On the spectroscopic nature of the cool evolved Am star HD 151878^★

Abstract

Recently, Tiwari, Chaubey & Pandey detected the bright component of the visual binary HD 151878 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 rapidly oscillating Ap (roAp) star known. As a new roAp star, HD 151878 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 FIbre‐fed Echelle 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−1. 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 roAp 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.

A 3D study of the photosphere of HD 99563 - I. Pulsation analysis

“…As they share the same region of the HR diagram with the (photometrically established) non‐oscillating Ap (noAp) stars, the selection of new roAp candidates is difficult and mostly leads to null results for both spectroscopic studies (see e.g. Elkin et al 2008a; Freyhammer et al 2008a; Freyhammer, Elkin & Kurtz 2008b) and for the photometric surveys listed above. The pulsations of roAp stars are described well by the oblique pulsator model, where the pulsation axis is assumed to be aligned with an oblique magnetic field axis (Kurtz 1982; Saio 2005), or derived to be offset from both the rotational and magnetic axes (Bigot & Dziembowski 2002).…”

The discovery of two new rapidly oscillating Ap stars, HD92499 and HD143487

“…Kochukhov et al (2008) found rapid oscillations in HD 115226 and Kochukhov et al (2009) detected low amplitude pulsation in HD 75445. Tiwari, Chaubey & Pandey (2007) reported rapid oscillations in HD 151878, but this was not confirmed by Freyhammer, Elkin & Kurtz (2008c). There are questions about the pulsations in another roAp candidate HD 965 (Elkin et al 2005a).…”