HD 24355 observed by the Kepler K2 mission: A rapidly oscillating Ap star pulsating in a distorted quadrupole mode

Abstract

We present an analysis of the first Kepler K2 mission observations of a rapidly oscillating Ap (roAp) star, HD 24355 ($V=9.65$). The star was discovered in SuperWASP broadband photometry with a frequency of 224.31 d${}^{-1}$, (2596.18 $\mu$Hz; $P=6.4$ min) and an amplitude of 1.51 mmag, with later spectroscopic analysis of low-resolution spectra showing HD 24355 to be an A5 Vp SrEu star. The high precision K2 data allow us to identify 13 rotationally split sidelobes to the main pulsation frequency of HD 24355. This number of sidelobes combined with an unusual rotational phase variation show this star to be the most distorted quadrupole roAp pulsator yet observed. In modelling this star, we are able to reproduce well the amplitude modulation of the pulsation, and find a close match to the unusual phase variations. We show this star to have a pulsation frequency higher than the critical cut-off frequency. This is currently the only roAp star observed with the Kepler spacecraft in Short Cadence mode that has a photometric amplitude detectable from the ground, thus allowing comparison between the mmag amplitude ground-based targets and the $\mu$mag spaced-based discoveries. No further pulsation modes are identified in the K2 data, showing this star to be a single-mode pulsator.

18 pages, accepted for publication in MNRAS

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• Whole Earth Telescope discovery of a strongly distorted quadrupole pulsation in the largest amplitude rapidly oscillating Ap star
• The roAp stars observed by the Kepler Space Telescope
• Tidal perturbations and eclipse mapping in the pulsations in the hierarchical triple system U~Gru
• Rotational and pulsational variability in the TESS light curve of HD27463
• Discovery of multiple p-mode pulsation frequencies in the roAp star, HD 86181
• OGLE-BLAP-001 and ZGP-BLAP-08: two possible magnetic blue large-amplitude pulsators
• A conclusive non-detection of magnetic field in the Am star o Peg with high-precision near-infrared spectroscopy