Suppressed phase variations in a high amplitude rapidly oscillating Ap star pulsating in a distorted quadrupole mode

TL;DR

This study observes a high amplitude rapidly oscillating Ap star pulsating in a distorted quadrupole mode with suppressed phase variations, revealing new insights into pulsation modes and stellar surface phenomena.

Contribution

It provides the first detailed analysis of phase suppression in a quadrupole pulsating roAp star, challenging existing models of stellar pulsation behavior.

Findings

Star pulsates at 151.93 d$^{-1}$ with 20.68 mmag amplitude

Pulsation in a distorted quadrupole mode with strong spherically symmetric component

Strong pulsation phase suppression observed in this star

Abstract

We present the results of a multisite photometric observing campaign on the rapidly oscillating Ap (roAp) star 2MASS 16400299-0737293 (J1640; $V=12.7$). We analyse photometric $B$ data to show the star pulsates at a frequency of $151.93$ d$^{-1}$ ($1758.45 \mu$Hz; $P=9.5$ min) with a peak-to-peak amplitude of 20.68 mmag, making it one of the highest amplitude roAp stars. No further pulsation modes are detected. The stellar rotation period is measured at $3.6747\pm0.0005$ d, and we show that rotational modulation due to spots is in anti-phase between broadband and $B$ observations. Analysis and modelling of the pulsation reveals this star to be pulsating in a distorted quadrupole mode, but with a strong spherically symmetric component. The pulsational phase variation in this star is suppressed, leading to the conclusion that the contribution of $\ell>2$ components dictate the shape of phase variations in roAp stars that pulsate in quadrupole modes. This is only the fourth time such a strong pulsation phase suppression has been observed, leading us to question the mechanisms at work in these stars. We classify J1640 as an A7 Vp SrEu(Cr) star through analysis of classification resolution spectra.