White Dwarf Mass Research Articles

The Optically Thick Rotating Magnetic Wind from a Massive White Dwarf Merger Product. II. Axisymmetric Magnetohydrodynamic Simulations

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.

Measuring the Mass–Radius Relation of White Dwarfs Using Wide Binaries

Measuring the mass–radius relation of individual white dwarfs is an empirically challenging task that has been performed for only a few dozen stars. We measure the white dwarf mass–radius relation using the gravitational redshifts and radii of 135 white dwarfs in wide binaries with main-sequence companions. We obtain the radial velocities of these systems using the main-sequence companion, and subtract these Doppler redshifts from the white dwarfs’ apparent motions, isolating their gravitational redshifts. We use Gaia data to calculate the surface temperatures and radii of these white dwarfs, thereby deriving an empirical gravitational redshift–radius relation. This work demonstrates the utility of low-resolution Galactic surveys to measure the white dwarf equation of state. Our results are consistent with theoretical models, and represent the largest sample of individual white dwarf gravitational redshift measurements to date.

A hidden population of massive white dwarfs: two spotted K+WD binaries

Abstract The identification and characterization of massive (≳ 0.8 M⊙) white dwarfs is challenging in part due to their low luminosity. Here we present two candidate single-lined spectroscopic binaries, Gaia DR3 4014708864481651840 and 5811237403155163520, with K-dwarf primaries and optically dark companions. Both have orbital periods of P ∼ 0.45 days and show rotational variability, ellipsoidal modulations, and high-amplitude radial velocity variations. Using light curves from the Transiting Exoplanet Survey Satellite (TESS), radial velocities from ground-based spectrographs, and spectral energy distributions, we characterize these binaries to describe the nature of the unseen companion. We find that both systems are consistent with a massive white dwarf companion. Unlike simple ellipsoidal variables, star spots cause the light curve morphology to change between TESS sectors. We attempt to constrain the orbital inclination using PHOEBE binary light curve models, but degeneracies in the light curves of spotted stars prevent a precise determination. Finally, we search for similar objects using Gaia DR3 and TESS, and comment on these systems in the context of recently claimed compact object binaries.

Formation of PSR J1012+5307 with an Extremely Low-mass White Dwarf: Testing Magnetic Braking Models

PSR J1012+5307 is a millisecond pulsar with an extremely low-mass white dwarf (WD) companion in an orbit of 14.5 hr. Magnetic braking (MB) plays an important role in influencing the orbital evolution of binary systems with a low-mass (≲1–2 M ⊙) donor star. At present, there exist several different MB descriptions. In this paper, we investigate the formation of PSR J1012+5307 as a probe to test plausible MB models. Employing a detailed stellar evolution model with the MESA code, we find that the convection- and rotation-boosted MB and “Intermediate” MB models can reproduce the WD mass, WD radius, WD surface gravity, neutron star mass, and orbital period observed in PSR J1012+5307. However, our simulated WD has a higher effective temperature than the observations. Three other MB mechanisms, including the standard MB model, are too weak to account for the observed orbital period in the Hubble time. A long cooling timescale caused by H-shell flashes of the WD may alleviate the discrepancy between the simulated effective temperature and the observed value.

Rotation plays a role in the generation of magnetic fields in single white dwarfs

ABSTRACT Recent surveys of close white dwarf binaries as well as single white dwarfs have provided evidence for the late appearance of magnetic fields in white dwarfs, and a possible generation mechanism, a crystallization and rotation-driven dynamo has been suggested. A key prediction of this dynamo is that magnetic white dwarfs rotate, at least on average, faster than their non-magnetic counterparts and/or that the magnetic field strength increases with rotation. Here we present rotation periods of ten white dwarfs within 40 pc measured using photometric variations. Eight of the light curves come from TESS observations and are thus not biased towards short periods, in contrast to most period estimates that have been reported previously in the literature. These TESS spin periods are indeed systematically shorter than those of non-magnetic white dwarfs. This means that the crystallization and rotation-driven dynamo could be responsible for a fraction of the magnetic fields in white dwarfs. However, the full sample of magnetic white dwarfs also contains slowly rotating strongly magnetic white dwarfs which indicates that another mechanism that leads to the late appearance of magnetic white dwarfs might be at work, either in addition to or instead of the dynamo. The fast-spinning and massive magnetic white dwarfs that appear in the literature form a small fraction of magnetic white dwarfs, and probably result from a channel related to white dwarf mergers.

