White Dwarf Mergers Research Articles

Rapid Variability in the Wind from the White Dwarf Merger Candidate J005311

Abstract We analyze time-series spectroscopy of the white dwarf merger candidate J005311 and confirm the unique nature of its optical spectrum. We detect an additional broad emission feature peaking at 343 nm that was predicted in the Gvaramadze et al. (2019) models. Comparing ten spectra taken with the Large Binocular Telescope, we find significant variability in the profile of the strong O vi 381.1/383.4 nm emission feature. This appears to be caused by rapidly shifting subpeaks generated by clumpiness in the stellar wind of J005311. This line variability is similar to what is seen in many Wolf-Rayet stars. However, in J005311, the rate of motion of the subpeaks appears exceedingly high as they can reach 16,000 km s−1 in less than two hours.

The gravitational wave emission of double white dwarf coalescences

Type Ia Supernovae (SNe Ia) are widely used as standard candles to probe the Universe. However, how these fierce explosions are produced itself is still a highly debated issue. There are mainly two popular models for SNe Ia: the double-degenerate scenario and the single-degenerate scenario. The double-degenerate scenario suggests that SNe Ia are produced by the coalescence of two degenerate white dwarfs, while the single-degenerate scenario suggests that the continuous accretion of a single degenerate white dwarf from its normal stellar companion will finally lead to a disastrous explosion when it is over-massive, resulting in an SN Ia. The rapid development of the gravitational wave astronomy sheds new light on the nature of SNe Ia. In this study, we calculate the gravitational wave emissions of double white dwarf coalescences and compare them with the sensitivities of several upcoming detectors. It is found that the gravitational wave emissions from double white dwarf mergers in the local universe are strong enough to be detected by LISA. We argue that LISA-like gravitational wave detectors sensitive in the frequency range of 0.01 — 0.1 Hz will be a powerful tool to test the double-degenerate model of SNe Ia, and also to probe the Universe.

Modelling R Coronae Borealis stars: effects of He-burning shell temperature and metallicity

ABSTRACT The R Coronae Borealis (RCB) stars are extremely hydrogen-deficient carbon stars that produce large amounts of dust, causing sudden deep declines in brightness. They are believed to be formed primarily through white dwarf mergers. In this paper, we use mesa to investigate how post-merger objects with a range of initial He-burning shell temperatures from 2.1 to 5.4 × 108 K with solar and subsolar metallicities evolve into RCB stars. The most successful model of these has subsolar metallicity and an initial temperature near 3 × 108 K. We find a strong dependence on initial He-burning shell temperature for surface abundances of elements involved in the CNO cycle, as well as differences in effective temperature and radius of RCBs. Elements involved in nucleosynthesis present around 1 dex diminished surface abundances in the 10 per cent solar metallicity models, with the exception of carbon and lithium that are discussed in detail. Models with subsolar metallicities also exhibit longer lifetimes than their solar counterparts. Additionally, we find that convective mixing of the burned material occurs only in the first few years of post-merger evolution, after which the surface abundances are constant during and after the RCB phase, providing evidence for why these stars show a strong enhancement of partial He-burning products.

A New Rusanov-Type Solver with a Local Linear Solution Reconstruction for Numerical Modeling of White Dwarf Mergers by Means Massive Parallel Supercomputers

The results of numerical modeling of white dwarf mergers on massive parallel supercomputers using a AVX-512 technique are presented. A hydrodynamic model of white dwarfs closed by a star equation of state and supplemented by a Poisson equation for the gravitational potential is constructed. This paper presents a modification based on a local linear reconstruction of the solution of the Rusanov scheme for the hydrodynamic equations. This reconstruction makes it possible to considerably decrease the numerical dissipation of the scheme for weak shock waves without any external piecewise polynomial reconstruction. The scheme is efficient for unstructured grids, when it is difficult to construct a piecewise polynomial solution, and also in parallel implementations of structured nested or adaptive grids, when the costs of interprocess interactions increase significantly. As input data, piecewise constant values of the physical variables in the left and right cells of a discontinuity are used. The smoothness of the solution is measured by the discrepancy between the maximum left and right eigenvalues. This discrepancy is used for a local piecewise polynomial reconstruction in the left and right cells. Then the solutions are integrated along the characteristics taking into account the piecewise linear representation of the physical variables. A performance of 234 gigaflops and 33-fold speedup are obtained on two Intel Skylake processors on the cluster NKS-1P of the Siberian Supercomputer Center ICM & MG SB RAS.

