[en] We report magnetic and spectroscopic observations and modelling of the Of?p star CPD -28° 2561. Using more than 75 new spectra, we have measured the equivalent width variations and examined the dynamic spectra of photospheric and wind-sensitive spectral lines. A period search results in an unambiguous 73.41 d variability period. High-resolution spectropolarimetric data analysed using least-squares deconvolution yield a Zeeman signature detected in the mean Stokes V profile corresponding to phase 0.5 of the spectral ephemeris. Interpreting the 73.41 d period as the stellar rotational period, we have phased the equivalent widths and inferred longitudinal field measurements. The phased magnetic data exhibit a weak sinusoidal variation, with maximum of about 565 G at phase 0.5, and a minimum of about -335 G at phase 0.0, with extrema approximately in phase with the (double-wave) Hα equivalent width variation. Modelling of the Hα equivalent width variation assuming a quasi-3D magnetospheric model produces a unique solution for the ambiguous couplet of inclination and magnetic obliquity angles: (i, β) or (β, i) = (35°, 90°). Adopting either geometry, the longitudinal field variation yields a dipole polar intensity B[SUB]d[/SUB] = 2.6 ± 0.9 kG, consistent with that obtained from direct modelling of the Stokes V profiles. We derive a wind magnetic confinement parameter η[SUB]*[/SUB] ≃ 100, leading to an Alfvén radius R[SUB]A[/SUB] ≃ 3-5R[SUB]*[/SUB], and a Kepler radius R[SUB]K[/SUB] ≃ 20R[SUB]*[/SUB]. This supports a physical scenario in which the Hα emission and other line variability have their origin in an oblique, corotating `dynamical magnetosphere' structure resulting from a magnetically channelled wind. Nevertheless, the details of the formation of spectral lines and their variability within this framework remain generally poorly understood.
Research Center/Unit :
LiSRI - Liège Space Research Institute - ULiège
Disciplines :
Space science, astronomy & astrophysics
Author, co-author :
Wade, G. A.; Department of Physics, Royal Military College of Canada, PO Box 17000 Station Forces, Kingston, ON K7K 7B4, Canada
Barbá, R. H.; Departamento de Física, Universidad de La Serena, Av. Cisternas 1200 Norte, La Serena, Chile
Grunhut, J.; European Southern Observatories, Karl-Schwarzschild-Str. 2, D-85748 Garching, Germany
Martins, F.; LUPM-UMR5299, CNRS & Université Montpellier II, Place Eugène Bataillon, F-34095 Montpellier, France
Petit, V.; Department of Physics and Astronomy, Bartol Research Institute, University of Delaware, Newark, DE 19716, USA
Sundqvist, J. O.; Institut für Astronomie und Astrophysik der Universität München, Scheinerstr. 1, D-81679 München, Germany
Townsend, R. H. D.; Department of Astronomy, University of Wisconsin-Madison, 475 N Charter Street, Madison, WI 53706, USA
Walborn, N. R.; Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
Alecian, E.; UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d'Astrophysique de Grenoble (IPAG) UMR 5274, F-38041 Grenoble, France
Alfaro, E. J.; Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain
sMaíz Apellániz, J.; Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain
Arias, J. I.; Departamento de Física, Universidad de La Serena, Av. Cisternas 1200 Norte, La Serena, Chile
Gamen, R.; Instituto de Astrofísica de La Plata (CCT La Plata-CONICET, Universidad Nacional de La Plata), Paseo del Bosque s/n, 1900 La Plata, Argentina
Morrell, N.; Las Campanas Observatory, Observatories of the Carnegie Institution of Washington, La Serena, Chile
Nazé, Yaël ; Université de Liège - ULiège > Groupe d'astrophysique des hautes énergies (GAPHE)
Sota, A.; Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain
ud-Doula, A.; Penn State Worthington Scranton, Dunmore, PA 18512, USA)
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