Pulsation of magnetic stars

Abstract

Abstract. Some Ap stars with strong magnetic fields pulsate in high-order p modes; they are called roAp (rapidly oscillating Ap) stars. The p-mode frequencies are modified by the magnetic fields. Although the large frequency separation is hardly affected, small separations are modified considerably. The magnetic field also affects the latitudinal amplitude distribution on the surface. We discuss the property of axisymmetric p-mode oscillations in roAp stars.Keywords. stars: magnetic fields, stars: oscillations Observational properties of roAp starsThe group of rapidly oscillating Ap (roAp) stars was discovered by Kurtz (1982) more than thirty years ago (it consisted of five members then). Since then, the number of known roAp stars has increased to around 40-50 (still increasing); a recent list is given by Kurtz et al. (2006). They are chemically-peculiar main-sequence stars having masses ranging from ∼ 1.5 to ∼ 2 M ⊙ , and effective temperatures from ∼ 6400 to ∼ 8400 K. On the H-R diagram (Fig 1, left panel), they lie in and around the δ Sct instability strip. However, the oscillation frequencies are much higher than those of δ Sct variables as seen in the right panel of Fig. 1, which plots dominant frequencies of roAp and δ Sct stars with respect to the effective temperature. The roAp stars pulsate in high-order p modes whose frequencies range from ∼ 1 to ∼ 3 mHz (periods; ∼ 6 − 21 min), while δ Sct stars

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

“…Mass cutoff frequency with pulsation phase changes much smaller than predicted. To put some of these characteristics into perspective, we plot these stars (in red) in the T eff − νL/M plane, after Saio (2014), in Fig. 12 alongside other roAp stars.…”

“…The numerical labels correspond to the stars in Table A1. Figure after Saio (2014). frequency parameters are combined to provide the νL/M parameter; this eliminates the mass dependence of the cutoff frequency and instability range due to H-ionisation zone at a given effective temperature.…”

Six new rapidly oscillating Ap stars in the Kepler long-cadence data using super-Nyquist asteroseismology

“…A further challenge to theoretical models of pulsations in magnetic stars are oscillations above the so-called acoustic cutoff frequency (Saio 2013;Holdsworth et al 2018b). In non-magnetic stars, oscillations above this frequency are not expected.…”

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

“…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.…”