Measuring magnetic fields of early-type stars with FORS1 at the VLT

This paper describes an experiment aimed at evaluating the capability of the FORS1 instrument at the VLT for measuring weak (similar or equal to0.1%) Zeeman circular polarization signatures in stellar H Balmer lines. We have obtained low-resolution polarized spectra at 3500-5800 Angstrom 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 Hbeta 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 4100 Angstrom, 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.