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Session 71 - Stellar Activity/Starspots.
Display session, Thursday, June 13
The Hubble Space Telescope, for the first time, provides us with the opportunity to obtain direct images of the surface of a star. Such images in two UV wavelength bands around 255 and 280 nm respectively have been obtained of the red supergiant \alpha Ori (Betelgeuse; HD 39801). They show a chromosphere that extends over a diameter of at least 125 milliarcsec (mas), far beyond the 55 mas optical disk, and they also show the presence of an unresolved bright spot in the South-West quadrant of the disk (Gilliland amp;\ Dupree, ApJ Letters, in press). In addition, spatially resolved spectra of the Mg II resonance doublet were obtained with the GHRS by scanning the small science aperture across the stellar disk in perpendicular directions with 27.5 mas steps. We have analyzed these spectra and present exciting new results here.
The chromosphere as observed in the Mg II hamp;k emission stretches over a diameter of at least 270 mas, even further than the filter band images suggest. The hamp;k lines (at 280.2 and 279.5 nm respectively) show blue/red asymmetries in the height of their emission; the h line has more emission in the blue peak, while the opposite is true for the k line. The signature of the bright spot is revealed as an asymmetry in the flux measured across the disk at constant wavelength. Such asymmetry is present both at continuum wavelengths and in the h-line emission, but not in the k-line emission. This together with the observed red/blue asymmetries prompts us to suggest that we are observing a non-spherically symmetric shock wave that propagates radially outward. The break in spherical symmetry may be induced by rotation in which case we expect the bright spot to coincide with one of the poles of the supergiant where the shock breaks out first. Shifts we measure in the position of photospheric lines indicate a rotation axis that is consistent with this proposition.
Program listing for Thursday