The viscous evolution of white dwarf merger remnants

We systematically explore the evolution of the merger of two carbon-oxygen (CO) white dwarfs. The dynamical evolution of a 0.9 Msun + 0.6 Msun CO white dwarf merger is followed by a three-dimensional SPH simulation. We use an elaborate prescription in which artificial viscosity is essentially absent, unless a shock is detected, and a much larger number of SPH particles than earlier calculations. Based on this simulation, we suggest that the central region of the merger remnant can, once it has reached quasi-static equilibrium, be approximated as a differentially rotating CO star, which consists of a slowly rotating cold core and a rapidly rotating hot envelope surrounded by a centrifugally supported disc. We construct a model of the CO remnant that mimics the results of the SPH simulation using a one-dimensional hydrodynamic stellar evolution code and then follow its secular evolution. The stellar evolution models indicate that the growth of the cold core is controlled by neutrino cooling at the interface between the core and the hot envelope, and that carbon ignition in the envelope can be avoided despite high effective accretion rates. This result suggests that the assumption of forced accretion of cold matter that was adopted in previous studies of the evolution of double CO white dwarf merger remnants may not be appropriate. Our results imply that at least some products of double CO white dwarfs merger may be considered good candidates for the progenitors of Type Ia supernovae. In this case, the characteristic time delay between the initial dynamical merger and the eventual explosion would be ~10^5 yr. (Abridged).

We present a large parameter study where we investigate the structure of white dwarf (WD) merger remnants after the dynamical phase. A wide range of WD masses and compositions are explored and we also probe the effect of different initial conditions. We investigated the degree of mixing between the WDs, the conditions for detonations as well as the amount of gas ejected. We find that systems with lower mass ratios have more total angular momentum and as a result more mass is flung out in a tidal tail. Nuclear burning can affect the amount of mass ejected. Many WD binaries that contain a helium-rich WD achieve the conditions to trigger a detonation. In contrast, for carbon-oxygen transferring systems only the most massive mergers with a total mass above ~2.1 solar masses detonate. Even systems with lower mass may detonate long after the merger if the remnant remains above the Chandrasekhar mass and carbon is ignited at the centre. Finally, our findings are discussed in the context of several possible observed astrophysical events and stellar systems, such as hot subdwarfs, R Coronae Borealis stars, single massive white dwarfs, supernovae of type Ia and other transient events. A large database containing 225 white dwarf merger remnants is made available via a dedicated web page.

Mergers of two carbon-oxygen (CO) white dwarfs (WDs) have been considered as progenitors of Type Ia supernovae (SNe Ia). Based on smoothed particle hydrodynamics (SPH) simulations, previous studies claimed that mergers of CO WDs lead to an SN Ia explosion either in the dynamical merger phase or stationary rotating merger remnant phase. However, the mass range of CO WDs that lead to an SN Ia has not been clearly identified yet. In the present work, we perform systematic SPH merger simulations for the WD masses ranging from $0.5~M_{\odot}$ to $1.1~M_{\odot}$ with higher resolutions than the previous systematic surveys and examine whether or not carbon burning occurs dynamically or quiescently in each phase. We further study the possibility of SN Ia explosion and estimate the mass range of CO WDs that lead to an SN Ia. We found that when the both WDs are massive, i.e., in the mass range of $0.9~M_{\odot} {\le} M_{1,2} {\le} 1.1~M_{\odot}$, they can explode as an SN Ia in the merger phase. On the other hand, when the more massive WD is in the range of $0.7~M_{\odot} {\le} M_{1} {\le} 0.9~M_{\odot}$ and the total mass exceeds $1.38~M_{\odot}$, they can finally explode in the stationary rotating merger remnant phase. We estimate the contribution of CO WD mergers to the entire SN Ia rate in our galaxy to be of ${\lt} 9\%$. So, it might be difficult to explain all galactic SNe Ia by CO WD mergers.

