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
Martins, F.; Owocki, S. P.; Wade, G. A.; Nazé, Y.; Maíz Apellániz, J.; Fullerton, A. W.; Barbá, R. H.; Petit, V.; ud-Doula, A.; Cohen, D. H.; Walborn, N. R.; Sundqvist, J. O.; Erba, C.; David-Uraz, A.; MacInnis, R.
United States, France, Chile, Spain, Belgium, Canada
Abstract
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.