White Dwarf Research Articles

Local 2D Simulations of the Ignition of a Helium Shell Detonation on a White Dwarf by an Impacting Stream

Local 2D Simulations of the Ignition of a Helium Shell Detonation on a White Dwarf by an Impacting Stream

A highly magnetized long-period radio transient exhibiting unusual emission features.

Long-period radio transients are a new class of astrophysical objects that exhibit periodic radio emission on timescales of tens of minutes. Their true nature remains unknown; possibilities include magnetic white dwarfs, binary systems, or long-period magnetars; the latter class is predicted to produce fast radio bursts (FRBs). Using the MeerKAT radio telescope, we conducted follow-up observations of the long-period radio transient GPM J1839-10. Here we report that the source exhibits a wide range of unusual emission properties, including polarization characteristics indicative of magnetospheric origin, linear-to-circular polarization conversion, and drifting substructures closely resembling those observed in repeating FRBs. These radio characteristics provide evidence in support of the long-period magnetar model and suggest a possible connection between long-period radio transients, magnetars, and FRBs.

Increased Surface Temperatures of Habitable White Dwarf Worlds Relative to Main-sequence Exoplanets

Abstract Discoveries of giant planet candidates orbiting white dwarf (WD) stars and the demonstrated capabilities of the James Webb Space Telescope bring the possibility of detecting rocky planets in the habitable zones (HZs) of WDs into pertinent focus. We present simulations of an aqua planet with an Earth-like atmospheric composition and incident stellar insolation orbiting in the HZ of two different types of stars—a 5000 K WD and main-sequence K-dwarf star Kepler-62 (K62) with a similar effective temperature—and identify the mechanisms responsible for the two differing planetary climates. The synchronously rotating WD planet's global mean surface temperature is 25 K higher than that of the synchronously rotating planet orbiting K62, due to its much faster (10 hr) rotation and orbital period. This ultrafast rotation generates strong zonal winds and meridional flux of zonal momentum, stretching out and homogenizing the scale of atmospheric circulation, and preventing an equivalent buildup of thick, liquid water clouds on the dayside of the planet compared to the synchronous planet orbiting K62, while also transporting heat equatorward from higher latitudes. White dwarfs may therefore present amenable environments for life on planets formed within or migrated to their HZs, generating warmer surface environments than those of planets with main-sequence hosts to compensate for an ever shrinking incident stellar flux.

Discovery and characterization of ZTF J0112+5827: An 80.9-minute polar with strong cyclotron features

A new X-ray Cataclysmic variable (CV) candidate exhibits distinct light-curve characteristics in the ZTF's g, r, and i bands. The paper includes the optical identification and multiwavelength analysis of this CV candidate. This work aims to determine if a previously identified CV candidate, ZTF J0112+5827, is a polar system by examining its X-ray and cyclotron radiation characteristics. We characterized the X-ray emission of ZTF J0112+5827 using the ROSAT observations. The gri-band optical light curves were obtained from the Zwicky Transient Facility. After two nights of time-domain spectroscopic observations with the Palomar 200-inch telescope, we mapped the accretion structures using Doppler tomography. ZTF J0112+5827 exhibits an orbital period of 80.9 minutes, determined from the ZTF light curves, and an average X-ray flux of $(68.4 ± 15.7) $ erg s^-1 cm^-2 in the 0.1–2.4 keV range. It shows an ellipsoidal-like variability curve in the g band, with two prominent humps around phases of ∼0.0 and ∼0.7 in the i and r bands. In the spectra corresponding to these phases, a redward-increasing power-law continuum appears, which is accompanied by prominent features of cyclotron emission humps. Emission lines of He II and Balmer series were observed. The magnetic field strength of ZTF J0112+5827 was determined from the cyclotron harmonics. Its tomography map revealed the presence of accretion streams, but there was no evidence of an accretion disk structure. The line-of-sight velocity of the Balmer emission was measured at about 500 km s^-1, the majority of which was contributed by accretion streams and accretion spots. Our result confirms that ZTF J0112+5827 is a polar system. It contains a magnetic white dwarf with a magnetic field strength of $ 38.7_ MG$.

