Astronomy and Astrophysics – Astrophysics
Scientific paper
2001-06-28
Astronomy and Astrophysics
Astrophysics
24 pages including 6 figures, to appear in The Astrophysical Journal, Main Journal, 558, Sept. 2001, for the related Press Rel
Scientific paper
10.1086/322284
Four spatially resolved near-UV raster scans across the chromospheric disk of alpha Ori, obtained with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, reveal mean chromospheric infall from 1998 January to 1998 April, which reversed to upflow in deeper layers between 1998 September and 1999 March. In 1998 September we detect systematic reversals in the component maxima of four double-peaked emission lines of Si I (UV 1), Fe II (UV 36), Fe II (UV 61), and Al II] (UV 1), when scanning across the UV disk. Detailed modeling of the Si I lambda 2516 resonance line with radiative transport calculations in spherical geometry constrain the mean radial velocity structure in the projected slit area (25 by 100 mas) for different aperture positions, observed off-limb to 157.5 mas. Hence we determine with semi-empirical models that these spatial reversals of emission line components correspond with average opposite flow velocities of ~2 km/s across the chromospheric disk. We determine that the chromospheric velocity field can not be represented by a unique radial velocity structure across the stellar disk in order to match the observed peak ratios of this raster scan. These sub-sonic velocities indicate (local) non-radial movements of chromospheric fluid in confined regions during a chromospheric oscillation phase, which reverses from global contraction into expansion over this monitoring period of 15 months.
Dupree Andrea K.
Lobel Alex
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