Magnetism in Isolated and Binary White Dwarfs

Abstract

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...

Zeeman tomography of magnetic white dwarfs

“…Both phenomena are characteristic of a high-field object with B > ∼ 50 MG (e.g. Wickramasinghe & Ferrario 2000).…”

“…In about 170 of the 5448 white dwarfs (WDs) listed in the Web Version 1 of the Villanova White Dwarf Catalog magnetic fields between 2 kG-1000 MG have been detected, corresponding to a fraction of 3% (McCook & Sion 1999;Wickramasinghe & Ferrario 2000;Vanlandingham et al 2005). Most of the WDs have not been scrutinisingly examined for the presence of a magnetic field, however, and a statistical study suggests that the true fractional incidence could be as high as 20% Schmidt et al 2003).…”

Joint Discussion 6 Neutron stars and black holes in star clusters

“…This result suggests that the brightest globular cluster CVs in Ter 5s should be intermediate polars. have also proposed that the lack of outbursts is due to higher white dwarf masses (higher mean masses are observed amongst strongly magnetic isolated white dwarfs; Wickramasinghe & Ferrario 2000). This is likely to be due to the difference in the formation mechanisms of GC and field CVs, since a substantial fraction of cluster CVs are likely to be formed through encounters, rather than from their primordial binaries .…”

Magnetic Field Evolution in Accreting White Dwarfs

“…Detailed calculations of this process remain to be carried out. Orbital period distribution for 53 AM Her systems from Wickramasinghe & Ferrario (2000) and for 20 intermediate polars from the catalogue of Ritter & Kolb (1998).…”

“…at P Q rb ~ 3-4h would allow the magnetic field to reemerge, on a timescale « 3 x 10 8 yr (AM/0.1 MQ) 7 / 5 (AM is the amount of accreted mass), similar to the time for a non-magnetic CV to cross the period gap. The picture is probably more complicated than this: for example, no correlation is seen between B and orbital period within AM Her systems (Wickramasinghe & Ferrario 2000). However, the fact that the magnetic field may change with time is a new ingredient to throw into the evolutionary models.…”