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

TL;DR

This study reports the discovery of very low amplitude, multiperiodic 9-minute pulsations in the magnetic Ap star HD 75445, suggesting many similar stars may host undetected non-radial pulsations.

Contribution

It provides the first detection of extremely low amplitude pulsations in HD 75445, indicating that roAp stars can have variability below previous detection thresholds.

Findings

Detected 20-30 m/s pulsations in Nd lines

Identified at least three non-radial pulsation modes

Supports the hypothesis of widespread low-amplitude pulsations in roAp stars

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 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 III lines with a period close to 9 min and amplitudes of 20-30 m/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 HD 75445 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.