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Multiple short-lived stellar prominences on O stars: The O6.5I(n)fp star lambda Cephei | N.P. Sudnik
; H.F. Henrichs
; | Date: |
1 Jun 2016 | Abstract: | Most O and many B stars show unexplained cyclical variability in their
spectral lines, i.e. modulation on the rotational timescale, but not strictly
periodic. The variability occurs in the so-called discrete absorption
components (DACs) that accelerate through the UV-wind line profiles and also in
many optical lines. For such OB stars no dipolar magnetic fields have been
detected with upper limits of ~300G. We investigate whether multiple magnetic
loops on the surface rather than non-radial pulsations or a dipolar magnetic
field can explain the observed cyclical UV and optical spectral line
variability. We present time-resolved, high-resolution optical spectroscopy of
the O6.5I(n)fp star lambda Cep. We apply a simplified phenomenological model in
which multiple spherical blobs attached to the surface represent magnetic-loop
structures, which we call stellar prominences, by analogy with solar
prominences. We compare the calculated line profiles as a function of
rotational phase, adopting a rotation period of 4.1d, with observed relative
changes in subsequent quotient spectra. We identify many periodicities in
spectral lines, almost none of which is stable over timescales from months to
years. We show that the relative changes in various optical absorption and
emission lines are often very similar. Our proposed model applied to the HeII
4686 line can typically be fitted with 2-5 equatorial blobs with lifetimes
between ~1 and 24h. Given the irregular timescales involved, we propose that
the azimuthal distribution of DACs correspond to the locations of stellar
prominences attached to the surface. This could explain the observed
variability of optical and UV lines, and put constraints on the strength and
lifetime of these structures, which can be compared with recent theoretical
predictions, in which bright magnetic surface spots are formed by the action of
the subsurface convection zone. | Source: | arXiv, 1606.0404 | Services: | Forum | Review | PDF | Favorites |
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