In this paper we review the current status of research on the observational and theoretical characteristics of isolated and binary magnetic white dwarfs (MWDs). Magnetic fields of isolated MWDs are observed to lie in the range 10^3-10^9G. While the upper limit cutoff appears to be real, the lower limit is more difficult to investigate. The incidence of magnetism below a few 10^3G still needs to be established by sensitive spectropolarimetric surveys conducted on 8m class telescopes. Highly magnetic WDs tend to exhibit a complex and non-dipolar field structure with some objects showing the presence of higher order multipoles. There is no evidence that fields of highly magnetic WDs decay over time, which is consistent with the estimated Ohmic decay times scales of ~10^11 yrs. MWDs, as a class, also appear to be more massive than their weakly or non-magnetic counterparts. MWDs are also found in binary systems where they accrete matter from a low-mass donor star. These binaries, called magnetic Cataclysmic Variables (MCVs) and comprise about 20-25\% of all known CVs. Zeeman and cyclotron spectroscopy of MCVs have revealed the presence of fields in the range $\sim 7-230$\,MG. Complex field geometries have been inferred in the high field MCVs (the polars) whilst magnetic field strength and structure in the lower field group (intermediate polars, IPs) are much harder to establish. The origin of fields in MWDs is still being debated. While the fossil field hypothesis remains an attractive possibility, field generation within the common envelope of a binary system has been gaining momentum, since it would explain the absence of MWDs paired with non-degenerate companions and also the lack of relatively wide pre-MCVs.Comment: 73 pages, 22 figures, 2 large tables. Invited review chapter on Magnetic White Dwarfs to appear in Space Science Reviews, Springe

ABSTRACT. Since the discovery of the Ðrst isolated magnetic white dwarf (MWD) Grw ]70¡8047 nearly 60 years ago, the number of stars belonging to this class has grown steadily. There are now some 65 isolated white dwarfs classiÐed as magnetic, and a roughly equal number of MWDs are found in the close interacting binaries known as the magnetic cataclysmic variables (MCVs). The isolated MWDs comprise D5% of all WDs, while the MCVs comprise D25% of all CVs. The magnetic Ðelds range from D3 ] 104È109 G in the former group with a distribution peaking at 1.6 ] 107 G, and D107È3 ] 108 G in the latter group. The space density of isolated magnetic white dwarfs with Ðelds in the range D3 ] 104È109 G is estimated to be D1.5 ] 10~4 pc~3. The MCVs have a space density that is about a hundred times smaller.About 80% of the isolated MWDs have almost pure H atmospheres and show only hydrogen lines in their spectra (the magnetic DAs), while the remainder show He I lines (the magnetic DBs) or molecular bands of and CH (magnetic DQs) and have helium as the dominant atmospheric constituent, mirroring the C 2 situation in the nonmagnetic white dwarfs. The incidence of stars of mixed composition (H and He) appears to be higher among the MWDs.There is growing evidence based on trigonometric parallaxes, space motions, and spectroscopic analyses that the isolated MWDs tend as a class to have a higher mass than the nonmagnetic white dwarfs. The mean mass for 16 MWDs with well-constrained masses is Magnetic Ðelds may therefore play a Z0.95 M _ . signiÐcant role in angular momentum and mass loss in the postÈmain-sequence phases of single star evolution a †ecting the initial-Ðnal mass relationship, a view supported by recent work on cluster MWDs. The progenitors of the vast majority of the isolated MWDs are likely to be the magnetic Ap and Bp stars. However, the discovery of two MWDs with masses within a few percent of the Chandrasekhar limit, one of which is also rapidly rotating minutes), has led to the proposal that these may be the result of (P spin \ 12 double-degenerate (DD) mergers. An intriguing possibility is that magnetism, through its e †ect on the initial-Ðnal mass relationship, may also favor the formation of more massive double degenerates in close binary evolution. The magnetic DDs may therefore be more likely progenitors of Type Ia supernovae.A subclass of the isolated MWDs appear to rotate slowly with no evidence of spectral or polarimetric variability over periods of tens of years, while others exhibit rapid rotation with coherent periods in the range of tens of minutes to hours or days. There is a strong suggestion of a bimodal period distribution. The "" rapidly ÏÏ rotating isolated MWDs may include as a subclass stars which have been spun up during a DD merger or a previous phase of mass transfer from a companion star.Zeeman spectroscopy and polarimetry, and cyclotron spectroscopy, have variously been used to estimate magnetic Ðelds of the isolated MWDs and the MWDs in MCVs and to place strong constraints on the Ðeld str...

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 (CVs). Some 1253 white dwarfs with a detached low-mass main-sequence companion are identified in the Sloan Digital Sky Survey (SDSS) but none of these is observed to show evidence for Zeeman splitting of hydrogen lines associated with a magnetic field in excess of 1 MG. 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 magnetic 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 CVs. The formation of a CV must involve orbital shrinkage from giant star to main-sequence star dimensions. It is believed that this shrinkage occurs as the low-mass companion and the white dwarf spiral together inside a common envelope. CVs emerge 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 CVs 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 magnetic white dwarfs. Thus, all highly magnetic white dwarfs, be they single stars or the components of magnetic CVs, have a binary origin. This hypothesis also accounts for the relative dearth of single white dwarfs with fields of 10 4 -10 6 G. Such intermediate-field white dwarfs are found preferentially in CVs. 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. From the space density of single highly magnetic white dwarfs we estimate that about three times as many common envelope events lead to a merged core as to a CV.

