Feige 7: A hot, rotating magnetic white dwarf

Journal Article · Wed Jun 01 04:00:00 EDT 1977 · Astrophys. J.; (United States)

DOI:https://doi.org/10.1086/155271· OSTI ID:7259627

Liebert, J; Angel, J R.P.; Stockman, H S; Spinrad, H; Beaver, E A

The blue white dwarf suspect Feige 7 (=L795-7=GR 267) has been found to be magnetic with a rich optical spectrum and variable circular polarization. The polarization observations show a variation that is sinusoidal with amplitude 0.3%, a mean of nearly zero, and a period of 2.2 hours; an ephemeris is provided. The mean longitudinal field at peak polarization is estimated to be 5 megagauss. Spectra obtained with the Lick Observatory scanner and the UCSD Digicon at Steward Observatory are presented. The detailed absorption spectrum is shown to fit Zeeman patterns of hydrogen and neutral helium in the presence of a mean homogeneous surface field varying slightly with the polarization period from 18 to 20 megagauss. The star in fact provides the first confirmation of the theoretical spectra of hydrogen and helium in such high fields, inaccessible to laboratory measurement. It is argued that the period must be due to rotation, and an oblique rotator model with the rotation axis in the plane of the sky and at approx.24degree tilt angle to the magnetic axis is compatible with the available observations. The proper motion from Luyten and the Lowell Observatory is large enough to very nearly constrain the star to be degenerate, rather than a hot subdwarf; the blue continuum indicates that it is the hottest of the known magnetic degenerate stars. The observed combination of neutral helium and hydrogen lines (and at comparable intensities) may be unique among white dwarf stars. The star must have a helium-dominated atmosphere, but the relative H/He abundances could vary over the surface due to the influence of the magnetic field. Since Feige 7 is a recently formed white dwarf, calculations show that any quadrupole and octopole magnetic moments originally present would not yet have decayed; since there is no evidence for appreciable higher moments in the observed surface field, it is argued that the field was essentially dipole at the time of the white dwarf's formation.

• Research Organization: Steward Observatory, University of Arizona
• OSTI ID: 7259627
• Journal Information: Astrophys. J.; (United States), Journal Name: Astrophys. J.; (United States) Vol. 214:2; ISSN ASJOA
• Country of Publication: United States
• Language: English

Related Subjects

640102* -- Astrophysics & Cosmology-- Stars & Quasi-Stellar
Radio & X-Ray Sources
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
ABSORPTION SPECTRA
ATMOSPHERES
CHEMICAL COMPOSITION
DWARF STARS
MAGNETIC FIELDS
MAGNETIC STARS
MOTION
POLARIZATION
ROTATION
SPECTRA
STAR EVOLUTION
STARS
STELLAR ATMOSPHERES
VARIATIONS
WHITE DWARF STARS