Dynamical masses across the Hertzsprung–Russell diagram

ABSTRACT We infer the dynamical masses of stars across the Hertzsprung–Russell (H–R) diagram using wide binaries from the Gaia survey. Gaia’s high-precision astrometry measures the wide binaries’ orbital motion, which contains the mass information. Using wide binaries as the training sample, we measure the mass of stars across the 2D H–R diagram using the combination of statistical inference and neural networks. Our results provide the dynamical mass measurements for main-sequence stars from 0.1 to 2 M⊙, unresolved binaries, and unresolved triples on the main sequence, and the mean masses of giants and white dwarfs. Two regions in the H–R diagram show interesting behaviours in mass, where one of them is pre-main-sequence stars, and the other one may be related to close compact object companions like M dwarf-white dwarf binaries. These mass measurements depend solely on Newtonian dynamics with minimal assumptions on eccentricities, providing independent constraints on stellar evolutionary models, and the occurrence rate of compact objects.

Classification and parameterization of a large Gaia sample of white dwarfs using XP spectra

Context. The latest Gaia data release in July 2022, DR3, in addition to the refinement of the astrometric and photometric parameters from DR2, added a number of important data products to those available in earlier releases, including radial velocity data, information on stellar multiplicity, and XP spectra of a selected sample of stars. Gaia has proved to be an important search tool for white dwarf stars, which are readily identifiable from their absolute G magnitudes as low luminosity objects in the Hertzsprung–Russell (H–R) diagram. Each data release has yielded large catalogs of white dwarfs, containing several hundred thousand objects, far in excess of the numbers known from all previous surveys (∼40 000). While the normal Gaia photometry (G, GBP, and GRP bands) and astrometry can be used to identify white dwarfs with high confidence, it is much more difficult to parameterize the stars and determine the white dwarf spectral type from this information alone. Observing all stars in these catalogs with follow-up spectroscopy and photometry is also a huge logistical challenge with current facilities. Aims. The availability of the XP spectra and synthetic photometry presents an opportunity for a more detailed spectral classification and measurement of the effective temperature and surface gravity of Gaia white dwarfs. Methods. A magnitude limit of G < 17.6 was applied to the routine production of XP spectra for Gaia sources, which would have excluded most white dwarfs. Therefore, we created a catalog of 100 000 high-quality white dwarf identifications for which XP spectra were processed, with a magnitude limit of G < 20.5. Synthetic photometry was computed for all these stars, from the XP spectra, in Johnson, SDSS, and J-PAS, published as the Gaia Synthetic Photometry Catalog – White Dwarfs (GSPC-WD). We took this catalog and applied machine learning techniques to provide a classification of all the stars from the XP spectra. We have then applied an automated spectral fitting program, with χ-squared minimization, to measure their physical parameters (effective temperature and log g) from which we could estimate the white dwarf masses and radii. Results. We present the results of this work, demonstrating the power of being able to classify and parameterize such a large sample of ≈100 000 stars. We describe what we can learn about the white dwarf population from this dataset. We also explored the uncertainties in the process and the limitations of the dataset.

The Common Envelope Evolution Outcome. II. Short-orbital-period Hot Subdwarf B Binaries Reveal a Clear Picture

Common envelope evolution (CEE) is vital for forming short-orbital-period compact binaries. It covers many objects, such as double compact merging binaries, Type Ia supernovae progenitors, binary pulsars, and X-ray binaries. Knowledge of the common envelope (CE) ejection efficiency still needs to be improved, though progress has been made recently. Short-orbital-period hot subdwarf B star (sdB) plus white dwarf (WD) binaries are the most straightforward samples with which to constrain CEE physics. We apply the known orbital period–WD mass relation to constrain the sdB progenitors of seven sdB+WD binaries with a known inclination angle. The average CE efficiency parameter is 0.32. This is consistent with previous studies. However, the CE efficiency need not be constant, but a function of the initial mass ratio, based on well-constrained sdB progenitor mass and evolutionary stage. Our results can be used as physical inputs for binary population synthesis simulations of related objects. A similar method can also be applied to study other short-orbital-period WD binaries.

