Journal Article

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

Hideyuki Saio, Tanya Ryabchikova and Mikhail Sachkov

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 403, issue 4, pages 1729-1738
Published in print April 2010 | ISSN: 0035-8711
Published online April 2010 | e-ISSN: 1365-2966 | DOI: https://dx.doi.org/10.1111/j.1365-2966.2009.16235.x
Pulsations in the atmosphere of the roAp star HD 24712 – II. Theoretical models

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We discuss pulsations of the rapidly oscillating Ap (roAp) star HD 24712 (HR 1217) based on non-adiabatic 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 κ-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 3 modes, respectively. The large frequency separation (≈68 μHz) is reproduced by models which lay within the error box of HD 24712 on the Hertzsprung–Russell diagram. The nearly equally spaced frequencies of HD 24712 indicate the small frequency separation to be as small as ≈0.5 μHz. However, the small separation derived from theoretical l= 1 and 2 modes is found to be larger than ∼3 μHz. The problem of equal spacing could be resolved by assuming that the spacings correspond to pairs of l= 2 and 0 modes; this is possible because magnetic fields significantly modify the frequencies of 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. The modified amplitude distribution of a quasi-quadrapole mode predicts a rotational amplitude modulation consistent with the observed one.

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 τ∼−3.5, which supports a T–τ relation having a small temperature inversion there.

Keywords: stars: individual: HD 24712; stars: magnetic fields; stars: oscillations

Journal Article.  7118 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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