Detection of anomalous element distribution in the extremely slowly rotating magnetic O9.7 V star HD 54879

TL;DR

This study investigates the magnetic field geometry and rotation period of the magnetic O9.7 V star HD 54879, revealing extremely slow rotation, element-dependent magnetic field differences, and insights into the star's atmospheric structure.

Contribution

First differential analysis of magnetic field strength in a magnetic O-type star based on element-specific line formation depths and NLTE effects.

Findings

Detected very slow magnetic field variability indicating extremely slow rotation.

Discovered element-dependent differences in measured magnetic field strengths.

Suggested presence of enhanced gas density inside magnetic field lines.

Abstract

The O9.7 V star HD 54879 is currently the only massive magnetic star whose magnetic field geometry and rotation period are not constrained. Over the last three years, we gathered additional observations of this star, obtained using various instruments at several astronomical facilities with, the aim to constrain the rotation period and the magnetic field geometry. The new data include the first full Stokes vector observations with the PEPSI spectropolarimeter, installed at the Large Binocular Telescope. The acquired spectropolarimetric observations show a very slow magnetic field variability related to the extremely slow rotation of HD 54879, which is also indicated in a dynamical spectrum, displaying variability of the H$\alpha$ line. The most intriguing result of our study is the discovery of differences in longitudinal magnetic field strengths measured using different LSD masks containing lines belonging to different elements. It is the first time that such a differential analysis of the field strength in dependence of the used lines is carried out for a magnetic O-type star. Since the LSD Stokes $I$ profiles of the studied O, Si, and He line masks remain stable over all observing epochs, we conclude that the detection of different field strengths using lines belonging to these elements is related to the different formation depths, with the He lines formed much higher in the stellar atmosphere compared to the silicon and the oxygen lines, and NLTE effects. Our numerical magnetospherical model suggests the presence of enhanced gas density that fills the volume inside the field lines close to the star.