Predicting the Rate of Fast Radio Bursts in Globular Clusters from Binary Black Hole Observations

Abstract The repeating fast radio burst (FRB) source in an old globular cluster (GC) in M81 proves that FRBs, which are typically associated with young magnetars, can also occur in old stellar populations. A potential explanation is super-Chandrasekhar binary white dwarf (BWD) coalescences, which may produce FRB-emitting neutron stars. GCs can also give rise to binary black hole (BBH) mergers detectable with gravitational waves, and the BWD coalescence rate from GCs is correlated with their BBH merger rate. For the first time, we combine independent observations of gravitational waves and FRBs to infer the origins of FRB sources. We use GC formation histories inferred from BBH observations to predict the rate of super-Chandrasekhar BWD coalescences originating from GCs as a function of redshift. We explore mass-loss and mass-conserved scenarios for BWD coalescences and find that the coalescence rates evolve differently across redshift in these two cases. In the mass-loss scenario, the BWD coalescence rates decrease with increasing redshift, similar to some recent measurements of the FRB rate as a function of redshift. We show that GCs could contribute ≲1% to the total FRB source formation rates in the local Universe. Our multimessenger approach also offers a novel method to better constrain the GC population using both FRB and gravitational-wave observations.

Distinguishing Compact Objects in Extreme-Mass-Ratio Inspirals by Gravitational Waves

Extreme-mass-ratio inspirals (EMRIs) are promising gravitational-wave (GW) sources for space-based GW detectors. EMRI signals typically have long durations, ranging from several months to several years, necessitating highly accurate GW signal templates for detection. In most waveform models, compact objects in EMRIs are treated as test particles without accounting for their spin, mass quadrupole, or tidal deformation. In this study, we simulate GW signals from EMRIs by incorporating the spin and mass quadrupole moments of the compact objects. We evaluate the accuracy of parameter estimation for these simulated waveforms using the Fisher Information Matrix (FIM) and find that the spin, tidal-induced quadruple, and spin-induced quadruple can all be measured with precision ranging from 10−2 to 10−1, particularly for a mass ratio of ∼10−4. Assuming the “true” GW signals originate from an extended body inspiraling into a supermassive black hole, we compute the signal-to-noise ratio (SNR) and Bayes factors between a test-particle waveform template and our model, which includes the spin and quadrupole of the compact object. Our results show that the spin of compact objects can produce detectable deviations in the waveforms across all object types, while tidal-induced quadrupoles are only significant for white dwarfs, especially in cases approaching an intermediate-mass ratio. Spin-induced quadrupoles, however, have negligible effects on the waveforms. Therefore, our findings suggest that it is possible to distinguish primordial black holes from white dwarfs, and, under certain conditions, neutron stars can also be differentiated from primordial black holes.

The Blue Lurker WOCS 14020: A Long-period Post-common-envelope Binary in M67 Originating from a Merger in a Triple System

Abstract We present Hubble Space Telescope far-ultraviolet (FUV) spectra of a blue lurker–white dwarf (BL–WD) binary system in the 4 Gyr open cluster M67. We fit the FUV spectrum of the WD, determining it is a C/O WD with a mass of 0.7 2 − 0.04 + 0.05 M ⊙ and a cooling age of ~400 Myr. This requires a WD progenitor of ~3 M ⊙, significantly larger than the current cluster turnoff mass of 1.3 M ⊙. We suggest the WD progenitor star formed several hundred megayears ago via the merger of two stars near the turnoff of the cluster. In this scenario, the original progenitor system was a hierarchical triple consisting of a close, near-equal-mass inner binary, with a tertiary companion with an orbit of a few thousand days. The WD is descended from the merged inner binary, and the original tertiary is now the observed BL. The likely formation scenario involves a common envelope while the WD progenitor is on the AGB, and thus the observed orbital period of 359 days requires an efficient common envelope ejection. The rapid rotation of the BL indicates it accreted some material during its evolution, perhaps via a wind prior to the common envelope. This system will likely undergo a second common envelope in the future and thus could result in a short-period double WD binary or merger of a 0.72 M ⊙ C/O WD and a 0.38 M ⊙ helium WD, making this a potential progenitor of an interesting transient such as a sub-Chandrasekhar Type Ia supernova.

