Doppler imaging of stellar surface structure

XV. A possible detection of differential rotation and local meridional flows on the rapidly-rotating giant HD 218153 = KU Pegasi

Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany e-mail: kstrassmeier@aip.de

Corresponding author: M. Weber, mweber@aip.de

Received: 21 February 2001
Accepted: 11 April 2001

Abstract

Time-resolved Doppler images of the rapidly rotating, but long-period (25 days), giant KU Pegasi show several cool low-to-medium latitude spots as well as an asymmetric polar feature. The average spot temperature is about 700 K below the photospheric temperature of 4700 K. KU Peg is one of the most massive, and currently the most evolved, late-type star with a Doppler image. We obtained two independent images from two consecutive stellar rotations covering 50 nights with a total of 43 spectra. From a cross-correlation analysis of the two maps, we detect systematic longitudinal and latitudinal shifts that we tentatively interpret as latitude-dependent differential rotation and local meridional flows, respectively. The differential-rotation pattern is more complex than on the Sun, but on average in the sense that the poles rotate slower than the stellar equator, i.e. in the same direction and also of the same order than on the Sun. The latitudinal shifts are of the order of 0.4°day^-1 towards the stellar pole and occur at longitudes of around 40° and 330°. The residual Hα profiles show a stationary emission component at rest wavelength and a blue-shifted absorption. The latter suggests an outward pointed velocity field with a flow velocity of approximately 35 km s^-1.