The FORS1 catalogue of stellar magnetic field measurements

Abstract

Context. The FORS1 instrument on the ESO Very Large Telescope was used to obtain low-resolution circular polarised spectra of nearly a thousand 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. Our understanding of the instrument and our data reduction techniques have both 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 approximately 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.

Monitoring and modelling of white dwarfs with extremely weak magnetic fields

“…The presence of a field was confirmed using low-resolution spectropolarimetry with the ESO FORS1 low-resolution spectropolarimeter by Aznar Cuadrado et al (2004), who detected fields of B z ≈ +3 and +4 kG at the 5 − 6σ level of significance on two different nights. (Note that these data were re-reduced as described by Bagnulo et al (2015), and the sign of the observations changed to conform to the usual convention.) The value of |B| was subsequently estimated from the UVES spectra to be about 110 kG (Koester et al 2009).…”

“…They have a typical S/N ratio per pixel of between 270 and 400 per Å. These data were reduced, and the mean longitudinal field B z deduced, as described in detail by Bagnulo et al (2015): for these spectra B z was determined by measuring the slope of the correlation between the circular polarisation V(λ) and (1/I)(dI(λ)/dλ) (see Sect. 4.3).…”

First constraints on the magnetic field strength in extra-Galactic stars: FORS2 observations of Of?p stars in the Magellanic Clouds

“…5 of Bagnulo et al 2015). Of course, chances to detect a magnetic field crucially depend not only on the intrinsic strength of the magnetic fields of the targets, but also on the geometrical view of the stellar field at the time of the observations.…”

“…Data were reduced as explained by Bagnulo et al (2015). The mean longitudinal magnetic field B z (i.e., the component of the magnetic field averaged over the visible stellar disk) was calculated by minimising the expression…”

“…Measurements have been phased with the rotation period of 224.78 d (Landstreet & Mathys 2000). Red solid triangles refer to previous field measurement obtained with FORS1 and with different grisms (see Landstreet et al 2014;Bagnulo et al 2015, and references therein).…”