Extreme resonance line profile variations in the ultraviolet spectra of NGC 1624-2: probing the giant magnetosphere of the most strongly magnetized known O-type star

David-Uraz, A.; Erba, C.; Petit, V.; Fullerton, A. W.; Martins, F.; Walborn, N. R.; MacInnis, R.; Barbá, R. H.; Cohen, D. H.; Maíz Apellániz, J.; Nazé, Y.; Owocki, S. P.; Sundqvist, J. O.; ud-Doula, A.; Wade, G. A.
Monthly Notices of the Royal Astronomical Society, Volume 483, Issue 2, p.2814-2824 (2019).


In this paper, we present high-resolution HST/COS observations of the extreme magnetic O star NGC 1624-2. These represent the first ultraviolet spectra of this archetypal object. We examine the variability of its wind-sensitive resonance lines, comparing it to that of other known magnetic O stars. In particular, the observed variations in the profiles of the C IV and Si IV doublets between low state and high state are the largest observed in any magnetic O-type star, consistent with the expected properties of NGC 1624-2's magnetosphere. We also observe a redshifted absorption component in the low state, a feature not seen in most stars. We present preliminary modelling efforts based on the Analytic Dynamical Magnetosphere (ADM) formalism, demonstrating the necessity of using non-spherically symmetric models to determine wind/magnetospheric properties of magnetic O stars.