Comprehensive and Critical Reanalysis of FORS Spectropolarimetric Observations: the Case of HD 92207

摘要

Stellar spectropolarimetry has become extremely popular during the past decade, and has led to major advances in our understanding of stellar magnetic fields and their impact on stellar structure and evolution.Many important discoveries have been made thanks to the observations performed with the FORS low-resolution spec- tropolarimeters of the ESO Very Large Telescope.We first review and summarize the major results of a homogeneous re-reduction and analysis of all single-slit FORSl spectropolarimetric observations.This work revealed a non-negligible dependence of the results upon the adopted reduction and analysis procedure, as well as the presence of instabilities, revealing that photon noise is not the only source of uncertainty.As a consequence of our new analysis and assessment of uncertainties, we are not able to confirm a large number of magnetic field detections presented in the past for a variety of stars.We further summarize the results of FORS2 spectropolarimetric observations of the AO supergiant HD 92207,which allowed us to explore further the nature of the instabilities, roughly constraining their maximum impact on the derived Stokes profiles and magnetic field values.We finally present new results obtained with a further independent pipeline on the FORS2 data of HD 92207,confirming our previous analysis, and discuss simple quality-check controls, which can be performed on the data in order to distinguish between the genuine and spurious signals.All together, our results reveal that the FORS spectropolarimeters are indeed reliable instruments when their capabilities are not pushed beyond the limits of the mounted Cassegrain low-resolution spectrograph.