S. Bagnulo, J.D. Landstreet, L. Fossati and O. Kochukhov
Figure 1. The polarized spectrum of the magnetic Ap star HD 94660. The top panel shows Stokes I (black solid line, in arbitrary units, and not corrected for the instrument response), PV (red solid line centred about 0), and the null profile (blue solid line, offset by -0.75 % for display purpose). Light blue bars represent the error bars of PV . The slope of the red interpolating lines in the bottom panels gives the mean longitudinal field from PV (left panel)and from the null profile (right panel, the latter being expected zero), both calculated using the H Balmer lines only. (After Bagnulo et al. 2011).
During the last decade,the FORS1 instrument of the ESO Very Large Telescope has been used to obtain low resolution circular polarized spectra for about 500 stars, with the aim of measuring their mean longitudinal magnetic fields. Magnetic field estimates were obtained by different authors, using different software tools. Several interesting detections were obtained at a 3σ level; some of them were eventually confirmed by follow-up investigations, some of them were not. This raises issues about the reliability of the stated uncertainties of some of the published field values. To investigate these problems, we have developed a semi-automatic procedure for magnetic field determination, which includes self-consistent checks for field detection reliability. We have applied our procedure to the full content of single star ("fast mode") circular spectropolarimetric measurements of the FORS1 archive, and explored the details and interagreement of various methods for data reduction. We have finally produced a catalogue of FORS1 longitudinal field measurements which includes about 1000 entries. Here we critically review the previously published FORS1 measurements, and, based on our results, we suggest that the incidence of the magnetic field in various classes of stars should be revised.
Last Revised: 2011 September 23rd