Thirty Years of Radio Observations of Type Ia SN 1972E and SN 1895B: Constraints on Circumstellar Shells

Abstract We have imaged 35 yr of archival Very Large Array observations of the nearby (d L = 3.15 Mpc) Type Ia supernovae SN 1972E and SN 1895B between 9 and 121 yr post-explosion. No radio emission is detected, constraining their radio luminosities to be L ν,8.5GHz < 6.0 × 1023 erg s−1 Hz−1 45 yr post-explosion and L ν,8.5GHz < 8.9 × 1023 erg s−1 Hz−1 121 yr post-explosion, respectively. These limits imply a clean circumstellar medium (CSM), with n < 0.9 cm−3 out to radii of a few ×1018 cm, if the SN blast wave is expanding into uniform density material. We also constrain the presence of CSM shells surrounding the progenitor of SN 1972E. We rule out essentially all medium and thick shells with masses of 0.05–0.3 M ⊙ at radii between ∼1017 and 1018 cm, and thin shells at specific radii with masses down to ≲0.01 M ⊙. These constraints rule out swaths of parameter space single and double degenerate progenitor scenarios, including recurrent nova, core-degenerate objects, ultra-prompt explosions, and white dwarf (WD) mergers with delays of a few hundred years between the onset of merger and explosion. Allowed progenitors include WD–WD systems with a significant (>104 yr) delay from the last episode of common envelope evolution and single degenerate systems undergoing recurrent nova—provided that the system has been in the nova phase for ≳104 yr, such that a large (>1018 cm) cavity has been evacuated. Future multi-epoch observations of additional intermediate-aged SNe Ia will provide a comprehensive view of the large-scale CSM around these explosions.

An Isolated White Dwarf with 317 s Rotation and Magnetic Emission

Abstract We report the discovery of short-period photometric variability and modulated Zeeman-split hydrogen emission in SDSSJ125230.93−023417.72 (EPIC 228939929), a variable white dwarf star observed at long cadence in K2 Campaign 10. The behavior is associated with a magnetic (B = 5.0 MG) spot on the stellar surface, making the 317.278 s period a direct measurement of the stellar rotation rate. This object is therefore the fastest-rotating, apparently isolated (without a stellar companion) white dwarf yet discovered and the second found to exhibit chromospheric Balmer emission after GD 356, in which the emission has been attributed to a unipolar inductor mechanism driven by a possible rocky planet. We explore the properties and behavior of this object, and consider whether its evolution may hold implications for white dwarf mergers and their remnants.

The formation of single neutron stars from double white-dwarf mergers via accretion-induced collapse

ABSTRACT The merging of double white dwarfs (WDs) may produce the events of accretion-induced collapse (AIC) and form single neutron stars (NSs). Meanwhile, it is also notable that the recently proposed WD+He subgiant scenario has a significant contribution to the production of massive double WDs, in which the primary WD grows in mass by accreting He-rich material from a He subgiant companion. In this work, we aim to study the binary population synthesis (BPS) properties of AIC events from the double WD mergers by considering the classical scenarios and also the contribution of the WD+He subgiant scenario to the formation of double WDs. First, we provided a dense and large model grid of WD+He star systems for producing AIC events through the double WD merger scenario. Secondly, we performed several sets of BPS calculations to obtain the rates and single NS number in our Galaxy. We found that the rates of AIC events from the double WD mergers in the Galaxy are in the range of $1.4{-}8.9\times 10^{\rm -3}\, \rm yr^{\rm -1}$ for all ONe/CO WD+ONe/CO WD mergers, and in the range of $0.3{-}3.8\times 10^{\rm -3}\, \rm yr^{\rm -1}$ when double CO WD mergers are not considered. We also found that the number of single NSs from AIC events in our Galaxy may range from 0.328 × 107 to 1.072 × 108. The chirp mass of double WDs for producing AIC events distribute in the range of $0.55{-}1.25\, \rm M_{\odot }$. We estimated that more than half of double WDs for producing AIC events are capable to be observed by the future space-based gravitational wave detectors.

