HD 181906: A star with two systems of active longitudes on its surface

Abstract

We use continuous 156-day COROT photometric observations of the F dwarf HD 181906 to analyze temperature inhomogeneities on the stellar surface and follow their evolution. The analysis used the iPH code, which solves for the temperature inhomogeneities in a two-temperature approximation without any assumptions about the shape or number of spots on the surface. For the first time in studies of active regions and active longitudes, we find that the phases of the active longitudes on the surface of HD 181906 are concentrated close to two systems of active longitudes. In each system, the active longitudes are separated by 180°, with the shift between the systems being 100°. During the observing period, switches between the systems of active longitudes occurred quasi-periodicly on a time scale of 30–35 day about two-thirds of the time, while these switches occurred less frequently about one-third of the time. The positions of active regions switched either every 20–25 day or every 40–45 day. The periodicity of brightness and amplitude variations is of the order of 110 day. Variations in the spot coverage and changes in the active longitudes have shorter time scales (about 55–75 day). All these parameters are variable on time scales of 25–38 day. A wavelet analysis of the periodicity of the brightness variations indicates that all the above processes are quasi-periodic; activity on all time scales became less pronounced in the last third of the observing period.