Magnetic white dwarf stars

by Putney, Angela

Abstract (Summary)
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Three aspects of magnetic white dwarf stars are studied to aid in the understanding of stellar evolution. A survey of ~ 50 DC white dwarf stars was conducted in circular spectropolarimetry to search for magnetic fields [...] 30 kG. Four DC stars were discovered with magnetic fields above 30 kG: G 111-49 with B[subscript e] ~ â220 MG, G 183-35 with B[subscript e] = +6.8 ± 0.5 MG, G 256-7 with B[subscript e] = +4.9 ± 0.5 MG, and G 234-4 with B[subscript e] = +39.6 ± 11.6 kG. A new magnetic DB white dwarf was also discovered, LB 8827 with B[subscript e] = 1.0 ± 0.5 MG. A total of 15% of the white dwarfs in the survey have a magnetic field > 30 kG. This value is far larger than the 2% of DA stars, but more than half of the DC stars were originally misclassified. Only 5% of the re-classified DC stars have magnetic fields above 30 kG. Three magnetic stars from the DC white dwarf survey were re-observed to investigate the possibility of rotation. Two are definitely rotating: LHS 1734 with 16 min [...] P < 1 yr and G 158-45 (=LHS 1044) with a probable period P ~ 11 hr but a definite period P [...] 1 d or P ~ a few days. G 183-35 might be rotating with 50 min [...] P < a few yr. From all the white dwarf rotations known, it is clear that angular momentum is lost before a star becomes a white dwarf, but not clear that the loss is greatly enhanced by magnetic fields. The isolated magnetic white dwarfs G 99-47, KUV 813-14 (KUV 23162-1220), and G 227-35 were observed in linear and circular spectropolarimetry and then compared to calculated theoretical spectra to find a model for the magnetic field strength and structure. The comparisons were to Stokes' V/I (circular polarization) spectra in addition to total flux F[...], and these add many constraints to the possible solutions. An off-centered dipole or a dipole+quadrupole configuration best fits the observations. The results of the survey and the modeling are consistent with the theory that the magnetic Ap stars are the predecessors of magnetic white dwarfs.
Bibliographical Information:

Advisor:Marshall Cohen

School:California Institute of Technology

School Location:USA - California

Source Type:Master's Thesis



Date of Publication:10/30/1995

© 2009 All Rights Reserved.