High Field Magnetic White Dwarfs Research Articles

The Double White Dwarf Merger Progenitors of SDSS J2211+1136 and ZTF J1901+1458

Double white dwarf (DWD) mergers are possibly the leading formation channel of massive, rapidly rotating, high-field magnetic white dwarfs (HFMWDs). However, a direct link connecting a DWD merger to any observed HFMWD is still missing. We here show that the HFMWDs SDSS J221141.80+113604.4 (hereafter J2211+1136) and ZTF J190132.9+145808.7 (hereafter J1901+1458) might be DWD merger products. J2211+1136 is a 1.27 M ⊙ white dwarf (WD) with a rotation period of 70.32 s and a surface magnetic field of 15 MG. J1901+1458 is a 1.327–1.365 M ⊙ WD with a rotation period of 416.20 s, and a surface magnetic field in the range 600–900 MG. With the assumption of single-star evolution and the currently measured WD masses and surface temperatures, the cooling ages of J2211+1136 and J1901+1458 are, respectively, 2.61–2.85 Gyr and 10–100 Myr. We hypothesize that these WDs are DWD merger products and compute the evolution of the postmerged configuration formed by a central WD surrounded by a disk. We show that the postmerger system evolves through three phases depending on whether accretion, mass ejection (propeller), or magnetic braking dominates the torque onto the central WD. We calculate the time the WD spends in each of these phases and obtain the accretion rate and disk mass for which the WD rotational age, i.e., the total time elapsed since the merger to the instant where the WD central remnant reaches the current measured rotation period, agrees with the estimated WD cooling age. We infer the mass values of the primary and secondary WD components of the DWD merger that lead to a postmerger evolution consistent with the observations.

The pulsational properties of ultra-massive DB white dwarfs with carbon-oxygen cores coming from single-star evolution

Context. Ultra-massive white dwarfs are relevant for many reasons: their role as type Ia supernova progenitors, the occurrence of physical processes in the asymptotic giant branch phase, the existence of high-field magnetic white dwarfs, and the occurrence of double white dwarf mergers. Some hydrogen-rich ultra-massive white dwarfs are pulsating stars and, as such, they offer the possibility of studying their interiors through asteroseismology. On the other hand, pulsating helium-rich ultra-massive white dwarfs could be even more attractive objects for asteroseismology if they were found, as they should be hotter and less crystallized than pulsating hydrogen-rich white dwarfs, something that would pave the way for probing their deep interiors. Aims. We explore the pulsational properties of ultra-massive helium-rich white dwarfs with carbon-oxygen and oxygen-neon cores resulting from single stellar evolution. Our goal is to provide a theoretical basis that could eventually help to discern the core composition of ultra-massive white dwarfs and the scenario of their formation through asteroseismology, anticipating the possible future detection of pulsations in helium-rich ultra-massive white dwarfs. Methods. We focus on three scenarios for the formation of helium-rich ultra-massive white dwarfs. First, we consider stellar models coming from two recently proposed single-star evolution scenarios for the formation of ultra-massive white dwarfs with carbon-oxygen cores that involve the rotation of the degenerate core after core helium burning and reduced mass-loss rates in massive asymptotic giant branch stars. Finally, we contemplate ultra-massive oxygen-neon core white-dwarf models resulting from standard single-star evolution. We compute the adiabatic pulsation gravity-mode periods for models in a range of effective temperatures, embracing the instability strip of average-mass pulsating helium-rich white dwarfs, and we compare the characteristics of the mode-trapping properties for models of different formation scenarios through the analysis of the period spacing. Results. Given that the white dwarf models coming from the three scenarios considered are characterized by distinct core chemical profiles, we find that their pulsation properties are also different, thus leading to distinctive signatures in the period-spacing and mode-trapping properties. Conclusions. Our results indicate that in the case of an eventual detection of pulsating ultra-massive helium-rich white dwarfs, it would be possible to derive valuable information encrypted in the core of these stars in connection with the origin of such exotic objects. This is of the utmost importance regarding recent evidence for the existence of a population of ultra-massive white dwarfs with carbon-oxygen cores. There will soon be many opportunities to detect pulsations in these stars through observations collected with ongoing space missions.

