Pulsations in the atmosphere of the roAp star HD 24712 II. Theoretical models

Abstract

We discuss pulsations of the rapidly oscillating Ap (roAp) star HD 24712 (HR 1217) based on nonadiabatic analyses taking into account the effect of dipole magnetic fields. We have found that all the pulsation modes appropriate for HD 24712 are damped; i.e., the kappa-mechanism excitation in the hydrogen ionization layers is not strong enough to excite high-order p-modes with periods consistent with observed ones, all of which are found to be above the acoustic cut-off frequencies of our models. The main (2.721 mHz) and the highest (2.806 mHz) frequencies are matched with modified $l=2$ and $l=3$ modes, respectively. The large frequency separation ($\approx 68\mu$Hz) is reproduced by models which lay within the error box of HD 24712 on the HR diagram. The nearly equally spaced frequencies of HD 24712 indicate the small frequency separation to be as small as $\approx 0.5\mu$Hz. However, the small separation derived from theoretical $l=1$ and 2 modes are found to be larger than $\sim 3\mu$Hz. The problem of equal spacings could be resolved by assuming that the spacings correspond to pairs of $l=2$ and $l=0$ modes. The amplitude distribution on the stellar surface is strongly affected by the magnetic field resulting in the predominant concentration at the polar regions. Amplitudes and phases of radial-velocity variations for various spectral lines are converted to relations of amplitude/phase versus optical depth in the atmosphere. Oscillation phase delays gradually outward in the outermost layers indicating the presence of waves propagating outward. The phase changes steeply around $\log \tau \sim -3.5$, which supports a $T-\tau$ relation having a small temperature inversion there.

11 pages, 13 figures, accepted for publication in MNRAS

References in corpus (6)

• Improved pulsating models of magnetic Ap stars I: exploring different magnetic field configurations
• Pulsation in the atmosphere of the roAp star HD 24712. I. Spectroscopic observations and radial velocity measurements
• Non-LTE line formation for Pr II and Pr III in A and Ap stars
• Pulsations in the atmosphere of the rapidly oscillating Ap star 10 Aquilae
• MOST photometry and modeling of the rapidly oscillating (roAp) star gamma Equ
• MOST photometry of the roAp star 10 Aql

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• On the understanding of pulsations in the atmosphere of roAp stars: phase diversity and false nodes
• Solar-like oscillations and magnetic activity of the slow rotator EK Eri
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• K2 observations of the rapidly oscillating Ap star 33 Lib (HD 137949): new frequencies and unique non-linear interactions
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• MOST observations of the roAp stars HD 9289, HD 99563, and HD 134214
• Numerical simulations of line-profile variation beyond a single-surface approximation for oscillations in roAp stars