Rotation, spectral variability, magnetic geometry and magnetosphere of the Of?p star CPD -28° 2561

WADE, G.; BARBÁ, R.; GRUNHUT, J.; MARTINS, F.; PETIT, V.; SUNDQVIST, J. O.; TOWNSEND, R. H. D.; WALBORN, N. R.; ALECIAN, E.; ALFARO, E. J.; MAÍZ APELLÁNIZ, J.; ARIAS, J.; GAMEN, R. C.; MORRELL, N. I.; NAZÉ, Y.; SOTA, A.; UD-DOULA, A.; MIMES COLLABORATION

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2015 vol. 447 p. 2551 - 2567

0035-8711

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) Halpha equivalent width variation. Modelling of the Halpha equivalent width variation assuming a quasi-3D magnetospheric model produces a unique solution for the ambiguous couplet of inclination and magnetic obliquity angles: (i, beta) or (beta, i) = (35°, 90°). Adopting either geometry, the longitudinal field variation yields a dipole polar intensity Bd = 2.6 ± 0.9 kG, consistent with that obtained from direct modelling of the Stokes V profiles. We derive a wind magnetic confinement parameter eta* ~= 100, leading to an Alfvén radius RA ~= 3-5R*, and a Kepler radius RK ~= 20R*. This supports a physical scenario in which the Halpha 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.