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Title: NGC 1624-2: a slowly rotating, X-ray luminous Of?cp star with an extraordinarily strong magnetic field
Authors: Wade, G. A
Maiz, J. A
Martins, F
Petit, V
Grunhut, J
Walborn, N. R
Barba, R. H
Gagne, M
Garcia-Melendo, E
Jose, J
Moffat, A. F. J
Naze, Y
Neiner, C
Pellerin, A
Penades, O. M
Shultz, M
Simon-Diaz, S
Sota, A
Keywords: Binaries: spectroscopic;Stars: early-type;Stars: magnetic fields;Stars: individual: NGC 1624-2
Issue Date: Sep-2012
Publisher: Wiley-Blackwell
Citation: Monthly Notices of the Royal Astronomical Society, Vol. 425, No. 2, pp. 1278–1293
Abstract: This paper presents a first observational investigation of the faint Of?p star NGC 1624-2, yielding important new constraints on its spectral and physical characteristics, rotation, magnetic field strength, X-ray emission and magnetospheric properties. Modelling the spectrum and spectral energy distribution, we conclude that NGC 1624-2 is a main-sequence star of mass M ≃ 30 M⊙, and infer an effective temperature of 35 ± 2 kK and log g = 4.0 ± 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 ± 0.94 d which we interpret as the rotational period of the star. We report the detection of a very strong (5.35 ± 0.5 kG) longitudinal magnetic field 〈B/subz/〉, coupled with probable Zeeman splitting of the Stokes I profiles of metal lines confirming a surface field modulus 〈B〉 of 14 ± 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 analyse archival Chandra ACIS-I X-ray data, inferring a very hard spectrum with an X-ray efficiency of log L/sub x/ /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 θ/sup1/ 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 η/sub*/ ≃ 1.5 × 10/sup4/, and a very large Alfvén radius, R/subAlf/ = 11.4 R/sub*/
Description: Restricted Access
ISSN: 0035-8711
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Appears in Collections:Publications based on data from IAO, Hanle
IIAP Publications

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