Magnetic field geometry and magnetospheric environment of the strongly magnetic Of?p star NGC 1624-2

TL;DR

This study characterizes the magnetic field geometry of the strongly magnetic O-type star NGC 1624-2, revealing a predominantly dipolar field with complex deviations, and presents the first MHD simulations of its magnetosphere.

Contribution

It provides detailed magnetic field measurements and modeling of NGC 1624-2, including the first MHD simulations of its magnetospheric gas flow, highlighting its unique magnetic properties.

Findings

Magnetic field shows sinusoidal variation indicating a dipolar component.

Field strength ranges from 9 to 12 kG, with longitudinal measurements from -0.2 to 4.5 kG.

Magnetosphere modeling reveals significant departure from a centered dipole.

Abstract

NGC 1624-2 is an O7f?p star with a reported probable polar magnetic field strength $\ge$ 20 kG, which is the strongest magnetic field ever measured in an O-type star. We study the variability of the mean longitudinal magnetic field $\langle B_z \rangle$ and the mean field modulus $\langle B \rangle$ to obtain constraints on its field geometry. Only one magnetic pole is observable over the rotation cycle. The approximately sinusoidal variation of $\langle B_z \rangle$ and the ratio of the values of the extrema of $\langle B \rangle$ indicate that there is an important component of the field that is dipolar. The $\langle B_z \rangle$ values measured over the rotation cycle are in the range from $-0.2$ to 4.5 kG, whereas the values for $\langle B \rangle$ vary between 9 and 12 kG. The $\langle B_z \rangle$ values obtained using the O III $\lambda$7455 emission line are in the range from 0.4 to 2.3 kG and show a variability pattern similar to that detected for the absorption lines. The fact that the phase of the $\langle B_z \rangle$ minimum coincides with the phase of the $\langle B \rangle$ maximum, indicates that the field structure must significantly depart from a centred dipole. Further, we discuss the nature of the observed variable Stokes $V$ profiles corresponding to a longitudinal field of negative polarity detected in the emission He I lines and present the first MHD numerical simulations of the gas flow in the magnetosphere of this star.