An observational evaluation of magnetic confinement in the winds of BA supergiants

作者：Shultz M^*; Wade G A; Petit V; Grunhut J; Neiner C; Hanes D

来源：Monthly Notices of the Royal Astronomical Society, 2014, 438(2): 1114-1126.

DOI：10.1093/mnras/stt2260

• circumstellar matter
• stars: early type
• stars: magnetic field
• stars: mass-loss
• supergiants
• stars: winds, outflows

Magnetic wind confinement has been proposed as one explanation for the complex wind structures of supergiant stars of spectral types B and A. Observational investigation of this hypothesis was undertaken using high-resolution (lambda/Delta lambda similar to 65 000) circular polarization (Stokes V) spectra of six late B- and early A-type supergiants (beta Ori, B8Iae; 4 Lac, B9Iab; eta Leo, A0Ib; HR1040, A0Ib; alpha Cyg, A2Iae; nu Cep, A2Iab), obtained with the instruments ESPaDOnS and Narval at the Canada-France-Hawaii Telescope and the Bernard Lyot Telescope. Least-squares deconvolution (LSD) analysis of the Stokes V spectra of all stars yields no evidence of a magnetic field, with best longitudinal field 1 sigma error bars ranging from similar to 0.5 to similar to 4.5G for most stars. Spectrum synthesis analysis of the LSD profiles using Bayesian inference yields an upper limit with 95.4 per cent credibility on the polar strength of the (undetected) surface dipole fields of individual stars ranging from 3 to 30 G. These results strongly suggest that magnetic wind confinement due to organized dipolar magnetic fields is not the origin of the wind variability of BA supergiant stars. Upper limits for magnetic spots may also be inconsistent with magnetic wind confinement in the limit of large spot size and filling factor, depending on the adopted wind parameters. Therefore, if magnetic spots are responsible for the wind variability of BA supergiant stars, they likely occupy a small fraction of the photosphere.

• 出版日期2014-2