A possible critical heating from decay reaction and new upper limit of the magnetic monopole flux for the supermassive white dwarfs

We present the new upper limit of the magnetic monopole (MM) flux and discuss three MM models of the heating resource for supermassive white dwarfs (WDs) by considering the effect of temperature on thermonuclear reaction and mass radius relation of WDs based on the catalytic nuclear decay by MM. We discuss the luminosity and compared it with the observations to apply to 25 supermassive WDs. We find the maxnium of the number of MM captured can be 9.6943×1011\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$9.6943\ imes 10^{11}$$\\end{document}, and 9.0671×1011\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$9.0671\ imes 10^{11}$$\\end{document} for O+Ne core high mass WDs (e.g., WD J055631.17+130639.78), and C+O core high mass WDs (e.g., WD J055631.17+130639.78), respectively. The luminosities increase with the increasing of the temperature and are agreed well with the observations for model (III). The differences are no more than one, and three orders of magnitude higher than observations for model (III), and (I, II), respectively. Finaly, we find that the maxnium of the upper limits of the MM flux ϕm\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\phi _{m}$$\\end{document} due to RC effect can be 9.1071×10-15\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$9.1071\ imes 10^{-15}$$\\end{document}, and 2.7670×10-14\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$2.7670\ imes 10^{-14}$$\\end{document} for O+Ne and C+O core high mass WDs, respectively. Our results are about one and two orders of magnitude higher than those of Abbasi et al. (EPJC 69:361, 2010) (Albert et al. in JHEP 07:054, 2017) for O+Ne, and C+O core mass WDs, respectively, and can be about three and four orders of magnitude higher than those of Aartsen et al. (EPJC 76:133, 2016) (Ic40, Ic86), respectively. Our results show that the monopole-catalyzed nucleon decay could prevent WDs from cooling down into a stellar graveyard by keeping them hot.

Study of Blue Metal-poor Stars Using UVIT/AstroSat

Blue metal-poor stars are main-sequence stars that are bluer and brighter than typical turn-off stars in metal-poor globular clusters. They are thought to have either evolved through post-mass transfer mechanisms as field blue straggler stars or have accreted from Milky Way dwarf satellite galaxies. It has been found that a considerable fraction of blue metal poor stars are binaries, possibly with a compact companion. We observed 27 blue metal poor stars using UV imaging telescope of AstroSat in two far-UV filters, F148W and F169M. In this work, we explain the possible formation channels of two stars, BMP17 and BMP37. We fit BMP17 to a single-component spectral energy distribution whereas BMP37 with a binary-component spectral energy distribution. As both of them are known SB1s, we suggest that the WD companion of BMP17 may have cooled down so that it is out of UV imaging telescope detection limit. On the other hand, we discover a normal mass white dwarf as the hot companion of BMP37, indicating mass transfer as the possible formation channel.

Discovery of Four Pulsars in a Pilot Survey at Intermediate Galactic Latitudes with FAST

We present the discovery and timing results of four pulsars discovered in a pilot survey at intermediate Galactic latitudes with the Five-hundred Aperture Spherical Telescope (FAST). Among these pulsars, two belong to the category of millisecond pulsars (MSPs) with spin periods of less than 20 ms. The other two fall under the classification of “mildly recycled” pulsars, with massive white dwarfs as companions. Remarkably, this small survey, covering an area of 4.7 deg2, led to the discovery of four recycled pulsars. Such success underscores the immense potential of future surveys at intermediate Galactic latitudes. In order to assess the potential yield of MSPs, we conducted population simulations and found that both FAST and Parkes new phased array feed surveys, focusing on intermediate Galactic latitudes, have the capacity to uncover several hundred new MSPs.

Wide post-common envelope binaries containing ultramassive white dwarfs: evidence for efficient envelope ejection in massive asymptotic giant branch stars

ABSTRACT Post-common envelope binaries (PCEBs) containing a white dwarf (WD) and a main-sequence (MS) star can constrain the physics of common envelope evolution and calibrate binary evolution models. Most PCEBs studied to date have short orbital periods (Porb ≲ 1 d), implying relatively inefficient harnessing of binaries’ orbital energy for envelope expulsion. Here, we present follow-up observations of five binaries from 3rd data release of Gaia mission containing solar-type MS stars and probable ultramassive WDs ($M\gtrsim 1.2\ {\rm M}_{\odot}$) with significantly wider orbits than previously known PCEBs, Porb = 18–49 d. The WD masses are much higher than expected for systems formed via stable mass transfer at these periods, and their near-circular orbits suggest partial tidal circularization when the WD progenitors were giants. These properties strongly suggest that the binaries are PCEBs. Forming PCEBs at such wide separations requires highly efficient envelope ejection, and we find that the observed periods can only be explained if a significant fraction of the energy released when the envelope recombines goes into ejecting it. Our one-dimensional stellar models including recombination energy confirm prior predictions that a wide range of PCEB orbital periods, extending up to months or years, can potentially result from Roche lobe overflow of a luminous asymptotic giant branch (AGB) star. This evolutionary scenario may also explain the formation of several wide WD + MS binaries discovered via self-lensing, as well as a significant fraction of post-AGB binaries and barium stars.