The formation of ultra-massive carbon-oxygen core white dwarfs and their evolutionary and pulsational properties

Context.The existence of ultra-massive white dwarf stars,MWD ≳ 1.05 M⊙, has been reported in several studies. These white dwarfs are relevant for the role they play in type Ia supernova explosions, the occurrence of physical processes in the asymptotic giant-branch phase, the existence of high-field magnetic white dwarfs, and the occurrence of double-white-dwarf mergers.Aims.We aim to explore the formation of ultra-massive, carbon-oxygen core white dwarfs resulting from single stellar evolution. We also intend to study their evolutionary and pulsational properties and compare them with those of the ultra-massive white dwarfs with oxygen-neon cores resulting from carbon burning in single progenitor stars, and with binary merger predictions. The aim is to provide a theoretical basis that can eventually help to discern the core composition of ultra-massive white dwarfs and the circumstances of their formation.Methods.We considered two single-star evolution scenarios for the formation of ultra-massive carbon-oxygen core white dwarfs, which involve the rotation of the degenerate core after core helium burning and reduced mass-loss rates in massive asymptotic giant-branch stars. We find that reducing standard mass-loss rates by a factor larger than 5−20 yields the formation of carbon-oxygen cores more massive than 1.05 M⊙as a result of the slow growth of carbon-oxygen core mass during the thermal pulses. We also performed a series of evolutionary tests of solar-metallicity models with initial masses between 4 and 9.5M⊙and with different core rotation rates. We find that ultra-massive carbon-oxygen core white dwarfs are formed even for the lowest rotation rates we analyzed, and that the range of initial masses leading to these white dwarfs widens as the rotation rate of the core increases, whereas the initial mass range for the formation of oxygen-neon core white dwarfs decreases significantly. Finally, we compared our findings with the predictions from ultra-massive white dwarfs resulting from the merger of two equal-mass carbon-oxygen core white dwarfs, by assuming complete mixing between them and a carbon-oxygen core for the merged remnant.Results.These two single-evolution scenarios produce ultra-massive white dwarfs with different carbon-oxygen profiles and different helium contents, thus leading to distinctive signatures in the period spectrum and mode-trapping properties of pulsating hydrogen-rich white dwarfs. The resulting ultra-massive carbon-oxygen core white dwarfs evolve markedly slower than their oxygen-neon counterparts.Conclusions.Our study strongly suggests the formation of ultra-massive white dwarfs with carbon-oxygen cores from a single stellar evolution. We find that both the evolutionary and pulsation properties of these white dwarfs are markedly different from those of their oxygen-neon core counterparts and from those white dwarfs with carbon-oxygen cores that might result from double-degenerate mergers. This can eventually be used to discern the core composition of ultra-massive white dwarfs and their formation scenario.

Hydrodynamic simulations of disrupted planetary accretion discs inside the core of an AGB star

ABSTRACT Volume complete sky surveys provide evidence for a binary origin for the formation of isolated white dwarfs with magnetic fields in excess of a MegaGauss. Interestingly, not a single high-field magnetic white dwarf has been found in a detached system, suggesting that if the progenitors are indeed binaries, the companion must be removed or merge during formation. An origin scenario consistent with observations involves the engulfment, inspiral, and subsequent tidal disruption of a low-mass companion in the interior of a giant star during a common envelope phase. Material from the shredded companion forms a cold accretion disc embedded in the hot ambient around the proto-white dwarf. Entrainment of hot material may evaporate the disc before it can sufficiently amplify the magnetic field, which typically requires at least a few orbits of the disc. Using three-dimensional hydrodynamic simulations of accretion discs with masses between 1 and 10 times the mass of Jupiter inside the core of an Asymptotic Giant Branch star, we find that the discs survive for at least 10 orbits (and likely for 100 orbits), sufficient for strong magnetic fields to develop.

Origin of magnetic fields in cataclysmic variables

In a series of recent papers, it has been proposed that high field magnetic white dwarfs are the result of close binary interaction and merging. Population synthesis calculations have shown that the origin of isolated highly magnetic white dwarfs is consistent with the stellar merging hypothesis. In this picture, the observed fields are caused by an alpha-Omega dynamo driven by differential rotation. The strongest fields arise when the differential rotation equals the critical break-up velocity and result from the merging of two stars (one of which has a degenerate core) during common envelope evolution or from the merging of two white dwarfs. We now synthesise a population of binary systems to investigate the hypothesis that the magnetic fields in the magnetic cataclysmic variables also originate during stellar interaction in the common envelope phase. Those systems that emerge from common envelope more tightly bound form the cataclysmic variables with the strongest magnetic fields. We vary the common envelope efficiency parameter and compare the results of our population syntheses with observations of magnetic cataclysmic variables. We find that common envelope interaction can explain the observed characteristics of these magnetic systems if the envelope ejection efficiency is low.

