The pulsationally modulated radial crossover signature of the slowly rotating magnetic B-type star $\xi^1$ CMa

TL;DR

This study presents spectropolarimetric observations of the magnetic B-type star ξ¹ CMa, revealing pulsation-modulated crossover signatures caused by radial pulsations and complex magnetic topology, confirming a long-period rotational model.

Contribution

It introduces the concept of radial crossover signatures in magnetic pulsating stars and links them to pulsations and non-dipolar magnetic field structures.

Findings

Confirmation of a 30-year rotational period for ξ¹ CMa.

Detection of pulsation-modulated crossover signatures in Stokes V profiles.

Explanation of phenomena through radial crossover effect and non-dipolar magnetic topology.

Abstract

We report the latest set of spectropolarimetric observations of the magnetic $\beta$ Cep star $\xi^1$ CMa. The new observations confirm the long-period model of Shultz et al.\ (2017), who proposed a rotational period of about 30 years and predicted that in 2018 the star should pass through a magnetic null. In perfect agreement with this projection, all longitudinal magnetic field $\langle B_z \rangle$ measurements are close to 0 G. Remarkably, individual Stokes $V$ profiles all display a crossover signature, which is consistent with $\langle B_z \rangle \sim 0$ but is {\em not} expected when $v\sin{i} \sim 0$. The crossover signatures furthermore exhibit pulsationally modulated amplitude and sign variations. We show that these unexpected phenomena can all be explained by a `radial crossover' effect related to the star's radial pulsations, together with an important deviation of the global field topology from a purely dipolar structure, which we explore via a dipole+quadrupole configuration as the simplest non-dipolar field.