2009A&A...493L..45K

We present our discovery of pulsational radial-velocity variations in the cool Ap star HD75445, an object spectroscopically similar to the bright, rapidly-oscillating Ap (roAp) star γEqu. Based on high-resolution time-series spectroscopy obtained with the HARPS spectrometer at the European Southern Observatory 3.6-m telescope, we detected oscillations in NdII and NdIII lines with a period close to 9min and amplitudes of 20-30m/s. Substantial variation in the pulsational amplitude during our 3.8 h observing run reveals the presence of at least three excited non-radial modes. The detection of extremely low amplitude pulsations in HD75445 indicates that the roAp excitation mechanism produces variability in the radial velocity amplitude of between a few tens m/s and several km/s. This supports the idea that many, if not all, cool Ap stars occupying the roAp instability strip may harbour non-radial pulsations, which currently remain undetected due to their small photometric and radial-velocity amplitudes.