New multiwavelength observations of the Of?p star CPD -28 2561

Abstract

A rather strong mean longitudinal magnetic field of the order of a few hundred Gauss was detected a few years ago in the Of?p star CPD -28 2561 using FORS2 low-resolution spectropolarimetric observations. In this work we present additional low-resolution spectropolarimetric observations obtained during several weeks in 2013 December using FORS2 (FOcal Reducer low dispersion Spectrograph) mounted at the 8-m Antu telescope of the VLT. These observations cover a little less than half of the stellar rotation period of 73.41d mentioned in the literature. The behaviour of the mean longitudinal magnetic field is consistent with the assumption of a single-wave variation during the stellar rotation cycle, indicating a dominant dipolar contribution to the magnetic field topology. The estimated polar strength of the surface dipole B_d is larger than 1.15kG. Further, we compared the behaviour of the line profiles of various elements at different rotation phases associated with different magnetic field strengths. The strongest contribution of the emission component is observed at the phases when the magnetic field shows a negative or positive extremum. The comparison of the spectral behaviour of CPD -28 2561 with that of another Of?p star, HD148937 of similar spectral type, reveals remarkable differences in the degree of variability between both stars. Finally, we present new X-ray observations obtained with the Suzaku X-ray Observatory. We report that the star is X-ray bright with log L_X/L_bol ~= -5.7. The low resolution X-ray spectra reveal the presence of a plasma heated up to 24MK. We associate the 24MK plasma in CPD -28 2561 with the presence of a kG strong magnetic field capable to confine stellar wind.

10 pages, 11 figures, 2 tables, accepted for publication in MNRAS

References in corpus (6)

• Discovery of magnetic fields in the beta Cephei star xi^1 CMa and in several Slowly Pulsating B stars
• Magnetic field measurements of O stars with FORS1 at the VLT
• An Extensive Collection of Stellar Wind X-ray Source Region Emission Line Parameters,Temperatures, Velocities, and Their Radial Distributions as Obtained from Chandra Observations of 17 OB Stars
• New magnetic field measurements of beta Cephei stars and Slowly Pulsating B stars
• HD148937: a multiwavelength study of the third Galactic member of the Of?p class
• Short time scale spectral variability in the A0 supergiant HD92207 and the importance of line profile variations for the interpretation of FORS2 spectropolarimetric observations

Cited by in corpus (7)

• The MiMeS survey of magnetism in massive stars: Magnetic analysis of the O-type stars
• B fields in OB stars (BOB): FORS2 spectropolarimetric follow-up of the two rare rigidly rotating magnetosphere stars HD23478 and HD345439
• A combined HST and XMM-Newton campaign for the magnetic O9.7 V star HD 54879: towards constraining the weak-wind problem of massive stars
• The search for magnetic fields in two Wolf-Rayet stars and the discovery of a variable magnetic field in WR55
• Constraints on Gamma-ray Burst Inner Engines in a Blandford-Znajek Framework
• The Bubble Nebula NGC 7635 -- testing the wind-blown bubble theory
• The very slow rotation of the magnetic O9.7V star HD54879