Zeeman tomography of magnetic white dwarfs II. The quadrupole-dominated magnetic field of HE 1045-0908

TL;DR

This study uses Zeeman tomography to analyze the complex magnetic field structure of the white dwarf HE 1045-0908, revealing a quadrupole-dominated field with additional multipole components and estimating its surface magnetic strength and temperature.

Contribution

First detailed Zeeman tomographic analysis of HE 1045-0908 showing a quadrupole-dominated magnetic field with multipole contributions and improved modeling techniques.

Findings

Magnetic field dominated by quadrupole, dipole, and octupole components.

Surface magnetic field strength varies between 10 and 75 MG.

Effective temperature of the white dwarf is approximately 10,000 K.

Abstract

We report time-resolved optical flux and circular polarization spectroscopy of the magnetic DA white dwarf HE 1045-0908 obtained with FORS1 at the ESO VLT. Considering published results, we estimate a likely rotational period of Prot ~ 2.7 h, but cannot exclude values as high as about 9 h. Our detailed Zeeman tomographic analysis reveals a field structure which is dominated by a quadrupole and contains additional dipole and octupole contributions, and which does not depend strongly on the assumed value of the period. A good fit to the Zeeman flux and polarization spectra is obtained if all field components are centred and inclinations of their magnetic axes with respect to each other are allowed for. The fit can be slightly improved if an offset from the centre of the star is included. The prevailing surface field strength is 16 MG, but values between 10 and ~75MG do occur. We derive an effective photospheric temperature of HE 1045-0908 of Teff = 10000 +/- 1000 K. The tomographic code makes use of an extensive database of pre-computed Zeeman spectra (Paper I).