Extreme Magnetic Field Modulus Variability of the Bp star HD 57372

TL;DR

This study reveals that the Bp star HD 57372 exhibits an extraordinary magnetic field modulus variability of about 10 kG, indicating a very unusual magnetic field geometry and providing new insights into magnetic field structures in early-type stars.

Contribution

The paper presents the first detailed analysis of the extreme magnetic field modulus variability in HD 57372, highlighting its unusual magnetic geometry and phase-correlated spectral and photometric variations.

Findings

Magnetic field modulus varies by about 10 kG.

Field ratio of 3 indicates unusual magnetic geometry.

Observable quantities vary in phase with the 7.889-day rotation period.

Abstract

Context. In chemically peculiar Ap/Bp stars with large-scale organised magnetic fields with a simple centred dipole configuration, the ratio between the maximum and the minimum of the mean magnetic field modulus is of the order of 1.25. Values of 2 or more are observed only for very few Ap/Bp stars and are indicative of a very unusual magnetic field geometry. Aims. 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 are observed in HARPS and APOGEE spectra. Methods. 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. Results. The mean magnetic field modulus <B> 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 of about 10 kG. The exceptional value of 3 for the ratio between the maximum and the minimum of the field modulus is indicative of a very unusual geometry of HD 57372's magnetic field. All observable quantities are 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% 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.