A short and sudden increase of the magnetic field strength and the accompanying spectral variability in the O9.7V star HD54879

Abstract

Only eleven O-type stars have been confirmed to possess large-scale organized magnetic fields. The presence of a −600 G longitudinal magnetic field in the O9.7 V star HD 54879 with a lower limit of the dipole strength of ∼2 kG, was discovered a few years ago in the framework of the ESO large program "B-fields in OB stars". Our FORS 2 spectropolarimetric observations from 2017 October 4 to 2018 February 21 reveal the presence of short and long-term spectral variability and a gradual magnetic field decrease from about −300 G down to about −90 G. Surprisingly, we discover on the night of 2018 February 17 a sudden, short-term increase of the magnetic field strength and measure a longitudinal magnetic field of −833 G. The inspection of the FORS 2 spectrum acquired during the observed magnetic field increase indicates a very strong change in spectral appearance with a significantly lower photospheric temperature and a decrease of the radial velocity by several 10 km s −1 . Different scenarios are discussed in an attempt to interpret our observations. The FORS 2 radial velocity measurements indicate that HD 54879 is a member of a long-period binary.