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

Abstract

ABSTRACT NGC 1624-2 is an O7f?p star with a reported probable polar magnetic field strength ≥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 〈Bz〉 and the mean field modulus 〈B〉 to obtain constraints on its field geometry. Only one magnetic pole is observable over the rotation cycle. The approximately sinusoidal variation of 〈Bz〉 and the ratio of the values of the extrema of 〈B〉 indicate that there is an important component of the field that is dipolar. The 〈Bz〉 values measured over the rotation cycle are in the range from −0.2 to 4.5 kG, whereas the values for 〈B〉 vary between 9 and 12 kG. The 〈Bz〉 values obtained using the O iii λ7455 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 〈Bz〉 minimum coincides with the phase of the 〈B〉 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 magnetohydrodynamical numerical simulations of the gas flow in the magnetosphere of this star.