On the first δ Sct–roAp hybrid pulsator and the stability of p and g modes in chemically peculiar A/F stars

Murphy, Simon J. and Saio, Hideyuki and Takada-Hidai, Masahide and Kurtz, Donald W. and Shibahashi, Hiromoto and Takata, Masao and Hey, Daniel R. (2020) On the first δ Sct–roAp hybrid pulsator and the stability of p and g modes in chemically peculiar A/F stars. Monthly Notices of the Royal Astronomical Society, 498 (3). pp. 4272-4286. ISSN 0035-8711

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Abstract

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 ∼5×104 K caused by the discontinuous H-ionization edge in bound-free opacity explains the observation of δ Sct pulsations in Am stars.


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Item Type: Article (Commonwealth Reporting Category C)
Refereed: Yes
Item Status: Live Archive
Additional Information: File reproduced in accordance with the copyright policy of the publisher/author.
Faculty/School / Institute/Centre: No Faculty
Faculty/School / Institute/Centre: No Faculty
Date Deposited: 14 Jul 2022 00:21
Last Modified: 18 Jul 2022 02:05
Uncontrolled Keywords: asteroseismology; stars: chemically peculiar; stars: interiors; stars: magnetic field; stars: oscillations; stars: variables: δ scuti; Astrophysics - Solar and Stellar Astrophysics
Fields of Research (2020): 51 PHYSICAL SCIENCES > 5101 Astronomical sciences > 510109 Stellar astronomy and planetary systems
Socio-Economic Objectives (2020): 28 EXPANDING KNOWLEDGE > 2801 Expanding knowledge > 280120 Expanding knowledge in the physical sciences
Identification Number or DOI: https://doi.org/10.1093/mnras/staa2667
URI: http://eprints.usq.edu.au/id/eprint/47932

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