Confirmation of ξ1 CMa’s ultra-slow rotation: Magnetic polarity reversal and a dramatic change in magnetospheric UV emission lines

Abstract

Abstract The magnetic β Cep pulsator ξ1 CMa has the longest rotational period of any known magnetic B-type star. It is also the only magnetic B-type star with magnetospheric emission that is known to be modulated by both rotation and pulsation. We report here the first unambiguous detection of a negative longitudinal magnetic field in ξ1 CMa (〈Bz〉 = −87 ± 2 G in 2019 and 〈Bz〉 = −207 ± 3 G in 2020), as well as the results of ongoing monitoring of the star’s Hα variability. We examine evidence for deviation from a purely dipolar topology. We also report a new HST UV spectrum of ξ1 CMa obtained near magnetic null that is consistent with an equatorial view of the magnetosphere, as evidenced by its similarity to the UV spectrum of β Cep obtained near maximum emission. The new UV spectrum of ξ1 CMa provides additional evidence for the extremely long rotation period of this star via comparison to archival data.