Extreme magnetic field modulus variability of the Bp star HD 57372

Abstract

In chemically peculiar Ap/Bp stars with large-scale organised magnetic fields of simple centred dipole configuration, the ratio between the maximum and the minimum of the mean magnetic field modulus is on the order of 1.25. Values of two or more are observed only for very few Ap/Bp stars and are indicative of a very unusual magnetic field geometry. Determining the magnetic field structure of Ap/Bp stars is bound to provide a different insight into the physics and the origin of the magnetic fields in early-type stars. In this respect, the Bp star HD\,57372 is of particular interest because strongly variable magnetically split lines have been observed in HARPS and APOGEE spectra. We obtained and analysed measurements of the mean magnetic field modulus and of the mean longitudinal magnetic field using near-infrared spectra and optical polarimetric spectra distributed over the stellar rotation period. The mean magnetic field modulus bm of HD\,57372, as estimated from absorption lines that are split via the Zeeman effect and resolved in both optical and near-infrared spectra, is found to vary by an extraordinary amount: about 10\,kG. The exceptional value of three for the ratio between the maximum and the minimum of the field modulus is indicative of a very unusual geometry for HD\,57372's magnetic field. All observable quantities were found to vary in phase with the photometric period of 7.889\,days. This includes the longitudinal magnetic field bz , which varies from $-6$\,kG up to $1.7$\,kG in FORS2 spectra, as well as the metal line strengths, whose equivalent widths change by up to 50<!PCT!> of their mean values over the course of the rotation period. The B8 temperature class of HD\,57372 also places it among the hottest stars known to exhibit resolved, magnetically split lines.