Recent studies of white dwarfs in open clusters have provided new constraints on the initial–final mass relationship (IFMR) for main‐sequence stars with masses in the range 2.5–6.5 M⊙. We re‐evaluate the ensemble of data that determines the IFMR and argue that the IFMR can be characterized by a mean IFMR about which there is an intrinsic scatter. We investigate the consequences of the IFMR for the observed mass distribution of field white dwarfs using population synthesis calculations. We show that while a linear IFMR predicts a mass distribution that is in reasonable agreement with the recent results from the Palomar–Green survey, the data are better fitted by an IFMR with some curvature. Our calculations indicate that a significant (∼28) percentage of white dwarfs originating from a single star evolution has masses in excess of ∼0.8 M⊙, obviating the necessity for postulating the existence of a dominant population of high‐mass white dwarfs that arise from binary star mergers.

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Observations of magnetic A, B and O stars show that the poloidal magnetic flux per unit mass Φ p /M appears to have an upper bound of approximately 10 −6.5 G cm 2 g −1 . A similar upper bound to the total flux per unit mass is found for the magnetic white dwarfs even though the highest magnetic field strengths at their surfaces are much larger. For magnetic A and B stars there also appears to be a well defined lower bound below which the incidence of magnetism declines rapidly. According to recent hypotheses, both groups of stars may result from merging stars and owe their strong magnetism to fields generated by a dynamo mechanism as they merge. We postulate a simple dynamo that generates magnetic field from differential rotation. We limit the growth of magnetic fields by the requirement that the poloidal field stabilizes the toroidal and vice versa. While magnetic torques dissipate the differential rotation, toroidal field is generated from poloidal by an Ω dynamo. We further suppose that mechanisms that lead to the decay of toroidal field lead to the generation of poloidal. Both poloidal and toroidal fields reach a stable configuration which is independent of the size of small initial seed fields but proportional to the initial differential rotation. We pose the hypothesis that strongly magnetic stars form from the merging of two stellar objects. The highest fields are generated when the merge introduces differential rotation that amounts to critical break up velocity within the condensed object. Calibration of a simplistic dynamo model with the observed maximum flux per unit mass for main-sequence stars and white dwarfs indicates that about 1.5 × 10 −4 of the decaying toroidal flux must appear as poloidal. The highest fields in single white dwarfs are generated when two degenerate cores merge inside a common envelope or when two white dwarfs merge by gravitational-radiation angular momentum loss. Magnetars are the most magnetic neutron stars. Though these are expected to form directly from single stars, their magnetic flux to mass ratio indicates that a similar dynamo, driven by differential rotation acquired at their birth, may also be the source of their strong magnetism.

We explore the hypothesis that the magnetic fields of neutron stars are of fossil origin. For parametrized models of the distribution of magnetic flux on the main sequence and of the birth spin period of the neutron stars, we calculate the expected properties of isolated radio pulsars in the Galaxy using as our starting point the initial mass function and star formation rate as a function of Galactocentric radius. We then use the 1374‐MHz Parkes Multi‐Beam Survey of isolated radio pulsars to constrain the parameters in our model and to deduce the required distribution of magnetic fields on the main sequence. We find agreement with observations for a model with a star formation rate that corresponds to a supernova rate of 2 per century in the Galaxy from stars with masses in the range 8–45 M⊙ and predict 447 000 active pulsars in the Galaxy with luminosities greater than 0.19 mJy kpc2. The progenitor OB stars have a field distribution which peaks at ∼46 G with ∼8 per cent of stars having fields in excess of 1000 G. The higher‐field progenitors yield a population of 24 neutron stars with fields in excess of 1014 G, periods ranging from 5 to 12 s, and ages of up to 100 000 yr, which we identify as the dominant component of the magnetars. We also predict that high‐field neutron stars (log B > 13.5) originate preferentially from higher‐mass progenitors and have a mean mass of 1.6 M⊙, which is significantly above the mean mass of 1.4 M⊙ calculated for the overall population of radio pulsars.

Subluminous Type Ia supernovae, such as the Type Iax-class prototype SN 2002cx, are described by a variety of models such as the failed detonation and partial deflagration of an accreting carbon-oxygen white dwarf star or the explosion of an accreting, hybrid carbon-oxygen-neon core. These models predict that bound remnants survive such events with, according to some simulations, a high kick velocity. We report the discovery of a high proper motion, low-mass white dwarf (LP 40-365) that travels at a velocity greater than the Galactic escape velocity and whose peculiar atmosphere is dominated by intermediate-mass elements. Strong evidence indicates that this partially burnt remnant was ejected following a subluminous Type Ia supernova event. This supports the viability of single-degenerate supernova progenitors.