Spatially Resolved STIS Spectroscopy of α Orionis: Evidence for Nonradial Chromospheric Oscillation from Detailed Modeling

Four spatially resolved near-UV raster scans across the chromospheric disk ofalpha Ori, obtained with the Space Telescope Imaging Spectrograph on the HubbleSpace Telescope, reveal mean chromospheric infall from 1998 January to 1998April, which reversed to upflow in deeper layers between 1998 September and1999 March. In 1998 September we detect systematic reversals in the componentmaxima of four double-peaked emission lines of Si I (UV 1), Fe II (UV 36), FeII (UV 61), and Al II] (UV 1), when scanning across the UV disk. Detailedmodeling of the Si I lambda 2516 resonance line with radiative transportcalculations in spherical geometry constrain the mean radial velocity structurein 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 modelsthat these spatial reversals of emission line components correspond withaverage opposite flow velocities of ~2 km/s across the chromospheric disk. Wedetermine that the chromospheric velocity field can not be represented by aunique radial velocity structure across the stellar disk in order to match theobserved peak ratios of this raster scan. These sub-sonic velocities indicate(local) non-radial movements of chromospheric fluid in confined regions duringa chromospheric oscillation phase, which reverses from global contraction intoexpansion over this monitoring period of 15 months.