Mergers of two carbon-oxygen white dwarfs have long been suspected to be progenitors of Type Ia Supernovae. Here we present our modifications to the cosmological smoothed particle hydrodynamics code Gadget to apply it to stellar physics including but not limited to mergers of white dwarfs. We demonstrate a new method to map a one-dimensional profile of an object in hydrostatic equilibrium to a stable particle distribution. We use the code to study the effect of initial conditions and resolution on the properties of the merger of two white dwarfs. We compare mergers with approximate and exact binary initial conditions and find that exact binary initial conditions lead to a much more stable binary system but there is no difference in the properties of the actual merger. In contrast, we find that resolution is a critical issue for simulations of white dwarf mergers. Carbon burning hotspots which may lead to a detonation in the so-called violent merger scenario emerge only in simulations with sufficient resolution but independent of the type of binary initial conditions. We conclude that simulations of white dwarf mergers which attempt to investigate their potential for Type Ia supernovae should be carried out with at least 10^6 particles.

Binaries have received much attention as possible progenitors of Type Ia supernova (SNIa) explosions, but long-term gravitational effects in tight triple or quadruple systems could also play a key role in producing SNIa. Here we report on the properties of a spectroscopic quadruple (SB4) found within a star cluster: the 2+2 hierarchical system HD 74438. Its membership in the open cluster IC 2391 makes it the youngest (43 My) SB4 discovered so far and among the quadruple systems with the shortest outer orbital period. The eccentricity of the 6 y outer period is 0.46 and the two inner orbits, with periods of 20.5 d and 4.4 d, and eccentricities of 0.36 and 0.15, are not coplanar. Using an innovative combination of ground-based high resolution spectroscopy and Gaia/Hipparcos astrometry, we show that this system is undergoing secular interaction that likely pumped the eccentricity of one of the inner orbits higher than expected for the spectral types of its components. We compute the future evolution of HD 74438 and show that this system is an excellent candidate progenitor of sub-Chandrasekhar SNIa through white dwarf (WD) mergers. Taking into account the contribution of this specific type of SNIa better accounts for the chemical evolution of iron-peak elements in the Galaxy than considering only near Chandrasekhar-mass SNIa.

The double-degenerate model, involving the merger of double carbon-oxygen white dwarfs (CO WDs), is one of the two classic models for the progenitors of type Ia supernovae (SNe Ia). Previous studies suggested that off-centre carbon burning would occur if the mass-accretion rate (Macc) is relatively high during the merging process, leading to the formation of oxygen-neon (ONe) cores that may collapse into neutron stars. However, the off-centre carbon burning is still incompletely understood, especially when the inwardly propagating burning wave reaches the centre. In this paper, we aim to investigate the propagating characteristics of burning waves and the subsequently evolutionary outcomes of these CO cores. We simulated the long-term evolution of CO WDs that accrete CO-rich material by employing the stellar evolution code MESA on the basis of the thick-disc assumption. We found that the final outcomes of CO WDs strongly depend on Macc (Msun/yr) based on the thick-disc assumption, which can be divided into four regions: (1) explosive carbon ignition in the centre, then SNe Ia (Macc < 2.45*10^-6); (2) OSi cores, then neutron stars (2.45*10^-6 < Macc < 4.5*10^-6); (3) ONe cores, then e-capture SNe (4.5*10^-6 < Macc < 1.05*10^-5); (4) off-centre oxygen and neon ignition, then off-centre explosion or Si-Fe cores (Macc > 1.05*10^-5). Our results indicate that the final fates of double CO WD mergers are strongly dependent on the merging processes (e.g. slow merger, fast merger, composite merger, violent merger, etc.).

Double white dwarf (WD) merger process and their post-merger evolution are important in many fields of astronomy, such as supernovae, gamma-ray bursts, gravitational waves, and so on. The evolutionary outcomes of double ultra-massive WD merger remnants are still a subject of debate, though the general consensus is that the merger remnant will collapse to form a neutron star (NS). In this work, we investigate the evolution of a $2.20\, {\rm M}_{\odot }$ merger remnant stemmed from the coalescence of double $1.10\, {\rm M}_{\odot }$ ONe WDs. We find that the remnant ignites off-centre neon burning at the position near the surface of primary WD soon after the merger, resulting in the stable inwardly propagating oxygen/neon (O/Ne) flame. The final outcomes of the merger remnant are sensitive to the effect of convective boundary mixing. If the mixing cannot stall the O/Ne flame, the flame will reach the centre within 20 yr, leading to the formation of super Chandrasekhar mass silicon core, and its final fate probably be NS through iron-core-collapse supernova. In contrast, if the convective mixing is effective enough to prevent the O/Ne flame from reaching the centre, the merger remnant will undergo electron capture supernova to form an ONeFe WD. Meanwhile, we find that the wind mass loss process may hardly alter the final fate of the remnant due to its fast evolution. Our results imply that the coalescence of double ONe WDs can form short lived giant like object, but the final outcomes (NS or ONeFe WD) are influenced by the uncertain convective mixing in O/Ne flame.