Genesis of magnetic fields in isolated white dwarfs

A dynamo mechanism driven by differential rotation when stars merge has been proposed to explain the presence of strong fields in certain classes of magnetic stars. In the case of the high field magnetic white dwarfs (HFMWDs), the site of the differential rotation has been variously thought to be the common envelope, the hot outer regions of a merged degenerate core or an accretion disc formed by a tidally disrupted companion that is subsequently accreted by a degenerate core. We have shown previously that the observed incidence of magnetism and the mass distribution in HFMWDs are consistent with the hypothesis that they are the result of merging binaries during common envelope evolution. Here we calculate the magnetic field strengths generated by common envelope interactions for synthetic populations using a simple prescription for the generation of fields and find that the observed magnetic field distribution is also consistent with the stellar merging hypothesis. We use the Kolmogorov-Smirnov test to study the correlation between the calculated and the observed field strengths and find that it is consistent for low envelope ejection efficiency. We also suggest that field generation by the plunging of a giant gaseous planet on to a white dwarf may explain why magnetism among cool white dwarfs (including DZ white dwarfs) is higher than among hot white dwarfs. In this picture a super Jupiter residing in the outer regions of the planetary system of the white dwarf is perturbed into a highly eccentric orbit by a close stellar encounter and is later accreted by the white dwarf.

ON THE EVOLUTION OF MAGNETIC WHITE DWARFS

We present the first radiation magnetohydrodynamics simulations of the atmosphere of white dwarf stars. We demonstrate that convective energy transfer is seriously impeded by magnetic fields when the plasma-beta parameter, the thermal to magnetic pressure ratio, becomes smaller than unity. The critical field strength that inhibits convection in the photosphere of white dwarfs is in the range B = 1-50 kG, which is much smaller than the typical 1-1000 MG field strengths observed in magnetic white dwarfs, implying that these objects have radiative atmospheres. We have then employed evolutionary models to study the cooling process of high-field magnetic white dwarfs, where convection is entirely suppressed during the full evolution (B > 10 MG). We find that the inhibition of convection has no effect on cooling rates until the effective temperature (Teff) reaches a value of around 5500 K. In this regime, the standard convective sequences start to deviate from the ones without convection owing to the convective coupling between the outer layers and the degenerate reservoir of thermal energy. Since no magnetic white dwarfs are currently known at the low temperatures where this coupling significantly changes the evolution, effects of magnetism on cooling rates are not expected to be observed. This result contrasts with a recent suggestion that magnetic white dwarfs with Teff < 10,000 K cool significantly slower than non-magnetic degenerates.

ENIGMAS FROM THE SLOAN DIGITAL SKY SURVEY DR7 KLEINMAN WHITE DWARF CATALOG

We report results from a continuation of our searches for high field magnetic white dwarfs paired in a detached binary with non degenerate companions. We made use of the Sloan Digital Sky Survey DR7 catalog of Kleinman et al. (2013) with 19,712 spectroscopically-identified white dwarfs. These include 1,735 white dwarf plus M dwarf detached pairs (almost 10\% of the Kleinman at al.'s list). No new pairs were found, although we did recover the polar (AM~Herculis system) ST\,LMi in a low state of accretion. With the larger sample the original situation reported ten years ago remains intact now at a much higher level of statistical significance: in the selected SDSS sample, high field magnetic white dwarfs are not found in an apparently-detached pairing with an M dwarf, unless they are a magnetic CV in a low state of accretion. This finding strengthens the case that the fields in the isolated high field magnetic white dwarfs are generated by stellar mergers but also raises questions on the nature of the progenitors of the magnetic CVs.

Merging binary stars and the magnetic white dwarfs

A magnetic dynamo driven by differential rotation generated when stars merge can explain strong fields in certain classes of magnetic stars, including the high field magnetic white dwarfs (HFMWDs). In their case the site of the differential rotation has been variously proposed to be within a common envelope, the massive hot outer regions of a merged degenerate core or an accretion disc formed by a tidally disrupted companion that is subsequently incorporated into a degenerate core. We synthesize a population of binary systems to investigate the stellar merging hypothesis for observed single HFMWDs. Our calculations provide mass distribution and the fractions of white dwarfs that merge during a common envelope phase or as double degenerate systems in a post common envelope phase. We vary the common envelope efficiency parameter alpha and compare with observations. We find that this hypothesis can explain both the observed incidence of magnetism and the mass distribution of HFMWDs for a wide range of alpha. In this model, the majority of the HFMWDs are of the Carbon Oxygen type and merge within a common envelope. Less than about a quarter of a per cent of HFMWDs originate from double degenerate stars that merge after common envelope evolution and these populate the high-mass tail of the HFMWD mass distribution.

