National Research Council of Canada. National Science Infrastructure
National Research Council of Canada
A new time series of high-resolution Stokes I and V spectra of the magnetic B2V star HD 184927 has been obtained in the context of the Magnetism in Massive Stars Large Program with an Echelle SpectroPolarimetric Device for the Observation of Stars (ESPaDOnS) spectropolarimeter at the Canada-France-Hawaii Telescope and dimaPol liquid crystal spectropolarimeter at 1.8-m telescope of Dominion Astrophysical Observatory. We model the optical and UV spectrum obtained from the International Ultraviolet Explorer (IUE) archive to infer the stellar physical parameters. Using magnetic field measurements, we derive an improved rotational period of 9.531 02 ± 0.0007 d. We infer the longitudinal magnetic field from lines of H, He, and various metals, revealing large differences between the apparent field strength variations determined from different elements. Magnetic Doppler Imaging using He and O lines yields strongly non-uniform surface distributions of these elements. We demonstrate that the diversity of longitudinal field variations can be understood as due to the combination of element-specific surface abundance distributions in combination with a surface magnetic field that is comprised of dipolar and quadrupolar components. We have reanalysed IUE high-resolution spectra, confirming strong modulation of wind-sensitive C IV and S IV resonance lines. However, we are unable to detect any modulation of the Hα profile attributable to a stellar magnetosphere. We conclude that HD 184927 hosts a centrifugal magnetosphere (η~ 2.4+22 -1.1 × 104), albeit one that is undetectable at optical wavelengths. The magnetic braking time-scale of HD 184927 is computed to be τ J = 0.96 or 5.8 Myr. These values are consistent with the slow rotation and estimated age of the star.
Monthly Notices of the Royal Astronomical Society447, no. 2 (2 January 2015): 1418–1438.