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A. Brown, G. M. Harper, T. R. Ayres (CASA, University of Colorado), J. M. Brown (Dept. of Astronomy, Caltech)
We have investigated the outer atmospheric structure of the F0 Ib supergiant Canopus using HST, Chandra and FUSE and have discovered a number of unexpected properties, including the presence of an unusually fast stellar wind from this post-M supergiant.
Our observations consist of 96 ksec of Chandra HETGS spectroscopy obtained on 2000 July 21, a set of HST-STIS (E140M, E230M, E230H) UV spectra obtained on 2002 June 11, and three FUSE FUV spectra obtained in 2000 December and 2001 October. The STIS and FUSE spectra provide full coverage of the 900-3000 Åspectrum and the first UV emission line profiles from the transition region of an F supergiant. These high signal-to-noise profiles allow detailed study of the physical conditions in Canopus' outer atmosphere from photospheric temperatures up to 5 105 K. Of particular interest is the hydrogen Lyman alpha line at 1215 Å; this line shows clear evidence of high velocity (up to 400 km s-1) wind absorption, which is a surprisingly high value for a cool star wind. Investigating the winds from F supergiants, lying between the fast (thousands km s-1) hot star winds and the slow (tens km s-1) red supergiant winds, is particularly important because the wind structure and driving mechanisms for such stars is currently unknown. We see no evidence for wind absorptions in any other UV emission lines observed by STIS or FUSE. The Chandra spectrum shows the presence of a corona whose emission measure peaks at 7 MK and contains temperatures up to 20 MK. Other coronal properties including elemental abundances are presented. No coronal variability was seen.
This work was supported by HST grant GO-06783.01-A, SAO Chandra grant GO0-1087X, and NASA grant NAG5-4804 to the University of Colorado.
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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.