binaries: spectroscopic; stars: early-type; stars: magnetic fields; stars: individual: NGC 1624-2
Abstract :
[en] 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[SUB]&sun;[/SUB], 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[SUB]z[/SUB]>, coupled with probable Zeeman splitting of the Stokes I profiles of metal lines confirming a surface field modulus 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[/SUB]/L[SUB]bol[/SUB] = -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[SUP]1[/SUP] 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]*[/SUB] ≃ 1.5 × 10[SUP]4[/SUP], and a very large Alfvén radius, R[SUB]Alf[/SUB] = 11.4 R[SUB]*[/SUB]. Based on spectropolarimetric observations obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers (INSU) of the Centre National de la Recherche Scientifique of France, and the University of Hawaii, as well as on observations obtained using the Narval spectropolarimeter at the Observatoire du Pic du Midi (France), which is operated by the INSU. The spectroscopic data were gathered with five facilities: the 9.2-m Hobby-Eberly Telescope at McDonald Observatory (MDO), the 3.5-m Telescope at Calar Alto Observatory (CAHA), the 1.5-m Telescope at the Observatorio de Sierra Nevada (OSN), the 4.2-m William Herschel Telescope at the Observatorio del Roque de los Muchachos (ORM) and the 2-m Himalayan Chandra Telescope at Indian Astronomical Observatory (IAO).
Disciplines :
Space science, astronomy & astrophysics
Author, co-author :
Wade, G. A.; Department of Physics, Royal Military College of Canada, PO Box 17000, Stn Forces Kingston, Ontario K7K 7B4 Canada
Maíz Apellániz, J.; Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n E-18008 Granada, Spain
Martins, F.; LUPM-UMR5299, CNRS & Université Montpellier II, Place Eugène Bataillon F-34095 Montpellier, France
Petit, V.; Department of Geology & Astronomy, West Chester University, West Chester, PA 19383 USA
Grunhut, J.; Department of Physics, Royal Military College of Canada, PO Box 17000, Stn Forces Kingston, Ontario K7K 7B4 Canada
Walborn, N. R.; Space Telescope Science Institute, 3700 San Martin Drive Baltimore, MD 21218 USA
Barbá, R. H.; Instituto de Ciencias Astronómicas, de la Tierra y del Espacio, Casilla 467 5400 San Juan, Argentina
Gagné, M.; Department of Geology & Astronomy, West Chester University, West Chester, PA 19383 USA
García-Melendo, E.; Fundació Privada Observatori Esteve Duran, C/Montseny 46 E-08553 Seva, Barcelona Spain ; Institut de Ciències de l'Espai (CSIC-IEEC), Campus UAB, Facultat de Ciències, Torre C5, parell, 2a pl. E-08193 Bellaterra, Spain
Jose, J.; Indian Institute of Astrophysics, Koramangala Bangalore, 560034 India
Moffat, A. F. J.; Département de physique, Université de Montréal, CP 6128, Succ. Centre-Ville Montréal, QC H3C 3J7 Canada
Nazé, Yaël ; Université de Liège - ULiège > GAPHE : Astrophysique observationnelle (sol et espace)
Neiner, C.; LESIA, UMR 8109 du CNRS, Observatoire de Paris, UPMC, Université Paris Diderot, 5 place Jules Janssen 92195 Meudon Cedex, France
Pellerin, A.; Department of Physics and Astronomy, Texas A&M University, College Station TX 77843 USA
Penadés Ordaz, M.; Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n E-18008 Granada, Spain
Shultz, M.; Department of Physics, Royal Military College of Canada, PO Box 17000, Stn Forces Kingston, Ontario K7K 7B4 Canada
Simón-Díaz, S.; Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife Spain
Sota, A.; Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n E-18008 Granada, Spain)
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