Whole Earth Telescope discovery of a strongly distorted quadrupole pulsation in the largest amplitude rapidly oscillating Ap star

TL;DR

This paper reports the discovery and detailed analysis of a strongly distorted quadrupole pulsation mode in the largest amplitude rapidly oscillating Ap star, revealing complex magnetic and rotational interactions.

Contribution

It provides the first detailed characterization of a distorted quadrupole pulsation in a high-amplitude roAp star, including modeling of magnetic distortion effects.

Findings

Pulsation mode is a distorted quadrupole with unusual phase variations.

Magnetic field strength estimated at 1.5 kG influences pulsation.

Significant offset between spots and pulsation axis observed.

Abstract

We present a new analysis of the rapidly oscillating Ap (roAp) star, 2MASS J$19400781-4420093$ (J1940; $V=13.1$). The star was discovered using SuperWASP broadband photometry to have a frequency of 176.39 d$^{-1}$ (2041.55 $\mu$Hz; $P = 8.2$ min; Holdsworth et al. 2014a) and is shown here to have a peak-to-peak amplitude of 34 mmag. J1940 has been observed during three seasons at the South African Astronomical Observatory, and has been the target of a Whole Earth Telescope campaign. The observations reveal that J1940 pulsates in a distorted quadrupole mode with unusual pulsational phase variations. A higher signal-to-noise ratio spectrum has been obtained since J1940's first announcement, which allows us to classify the star as A7 Vp Eu(Cr). The observing campaigns presented here reveal no pulsations other than the initially detected frequency. We model the pulsation in J1940 and conclude that the pulsation is distorted by a magnetic field of strength $1.5$ kG. A difference in the times of rotational maximum light and pulsation maximum suggests a significant offset between the spots and pulsation axis, as can be seen in roAp stars.