Mapping the magnetic field of Plaskett's star

Plaskett's star is a spectroscopic binary, with two O star components in the spectrum, one of which has a strong magnetic field. This system represents one of the very few hot, massive, multiple star systems where a magnetic field is present. The narrow-line spectroscopic component shows a clear short orbital period of 14.396 days, while the motion of the broad-line component is less clear. Using spectropolarimetric observations of Plaskett's star from ESPaDOnS at the CFHT in circular polarization (Stokes V), we clearly detect a magnetic field in the broad-line component. We then model the observed Stokes V profiles using Zeeman Doppler Imaging to reconstruct the magnetic field of the broad-line component. We find the magnetic field has a strong dipolar component, however there are also important departures from a simple dipole. Our preliminary results find an average magnetic field strength of 430 G, and a strength of the dipolar component of 700 G at the pole. The field is almost completely poloidal (only 5% of the magnetic energy is toroidal), 65% of the poloidal energy is in the dipole, 22% in the quadrupole, and 13% in higher order multipoles. The dipolar component has an obliquity near 90 degrees and the magnetic field is dominantly non-symmetric about the rotation axis. The Stokes V observations can be fully explained by the magnetic field of the broad-line component, thus we find no evidence for a magnetic field in the narrow-line star. The Stokes V observations provide some constraints on the physical parameters of the broad-line star and its motion. In particular, matching the observations requires the broad-line component to have very little radial velocity variability, challenging the interpretation of this system as a binary with similar mass stars.