Measuring magnetic fields of early-type stars with $\vec{FORS{\it 1}}$ at the $\vec{VLT}$

Abstract

Abstract. This paper describes an experiment aimed at evaluating the capability of the FORS1 instrument at the VLT for measuring weak ( 0.1%) Zeeman circular polarization signatures in stellar H Balmer lines. We have obtained low-resolution polarized spectra at 3500-5800Å of two bright A-type stars, HD 94660, and HD 96441. The former is a well known magnetic star, the latter an apparently non-magnetic, unpolarized star that was observed for comparison purposes. In order to test the possibility of performing multi-object spectropolarimetric measurements (e.g., for cluster studies), observations were taken both on-axis (i.e., at the CCD field center) and off-axis (i.e., at the edges of the CCD). In HD 94660 (the magnetic star), we detected a clear signal of circular polarization in all observed hydrogen Balmer lines (i.e., from Hβ to the Balmer limit), with a typical peak-to-peak amplitude of about 0.8%. From the analysis of Stokes V under the weak-field approximation, we have estimated a mean longitudinal field of −2085 ± 85 G, a value fully consistent with previous studies of this star. Notably, we found that at wavelengths shorter than about 4100Å, the polarization signal detected off-axis is about 20-30% smaller than that detected on-axis. No polarization due to the Zeeman effect was detected in the comparison star HD 96441, and we estimate that the contribution due to instrumental circular polarization is limited to the order of 0.01%. This experiment demonstrates the effectiveness of FORS1 at the VLT as a tool for high-sensitivity diagnosis of the magnetic field in upper main sequence stars, providing the potential for measuring fields in fainter and more rapidly rotating stars than has previously been possible.

Detection of magnetic field in the B2 star ρ Ophiuchi A with ESO FORS2

“…The FORS2 data were reduced and analysed using the pipeline thoroughly described in Fossati et al (2015), which is based on the algorithms and recommendations given by Bagnulo et al (2012Bagnulo et al ( , 2013. The surface average longitudinal magnetic field B z was derived using the method first described in Bagnulo et al (2002), which is based on extracting the slope of the linear regression of V(λ)/I(λ) versus the quantity…”

Magnetic field measurements and their uncertainties: the FORS1 legacy

“…Bagnulo et al (2002) proposed to use a least-squares technique to derive the longitudinal field via Eq. (5), by minimising the expression…”

“…Figure 1 illustrates the technique discussed in this section, showing the results obtained for the FORS observations of the well known magnetic star HD 94660. Bagnulo et al (2002) have obtained the error bar of the longitudinal magnetic field from the inverse of the χ 2 matrix. This way, the "internal" error bar s B z was obtained by propagating the errors σ i of the P V profiles onto the analytical expression of B z that is obtained by minimising the χ 2 expression of Eq.…”

“…For more details on how field results change when Eq. (7) is applied to Balmer lines only, or only to He+metal lines, or to the full spectrum, see, e.g., Bagnulo et al (2002) and Bagnulo et al (2006). v) In some cases, using the rectified profiles of Eq.…”

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

“…FORS2 is a multipurpose instrument capable of imaging and low resolution spectroscopy, and equipped with polarimetric optics (a retarder waveplate and a Wollaston prism). For the measurement of the magnetic field we have used the technique described by Bagnulo et al (2002) and Bagnulo et al (2012). The five extra-Galactic Of?p stars were observed in multi-object mode, following the procedure already adopted by Bagnulo et al (2006) for a survey of magnetic stars in open clusters, and by Nazé et al (2012) for a magnetic survey of bright X-ray emitters.…”