We analyze the evolution of binary stars to calculate synthetic rates and delay times of the most promising Type Ia Supernovae (SNe Ia) progenitors. We present and discuss evolutionary scenarios in which a white dwarf (WD) reaches the Chandrasekhar mass and potentially explodes in a SNe Ia. We consider Double Degenerate (DDS; merger of two WDs), Single Degenerate (SDS; WD accreting from H-rich companion), and AM Canum Venaticorum (AM CVn; WD accreting from He-rich companion) scenarios. The results are presented for two different star formation histories: burst (elliptical-like galaxies) and continuous (spiral-like galaxies). It is found that delay times for the DDS in our standard model (with common envelope efficiency αCE = 1) follow a power-law distribution. For the SDS we note a wide range of delay times, while AM CVn progenitors produce a short burst of SNe Ia at early times. The DDS median delay time falls between ∼0.5 and 1 Gyr; the SDS between ∼2 and 3 Gyr; and the AM CVn between ∼0.8 and 0.6 Gyr depending on the assumed αCE. For a Milky-Way-like (MW-like) galaxy, we estimate the rates of SNe Ia arising from different progenitors as: ∼10−4 yr−1 for the SDS and AM CVn, and ∼10−3 yr−1 for the DDS. We point out that only the rates for two merging carbon–oxygen WDs, the only systems found in the DDS, are consistent with the observed rates for typical MW-like spirals. We also note that DDS progenitors are the dominant population in elliptical galaxies. The fact that the delay time distribution for the DDS follows a power law implies more SNe Ia (per unit mass) in young rather than in aged populations. Our results do not exclude other scenarios, but strongly indicate that the DDS is the dominant channel generating SNe Ia in spiral galaxies, at least in the framework of our adopted evolutionary models. Since it is believed that WD mergers cannot produce a thermonuclear explosion given the current understanding of accreting WDs, either the evolutionary calculations along with accretion physics are incorrect, or the explosion calculations are inaccurate and need to be revisited.

Recent observational evidence has demonstrated that white dwarf (WD) mergers are a highly efficient mechanism for mass accretion onto WDs in the galaxy. In this paper, we show that WD mergers naturally produce highly magnetized, uniformly rotating WDs, including a substantial population within a narrow mass range close to the Chandrasekhar mass (M Ch). These near-M Ch WD mergers subsequently undergo rapid spin up and compression on a ∼ 102 yr timescale, either leading to central ignition and a normal SN Ia via the DDT mechanism, or alternatively to a failed detonation and SN Iax through pure deflagration. The resulting SNe Ia and SNe Iax will have spectra, light curves, polarimetry, and nucleosynthetic yields similar to those predicted to arise through the canonical near-M Ch single degenerate (SD) channel, but with a t −1 delay time distribution characteristic of the double-degenerate channel. Furthermore, in contrast to the SD channel, WD merger near-M Ch SNe Ia and SNe Iax will not produce observable companion signatures. We discuss a range of implications of these findings, from SNe Ia explosion mechanisms, to galactic nucleosynthesis of iron peak elements including manganese.

