology, their progenitors and exact explosion mechanism remain unclear. The two standard scenarios for SNe Ia explosions, single-degenerate accretion and double-degenerate mergers, both have a number of theoretical and observational challenges, and there is still no consensus...
The discovery of hypervelocity white dwarfs strongly supports the Dynamical-Driven Double-Degenerate Double-Detonation ($D^6$) model for type Ia supernova scenario. We have performed high-resolution SPH simulations of $D^6$ explosions in various double-degenerate systems. We have found several explosion modes. In this presentation, we will show observational features of these explosion modes.
Type-Ia supernovae (SNe) are thought to originate from the thermonuclear explosions of carbon-oxygen (CO) white-dwarf (WD) stars. They produce most of the Iron-peak elements in the universe, and bright Ia-SNe serve as important “standard candle” cosmological distance-indicators. The proposed progenitors of standard type Ia-SNe had been studied for decades and can be generally divided between...
Type-Ia supernovae (SNe) are thought to originate from the thermonuclear explosions of carbon-oxygen (CO) white-dwarf (WD) stars. The proposed progenitors of standard type Ia-SNe can be generally divided between explosions of CO-WDs accreting material from stellar non-degenerate companions (single-degenerate; SD models), and those arising from the mergers of two CO-WDs (double-degenerate; DD...
In the D6 scenario for Type Ia supernovae, the lower mass white dwarf is in a close orbit while donating material to its companion that explodes as a supernova. This orbit leads to the high velocities of runaway stars thought to be candidate white dwarf donor remnants from D6 systems. It also implies that the donor star should experience significant interaction with the supernova ejecta, which...
The D6 scenario suggests that the white dwarf donor interacts significantly with the supernova ejecta, as a way to explain the luminosity and size of candidate remnant objects. As this is clearly a dynamical interaction, we use the self-gravity and newly developed EOS capabilities of Athena++ to explicitly evolve the hydrodynamical phase of the ejecta-donor interaction, with particular...
While the stellar progenitors of Type Ia supernovae (SNe Ia) remain a subject of active investigation, recent multi-wavelength observations of SNe Ia have tightly constrained near-Chandrasekhar mass (near-Mch) single-degenerate (SD) SNe Ia. In particular, the most extensive set of non-detections of nebular H-alpha from SNe Ia of 110 events of all classes (Tucker et al, 2019) guide us to...
Carbon-oxygen WDs accreting a helium shell have the potential to explode in the sub-Chandrasekhar mass regime through the double detonation scenario, when a helium shell ignition propagates a shock wave into the the core of the WD causing a central ignition. I will present the results of a recent numerical parameter survey of hydrodynamic and radiative transport models of sub-Chandrasekhar...
Traditionally it has been assumed that the collision of two white dwarfs was a low probability event confined to the surroundings of the Galactic centre or to the core of globular clusters. Depending on the nature of the two stars (mass and chemical composition) and on the parameters of the collision (relative velocity and impact parameter) the outcome of the encounter can go from a simple...
The merger of carbon-oxygen or oxygen-neon white dwarfs need not lead to a thermonuclear explosion, even when their total mass is in excess of the Chandrasekhar mass. I will discuss the post-merger evolution of these massive merger remnants, which may sometimes result in the production of a neutron star, and draw parallels to ultra-stripped core collapse supernovae. I will also describe the...
The final outcome of white dwarf mergers across the entire merger parameter space is still relatively uncertain. This is mainly because, as the evolutionary phases involved span several orders of magnitude in timescales, one needs different numerical approaches/codes such as smoothed-particle hydrodynamics (SPH), magnetohydrodynamics (MHD), and detailed stellar evolution for the dynamical...
We present the results of an investigation of the dredge-up and mixing during the merger of two white dwarfs (WDs) with different chemical compositions by conducting hydrodynamic simulations of binary mergers for three representative mass ratios. In all the simulations, the total mass of the two WDs is $≲1.0 {\mathrm M_☉}$. Mergers involving a CO and a He WD have been suggested as a possible...
The R Coronae Borealis (RCB) stars are rare hydrogen-deficient, carbon-rich supergiants. They undergo extreme, irregular declines in brightness of many magnitudes due to the formation of thick clouds of carbon dust. Two scenarios have been proposed for the origin of an RCB star: the merger of a CO/He white dwarf (WD) binary and a final helium-shell flash. We constructed post-merger spherical...
The merger of two white dwarfs with a combined mass below the Chandrasekhar-limit is expected to be non-explosive, resulting in a single massive white dwarf. However, because single star evolution can also produce massive white dwarfs, few known degenerate stars have categorically been confirmed as merger products.
We report the identification of a 1.15 $M_\odot$ white dwarf with a unique...
We propose a new category of low frequency gravitational wave sources
related to mergers of double white dwarfs. A remnant just after a merger
is a rapidly and differentially rotating objects, which may develop
non-axisymmetric instability of hydrodynamical origin. If the remnant
is susceptible to the so-called 'low T/W' instability, m=2 (bar)
or m=1 (spiral) density pattern may develop (here...
