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arXiv · 1206.2834

NGC 1624-2: A slowly rotating, X-ray luminous Of?cp star with an extraordinarily strong magnetic field

G. A. Wade·J. Maíz Apellániz·F. Martins·V. Petit·J. Grunhut·N. R. Walborn·R. H. Barbá·M. Gagné·E. García-Melendo·J. Jose·A. F. J. Moffat·Y. Nazé·C. Neiner·A. Pellerin·M. Penadés Ordaz·M. Shultz·S. Simón-Díaz·A. Sota·the MiMeS Collaboration
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Abstract

This paper presents a first observational investigation of the faint Of?cp star NGC 1624-2, yielding important new constraints on its spectral and physical characteristics, rotation, magnetic field strength, X-ray emission and magnetospheric properties. Modeling the spectrum and spectral energy distribution, we conclude that NGC 1624-2 is a main sequence star of mass M {\simeq} 30 M{\odot}, and infer an effective temperature of 35 {\pm} 2 kK and log g = 4.0 {\pm} 0.2. Based on an extensive time series of optical spectral observations we report significant variability of a large number of spectral lines, and infer a unique period of 157.99 {\pm} 0.94 d which we interpret as the rotational period of the star. We report the detection of a very strong - 5.35 {\pm} 0.5 kG - longitudinal magnetic field , coupled with probable Zeeman splitting of Stokes I profiles of metal lines confirming a surface field modulus of 14 {\pm} 1 kG, consistent with a surface dipole of polar strength >~ 20 kG. This is the largest magnetic field ever detected in an O-type star, and the first report of Zeeman splitting of Stokes I profiles in such an object. We also report the detection of reversed Stokes V profiles associated with weak, high-excitation emission lines of O iii, which we propose may form in the close magnetosphere of the star. We analyze archival Chandra ACIS-I X-ray data, inferring a very hard spectrum with an X-ray efficiency log Lx/Lbol = -6.4, a factor of 4 larger than the canonical value for O-type stars and comparable to that of the young magnetic O-type star {\theta}1 Ori C and other Of?p stars. Finally, we examine the probable magnetospheric properties of the star, reporting in particular very strong magnetic confinement of the stellar wind, with {\eta}* {\simeq} 1.5 {\times} 10^4, and a very large Alfven radius, RAlf = 11.4 R*.

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G. A. Wade, J. Maíz Apellániz, F. Martins, V. Petit, J. Grunhut, N. R. Walborn, R. H. Barbá, M. Gagné, E. García-Melendo, J. Jose, A. F. J. Moffat, Y. Nazé, C. Neiner, A. Pellerin, M. Penadés Ordaz, M. Shultz, S. Simón-Díaz, A. Sota, the MiMeS Collaboration. 2012-06-13. NGC 1624-2: A slowly rotating, X-ray luminous Of?cp star with an extraordinarily strong magnetic field. https://doi.org/10.1111/j.1365-2966.2012.21523.x

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