* Astronomy

Title: Discovery of very low amplitude 9-minute multiperiodic pulsations in the magnetic Ap star HD 75445
Authors: O. Kochukhov, S. Bagnulo, G. Lo Curto

We discovered pulsational radial velocity variations in the cool Ap star HD 75445 -- an object spectroscopically very similar to the bright rapidly oscillating Ap (roAp) star gamma Equ. 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 Nd II and Nd II lines with a period close to 9 min and amplitudes of 20-30 m/s. Substantial variation of 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 HD 75445 indicates that the roAp excitation mechanism produces variability with the radial velocity amplitude anywhere between 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 very small photometric and radial velocity amplitudes.

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