Astronomy & Celestial Mechanics
the sky, computed
Concept

Fossil field — where it appears

A magnetic field left over from an earlier stage of a star's history and no longer regenerated by a dynamo. Flux freezing keeps it in place for billions of years, so its strength and geometry record how it was made rather than anything happening now.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Field strength against radius, from the Sun to a magnetar. Surface magnetic field against radius, both on logarithmic scales, radius in solar radii and field in gauss, for five kinds of star: the Sun's dipole, Ap and Bp stars, magnetic white dwarfs, radio pulsars, magnetars. The diagonal lines are constant magnetic flux, πR²B = 10²³, 10²⁵, 10²⁷ gauss square centimetres, along which a star shrinking with its field frozen in would slide. The Ap and Bp stars, the magnetic white dwarfs and the magnetars all fall close to the upper lines: a field of a few kilogauss on a star of three solar radii, squeezed to the size of a white dwarf, becomes a hundred million gauss, and squeezed to twelve kilometres becomes a hundred trillion. The Sun's dipole and the ordinary pulsars carry a hundred to a thousand times less flux.

The same flux in three sizes

A few per cent of hot main-sequence stars carry kilogauss fields that do not change over decades. So do a similar share of white dwarfs, at a million times the strength, and of young neutron stars, at a hundred billion times it. Multiply each field by its star's area and the three classes carry nearly the same magnetic flux — what a field frozen into a star would keep as the star shrank by a factor of a hundred thousand. A collapse makes a field enormous, but it cannot make it important.

stars · Flux freezing The share of white dwarfs whose centres have begun to freeze. The fraction of a population of carbon–oxygen white dwarfs, with masses spread as a Gaussian of mean 0.62 and width 0.12 M☉, whose centres have begun to crystallise, against cooling age, overall and in three mass ranges, in the toy model of the other figures. If a field appears when freezing begins, this is the shape its incidence must take: it switches on first among the heaviest white dwarfs, half of which have begun freezing by 0.5 Gyr, and last among the typical ones near 0.6 M☉, by 2.5 Gyr. A fossil field, present from the white dwarf's birth, would show no such delay and no such ordering by mass.

A field that waits for the core to freeze

A field inherited from the parent star would be present from a white dwarf's birth. Yet magnetic white dwarfs of ordinary mass are rare among the young and common among the old, as if their fields were switched on later. The proposed switch is the freezing of the carbon–oxygen core, which stirs the liquid above it. If that is right, the delay is set by when each mass begins to freeze, and the heaviest should go first.

stars · Flux freezing

Named alongside it

The objects these essays reach for when they reach for this one.

Flux freezingStellar mergerWhite dwarfAp starsCooling ageCrystallisationDynamoMagnetarMagnetic diffusivityMagnetic fluxMagnetic reynolds numberPhase separation

All concepts