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

A David-Uraz, C Erba, V Petit, A W Fullerton, F Martins, N R Walborn, R MacInnis, R H Barbá, D H Cohen, J Maíz Apellániz, Y Nazé, S P Owocki, J O Sundqvist, A ud-Doula, G A Wade. 2019. 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. Monthly Notices of the Royal Astronomical Society 483, 2, 2814-2824 DOI: 10.1093/mnras/sty3227

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.

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