Luminosity–duration relations and luminosity functions of repeating and non-repeating fast radio bursts

ABSTRACT Fast radio bursts (FRBs) are mysterious radio bursts with a time-scale of approximately milliseconds. Two populations of FRB, namely repeating and non-repeating FRBs, are observationally identified. However, the differences between these two and their origins are still cloaked in mystery. Here we show the time-integrated luminosity–duration (Lν–wint, rest) relations and luminosity functions (LFs) of repeating and non-repeating FRBs in the FRB Catalogue project. These two populations are obviously separated in the Lν-wint, rest plane with distinct LFs, i.e. repeating FRBs have relatively fainter Lν and longer wint, rest with a much lower LF. In contrast with non-repeating FRBs, repeating FRBs do not show any clear correlation between Lν and wint, rest. These results suggest essentially different physical origins of the two. The faint ends of the LFs of repeating and non-repeating FRBs are higher than volumetric occurrence rates of neutron star (NS) mergers and accretion-induced collapse (AIC) of white dwarfs (WDs), and are consistent with those of soft gamma-ray repeaters (SGRs), Type Ia supernovae (SNe Ia), magnetars, and WD mergers. This indicates two possibilities: either (i) faint non-repeating FRBs originate in NS mergers or AIC and are actually repeating during the lifetime of the progenitor, or (ii) faint non-repeating FRBs originate in any of SGRs, SNe Ia, magnetars, and WD mergers. The bright ends of LFs of repeating and non-repeating FRBs are lower than any candidates of progenitors, suggesting that bright FRBs are produced from a very small fraction of the progenitors regardless of the repetition. Otherwise, they might originate in unknown progenitors.

Magnetars from Neutron Star–White Dwarf Mergers: Application to Fast Radio Bursts

Abstract It is widely believed that magnetars could be born in core-collapse supernovae (SNe), binary neutron star (BNS) or binary white dwarf (BWD) mergers, or accretion-induced collapse (AIC) of white dwarfs. In this paper, we investigate whether magnetars could also be produced from neutron star–white dwarf (NSWD) mergers, motivated by FRB 180924-like fast radio bursts (FRBs) possibly from magnetars born in BNS/BWD/AIC channels suggested by Margalit et al. (2019). By a preliminary calculation, we find that NSWD mergers with unstable mass transfer could result in the NS acquiring an ultra-strong magnetic field via the dynamo mechanism due to differential rotation and convection or possibly via the magnetic flux conservation scenario of a fossil field. If NSWD mergers can indeed create magnetars, then such objects could produce at least a subset of FRB 180924-like FRBs within the framework of flaring magnetars, since the ejecta, local environments, and host galaxies of the final remnants from NSWD mergers resemble those of BNS/BWD/AIC channels. This NSWD channel is also able to well explain both the observational properties of FRB 180924-like and FRB 180916.J0158+65-like FRBs within a large range in local environments and host galaxies.

Double White Dwarf Merger Products among High-mass White Dwarfs

Abstract Double white dwarf (double-WD) binaries may merge within a Hubble time and produce high-mass WDs. Compared to other high-mass WDs, the double-WD merger products have higher velocity dispersion because they are older. With the power of Gaia data, we show strong evidence for double-WD merger products among high-mass WDs by analyzing the transverse-velocity distribution of more than 1000 high-mass WDs (0.8–1.3 M ⊙). We estimate that the fraction of double-WD merger products in our sample is about 20%. We also obtain a precise double-WD merger rate and its mass dependence. Our merger rate estimates are close to binary population synthesis results and support the idea that double-WD mergers may contribute to a significant fraction of type Ia supernovae.

