On the first $\delta$ Sct--roAp hybrid pulsator and the stability of p and g modes in chemically peculiar A/F stars

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 $2.8\pm 0.5$ 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 $\delta$ 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 $\lesssim 4$ kG, while other low-overtone p modes are driven for polar field strengths up to $\sim$1.5 kG. We find that the high-order g modes commonly observed in $\gamma$ 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 $\gamma$ Dor stars. We use our helium-depleted models to calculate the $\delta$ Sct instability strip for metallic-lined A (Am) stars, and find that driving from a Rosseland mean opacity bump at $\sim$$5\times 10^4$ K caused by the discontinuous H-ionization edge in bound-free opacity explains the observation of $\delta$ Sct pulsations in Am stars.

arXiv commentAccepted for publication in MNRAS

References in corpus (53)

• The chemical composition of the Sun
• Gaia Data Release 2. Summary of the contents and survey properties
• MESA Isochrones and Stellar Tracks (MIST). I: Solar-Scaled Models
• MESA Isochrones and Stellar Tracks (MIST) 0: Methods for the construction of stellar isochrones
• Estimating distances from parallaxes IV: Distances to 1.33 billion stars in Gaia Data Release 2
• Revised Stellar Properties of Kepler Targets for the Quarter 1-16 Transit Detection Run
• Galactic Reddening in 3D from Stellar Photometry - An Improved Map
• The Kepler characterization of the variability among A- and F-type stars. I. General overview
• The r-Process Enriched Low Metallicity Giant HD 115444
• Rotational velocities of A-type stars IV. Evolution of rotational velocities
• Automatic Determination of Stellar Atmospheric Parameters and Construction of Stellar Spectral Templates of the Guoshoujing Telescope (LAMOST)
• {\em Gaia}-derived luminosities of {\em Kepler} A/F stars and the pulsator fraction across the δ Scuti instability strip
• Magnetic field measurements and their uncertainties: the FORS1 legacy
• Activity indicators and stellar parameters of the Kepler targets. An application of the ROTFIT pipeline to LAMOST-Kepler stellar spectra
• Characterising the observational properties of δ Sct stars in the era of space photometry from the Kepler mission
• Super-Nyquist asteroseismology with the Kepler Space Telescope
• The first view of $\delta$ Scuti and $\gamma$ Doradus stars with the TESS mission
• Spectroscopic survey of Kepler stars. I. HERMES/Mercator observations of A- and F-type stars
• Finding binaries from phase modulation of pulsating stars with Kepler: V. Orbital parameters, with eccentricity and mass-ratio distributions of 341 new binaries
• Finding binaries among Kepler pulsating stars from phase modulation of their pulsations
• The role of turbulent pressure as a coherent pulsational driving mechanism: the case of the delta Scuti star HD 187547
• Forward seismic modelling of the pulsating magnetic B-type star HD 43317
• Zodiacal Exoplanets in Time (ZEIT) VIII: A Two Planet System in Praesepe from K2 Campaign 16
• Asteroseismic signatures of evolving internal stellar magnetic fields
• Multisite spectroscopic seismic study of the beta Cep star V2052 Oph: inhibition of mixing by its magnetic field
• Gravity-mode period spacings and near-core rotation rates of 611 $\gamma$ Doradus stars with \textit{Kepler}
• SuperWASP observations of pulsating Am stars
• How to drive roAp stars
• Rotation and pulsation in Ap stars: first light results from TESS sectors 1 and 2
• Pulsation versus metallicism in Am stars as revealed by LAMOST and WASP
• Theoretical light curves of dipole oscillations in roAp stars
• The fundamental parameters of the roAp star alpha Circini
• Testing excitation models of rapidly oscillating Ap stars with interferometry
• First discovery of a magnetic field in a main sequence delta Scuti star: the Kepler star HD188774
• Discovery of the longest-period rapidly oscillating Ap star HD177765
• The new Toulouse-Geneva Stellar Evolution Code including radiative accelerations of heavy elements
• Hydrodynamical instabilities induced by atomic diffusion in A stars and their consequences
• Detecting axisymmetric magnetic fields using gravity modes in intermediate-mass stars
• Spectroscopic survey of Kepler stars. II. FIES/NOT observations of A- and F-type stars
• Rotation and surface abundance peculiarities in A-type stars
• Discovery of a magnetic field in $\rho$ Pup
• Suppressed phase variations in a high amplitude rapidly oscillating Ap star pulsating in a distorted quadrupole mode
• AO Velorum: a young quadruple system with a ZAMS eclipsing BpSi primary
• Long period Ap stars discovered with TESS data
• HD 99458: First time ever Ap-type star as a $\delta$ Scuti pulsator in a short period eclipsing binary?
• HD 41641: A classical $\delta$ Sct-type pulsator with chemical signatures of an Ap star
• Six new rapidly oscillating Ap stars in the Kepler long-cadence data using super-Nyquist asteroseismology
• Multiplicity of rapidly oscillating Ap stars
• A theoretical tool for the study of radial velocities in the atmospheres of roAp stars
• The determination of the rotation period and magnetic field geometry of the strongly magnetic roAp star HD154708
• Spectroscopic and asteroseismic analysis of the remarkable main-sequence A star KIC 11145123
• Probing sunspots with two-skip time-distance helioseismology
• A magnetic study of the $\delta$ Scuti variable HD 21190 and the close solar-type background star CPD -83${}^{\circ }$ 64B

