The very slow rotation of the magnetic O9.7V star HD54879

TL;DR

This paper reports on the extremely slow magnetic field variability of the O9.7V star HD54879, attributed to its very slow rotation, based on spectropolarimetric observations over several years.

Contribution

It provides a detailed analysis of the magnetic field evolution of HD54879, confirming its very slow rotation and correcting previous data analysis errors.

Findings

Magnetic field decreased from -600G to -90G over several years.

Spectropolarimetric data confirms very slow magnetic variability.

Previous data re-analysis corrected earlier apparent field increases.

Abstract

The first FORS2 spectropolarimetric observation of the longitudinal magnetic field of HD54879 of the order of -600G with a lower limit of the dipole strength of ~2kG dates back to 2014. Since then observations showed a gradual decrease of the absolute value of the mean longitudinal magnetic field. In the course of the most recent monitoring of HD54879 using FORS2 spectropolarimetric observations from 2017 October to 2018 February, a longitudinal magnetic field strength change from about -300G down to about -90G was reported. A sudden increase of the absolute value of the mean longitudinal magnetic field and an accompanying spectral variability was detected on 2018 February 17. New FORS2 spectropolarimetric data obtained from 2018 December to 2019 February confirm the very slow magnetic field variability, with the field decreasing from about 150G to -100G over two months. Such a slow magnetic field variability, related to the extremely slow rotation of HD54879, is also confirmed using high-resolution HARPSpol and ESPaDOnS spectropolarimetry. The re-analysis of the FORS2 polarimetric spectra from 2018 February indicates that the previously reported field increase and the change of the spectral appearance was caused by improper spectra extraction and wavelength calibration using observations obtained at an insufficient signal-to-noise ratio. The presented properties of HD54879 are discussed in the context of the Of?p spectral classification.