Detection of Fluorine in Hot Extreme Helium Stars

Abstract The main objective of this paper is to explore abundances of fluorine in hot extreme helium stars (EHes). Overabundance of fluorine is a characteristic feature for cool EHes and R Coronae Borealis stars and further enforces their close connection. For hot EHes this relationship with the cooler EHes, based on their fluorine abundance is unexplored. We present in this paper the first abundance estimates of fluorine determined from singly ionized fluorine lines (F ii) for 10 hot EHe stars from optical spectra. Fluorine abundances were determined using the F ii lines in two windows centered at 3505 Å and 3850 Å. Six of the 10 stars show significant enhancement of fluorine similar to the cool EHes. Two carbon-poor hot EHes show no signature of fluorine and have a significant low upper limit for the F abundance. These fluorine abundances are compared with the other elemental abundances observed in these stars, which provide an idea about the formation and evolution of these stars. The trends of fluorine with C, O, and Ne show that significant helium burning after a CO–He white dwarf merger can account for a majority of the observed abundances. Predictions from simulations of white dwarf mergers are discussed in light of the observed abundances.

New white dwarf envelope models and diffusion

Context.Recent studies of the atmospheres of carbon-rich (DQ) white dwarfs have demonstrated the existence of two different populations that are distinguished by the temperature range, but more importantly, by the extremely high masses of the hotter group. The classical DQ below 10 000 K are well understood as the result of dredge-up of carbon by the expanding helium convection zone. The high-mass group poses several problems regarding their origin and also an unexpected correlation of effective temperature with mass.Aims.We propose to study the envelopes of these objects to determine the total hydrogen and helium masses as possible clues to their evolution.Methods.We developed new codes for envelope integration and diffusive equilibrium that are adapted to the unusual chemical composition, which is not necessarily dominated by hydrogen and helium.Results.Using the new results for the atmospheric parameters, in particular, the masses obtained usingGaiaparallaxes, we confirm that the narrow sequence of carbon abundances withTeffin the cool classical DQ is indeed caused by an almost constant helium to total mass fraction, as found in earlier studies. This mass fraction is smaller than predicted by stellar evolution calculations. For the warm DQ above 10 000 K, which are thought to originate from double white dwarf mergers, we obtain extremely low hydrogen and helium masses. The correlation of mass withTeffremains unexplained, but another possible correlation of helium layer masses withTeffas well as the gravitational redshifts casts doubt on the reality of both and suggests possible shortcomings of current models.

Faint rapid red transients from neutron star–CO white dwarf mergers

ABSTRACT Mergers of neutron stars (NS) and white dwarfs (WD) may give rise to observable explosive transient events. We use 3D hydrodynamical (smoothed particle hydrodynamics – SPH) simulations and 2D hydrodynamical–thermonuclear simulations (using the flash adaptive mesh refinement code) to model the disruption of CO-WDs by NSs, which produce faint transient events. We post-process the simulations using a large nuclear network and make use of the SuperNu radiation transfer code to predict the observational signatures and detailed properties of these transients. We calculate the light curves and spectra for five models of NS–CO-WD mergers. The small yields of 56Ni (few $\times 10^{-3}\, {\rm M_{\odot }}$) result in faint, rapidly evolving reddened transients (RRTs) with B(R) peak magnitudes of at most ∼−12 (−13) to ∼−13 (−15), much shorter and fainter than both regular and faint/peculiar Type Ia supernovae. These transients are likely to be accompanied by several months long, 1–2 mag dimmer red/infrared afterglows. We show that the spectra of RRTs share some similarities with rapidly evolving transients such as SN 2010X, although RRTs are significantly fainter, especially in the I/R bands, and show far stronger Si lines. We estimate that the upcoming Large Synoptic Survey Telescope could detect RRTs at a rate of up to ∼10–70 yr−1 through observations in the R/I bands. The qualitative agreement between the SPH and flash approaches supports the earlier hydrodynamical studies of these systems.

Formation of Carbon-Enhanced Metal-Poor RR Lyrae Stars

Carbon-enhanced metal-poor (CEMP) stars are considered to be related to the first generation of stars, and responsible for the chemical evolution of the early Galaxy. More than half of them are in binaries, and could be explained by the binary evolution, but the formation channel of them is still not fully understood. Among the hundreds of CEMP stars, there are nine CEMP RR Lyrae stars identified, and at least seven of which are very likely not binaries. The usual binary star evolution channel is difficult to produce such a single star, particularly that of carbon enrichment. One way in which such a single star might be produced is the merger of a helium white dwarf with a Hertzsprung gap (HG) star. We use a stellar evolution program to calculate the models of the merger remnants, and find that the models can reproduce the observed distribution of these CEMP single RR Lyrae stars in terms of surface temperature, gravity, and carbon abundance. Hence, it is extremely possible that the helium white dwarf and HG star merger model is one of the formation channels of the metal-poor carbon-rich RR Lyrae stars.