Cited by in corpus (24)

• Periodic variable A-F spectral type stars in the northern TESS continuous viewing zone
• TESS Cycle 1 observations of roAp stars with 2-min cadence data
• Highlights of Discoveries for $\delta$ Scuti Variable Stars From the Kepler Era
• KIC5950759: a high-amplitude $\delta$ Sct star with amplitude and frequency modulation near the terminal age main sequence
• Asteroseismology of the young open cluster NGC 2516 I: Photometric and spectroscopic observations
• Asteroseismic g-mode period spacings in strongly magnetic rotating stars
• TIC 308396022: a $\delta$ Scuti--$\gamma$ Doradus hybrid with large-amplitude radial fundamental mode and regular g-mode period spacing
• Rapidly-oscillating {\it TESS} A--F main sequence stars: are the roAp stars a distinct class?
• Long-period Ap stars discovered with TESS data: The northern ecliptic hemisphere
• Frequency analysis of KIC 1573174 : shedding light on the nature of HADS stars
• An Ap star catalog based on LAMOST DR9
• Physical properties of the eclipsing binary KIC 9851944 and analysis of its tidally-perturbed p- and g-mode pulsations
• Variability and stellar pulsation incidence in Am/Fm stars using TESS and Gaia data
• Searching for magnetic fields in pulsating A-type stars: the discovery of a strong field in the probable delta Sct star HD340577 and a null result for the gamma Dor star HR8799
• Exploring the convective core of the high-amplitude $\delta$ Scuti star TIC 120857354 with asteroseismology
• Modelling $\delta$ Scuti pulsations: A new grid of p, g, and f modes across pre-main-sequence to post-main-sequence evolution
• A conclusive non-detection of magnetic field in the Am star o Peg with high-precision near-infrared spectroscopy
• Asteroseismology of the young open cluster NGC 2516 II. Constraining cluster age using gravity-mode pulsators
• Statistical analysis of asteroseismic indices and stellar parameters of TESS-observed $\delta$ Scuti stars
• Discovery of new magnetic δ Scuti stars and impact of magnetism on pulsation excitation
• KIC 10855535: An elegant Delta Scuti pulsator with Amplitude and Phase Modulation
• Characterizing bright $\delta$ Scuti pulsators using TESS lightcurves
• 50 Dra: Am-type twins with additional variability in a non-eclipsing system
• The $\beta$ Pictoris b Hill sphere transit campaign. Paper II: Searching for the signatures of the $\beta$ Pictoris exoplanets through time delay analysis of the $\delta$ Scuti pulsations