Astrophysical systematics on testing general relativity with gravitational waves from Galactic double white dwarfs

Astrophysical systematics on testing general relativity with gravitational waves from Galactic double white dwarfs

The Variable X-Ray Emission of the Cataclysmic Variable V1460 Her with a Rapidly Rotating White Dwarf

The Variable X-Ray Emission of the Cataclysmic Variable V1460 Her with a Rapidly Rotating White Dwarf

Gamma-Ray Bursts and Kilonovae from the Accretion-induced Collapse of White Dwarfs

Gamma-Ray Bursts and Kilonovae from the Accretion-induced Collapse of White Dwarfs

Strong signature of right-handed circularly polarized photoionization close to the cyclotron line in the atmosphere of magnetic white dwarfs

Magnetic fields break the symmetry of the interaction of atoms with photons with different polarizations, yielding chirality and anisotropy properties. The dependence of the absorption spectrum on the polarization, a phenomenon known as dichroism, is present in the atmosphere of magnetic white dwarfs. Its evaluation for processes in the continuum spectrum has been elusive so far due to the absence of appropriate ionization equilibrium models and incomplete data on photoionization cross sections. We combined rigorous solutions to the equilibrium of atomic populations with approximate cross sections to calculate the absolute opacity due to photoionization in a magnetized hydrogen gas. We predict a strong right-handed circularly polarized absorption (χ+) formed blueward of the cyclotron resonance for fields from about 14 to several hundred megagauss. In energies lower than the cyclotron fundamental, this absorption shows a deep trough with respect to linear and left-handed circular polarizations that steepens with the field strength. The jump in χ+ is due to the confluence of a large number of photoionization continua produced by right-handed circularly polarized transitions from atomic states with a nonnegative magnetic quantum number toward different Landau levels.

Detectability of Self-lensing Flares of White Dwarfs with Compact Companions

Binaries containing compact objects, viewed nearly edge on, can produce periodic brightening events under certain conditions on the masses, radii, and binary separation. Such flares are caused by one object gravitationally lensing another, in what is known as self-lensing flares. We present a simulation tool that efficiently reproduces the main features of self-lensing flares and facilitates a detection sensitivity analysis for various sky surveys. We estimate the detection prospects for a handful of representative surveys when searching for systems of either two white dwarfs or a white dwarf with other compact objects, i.e., neutron stars and black holes. We find only a marginal ability to detect such systems in existing surveys. However, we estimate many such systems could be detectable by surveys in the near future, including the Vera Rubin Legacy Survey of Space and Time (LSST). We provide a quantitative analysis of the detectability of double-compact object self-lensing flares across the landscape of system parameters, and a qualitative discussion of survey and follow-up approaches to distinguish such flares from confounding events, such as stellar flares, satellite glints, and cosmic rays. We estimate 0.3, 3 and 247 double white dwarf systems could be detected by Transiting Exoplanet Survey Satellite, Zwicky Transient Facility, and LSST, respectively. A similar number of systems with a neutron star or black hole companion could be detected, but we caution that the number densities of such binaries is model dependent and so are our detection estimates. Such binaries can be used to constrain models of the end states of binary evolution.

Producing Type Ia Supernovae from Hybrid CONe White Dwarfs with Main-sequence Binary Companions at Low Metallicity of Z = 0.0001

Abstract The nature of progenitors of Type Ia supernovae (SNe Ia) and their explosion mechanism remains unclear. It has been suggested that SNe Ia may have resulted from thermonuclear explosions of hybrid carbon–oxygen–neon white dwarfs (CONe WDs) when they grow in mass to approach the Chandrasekhar mass limit by accreting matter from a binary main-sequence (MS) companion. In this work, we combine the results of detailed binary evolution calculations with population synthesis models to investigate the rates and delay times of SNe Ia in the CONe WD + MS channel at a low metallicity environment of Z = 0.0001. For a constant star formation rate of 5 M ⊙ yr−1, our calculations predict that the SN Ia rates in the CONe WD + MS channel at low metallicity of Z = 0.0001 is about 0.11−3.89 × 10−4 yr−1. In addition, delay times in this channel cover a wide range of 0.05−2.5 Gyr. We further compare our results to those given by a previous study for the CONe WD + MS channel with a higher metallicity of Z = 0.02 to explore the influence of metallicity on the results. We find that these two metallicity environments give a slight difference in rates and delay times of SNe Ia from the CONe WD + MS channel, although SNe Ia produced at a low metallicity environment of Z = 0.0001 have relatively longer delay times.

