S. Jordan

Discovery of kilogauss magnetic fields in three DA white dwarfs

We have detected longitudinal magnetic fields between 2 and 4 kG in three (WD 0446−790, WD 1105−048, WD 2359−434) out of a sample of 12 normal DA white dwarfs by using optical spectropolarimetry done with the VLT Antu 8 m telescope equipped with FORS1. With the exception of 40 Eri B (4 kG) these are the first positive detections of magnetic fields in white dwarfs below 30 kG. Although suspected, it was not clear whether a significant fraction of white dwarfs contain magnetic fields at this level. These fields may be explained as relics from magnetic fields in the main-sequence progenitors considerably enhanced by magnetic flux conservation during the shrinkage of the core. A detection rate of 25% (3/12) may indicate now for the first time that a substantial fraction of white dwarfs have a weak magnetic field. This result, if confirmed by future observations, would form a cornerstone for our understanding of the evolution of stellar magnetic fields.

Zeeman tomography of magnetic white dwarfs - I. Reconstruction of the field geometry from synthetic spectra

We have computed optical Zeeman spectra of magnetic white dwarfs for field strengths between 10 and 200 MG and eective temperatures between 8000 and 40 000 K. They form a database containing 20 628 sets of flux and circular polarization spectra. A least-squares optimization code based on an evolutionary strategy can recover relatively complex magnetic field topologies from phase-resolved synthetic Zeeman spectra of rotating magnetic white dwarfs. We consider dipole and quadrupole components which are non-aligned and shifted o-centre. The model geometries include stars with a single high-field spot and with two spots separated by90. The accuracy of the recovered field structure increases with the signal-to-noise ratio of the input spectra and is significantly improved if circular polarization spectra are included in addition to flux spectra. We discuss the strategies proposed so far to unravel the field geometries of magnetic white dwarfs.

Magnetic white dwarfs in the Early Data Release of the Sloan Digital Sky survey

We have identified 7 new magnetic DA white dwarfs in the Early Data Release of the Sloan Digital Sky Survey. Our selection strategy has also recovered all the previously known magnetic white dwarfs contained in the SDSS EDR, KUV 03292+0035 and HE 0330-0002. Analysing the SDSS fibre spectroscopy of the magnetic DA white dwarfs with our state-of-the-art model spectra, we find dipole field strengths 1.5 MG ≤ B d ≤ 63 MG and effective temperatures 8500 ≤ T e f f < 39 000 K. As a conservative estimate, we expect that the complete SDSS will increase the number of known magnetic white dwarfs by a factor 3.

Zeeman tomography of magnetic white dwarfs. III, The 70–80 Megagauss magnetic field of PG 1015+014

Aims. We analyse the magnetic field geometry of the magnetic DA white dwarf PG 1015+014 with our Zeeman tomography method. nMethods. This study is based on rotation-phase resolved optical flux and circular polarization spectra of PG 1015+014 obtained with FORS1 nat 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. nResults. 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 nwith 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 eff = 10 000 ± 1000 K was found. nConclusions. 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.

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

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 P rot 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 ∼ 75 MG do occur. We derive an effective photospheric temperature of HE 1045−0908 of T eff = 10 000 ± 1000 K. The tomographic code makes use of an extensive database of pre-computed Zeeman spectra (Paper I).

An astronomical search for evidence of new physics: Limits on gravity-induced birefringence from the magnetic white dwarf RE J0317-853

The coupling of the electromagnetic field directly with gravitational gauge fields leads to new physical effects that can be tested using astronomical data. Here we consider a particular case for closer scrutiny, a specific nonminimal coupling of torsion to electromagnetism, which enters into a metric-affine geometry of space-time. We show that under the assumption of this nonminimal coupling, space-time is birefringent in the presence of such a gravitational field. This leads to the depolarization of light emitted from extended astrophysical sources. We use polarimetric data of the magnetic white dwarf RE J0317-853 to set, for the very first time, constraints on the essential coupling constant for this effect, giving

HE0241-0155 – Evidence for a large scale homogeneous field in a highly magnetic white dwarf

{k}^{2}ensuremath{lesssim}(19text{ }text{ }mathrm{m}{)}^{2}

Gravity-induced birefringence within the framework of Poincare gauge theory

.

Fast Object Detection for Use Onboard Satellites

In the course of the Hamburg/ESO survey we have discovered a white dwarf whose spectrum exhibits many similarities with the prototype of magnetic white dwarfs Grw+70°8247. In particular several stationary line components indicative for magnetic fields between aboutxa0150 and 400 MG are found in both objects. However, the features betweenxa05000 and 5500u2009Au2000in the spectrum of HE0241-0155 cannot be explained by stationary line components and demand a relatively homogeneous magnetic field with clustering around 200u2009MG. For this reason a pure dipole model failed to reproduce this spectral region. An offset-dipole configuration led to some improvement in the fit but a good agreement was only possible for a geometryxa0– described by an expansion into spherical harmonics – where most of the surface is covered with magnetic field strengths strongly clustered around 200u2009MG. This may indicate the presence of a large magnetic spot whose presence could be tested with time resolved spectro-polarimetry.

Zeeman Tomography of Magnetic White Dwarfs: General Method and Application to EF Eridani

Gauge theories of gravity provide an elegant and promising extension of general relativity. In this paper we show that the Poincare gauge theory exhibits gravity-induced birefringence under the assumption of a specific gauge invariant nonminimal coupling between torsion and Maxwells field. Furthermore we give for the first time an explicit expression for the induced phase shift between two orthogonal polarization modes within the Poincare framework. Since such a phase shift can lead to a depolarization of light emitted from an extended source this effect is, in principle, observable. We use white dwarf polarimetric data to constrain the essential coupling constant responsible for this effect.