A search for kilogauss magnetic fields in white dwarfs and hot subdwarf stars

We present new results of a survey for weak magnetic fields among DA white dwarfs, including some brighter hot subdwarf stars. We have detected variable circular polarization in the Hα line of the hot subdwarf star Feige 34 (spectroscopic type: sdO). From these data, we estimate that the longitudinal magnetic field of this star varies from -1.1 ± 3.2 to +9.6 ± 2.6 kG, with a mean of about +5 kG and a period longer than 2 hr. In this study, we also confirm the magnetic nature of white dwarf WD 1105-048, found earlier in a study by Aznar Cuadrado and coworkers, and present upper limits of kilogauss longitudinal magnetic fields of the five brightest DA white dwarfs. Our data support the finding of Aznar Cuadrado and coworkers that ~25% of white dwarfs have kilogauss magnetic fields. This frequency also confirms results of early estimates obtained using the magnetic field function of white dwarfs (Fabrika & Valyavin).

[1]  G. Wade,et al.  Measuring magnetic fields of early-type stars with $\vec{FORS{\it 1}}$ at the $\vec{VLT}$ , 2002 .

[2]  Discovery of magnetic fields in hot subdwarfs , 2004, astro-ph/0503243.

[3]  Paul S. Smith,et al.  The incidence of magnetism among white dwarfs: The first stars below 100 kilogauss , 1994 .

[4]  I. Iben,et al.  On the formation and evolution of a helium degenerate dwarf in a close binary , 1986 .

[5]  D. Wickramasinghe,et al.  The origin of the magnetic fields in white dwarfs , 2005 .

[6]  G. Wade,et al.  Rotation period and magnetic field morphology of the white dwarf WD 0009+501 , 2005 .

[7]  J. Liebert,et al.  The True Incidence of Magnetism Among Field White Dwarfs , 2002, astro-ph/0210319.

[8]  J. McGraw,et al.  A Search for Binary Hot Subdwarfs. I. BVRI Photometry of Palomar‐Green Survey sdO Stars , 2001 .

[9]  Paul S. Smith,et al.  A Search for Magnetic Fields among DA White Dwarfs , 1995 .

[10]  Discovery of kilogauss magnetic fields in three DA white dwarfs , 2004, astro-ph/0405308.

[11]  J. Liebert,et al.  Atmospheric parameters of field subdwarf B stars , 1994 .

[12]  S. Fabrika,et al.  Magnetic fields of white dwarfs , 2003 .

[13]  R. Hilditch,et al.  Spectroscopically and spatially resolving the components of close binary stars : proceedings of a meeting held in Dubrovnik, Croatia 20-24 October 2003 , 2004 .

[14]  Brian Warner,et al.  Whole earth telescope observations of the DBV white dwarf GD 358 , 1994 .

[15]  Anneila I. Sargent,et al.  The Nature of Faint Blue Stars in the Halo. II , 1974 .

[16]  A. Grauer,et al.  Upper Limits for Magnetic Fields on Pulsating White Dwarfs , 1997 .

[17]  J. Angel,et al.  The magnetic fields of white dwarfs , 1981 .

[18]  J. Greenstein,et al.  Emission lines in the magnetic white dwarf GD 356 , 1985 .

[19]  S. Vennes,et al.  Ap stars as progenitors of magnetic white dwarfs , 2004, Proceedings of the International Astronomical Union.

[20]  H. Horn,et al.  Magnetic field evolution in white dwarfs , 1987 .

[21]  J. A. Markiel,et al.  Dynamo generation of magnetic field in the white dwarf GD 358 , 1994 .

[22]  G. G. Valyavin,et al.  Magnetic survey of bright northern main sequence stars , 2002 .

[23]  Edward M. Sion,et al.  A Catalog of Spectroscopically Identified White Dwarfs , 1987 .

[24]  G. Wegner,et al.  Detection of H-alpha emission in the hot white dwarf G191-B2B , 1988 .