Transcript re0317original97

RE J 0317-853: A Magnetic White
Dwarf Holding Many Records
Authors
Institutes
RE J 0317-853 is a hydrogen rich strongly magnetic white dwarf discovered as an EUV source by the ROSAT Wide Field Camera (WFC). The average optical
spectrum of this star could be fitted by a dipole model which is off-centered along the dipole axis by 0.2 stellar radii in the direction of the southern magnetic
pole (Barstow, Jordan et al. 1995, MNRAS 277, 971). We concluded that the range of magnetic field strengths seen by an observer was about 170-660 MG,
so that RE J 0317-853 has one of the strongest magnetic fields detected on white dwarfs.
High speed photometry has shown that the optical brightness of RE J 0317-853 varies almost sinusoidal with a period of 725 sec and an amplitude of
more that 0.1 magnitudes. The only possible explanation was rotation so that RE J 0317-853 is rotating faster than any other known isolated white
dwarf. The photometric variation must be caused by differences in the brightness on various parts of the stellar surface. Since RE J 0317-853 (very probably)
has a DA white dwarf companion (LB 9802) 16” away, we could use its atmospheric parameters (Teff=16,030230 K, log g=8.190.05) as an additional
constraint for the parameters of RE J 0317-853, since both stars are at the same distance.
Moreover, the measurement of the colors B-V and U-B indicate a rather hot photosphere. We concluded that RE J 0317-853 is the hottest (50 000 K)
known magnetic white dwarf and probably the most massive ( 1.35 solar masses) isolated white dwarf discovered so far.
In order to analyze the magnetic field of RE J 0317-853 in detail we have obtained time resolved spectropolarimetric data with the 3.9 m Anglo-Australian
Telescope (shown in Fig. 1 and 2) and spectra with the HST (Fig. 3 and 4), EUVE and ORFEUS satellites. We concluded that the magnetic field structure is
extremely complicated and cannot be described by a simple dipole, or offset dipole which we have used to fit the average optical spectrum of the star.
A preliminary analysis of the observations with theoretical models for the transfer of polarized radiation through a magnetized stellar atmosphere shows that
the data can be best reproduced by a magnetic field with basically two different field strengths (200 and 500 MG) on different hemispheres, rather than a
continuous variation by a factor of two expected for a centered dipole model (see Fig. 3 and 4). One feature at 1170 Å is a dipole forbidden component of
Lyman  (1s0->2s0), only possible due to the simultaneous presence of a magnetic and electric field.
 circular polarization up to 22%
Fig. 1: Variation of the flux spectrum of RE J 0317-853 with time. Both,
the spectral features and the brightness ( m = ±0.1) change with phase
Fig. 2: Variation of the wavelength dependent circular polarization of
RE J 0317-853 shown as a surface and contour plot.
 component of Lyman 
200 MG



200 MG
interstellar
Lyman 

500 MG
+ component of Lyman 

1s0->2s0 dipole forbidden
Lyman  component
Observation
Fig. 3: Series of HST spectra of RE J 0317-853 showing the variation of the
Lyman   and + components due to the change of the magnetic field.
The fit with a theoretical model is plotted in red.
Theory
Fig. 4: Same HST spectra, but the variation with phase is
shown as a contour plot