A Time-dependent Spectral Analysis of γ Cassiopeiae

We investigated the temporal and spectral features of γ Cassiopeiae’s X-ray emission within the context of the white dwarf (WD) accretion hypothesis. We find that the variabilities present in the X-ray data show two different signals, one primarily due to absorption and the other due to flickering like in nonmagnetic cataclysmic variables. We then use this two-component insight to investigate previously unreported simultaneous XMM and NuSTAR data. The results of model fitting find WD properties consistent with optical studies alongside a significant secondary, thermal source. We propose a secondary shock between the Be decretion disk and WD accretion disk as the source. Finally, we analyzed a unique, low-count-rate event of the XMM light curve as potential evidence for the WD encountering Be decretion disk structures.

Bremsstrahlung emission from nucleon-nucleus reactions in dense medium of compact stars

Abstract Bremsstrahlung photons emitted during nucleon-nucleus reactions in compact star are investigated.
Influence of density of stellar medium on intensity of emission is studied at first time in quantum approach.
Bremsstrahlung model is generalized, where new term describing influence of stellar medium is added to interactions between nucleons of nucleus (in frameworks of nuclear model of deformed oscillatoric shells).
Polytropic EOS, Chandrasekar EOS and Harrison-Wheeler EOS are applied for calculations.
Unified EOS of neutron-star matter of Haensel and Potekhin based on FPS an SLy EOSs is used for tests.
Bremsstrahlung calculations are tested on existed measurements of bremsstrahlung in the scattering of protons off the \isotope[197]{Au} nuclei at energy of proton beam of $E_{\rm p}=190$~MeV.
Many properties of bremsstrahlung emitted from nuclear processes in stellar medium of compact stars are studied at first time.
In particular, the spectra of photons in the scattering of protons and neutrons off \isotope[4]{He}, \isotope[8]{Be}, \isotope[12]{C}, \isotope[16]{O}, \isotope[24]{Mg}, \isotope[40]{Ca}, \isotope[56]{Fe} are estimated in dependence on density of stellar medium.
Medium of white dwarfs has small influence on the bremsstrahlung emission from nuclear processes, while
bremsstrahlung emission is intensive in neutron stars and it is changed essentially in dependence on stellar density and structure.

Gravitational collapse of white dwarfs to neutron stars: From initial conditions to explosions with neutrino-radiation hydrodynamics simulations

Abstract Using general relativistic neutrino-radiation hydrodynamics simulations with the multi-group M1 scheme in one dimension, we investigate the collapse of massive, fully convective, and non-rotating white dwarfs (WDs), which are formed by accretion-induced collapse or merger-induced collapse, and the subsequent explosion. We produce initial WDs in hydrostatic equilibrium, which have super-Chandrasekhar mass and are about to collapse. The WDs have masses of $1.6\, M_{\odot }$ with different initial central densities specifically at $1.0\times 10^{10}$, $4.0\times 10^{9}$, $2.0\times 10^{9}$, and $1.0\times 10^{9}\:\mbox{g}\:\mbox{cm}^{-3}$. First, we examine the stability of initial WD in case weak interactions are turned off. Secondly, we calculate the collapse of WDs with weak interactions. We employ hydrodynamics simulations with Newtonian gravity in the first and second steps. Thirdly, we calculate the formation of neutron stars and accompanying explosions with general relativistic simulations. As a result, WDs with the highest density of $10^{10}\:\mbox{g}\:\mbox{cm}^{-3}$ collapse not by weak interactions but by the photodissociation of the iron, and three WDs with low central densities collapse by the electron capture as expected at the second step and succeed in the explosion with a small explosion energy of $\sim\! 10^{48}\:$erg at the third step. By changing the surrounding environment of WDs, we find that there is a minimum value of ejecta masses, which is $\sim\! 10^{-5}\, M_{\odot }$. With the most elaborate simulations of this kind so far, this value is one to two orders of magnitude smaller than previously reported values and is compatible with the estimated ejecta mass from FRB 121102.