Section I. Halo White Dwarfs and Galactic Structure. Old Ultracool White Dwarfs as Cosmological Probes. Number counts of white dwarfs: the impact of GAIA. Influence of Metallicity in the Determination of the Age of Halo White Dwarfs. Cool Halo White Dwarfs from GSCII. Dark halo baryons not in ancient white dwarfs? White Dwarfs in 2QZ and Sloan Surveys. White Dwarfs and Cataclysmic in the FBS. White Dwarfs in Globular Clusters. Core/Envelope Symmetry in Pulsating White Dwarfs Stars. A Search for Variability in Cool White Dwarf Stars. Planetary Nebulae and the Galactic Bulge. Section II. Type Ia Supernovae. Type Ia Supernovae and Cosmology. The Progenitors of Type Ia Supernovae. Progenitors of Supernovae Type Ia. Polulation synthesis for progenitors of type Ia supernovae. How far can we trust SNE Ia as Standard Candles? Kirshner on White Dwarfs. Supernovae. and Cosmology. Section III. Cataclysmic Variables and White Dwarf Accretion. Accretion in Cataclysmic Variable Stars. White Dwarfs in Cataclysmic Variables: Probes of Accretion History. Intermediate Polars in Low States. Chemical Abundances of WDs in CVs. AM CVn Stars in the UCT CCD CV Survey. TUG Observations of V2275 Cyg, RW Umi, PX And and FO Per. The Possible Identification of Two Hibernating Novae. Evidence for large superhumps in TX Col and V4742 Sgr. Concluding remarks.

I systematically investigate the formation of double degenerates (DDs) via binary interactions. I consider three evolutionary channels for their formation [stable Roche lobe overflow (RLOF) plus common envelope (CE), CE plus CE, exposed core plus CE], and carry out Monte Carlo simulations. I explore the effects of model parameters, such as the tidal-enhancement parameter for stellar wind, the mass transfer efficiency for stable RLOF and the CE ejection efficiency, on my results. I also explore the effects of various assumptions about age, metallicity, mass ratio distribution and wind velocity. My results show that the model is successful in the explanation of the formation of DDs. I explain satisfactorily the distributions of masses, mass ratios, orbital periods and birth rate of the observed DDs. The main conclusions are the following. (i) Stable RLOF plus CE and CE plus CE are the main evolutionary scenarios leading to the formation of DDs. (ii) The Galactic birth rate of DDs is 0.03 yr(-1), and the birth rate of DDs with helium (He) white dwarfs (WDs) as brighter components is 0.017 yr(-1). (iii) The number of detectable DDs in our Galaxy is 3 x 10(6), and DDs with brighter He WDs make up 56 per cent. (iv) The distribution of orbital periods for detectable DDs peaks around 6 h. (v) WD 0957-666 and WD 1101+364 are formed through the stable RLOF plus CE scenario, and WD 0135-052 is possibly a carbon-oxygen (CO) WD pair rather than a helium (He) WD pair. (vi) The Galactic birth rates of close i, respectively. (vii) The mergers of two WD binaries and DD mergers are 0.074 and 0.029 yr(-1), respectively. (vii) The merges of two He WDs and the mergers of He and CO WDs have masses of 0.61 +/- 0.09 and 0.96 +/- 0.13 M., respectively. (viii) Mass transfer during stable RLOF is not conservative. (ix) A tidally enhanced stellar wind exists. I also investigate the formation of Type Ia supernovae (SNe Ia), cataclysmic variables (CVs), subdwarf O-type (sdO) stars and R Coronae Borealis (R CrB) stars. The birth rates of SNe Ia and CVs are successfully explained in the above model. The model also shows that CVs with long orbital periods tend to have CO WDs. The birth rates of the mergers of two He WDs (sdO stars) and the mergers of He and CO WDs (R CrB stars) are 0.006 and 0.018 yr(-1) in the Galaxy, respectively. The birth rates of CVs, DDs, DD mergers and SNe Ia are more sensitive to the recent stellar formation history than to the past one.

