S. Bagnulo, L. Fossati, J.D. Landsreet, & C. Izzo

The FORS1 catalogue of stellar magnetic field measurements*

Figure 3. An example of data reduction: the case of the Ap star HD 94660. In the upper panel, the black solid line shows the intensity profile, the shape of which is heavily affected by the transmission function of the atmosphere + telescope optics + instrument. The red solid line is the PV profile (in % units) and the blue solid line is the null profile offset by −0.5 % for display purpose. Photon-noise error bars are centred around -0.5 % and appear as a light blue background. Spectral regions highlighted by green bars have been used to detemine the ⟨Bz⟩ value from H Balmer lines, and the magenta bars highlight the spectral regions used to estimate the magnetic field from metal lines. The four bottom panels show the best-fit obtained by minimising the χ² expression of Eq. (4) using the PV profiles (left panels) and the NV profiles (right panels) for H Balmer lines and metal lines as described. The field values (⟨Bz⟩ ∼ −2000 G and ⟨Nz⟩ ∼ 0 G) are determined with a formal accuracy of ∼ 40 G for Balmer lines and ∼ 25 G for metal lines.


Context. The FORS1 instrument of the ESO Very Large Telescope was used to obtain low resolution circular polarized spectra of nearly a thousand of different stars, with the aim of measuring their mean longitudinal magnetic fields. Magnetic fields were measured by different authors, and using different methods and software tools.

Aims. A catalogue of FORS1 magnetic measurements would provide a valuable resource with which to better understand the strengths and limitations of this instrument and of similar low-dispersion, Cassegrain spectropolarimeters. However, FORS1 data reduction has been carried out by a number of different groups using a variety of reduction and analysis techniques. Both our understanding of the instrument and our data reduction techniques have improved over time. A full re-analysis of FORS1 archive data using a consistent and fully documented algorithm would optimise the accuracy and usefulness of a catalogue of field measurements.

Methods. Based on the ESO FORS pipeline, we have developed a semi-automatic procedure for magnetic field determinations, which includes self-consistent checks for field detection reliability. We have applied our procedure to the full content of circular spectropolarimetric measurements of the FORS1 archive.

Results. We have produced a catalogue of spectro-polarimetric observations and magnetic field measurements for 1400 observations of 850 different objects. The spectral type of each object has been accurately classified. We have also been able to test different methods for data reduction is a systematic way. The resulting catalogue has been used to produce an estimator for an upper limit to the uncertainty in a field strength measurement of an early type star as a function of the signal-to-noise ratio of the observation.

Conclusions. While FORS1 is not necessarily an optimal instrument for the discovery of weak magnetic fields, it is very useful for the systematic study of larger fields, such as those found in Ap/Bp stars and in white dwarfs.

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Last Revised: 2015 August 17th