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

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 µHz; 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 ± 0.0005 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 ℓ > 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.

LCO observations of a super-critical distorted pulsation in the roAp star J0855 (TYC 2488-1241-1)

“…It is interesting that, even at the level of precision we have obtained here, we do not see harmonics of the pulsation in J0855. The pulsation amplitude is similar to that seen in J1640 where harmonics up to 3ν were detected (Holdsworth et al 2018b), albeit only one had an amplitude above our noise level here. It is still unclear what information can be extracted from the harmonics of the oscillations in some roAp stars, and indeed why some stars show them whilst others do not.…”

“…Many roAp stars show harmonics of their principal pulsation mode frequency (e.g. γ Equ, Gruberbauer et al 2008;HD 99563, Handler et al 2006; HR 3831, Kurtz et al 1993) which The plot is based on that of Saio (2014) and updated from Holdsworth et al (2018b) using Gaia DR2 parallaxes. The squares represent the principle pulsation frequency, with vertical bars showing the range of frequencies for multi-periodic stars.…”

“…Acoustic cutoff frequencies are represented by the dashed lines. The numerical labels correspond to the stars in Table A1 of Holdsworth et al (2018b). demonstrates the non-linearity of their pulsations. It is interesting that, even at the level of precision we have obtained here, we do not see harmonics of the pulsation in J0855.…”

HD 42659: the only known roAp star in a spectroscopic binary observed with B photometry, TESS, and SALT

“…The choice of B p is arbitrary due to the aforementioned reasons. We postulated, as a result of its similarity to distorted quadrupole mode pulsators (Holdsworth et al 2018b), that HD 42659 could rather be pulsating in a distorted quadrupole mode. Therefore, we attempted to model the amplitude and phase variations accordingly.…”

Magnetic OB[A] Stars with TESS: probing their Evolutionary and Rotational properties (MOBSTER) – I. First-light observations of known magnetic B and A stars

“…1, to account for the average difference in noise levels in the high and low frequency regimes (i.e. 'pink noise') often found in oversampled timeseries data (see Schwarzenberg-Czerny 2003;Degroote et al 2009;Holdsworth et al 2018;Bowman et al 2018). Finally, we perform a Lomb-Scargle analysis (Lomb 1976;Scargle 1982) of these detrended light curves of our nineteen targets by using the LombScargle class in the astropy.stats Python package.…”