Strong Carbon Features and a Red Early Color in the Underluminous Type Ia SN 2022xkq

We present optical, infrared, ultraviolet, and radio observations of SN 2022xkq, an underluminous fast-declining Type Ia supernova (SN Ia) in NGC 1784 (D ≈ 31 Mpc), from <1 to 180 days after explosion. The high-cadence observations of SN 2022xkq, a photometrically transitional and spectroscopically 91bg-like SN Ia, cover the first days and weeks following explosion, which are critical to distinguishing between explosion scenarios. The early light curve of SN 2022xkq has a red early color and exhibits a flux excess that is more prominent in redder bands; this is the first time such a feature has been seen in a transitional/91bg-like SN Ia. We also present 92 optical and 19 near-infrared (NIR) spectra, beginning 0.4 days after explosion in the optical and 2.6 days after explosion in the NIR. SN 2022xkq exhibits a long-lived C i 1.0693 μm feature that persists until 5 days post-maximum. We also detect C ii λ6580 in the pre-maximum optical spectra. These lines are evidence for unburnt carbon that is difficult to reconcile with the double detonation of a sub-Chandrasekhar mass white dwarf. No existing explosion model can fully explain the photometric and spectroscopic data set of SN 2022xkq, but the considerable breadth of the observations is ideal for furthering our understanding of the processes that produce faint SNe Ia.

Ultra-low-mass and small-radius white dwarfs made of heavy elements

Seven possible ultra-low-mass and small-radius white dwarfs have been recently identified, with masses ranging from ∼0.02 M⊙ to ∼0.08 M⊙ and radii ranging from ∼ 4,270 km to 10670 km. The mass–radius measurements of these white dwarfs pose challenges to traditional white dwarf models, assuming they are mostly made of nuclei lighter than 56Fe. In this work, we consider the possibility that those white dwarfs are made of heavier elements. Due to the small charge-to-mass ratios in heavy elements, the electron number density in white dwarf matter is effectively reduced, which reduces the pressure with additional contributions of lattice energy and electron polarization corrections. This consequently leads to white dwarfs with much smaller masses and radii, which coincide with the seven ultra-low-mass and small-radius white dwarfs. The mass of the most massive white dwarfs is effectively reduced and could possibly account for the sub-Chandrasekhar progenitors in underluminous Type Ia supernovae. The corresponding equation of state and matter contents of dense stellar matter with and without reaching the cold-catalyzed ground state are presented, which are obtained using the latest Atomic Mass Evaluation (AME 2020). Further observations are necessary to unveil the actual matter contents in those white dwarfs via, e.g., spectroscopy, asteroseismology, and the discoveries of other ultra-low-mass and small-radius white dwarfs.

Electromagnetic Characterization of the LISA Verification Binary ZTF J0526+5934

We present an analysis of new and archival data to the 20.506 minute LISA verification binary J052610.42+593445.32 (J0526+5934). Our joint spectroscopic and photometric analysis finds that the binary contains an unseen M 1 = 0.89 ± 0.11 M ⊙ CO-core white dwarf primary with an M 2 = 0.38 ± 0.07 M ⊙ post-core-burning subdwarf, or low-mass white dwarf, companion. Given the short orbital period and relatively large total binary mass, we find that LISA will detect this binary with signal-to-noise ratio 44 after 4 yr of observations. J0526+5934 is expected to merge within 1.8 ± 0.3 Myr and likely result in a D6 scenario Type Ia supernova or form a He-rich star that will evolve into a massive single white dwarf.

Spectra of V1405 Cas at the Very Beginning Indicate a Low-mass ONeMg White Dwarf Progenitor

The lowest possible mass of ONeMg white dwarfs (WDs) has not been clarified despite its importance in the formation and evolution of WDs. We tackle this issue by studying the properties of V1405 Cas (Nova Cassiopeiae 2021), which is an outlier given a combination of its very slow light-curve evolution and the recently reported neon-nova identification. We report its rapid spectral evolution in the initial phase, covering 9.88, 23.77, 33.94, 53.53, 71.79, and 81.90 hr after the discovery. The first spectrum is characterized by lines from highly ionized species, most noticeably He ii and N iii. These lines are quickly replaced by lower-ionization lines, e.g., N ii, Si ii, and O i. In addition, Al ii (6237 Å) starts emerging as an emission line at the second epoch. We perform emission-line strength diagnostics, showing that the density and temperature quickly decrease toward later epochs. This behavior, together with the decreasing velocity seen in Hα, Hβ, and He i, indicates that the initial nova dynamics is reasonably well described by an expanding fireball on top of an expanding photosphere. Interestingly, the strengths of the N iii and Al ii indicate large enhancement in abundance, pointing to a ONeMg WD progenitor as is consistent with its neon-nova classification. Given its low-mass nature inferred by the slow light-curve evolution and relatively narrow emission lines, it provides a challenge to the stellar evolution theory that predicts the lower limit of the ONeMg WD mass being ∼1.1 M ⊙.

