SPOT ACTIVITY AND THE DIFFERENTIAL ROTATION ON HD-106225 DERIVED FROMDOPPLER TOMOGRAPHY

Doppler images of the cool spot distribution on the RS CV n-type star HD 106225 are presented. These images, for Epoch 1995.14, were derived using four photospheric lines. The spot distribution is characterized by a large (radius = 20 degrees) spot at latitude 45 degrees and a we ak polar spot with an appendage. The polar spot is considerably smalle r than similar features found on other RS CVn stars. The rotation rate as a function of latitude was derived by comparing the Doppler images of this work to one found for Epoch 1991.3 by Strassmeier. This rotat ion law is consistent to the one found by Strassmeier in that the diff erential rotation on HD 106225 is in the opposite sense as on the Sun and at least a factor of 10 less extreme. A comparison of the differen tial rotation rates derived from Doppler imaging of other spotted star s yields a linear relationship with the logarithm of the stellar rotat ion period; the longer the rotation period the faster the polar region s are rotating with respect to the equatorial regions. The crossover p oint between polar acceleration and equatorial acceleration occurs at a rotation period of about 2 days. The full H alpha emission (after su btracting the absorption component of an inactive star) shows sinusoid al variations with an amplitude of about 10 km s(-1), significantly le ss than the rotation rate of 25 km s(-1). This modulation is interpret ed as arising from plage located at high stellar latitude and adjacent to the polar appendage seen in the photospheric spot distribution.