Mon. Not. R. Astron. Soc., 498, 4272-4286 (2020/November-1)
On the first δ Sct-roAp hybrid pulsator and the stability of p and g modes in chemically peculiar A/F stars.
MURPHY S.J., SAIO H., TAKADA-HIDAI M., KURTZ D.W., SHIBAHASHI H., TAKATA M. and HEY D.R.
Abstract (from CDS):
Strong magnetic fields in chemically peculiar A-type (Ap) stars typically suppress low-overtone pressure modes (p modes) but allow high-overtone p modes to be driven. KIC 11296437 is the first star to show both. We obtained and analysed a Subaru spectrum, from which we show that KIC 11296437 has abundances similar to other magnetic Ap stars, and we estimate a mean magnetic field modulus of 2.8 ± 0.5 kG. The same spectrum rules out a double-lined spectroscopic binary, and we use other techniques to rule out binarity over a wide parameter space, so the two pulsation types originate in one δ Sct-roAp hybrid pulsator. We construct stellar models depleted in helium and demonstrate that helium settling is second to magnetic damping in suppressing low-overtone p modes in Ap stars. We compute the magnetic damping effect for selected p and g modes, and find that modes with frequencies similar to the fundamental mode are driven for polar field strengths <=4 kG, while other low-overtone p modes are driven for polar field strengths up to ∼1.5 kG. We find that the high-order g modes commonly observed in γ Dor stars are heavily damped by polar fields stronger than 1-4 kG, with the damping being stronger for higher radial orders. We therefore explain the observation that no magnetic Ap stars have been observed as γ Dor stars. We use our helium-depleted models to calculate the δ Sct instability strip for metallic-lined A (Am) stars, and find that driving from a Rosseland mean opacity bump at ∼5 x 104 K caused by the discontinuous H-ionization edge in bound-free opacity explains the observation of δ Sct pulsations in Am stars.
© 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
asteroseismology - stars: chemically peculiar - stars: interiors - stars: magnetic field - stars: oscillations - stars: variables: d scuti
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