Asteroseismic Study of KUV03442+0719 with Parallax Constraints

Hydrogen atmosphere white dwarf KUV03442+0719 was first reported as a pulsator by Gianninas et al. in 2006. Follow-up campaigns by Su et al. revealed more periods. Some spectroscopic results suggest that KUV03442+0719 has a slightly below-average mass and an effective temperature of 11,000 K. However, Gaia data (parallax and magnitude) suggest that it may be a low-mass white dwarf. Such an object would have a helium core. We perform asteroseismic fitting of KUV03442+0719, modeling it both as a carbon/oxygen normal mass white dwarf, and a helium-core low-mass white dwarf. To perform the study, we perform a grid search with WDEC models, refined by simplex minimization of the best fits. Both analyses result in best-fit models that are comparable in terms of quality of fit. More pulsation data would be required to allow us to distinguish between the two scenarios. We present and contrast our results with expectations from stellar evolution. We also provide analytic formulae for a temperature-dependent mass–radius relationship for helium-core white dwarfs.

Eruptive novae in symbiotic systems

ABSTRACT We conduct numerical simulations of multiple nova eruptions in detached, widely separated symbiotic systems that include an asymptotic giant branch (AGB) companion to investigate the impact of white dwarf (WD) mass and binary separation on the evolution of the system. The accretion rate is determined using the Bondi–Hoyle–Lyttleton method, incorporating orbital momentum loss caused by factors such as gravitational radiation, magnetic braking, and drag. The WD in such a system accretes matter coming from the strong wind of an AGB companion until it finishes shedding its envelope. This occurs on an evolutionary time-scale of ≈3 × 105 yr. Throughout all simulations, we use a consistent AGB model with an initial mass of 1.0 M⊙ while varying the WD mass and binary separation, as they are the critical factors influencing nova eruption behaviour. We find that the accretion rate fluctuates between high and low rates during the evolutionary period, significantly impacted by the AGB’s mass loss rate. We show that unlike novae in cataclysmic variables, the orbital period may either increase or decrease during evolution, depending on the model, while the separation consistently decreases. Furthermore, we have identified cases in which the WDs produce weak, non-ejective novae and experience mass gain. This suggests that provided the accretion efficiency can be achieved by a more massive WD and maintained for long enough, they could potentially serve as progenitors for type Ia supernovae.

The donor star radial velocity curve in the cataclysmic variable GY Cnc confirms white dwarf eclipse modelling mass

ABSTRACT A large number of white dwarf and donor masses in cataclysmic variables have been found via modelling the primary eclipse, a method that relies on untested assumptions. Recent measurements of the mass of the white dwarf in the cataclysmic variable GY Cnc, obtained via modelling its ultraviolet spectrum, conflict with the mass obtained via modelling the eclipse light curve. Here we measure the radial velocity of the absorption lines from the donor star in GY Cnc to be Kabs = 280 ± 2 km s−1, in excellent agreement with the prediction based on the masses derived from modelling the eclipse light curve. It is possible that the white dwarf mass derived from the ultraviolet spectrum of GY Cnc is affected by the difficulty of disentangling the white dwarf spectrum from the accretion disc spectrum.

Initial-final mass relation from white dwarfs within 40 pc

ABSTRACT We present an initial-final mass relation derived from the spectroscopically complete volume-limited 40 pc sample of white dwarfs. The relation is modelled using population synthesis methods to derive an initial stellar population which can be fit to the observed mass distribution of white dwarfs. The population synthesis accounts for binary evolution, where higher mass white dwarfs are more likely to be merger products than their lower mass counterparts. Uncertainties are accounted from the initial mass function, stellar metallicity, and age of the Galactic disc. We also consider biases induced by the spectral type of the white dwarf where pure-hydrogen atmosphere white dwarfs are likely to have more accurate masses, whilst the full white dwarf sample will have fewer biases arising from spectral evolution. We provide a four-piece segmented linear regression using Monte Carlo methods to sample the 1-σ range of uncertainty on the initial stellar population. The derived initial-final mass relation provides a self-consistent determination of the progenitor mass for white dwarfs in the Solar neighbourhood which will be useful to study the local stellar formation history.