Si:P as a laboratory analogue for hydrogen on high magnetic field white dwarf stars

Laboratory spectroscopy of atomic hydrogen in a magnetic flux density of 10(5) T (1 gigagauss), the maximum observed on high-field magnetic white dwarfs, is impossible because practically available fields are about a thousand times less. In this regime, the cyclotron and binding energies become equal. Here we demonstrate Lyman series spectra for phosphorus impurities in silicon up to the equivalent field, which is scaled to 32.8 T by the effective mass and dielectric constant. The spectra reproduce the high-field theory for free hydrogen, with quadratic Zeeman splitting and strong mixing of spherical harmonics. They show the way for experiments on He and H(2) analogues, and for investigation of He(2), a bound molecule predicted under extreme field conditions.

High-field magnetic white dwarfs as the progeny of early-type stars?

Abstract We present an analysis of the newly resolved components of two hot, double-degenerate systems, SDSS J074853.07+302543.5 + J074852.95+302543.4 and SDSS J150813.24+394504.9 + J150813.31+394505.6 (CBS 229). We confirm that each system has widely separated components (a &amp;gt; 100 au) consisting of a H-rich, non-magnetic white dwarf and a H-rich, high-field magnetic white dwarf (HFMWD). The masses of the non-magnetic degenerates are found to be larger than typical of field white dwarfs. We use these components to estimate the total ages of the binaries and demonstrate that both magnetic white dwarfs are the progeny of stars with Minit &amp;gt; 2 M⊙. We briefly discuss the traits of all known hot, wide, magnetic + non-magnetic double degenerates in the context of HFMWD formation theories. These are broadly consistent (chance probability, P ≈ 0.065) with HFMWDs forming primarily from early-type stars and, in the most succinct interpretation, link their magnetism to the fields of their progenitors. Our results do not, however, rule out that HFMWDs can form through close binary interactions and studies of more young, wide double degenerates are required to reach firm conclusions on these formation pathways.

DOUBLE DEGENERATE MERGERS AS PROGENITORS OF HIGH-FIELD MAGNETIC WHITE DWARFS

High-field magnetic white dwarfs have been long suspected to be the result of stellar mergers. However, the nature of the coalescing stars and the precise mechanism that produces the magnetic field are still unknown. Here we show that the hot, convective, differentially rotating corona present in the outer layers of the remnant of the merger of two degenerate cores is able to produce magnetic fields of the required strength that do not decay for long timescales. We also show, using an state-of-the-art Monte Carlo simulator, that the expected number of high-field magnetic white dwarfs produced in this way is consistent with that found in the Solar neighborhood.

Two new young, wide, magnetic + non-magnetic double-degenerate binary systems

Abridged: We report the discovery of two, new, rare, wide, double-degenerate binaries that each contain a magnetic and a non-magnetic star. The components of SDSSJ092646.88+132134.5 + J092647.00+132138.4 and SDSSJ150746.48+521002.1 + J150746.80+520958.0 have angular separations of only 4.6 arcsec (a~650AU) and 5.1 arcsec (a~750AU), respectively. They also appear to share common proper motions. Follow-up optical spectroscopy reveals each system to consist of a DA and a H-rich high-field magnetic white dwarf (HFMWD). Our measurements of the effective temperatures and the surface gravities of the DA components reveal both to have larger masses than are typical of field white dwarfs. By assuming that these degenerates have evolved essentially as single stars, due to their wide orbital separations, we use them to place limits on the total ages of our stellar systems. These argue that in each case the HFMWD is probably associated with an early type progenitor (M_init > 2M_solar). We find that the cooling time of SDSSJ150746.80+520958.0 (DAH) is somewhat lower than might be expected had it followed the evolutionary path of a typical single star. This mild discord is in the same sense as that observed for two of the small number of other HFMWDs for which progenitor mass estimates have been made, REJ0317-853 and EG59. The mass of the other DAH, SDSSJ092646.88+132134.5, appears to be smaller than expected on the basis of single star evolution. If this object was/is a member of a hierarchical triple system it may have experienced greater mass loss during an earlier phase of its life as a result of it having a close companion. The large uncertainties on our estimates of the parameters of the HFMWDs suggest a larger sample of these objects is required to firmly identify any trends in their inferred cooling times and progenitor masses.