The impact of the environment of white dwarf mergers on fast radio bursts

ABSTRACT Fast radio bursts (FRBs) are transient intense radio pulses with duration of milliseconds. Although, the first FRB was detected more than a decade ago, the progenitors of these energetic events are not yet known. The currently preferred formation channel involves the formation of a neutron star (NS)/magnetar. While these objects are often the end product of the core-collapse (CC) explosion of massive stars, they could also be the outcome of the merging of two massive white dwarfs. In the merger scenario the ejected material interacts with a constant-density circumbinary medium and creates supersonic shocks. We found that when a radio pulse passes through these shocks the dispersion measure (DM) increases with time during the free expansion phase. The rotation measure (RM) displays a similar trend if the power-law index, n, of the outer part of the ejecta is >6. For n = 6, the RM remains constant during this phase. Later, when the ejecta move into the Sedov–Taylor phase while the DM still increases, however, with a different rate, the RM reduces. This behaviour is somewhat similar to that of FRB 121102 for which a marginal increase of DM and a 10 per cent decrease of RM have been observed over time. These features are in contrast to the CC scenario, where the DM and RM contributions to the radio signal always diminish with time.

The Optically Thick Rotating Magnetic Wind from a Massive White Dwarf Merger Product

Abstract WD J005311 is a newly identified white dwarf (WD) in a mid-infrared nebula. The spectroscopic observation indicates the existence of a neon-enriched carbon/oxygen wind with a terminal velocity of ∼ 16,000 and a mass-loss rate of . Here we consistently explain the properties of WD J005311 using a newly constructed wind solution, where the optically thick outflow is launched from the carbon-burning shell on an oxygen–neon core and accelerated by the rotating magnetic field to become supersonic and unbound well below the photosphere. Our model implies that WD J005311 has a mass of M * ∼ 1.1–1.3 M ⊙, a magnetic field of B * ∼ (2–5) × 107 G, and a spin angular frequency of Ω ∼ 0.2–0.5 s−1. The large magnetic field and fast spin support the carbon–oxygen WD merger origin. WD J005311 will neither explode as a type Ia supernova nor collapse into a neutron star. If the wind continues to blow another few kyr, WD J005311 will spin down significantly and join to the known sequence of slowly rotating magnetic WDs. Otherwise it may appear as a fast-spinning magnetic WD and could be a new high-energy source.

Detectability of neutron star — White dwarf coalescences by eROSITA and ART-XC

Coalescences of neutron stars and white dwarfs are relatively frequent phenomena, outnumbering other types of compact object mergers (neutron stars and black holes without involving white dwarfs) altogether. Such event potentially can produce not only optical, but also an X-ray burst. Transient source CDF-S XT2 (Xue et al., 2019) can be an example of this type of events as suggested by Fernández et al. (2019). In this note we estimate the rate of these transients in the field of view of X-ray instruments on-board Spectrum-RG satellite. We demonstrate that during four years of the survey program several thousand of events related to neutron star — white dwarf mergers might appear in the field of view of eROSITA. Collimation of X-ray emission can reduce this number. Smaller, but comparable number of transients is expected in the case of ART-XC telescope. However, due to relatively short duration — ≲104 s, — mostly such transients might be visible just in one scan of telescopes (∼40 s), and so only a few photons are expected to be detected which makes definite identification without additional information very problematic.

The Impact of White Dwarf Natal Kicks and Stellar Flybys on the Rates of Type Ia Supernovae in Triple-star Systems

