Oscillations in beta Ursae Minoris - Observations with SMEI

Aims. From observations of the K4III star β UMi we attempt to determine whether oscillations or any other form of variability is present. Methods. A high-quality photometric time series of ≈1000 days in length obtained from the SMEI instrument on the Coriolis satellite is analysed. Various statistical tests were performed to determine the significance of features seen in the power density spectrum of the light curve. Results. Two oscillations with frequencies 2.44 and 2.92 μHz have been identified. We interpret these oscillations as consecutive overtones of an acoustic spectrum, implying a large frequency spacing of 0.48 μHz. Using derived asteroseismic parameters in combination with known astrophysical parameters, we estimate the mass of β UMi to be 1.3 ± 0.3 M. Peaks of the oscillations in the power density spectrum show width, implying that modes are stochastically excited and damped by convection. The mode lifetime is estimated at 18 ± 9 days.