Abstract
<b>Aims:</b> We analyse the magnetic field geometry of the magnetic DA white dwarf PG 1015+014 with our Zeeman tomography method.<br> <b>Methods:</b> This study is based on rotation-phase resolved optical flux and circular polarization spectra of PG 1015+014 obtained with FORS1 at the ESO VLT. Our tomographic code makes use of an extensive database of pre-computed Zeeman spectra. The general approach has been described in Papers I and II of this series.<br> <b>Results:</b> The surface field strength distributions for all rotational phases of PG 1015+014 are characterised by a strong peak at 70 MG. A separate peak at 80 MG is seen for about one third of the rotation cycle. Significant contributions to the Zeeman features arise from regions with field strengths between 50 and 90 MG. We obtain equally good simultaneous fits to the observations, collected in five phase bins, for two different field parametrizations: (i) a superposition of individually tilted and off-centred zonal multipole components; and (ii) a truncated multipole expansion up to degree l = 4 including all zonal and tesseral components. The magnetic fields generated by both parametrizations exhibit a similar global structure of the absolute surface field values, but differ considerably in the topology of the field lines. An effective photospheric temperature of T<sub>eff</sub> = 10000 ± 1000 K was found.<br> <b>Conclusions:</b> Remaining discrepancies between the observations and our best-fit models suggest that additional small-scale structure of the magnetic field exists which our field models are unable to cover due to the restricted number of free parameters.<br>
Highlights
In about 170 of the 5448 white dwarfs (WDs) listed in the Web Version1 of the Villanova White Dwarf Catalog magnetic fields between 2 kG–1000 MG have been detected, corresponding to a fraction of 3% (McCook & Sion 1999; Wickramasinghe & Ferrario 2000; Vanlandingham et al 2005)
In the present work – the third paper of our series on Zeeman tomography – we present an application of our code to observations of the non-accreting white dwarf PG 1015+014 and find a field geometry that deviates strongly from centred dipoles or quadrupoles
We have analysed high-quality spectropolarimetric data of PG 1015+014 covering a whole rotational period with the Zeeman tomography code described in Papers I and II
Summary
In about 170 of the 5448 white dwarfs (WDs) listed in the Web Version of the Villanova White Dwarf Catalog magnetic fields between 2 kG–1000 MG have been detected, corresponding to a fraction of 3% (McCook & Sion 1999; Wickramasinghe & Ferrario 2000; Vanlandingham et al 2005). The magnetic white dwarfs (MWDs) are widely believed to be the successors of the chemically peculiar magnetic Ap stars, which are the only main sequence stars to show substantial globally organised magnetic fields. The magnetic DA white dwarf PG 1015+014, discovered in the Palomar Green survey (Green et al 1987), was observed by Wickramasinghe & Cropper (1988, hereafter WC88) with the RGO spectrograph at the AAT in the wavelength range 4000–7000 Å Their phase-resolved spectroscopy and low-resolution circular polarimetry revealed significant modulations in flux and circular polarization (V/I) over the rotation cycle. In the individual polarization spectra of WC88, |V/I| is 5% in the continuum and up to 10% in individual features They fitted theoretical MWD model spectra to the observations and found an obliquely rotating magnetic dipole model with a polar field strength of Bdpol = 120 ± 10 MG and an almost equator-on view to be the best-fitting field geometry. Our analysis provides a substantially improved insight into the field structure of PG 1015+014
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