Letter to the Editor

Discovery of very low amplitude 9-minute multiperiodic pulsations in the magnetic Ap star HD 75445^*

¹ Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden e-mail: oleg@fysast.uu.se
² Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland, UK
³ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Casilla 19001 Santiago 19, Chile
⁴ Institute of Astronomy, Russian Academy of Sciences, Pyatnitskaya 48, 109017 Moscow, Russia
⁵ Department of Astronomy, University of Vienna, Türkenschanzstraße 17, 1180 Vienna, Austria

Received: 25 November 2008
Accepted: 8 December 2008

Abstract

We present our discovery of pulsational radial-velocity variations in the cool Ap star HD 75445, an object spectroscopically similar to the bright, rapidly-oscillating Ap (roAp) star γ 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 III lines with a period close to 9 min and amplitudes of 20-30 m s^-1. 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 HD 75445 indicates that the roAp excitation mechanism produces variability in the radial velocity amplitude of between a few tens m s^-1 and several km s^-1. 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.