The nature of the progenitors of Type Ia supernovae (SNe Ia) is still unclear. In this paper, by considering the effect of the instability of accretion disc on the evolution of white dwarf (WD) binaries, we performed binary evolution calculations for about 2400 close WD binaries, in which a carbon–oxygen WD accretes material from a main-sequence (MS) star or a slightly evolved subgiant star (WD + MS channel), or a red-giant star (WD + RG channel) to increase its mass to the Chandrasekhar (Ch) mass limit. According to these calculations, we mapped out the initial parameters for SNe Ia in the orbital period–secondary mass (log Pi−Mi2) plane for various WD masses for these two channels, respectively. We confirm that WDs in the WD + MS channel with a mass as low as 0.61 M⊙ can accrete efficiently and reach the Ch limit, while the lowest WD mass for the WD + RG channel is 1.0 M⊙. We have implemented these results in a binary population synthesis study to obtain the SN Ia birthrates and the evolution of SN Ia birthrates with time for both a constant star formation rate and a single starburst. We find that the Galactic SN Ia birthrate from the WD + MS channel is ∼1.8 × 10−3 yr−1 according to our standard model, which is higher than the previous results. However, similar to the previous studies, the birthrate from the WD + RG channel is still low (∼3 × 10−5 yr−1). We also find that about one-third of SNe Ia from the WD + MS channel and all SNe Ia from the WD + RG channel can contribute to the old populations (≳1 Gyr) of SN Ia progenitors.

Double white dwarf (DWD) mergers are relevant astrophysical sources expected to produce massive, highly magnetized white dwarfs (WDs), supernovae (SNe) Ia, and neutron stars (NSs). Although they are expected to be numerous sources in the sky, their detection has evaded the most advanced transient surveys. This article characterizes the optical transient expected from DWD mergers in which the central remnant is a stable (sub-Chandrasekhar) WD. We show that the expansion and cooling of the merger’s dynamical ejecta lead to an optical emission peaking at 1–10 days postmerger, with luminosities of 1040–1041 erg s−1. We present simulations of the light curves, spectra, and the color evolution of the transient. We show that these properties, together with the estimated rate of mergers, are consistent with the absence of detection, e.g., by the Zwicky Transient Facility. More importantly, we show that the Legacy Survey of Space and Time of the Vera C. Rubin Observatory will likely detect a few/several hundred per year, opening a new window to the physics of WDs, NSs, and SNe Ia.

We numerically construct a series of axisymmetric rotating magnetic wind solutions, aiming at exploring the observation properties of massive white dwarf (WD) merger remnants with a strong magnetic field, a fast spin, and an intense mass loss, as inferred for WD J005311. We investigate the magnetospheric structure and the resultant spin-down torque exerted to the merger remnant with respect to the surface magnetic flux Φ*, spin angular frequency Ω* and the mass-loss rate Ṁ . We confirm that the wind properties for σ≡Φ*2Ω*2/Ṁvesc3≳1 significantly deviate from those of the spherical Parker wind, where v esc is the escape velocity at stellar surface. For such a rotating magnetic wind sequence, we find (i) a quasiperiodic mass eruption triggered by magnetic reconnection along with the equatorial plane and (ii) a scaling relation for the spin-down torque T≈(1/2)×ṀΩ*R*2σ1/4 . We apply our results to discuss the spin-down evolution and wind anisotropy of massive WD merger remnants, the latter of which could be probed by a successive observation of WD J005311 using Chandra.

Many double white dwarf (WD) mergers likely do not lead to a prompt thermonuclear explosion. We investigate the prospects for observationally detecting the surviving remnants of such mergers, focusing on the case of mergers of double Carbon–Oxygen WDs. For ∼104 yr, the merger remnant is observationally similar to an extreme AGB star evolving to become a massive WD. Identifying merger remnants is thus easiest in galaxies with high-stellar masses (high WD merger rate) and low star formation rates (low birth rate of ∼6–10 M⊙ stars). Photometrically identifying merger remnants is challenging even in these cases because the merger remnants appear similar to He stars and post-outburst classical novae. We propose that the most promising technique for discovering WD merger remnants is through their unusual surrounding photoionized nebulae. We use CLOUDY photoionization calculations to investigate their unique spectral features. Merger remnants should produce weak hydrogen lines, strong carbon and oxygen recombination, and fine-structure lines in the UV, optical and IR. With narrow-band imaging or integral field spectrographs, we predict that multiple candidates are detectable in the bulge of M31, the outskirts of M87 and other nearby massive galaxies, and the Milky Way. Our models roughly reproduce the WISE nebula surrounding the Galactic WD merger candidate IRAS 00500+6713; we predict detectable [Ne vi] and [Mg vii] lines with JWST but that the mid-IR WISE emission is dominated by dust not fine-structure lines.