On the spectroscopic nature of the cool evolved Am star HD151878

TL;DR

This study investigates the star HD151878, initially thought to be a rapidly oscillating Ap star, but high-resolution spectroscopy reveals it is an Am star without the expected pulsations, suggesting the original detection was an artifact.

Contribution

The paper provides high-resolution spectroscopic analysis that refutes previous claims of rapid oscillations in HD151878, classifying it as an Am star rather than a pulsating Ap star.

Findings

HD151878 shows no rapid oscillations at high resolution

Spectroscopy classifies HD151878 as an Am star, not Ap

Original pulsation detection likely due to instrumental artifact

Abstract

Recently, Tiwari, Chaubey, & Pandey (2007) detected the bright component of the visual binary HD151878 to exhibit rapid photometric oscillations through a Johnson B filter with a period of 6 min (2.78 mHz) and a high, modulated amplitude up to 22 mmag peak-to-peak, making this star by far the highest amplitude roAp star known. As a new roAp star, HD151878 is of additional particular interest as a scarce example of the class in the northern sky, and only the second known case of an evolved roAp star - the other being HD 116114. We used the FIES spectrograph at the Nordic Optical Telescope to obtain high time resolution spectra at high dispersion to attempt to verify the rapid oscillations. We show here that the star at this epoch is spectroscopically stable to rapid oscillations of no more than a few tens of m/s. The high-resolution spectra furthermore show the star to be of type Am rather than Ap and we show the star lacks most of the known characteristics for rapidly oscillating Ap stars. We conclude that this is an Am star that does not pulsate with a 6-min period. The original discovery of pulsation is likely to be an instrumental artefact.