The most magnetic stars

Abstract

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.

The VLT-FLAMES Tarantula Survey

“…One possibility is that of a fossil field that is frozen into a stable large-scale configuration during the star-formation process (Duez & Mathis 2010). Another is that the fields are formed during a strong binary interaction, e.g., a stellar merger (e.g., Wickramasinghe et al 2014). Merger products are not expected to be found in close binaries, as then the pre-merger triple star would have been highly unstable.…”

A novel and sensitive method for measuring very weak magnetic fields of DA white dwarfs

“…It has been proposed that they may be the fossil remmnants of magnetism inherited from an earlier stage of evolution, for example from the main sequence (Landstreet 1992). Alternatively, the fields of white dwarfs may have a more specific origin, such as being produced during the mergers of close binary systems Tout et al 2008;Wickramasinghe et al 2014). …”

Merging binary stars and the magnetic white dwarfs

“…Potter & Tout (2010) attempted to model this phenomenon and found a potential problem in that the time-scale for the diffusion of the field into the white dwarf is generally significantly longer than the expected common envelope lifetime. Wickramasinghe, Tout & Ferrario (2014) suggested that strong magnetic fields in white dwarfs are generated by a dynamo process that feeds on the differential rotation in the merged object as it forms. A weak poloidal seed field that is already present in the pre-white dwarf core is amplified by the dynamo to a strong field that is independent of its initial strength but depends on the amount of the initial differential rotation.…”