Spot activity and the differential rotation on HD 106225 derived from Doppler tomography

Doppler images of the cool spot distribution on the RS CVn-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◦) spot at latitude 45◦ and a weak polar spot with an appendage. The polar spot is considerably smaller 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 rotation law is consistent to the one found by Strassmeier in that the differential 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 differential rotation rates derived from Doppler imaging of other spotted stars yields a linear relationshipwith the logarithm of the stellar rotation period; the longer the rotation period the faster the polar regions are rotatingwith respect to the equatorial regions. The crossover point between polar acceleration and equatorial acceleration occurs at a rotation period of about 2 days. The full Hα emission (after subtracting the absorption component of an inactive star) shows sinusoidal variations with an amplitude of about 10 km s−1 , significantly less than the rotation rate of 25 km s−1 . This modulation is interpreted as arising from plage located at high stellar latitude and adjacent to the polar appendage seen in the photospheric spot distribution.