SPECTROPOLARIMETRY OF MAGNETIC STARS .6. LONGITUDINAL-FIELD, CROSSOVER AND QUADRATIC FIELD - NEW MEASUREMENTS

New determinations of the mean longitudinal magnetic field, of the cro ssover, and of the mean quadratic magnetic field of Ap stars are prese nted. They are based on spectra recorded simultaneously in both circul ar polarizations at ESO with the CASPEC spectrograph fed by the 3.6 m telescope. This paper discusses 95 observations of 44 stars. A major r esult of this study is the discovery that HD 137509 has a predominantl y quadrupolar magnetic field, a strucuture previously found in only a couple of stars. Improvement or revision of the determination of the r otation period has been achieved for 3 stars. The stars studied in thi s work include 14 rapidly oscillating Ap stars (for 6 of which no prev ious attempt to detect a magnetic field had ever been made) and 21 Ap stars with spectral lines resolved into their magnetically split compo nents when observed at high enough dispersion in unpolarized light (fo r 9 of these stars, no determination of the longitudinal field had bee n performed before). The observations discussed in this paper have bee n performed between 1989 and 1994, a period during which CASPEC and it s Zeeman analyzer have progressively undergone various configuration c hanges. The results reported here demonstrate that the polarimetric pe rformance of the instrument has remained unaltered through these modif ications. Thanks to the latter, the achieved resolving power was incre ased, which resulted in improved magnetic measurement accuracies.