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    Magnetic fields and the variable wind of the early-type supergiant β Ori

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    Shultz_Matthew_201204_MsC.pdf (11.20Mb)
    Date
    2012-04-30
    Author
    Shultz, Matthew Eric
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    Abstract
    Supergiant stars of spectral types B and A are characterized by variable and structured winds, as revealed by variability of optical and ultraviolet spectral lines. Non-

    radial pulsations and magnetically supported loops have been proposed as explanations for these phenomena. The latter hypothesis is tested using a time series of 65

    high-resolution (λ/∆λ ∼ 65, 000) circular polarization (Stokes I and V ) spectra of

    the late B type supergiant Rigel (β Ori, B8 Iae), obtained with the instruments ESPaDOnS and Narval at the Canada-France-Hawaii Telescope and the Bernard Lyot Telescope, respectively. Examination of the unpolarized (Stokes I) spectra using standard spectral analysis tools confirms complex line profile variability during the 5 month period of observations; the high spectral resolution allows the identification of a weak, transient Hα feature similar in behaviour to a High Velocity Absorption event. Analysis of the Stokes V spectra using the cross-correlation technique Least Squares Deconvolution (LSD) yields no evidence of a magnetic field in either LSD Stokes V profiles or longitudinal field measurements, with longitudinal field 1σ error bars of ∼ 12 G for individual observations, and a mean field in the best observed period of 3 ± 2 G. Synthetic LSD profiles fit to the observations using a Monte Carlo approach yield an upper limit on the surface dipolar field strength of Bdip ≤ 50 G for most orientations of the rotational and magnetic axes, lowered to Bdip ≤ 35 G if the mean LSD profile from the most densely time-sampled epoch (with an LSD SNR of ∼80,000) is used. A simple two-spot geometry representing the footpoints of a magnetic loop emerging from the photosphere yields upper limits on the spot magnetic fields of 60–600 G, depending on the filling factor of the spots. Given existing measurements of the mass loss rate and the wind terminal velocity, these results

    cannot rule out a magnetically confined wind as, for Bdip ≤ 15 G, η∗ ≥ 1. However, the detailed pattern of line profile variability seems inconsistent with the periodic wind modulation characteristic of known magnetic early-type stars, suggesting that magnetic fields do not play a dominant role in Rigel’s variable winds.
    URI for this record
    http://hdl.handle.net/1974/7165
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