A BINARY SCENARIO FOR THE FORMATION OF STRONGLY MAGNETIZED WHITE DWARFS

Since their initial discovery, the origin of isolated white dwarfs (WDs) with magnetic fields in excess of ~1 MG has remained a mystery. Recently, the formation of these high-field magnetic WDs has been observationally linked to strong binary interactions incurred during post-main-sequence evolution. Planetary, brown dwarf or stellar companions located within a few AU of main-sequence stars may become engulfed during the primary's expansion off the main sequence. Sufficiently low-mass companions in-spiral inside a common envelope until they are tidally shredded near the natal white dwarf. Formation of an accretion disk from the disrupted companion provides a source of turbulence and shear which act to amplify magnetic fields and transport them to the WD surface. We show that these disk-generated fields explain the observed range of magnetic field strengths for isolated, high-field magnetic WDs. Additionally, we discuss a high-mass binary analogue which generates a strongly-magnetized WD core inside a pre-collapse, massive star. Subsequent core-collapse to a neutron star may produce a magnetar.

Formation of high-field magnetic white dwarfs from common envelopes

The origin of highly magnetized white dwarfs has remained a mystery since their initial discovery. Recent observations indicate that the formation of high-field magnetic white dwarfs is intimately related to strong binary interactions during post-main-sequence phases of stellar evolution. If a low-mass companion, such as a planet, brown dwarf, or low-mass star, is engulfed by a post-main-sequence giant, gravitational torques in the envelope of the giant lead to a reduction of the companion's orbit. Sufficiently low-mass companions in-spiral until they are shredded by the strong gravitational tides near the white dwarf core. Subsequent formation of a super-Eddington accretion disk from the disrupted companion inside a common envelope can dramatically amplify magnetic fields via a dynamo. Here, we show that these disk-generated fields are sufficiently strong to explain the observed range of magnetic field strengths for isolated, high-field magnetic white dwarfs. A higher-mass binary analogue may also contribute to the origin of magnetar fields.

Binary star origin of high field magnetic white dwarfs

White dwarfs with surface magnetic fields in excess of $1 $MG are found as isolated single stars and relatively more often in magnetic cataclysmic variables. Some 1,253 white dwarfs with a detached low-mass main-sequence companion are identified in the Sloan Digital Sky Survey but none of these is observed to show evidence for Zeeman splitting of hydrogen lines associated with a magnetic field in excess of 1MG. If such high magnetic fields on white dwarfs result from the isolated evolution of a single star then there should be the same fraction of high field white dwarfs among this SDSS binary sample as among single stars. Thus we deduce that the origin of such high magnetic fields must be intimately tied to the formation of cataclysmic variables. CVs emerge from common envelope evolution as very close but detached binary stars that are then brought together by magnetic braking or gravitational radiation. We propose that the smaller the orbital separation at the end of the common envelope phase, the stronger the magnetic field. The magnetic cataclysmic variables originate from those common envelope systems that almost merge. We propose further that those common envelope systems that do merge are the progenitors of the single high field white dwarfs. Thus all highly magnetic white dwarfs, be they single stars or the components of MCVs, have a binary origin. This hypothesis also accounts for the relative dearth of single white dwarfs with fields of 10,000 - 1,000,000G. Such intermediate-field white dwarfs are found preferentially in cataclysmic variables. In addition the bias towards higher masses for highly magnetic white dwarfs is expected if a fraction of these form when two degenerate cores merge in a common envelope. Similar scenarios may account for very high field neutron stars.

High-Resolution Spectra of Bright Central Stars of Bipolar Planetary Nebulae and the Question of Magnetic Shaping

We present ESO New Technology Telescope high-resolution echelle spectroscopy of the central stars (CSs) of eight southern bipolar planetary nebulae (PNe) selected for their asymmetry. Our aim was to determine or place limits on the magnetic fields of the CSs of these nebulae, and hence to explore the role played by magnetic fields in nebular morphology and PN shaping. If magnetic fields do play a role, we expect these CSs to have fields in the range 102-107 G from magnetic flux conservation on the reasonable assumption that they must evolve into the high-field magnetic white dwarfs. We were able to place an upper limit of ≈20,000 G on the magnetic fields of the central stars of He 2-64 and MyCn 18. The spectrum of He 2-64 also shows a P Cygni profile in He I λ5876 and λ6678, corresponding to an expanding photosphere with velocity ~100 km s-1. The detection of helium absorption lines in the spectrum of He 2-36 confirms the existence of a hot stellar component. We did not reach the necessary line detection for magnetic field analysis in the remaining objects. Overall, our results indicate that if magnetic fields are responsible for shaping bipolar planetary nebulae, these are not required to be greater than a few tens of kilogauss.