Constraining the Galactic Structure Using Time Domain Gravitational Wave Signal from Double White Dwarfs Detected by Space Gravitational Wave Detectors

The gravitational wave (GW) signals from a large number of double white dwarfs (DWDs) in the Galaxy are expected to be detected by space GW detectors, e.g., the Laser Interferometer Space Antenna (LISA), Taiji, and Tianqin in the millihertz band. In this paper, we present an alternative method by directly using the time-domain GW signal detected by space GW detectors to constrain the anisotropic structure of the Galaxy. The information of anisotropic distribution of DWDs is naturally encoded in the time-domain GW signal because of the variation of the detectors’ directions and consequently the pattern functions due to their annual motion around the Sun. The direct use of the time-domain GW signal enables simple calculations, such as utilizing an analytical method to assess the noise arising from the superposition of random phases of DWDs and using appropriate weights to improve the constraints. We investigate the possible constraints on the scale of the Galactic thin disk and bulge that may be obtained from LISA and Taiji by using this method with mock signals obtained from population synthesis models. We further show the different constraining capabilities of the low-frequency signal (foreground) and the high-frequency signal (resolvable sources) via the Markov Chain Monte Carlo method, and find that the scale height and length of the Galactic thin disk and the scale radius of the bulge can be constrained to a fractional accuracy of ∼30%, 30%, 40% (or 20%, 10%, 40%) by using the low-frequency (or high-frequency) signal detected by LISA or Taiji.

White Dwarf envelops and temperature corrections in exponential f(T) gravity

White Dwarf envelops and temperature corrections in exponential f(T) gravity

Merging white dwarf binaries produce Type Ia supernovae in elliptical galaxies

ABSTRACT I find that Type Ia supernovae (SNe Ia) with bimodal nebular emission profiles occur almost exclusively in massive (${\rm M_\star } \gtrsim 10^{11}~{\rm M_\odot }$) galaxies with low star formation rates (SFR $\lesssim 0.5~{\rm M_\odot }$ yr−1). The bimodal profiles are likely produced by two white dwarfs (WDs) that exploded during a merger or collision, supported by a correlation between the peak-to-peak velocity separation ($v_{\rm sep}$) and the SN Ia peak luminosity ($M_V$) which arises naturally from more massive WD binaries synthesizing more $^{56}$Ni during the explosion. The distributions of SNe Ia with and without bimodal nebular lines differ in host mass, SFR, and specific SFR with Kolmogorov–Smirnov test probabilities of $3.1{{\ \rm per\ cent}}$, $0.03{{\ \rm per\ cent}}$, and $0.02{{\ \rm per\ cent}}$, respectively. Viewing angle effects can fully explain the SNe Ia in quiescent hosts without bimodal emission profiles and the dearth of merger/collision driven SNe Ia in star-forming hosts requires at least two distinct progenitor channels for normal SNe Ia. $30\!-\!40{{\ \rm per\ cent}}$ of all SNe Ia originate from mergers or collisions depending on how cleanly host environment distinguishes progenitor scenarios. Existing models for WD mergers and collisions broadly reproduce the $v_{\rm sep}$–$M_V$ correlation and future analyses may be able to infer the masses/mass-ratios of merging WDs in external galaxies.

Search for hot subdwarf stars from SDSS images using a deep learning method: SwinBayesNet

Hot subdwarfs are essential for understanding the structure and evolution of low-mass stars, binary systems, astroseismology, and atmospheric diffusion processes. In recent years, deep learning has driven significant progress in hot subdwarf searches. However, most approaches tend to focus on modelling with spectral data, which are inherently more costly and scarce compared to photometric data. To maximise the reliable candidates, we used Sloan Digital Sky Survey (SDSS) photometric images to construct a two-stage hot subdwarf search model called SwinBayesNet, which combines the Swin Transformer and Bayesian neural networks. This model not only provides classification results but also estimates uncertainty. As negative examples for the model, we selected five classes of stars prone to confusion with hot subdwarfs, including O-type stars, B-type stars, A-type stars, white dwarfs (WDs), and blue horizontal branch stars (BHBs). On the test set, the two-stage model achieved F1 scores of 0.90 and 0.89 in the two-class and three-class classification stages, respectively. Subsequently, with the help of $Gaia$ DR3, a large-scale candidate search was conducted in SDSS DR17. We found 6804 hot-subdwarf candidates, including 601 new discoveries. Based on this, we applied a model threshold of 0.95 and Bayesian uncertainty estimation for further screening, refining the candidates to 3413 high-confidence objects, which include 331 new discoveries.