Abstract SNe Ia could arise from mergers of carbon–oxygen white dwarfs (WDs) triggered by Lidov–Kozai (LK) oscillations in hierarchical triple-star systems. However, predicted merger rates are several orders of magnitude lower than the observed SNe Ia rate. The low predicted rates can be attributed in part to the fact that many potential WD-merger progenitor systems, with high mutual orbital inclination, merge or interact before the WD stage. Recently, evidence was found for the existence of natal kicks imparted on WDs with a typical magnitude of 0.75 km s−1. In triples, kicks change the mutual inclination and in general increase the outer orbit eccentricity, bringing the triple into an active LK regime at late stages and avoiding the issue of pre-WD merger or interaction. Stars passing by the triple can result in similar effects. However, both processes can also disrupt the triple. In this paper, we quantitatively investigate the impact of WD kicks and flybys on the rate of WD mergers using detailed simulations. We find that WD kicks and flybys combine to increase the predicted WD merger rates by a factor of ∼2.5, resulting in a time-integrated rate of ≈1.1 × 10−4 M ⊙ −1. Despite the significant boost, the predicted rates are still more than one order of magnitude below the observed rate of ∼10−3 M ⊙ −1. However, many systematic uncertainties still remain in our calculations, in particular the potential contributions from tighter triples, dynamically unstable systems, unbound systems due to WD kicks, and quadruple systems.

Hot Subdwarf Stars Observed in Gaia DR2 and LAMOST DR5

Abstract Combing Gaia DR2 with LAMOST DR5, we spectroscopically identified 924 hot subdwarf stars, among which 32 stars exhibit strong double-lined composite spectra. We measured the effective temperature T eff, surface gravity , helium abundance y = nHe/nH, and radial velocities (RVs) of 892 non-composite spectra hot subdwarf stars by fitting LAMOST observations with Tlusty/Synspec non-LTE synthetic spectra. We outlined four different groups in the diagram with our helium abundance classification scheme and two nearly parallel sequences in the diagram. 3D Galactic space motions and orbits of 747 hot subdwarf stars with (G BP − G RP)0 < −0.36 mag were computed using LAMOST RVs and Gaia parallaxes and proper motions. Based on the U − V velocity diagram, J z −eccentricity diagram, and Galactic orbits, we derived Galactic population classifications and the fractional distributions of the four hot subdwarf helium groups in the halo, thin disk, and thick disk. Comparisons with the predictions of binary population synthesis calculations suggest that He-rich hot subdwarf stars with are from the double helium white dwarfs (HeWDs) merger, He-deficient hot subdwarf stars with from the common envelope ejection, and He-deficient hot subdwarf stars with from the stable Roche lobe overflow channels. The relative number of He-rich hot subdwarf stars with and in the halo is more than twice the prediction of Zhang et al. by more than six times in the thin disk, which implies that the mergers of HeWDs with low-mass main-sequence stars may not be the main formation channel for He-rich hot subdwarf stars with , especially in younger environments.

Nature of the unusual transient AT 2018cow from HI observations of its host galaxy

Context.Unusual stellar explosions represent an opportunity to learn about both stellar and galaxy evolution. Mapping the atomic gas in host galaxies of such transients can lead to an understanding of the conditions that trigger them.Aims.We provide resolved atomic gas observations of the host galaxy, CGCG137-068, of the unusual and poorly understood transient AT 2018cow, which we obtained in searching for clues to understand its nature. We test whether it is consistent with a recent inflow of atomic gas from the intergalactic medium, as suggested for host galaxies of gamma-ray bursts (GRBs) and some supernovae (SNe).Methods.We observed the HIhyperfine structure line of the AT 2018cow host with the Giant Metrewave Radio Telescope.Results.There is no unusual atomic gas concentration near the position of AT 2018cow. The gas distribution is much more regular than the distributions of GRB/SN hosts. The AT 2018cow host has an atomic gas mass lower by 0.24 dex than predicted from its star formation rate (SFR) and is at the lower edge of the galaxy main sequence. In the continuum we detected the emission of AT 2018cow and of a star-forming region in the north-eastern part of the bar (away from AT 2018cow). This region hosts a third of the galaxy’s SFR.Conclusions.The absence of atomic gas concentration close to AT 2018cow, along with a normal SFR and regular HIvelocity field, sets CGCG137-068 apart from GRB/SN hosts studied in HI. The environment of AT 2018cow therefore suggests that its progenitor may not have been a massive star. Our findings are consistent with an origin of the transient that does not require a connection between its progenitor and gas concentration or inflow: an exploding low-mass star, a tidal disruption event, a merger of white dwarfs, or a merger between a neutron star and a giant star. We interpret the recently reported atomic gas ring in CGCG 137-068 as a result of internal processes connected with gravitational resonances caused by the bar.