The origin of the magnetic fields in white dwarfs

Magnetic white dwarfs with fields in excess of ∼10 6 G (the high field magnetic white dwarfs; HFMWDs) constitute about ∼10 per cent of all white dwarfs and show a mass distribution with a mean mass of ∼0.93 M ○. compared to ∼0.56 M ○. for all white dwarfs. We investigate two possible explanations for these observations. First, that the initial-final mass relationship (IFMR) is influenced by the presence of a magnetic field and that the observed HFMWDs originate from stars on the main sequence that are recognized as magnetic (the chemically peculiar A and B stars). Secondly, that the IFMR is essentially unaffected by the presence of a magnetic field, and that the observed HFMWDs have progenitors that are not restricted to these groups of stars. Our calculations argue against the former hypothesis and support the latter. The HFMWDs have a higher than average mass because on the average they have more massive progenitors and not because the IFMR is significantly affected by the magnetic field. A requirement of our model is that ∼40 per cent of main-sequence stars more massive than ∼4.5 M ○. must either have magnetic fields in the range of ∼10-100 G, which is below the current level of detection, or generate fields during subsequent stellar evolution towards the white dwarf phase. In the former case, the magnetic fields of the HFMWDs could be fossil remnants from the main-sequence phase consistent with the approximate magnetic flux conservation.

Magnetic fields in white dwarfs and stellar evolution

The surface magnetic field strengths observed in the magnetic Ap, Bp stars (100–20 000 G) and in the high field magnetic white dwarfs (106–109 G) cover many decades but nevertheless the range of magnetic fluxes observed in each of these stellar groups is similar. An evolutionary link between them therefore appears plausible. For both groups of stars there is also information on field complexity. The magnetic white dwarfs in general show non-dipole field structures which can be best modelled if we assume contributions from higher order multipoles. The field structures of the Ap and Bp stars are similarly complex. We investigate the hypothesis that the magnetic fields of the white dwarfs could be fossil remnants from the main-sequence phase by focussing on the problem of how field complexity may arise and be maintained during evolution to the compact star state. We also address the question of to what extent magnetic fields seen in the early type stars could be fossil remnants from the pre-main-sequence phases of stellar evolution dating back perhaps to the time of star formation.

Magnetic fields and rotation in white dwarfs and neutron stars

Recent spectropolarimetric observations of Ap and Bp stars with improved sensitivity have suggested that most Ap and Bp stars are magnetic with dipolar fields of at least a few hundred gauss. These new estimates suggest that the range of magnetic fluxes found for the majority of magnetic white dwarfs is similar to that of main-sequence Ap‐Bp stars, thus strengthening the empirical evidence for an evolutionary link between magnetism on the main sequence and magnetism in white dwarfs. We draw parallels between the magnetic white dwarfs and the magnetic neutron stars and argue that the observed range of magnetic fields in isolated neutron stars (Bp ∼ 10 11 ‐10 15 G) could also be explained if their mainly O-type progenitors have effective dipolar fields in the range of a few gauss to a few kilogauss, assuming approximate magnetic flux conservation with the upper limit being consistent with the recent measurement of a field of Bp ∼ 1100 G for θ Orion C. In the magnetic field‐rotation diagram, the magnetic white dwarfs can be divided into three groups of different origin: a significant group of strongly magnetized slow rotators (P rot ∼ 50 ‐100 yr) that have originated from single-star evolution, a group of strongly magnetized fast rotators (P rot ∼ 700 s), typified by EUVE J0317‐853, that have originated from a merger, and a group of modest rotators (P rot ∼ hours‐days) of mixed origin (single-star and CV-type binary evolution). We propose that the neutron stars may similarly divide into distinct classes at birth, and suggest that the magnetars may be the counterparts of the slowly rotating high-field magnetic white dwarfs. Ke yw ords: stars: magnetic fields ‐ stars: neutron ‐ white dwarfs.