KIC 3868420: A high-amplitude $\delta$ Scuti-$\gamma$ Dor hybrid star crossing the Hertzsprung gap

Abstract

We report a photometric and asteroseismic analysis of KIC 3868420, a newly identified high-amplitude $\delta$ Scuti-$\gamma$ Doradus hybrid star located in the Hertzsprung gap - a short-lived and rarely observed post-main-sequence phase. Using four years of Kepler long-cadence photometry, we detect 36 significant frequencies, including 11 independent modes spanning both low- and high-frequency regimes. Grid-based modeling with MESA and GYRE, including rotation, shows that five independent frequencies match a combination of radial p-modes and non-radial g-modes, supporting its hybrid nature. The best-fit models yield an evolved post-main-sequence star ($M\sim 2.26-2.30M_{\odot }$, $R\sim 4.41-4.43R_{\odot }$, $\tau \sim 5.4\times 10^8$ yr), although degeneracies from rotation and unknown inclination preclude a unique solution. KIC 3868420 thus represents a rare example of a high-amplitude hybrid pulsator in the Hertzsprung gap, demonstrating that high-amplitude pulsation can coexist with mixed p/g-mode behavior in a rapidly evolving star. This finding highlights the value of space-based photometry for identifying such rare objects.

arXiv comment9 pages, 9 figures, Published in ApJ, 2026,1001,159

References in corpus (47)

• The chemical composition of the Sun
• Modules for Experiments in Stellar Astrophysics (MESA)
• Modules for Experiments in Stellar Astrophysics (MESA): Convective Boundaries, Element Diffusion, and Massive Star Explosions
• Estimating distances from parallaxes IV: Distances to 1.33 billion stars in Gaia Data Release 2
• Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation
• The Revised TESS Input Catalog and Candidate Target List
• Kepler Input Catalog: Photometric Calibration and Stellar Classification
• GYRE: An open-source stellar oscillation code based on a new Magnus Multiple Shooting Scheme
• The Kepler characterization of the variability among A- and F-type stars. I. General overview
• Probing the interior physics of stars through asteroseismology
• Probing the properties of convective cores through g modes: high-order g modes in SPB and gamma Doradus stars
• Hybrid gamma Doradus - delta Scuti pulsators: New insights into the physics of the oscillations from Kepler observations
• The interior rotation of a sample of gamma Doradus stars from ensemble modelling of their gravity mode period spacings
• General Cosmography Model with Spatial Curvature
• {\em Gaia}-derived luminosities of {\em Kepler} A/F stars and the pulsator fraction across the δ Scuti instability strip
• Internal mixing of rotating stars inferred from dipole gravity modes
• Metal Abundances, Radial Velocities and other Physical Characteristics for the RR Lyrae Stars in the Kepler Field
• A new asteroseismic diagnostic for internal rotation in $\gamma$ Doradus stars
• Characterising the observational properties of δ Sct stars in the era of space photometry from the Kepler mission
• On the sensitivity of gravito-inertial modes to differential rotation in intermediate-mass main-sequence stars
• Super-Nyquist asteroseismology with the Kepler Space Telescope
• Turbulent convection and pulsation stability of stars - II. Theoretical instability strip for $\delta$ Scuti and $\gamma$ Doradus stars
• A detection threshold in the amplitude spectra calculated from Kepler data obtained during K2 mission
• Detecting non-uniform period spacings in the Kepler photometry of gamma Doradus stars: methodology and case studies
• High frequency A-type pulsators discovered using SuperWASP
• Measuring mean densities of delta Scuti stars with asteroseismology. Theoretical properties of large separations using TOUCAN
• Period spacings of $\gamma$ Doradus pulsators in the Kepler field: detection methods and application to 22 slow rotators
• The effects of moderately fast shellular rotation on adiabatic oscillations
• An in-depth study of HD 174966 with CoRoT photometry and HARPS spectroscopy. Large separation as a new observable for δSct stars
• On magnetic inhibition of photospheric macro-turbulence generated in the iron-bump opacity zone of O-stars
• Observational consequences of turbulent pressure in the envelopes of massive stars
• The gamma Dor CoRoT target HD49434. I-Results from the ground-based campaign
• Confirmation of simultaneous p and g mode excitation in HD 8801 and Gamma Peg from time-resolved multicolour photometry of six candidate "hybrid" pulsators
• The role of rotation on Petersen Diagrams. The $P{i}_{1/0}(Omega)$ period ratios
• The Kepler Asteroseismic Investigation: Scientific goals and the first results
• A Volume-Limited Photometric Survey of 114 gamma Doradus Candidates
• The role of rotation on Petersen Diagrams. II The influence of near-degeneracy
• Spectroscopic survey of Kepler stars. II. FIES/NOT observations of A- and F-type stars
• A Weak Modulation Effect Detected in the Light Curves of KIC 5950759: Intrinsic or Instrumental Effect?
• 3D Simulations and MLT: I. Renzini's Critique
• Magnetic signatures on mixed-mode frequencies. II. Period spacings as a probe of the internal magnetism of red giants
• TIC 308396022: a $\delta$ Scuti--$\gamma$ Doradus hybrid with large-amplitude radial fundamental mode and regular g-mode period spacing
• Study of the low-order $\Delta \nu$-$\bar{\rho }$ relation for moderately-rotating $\delta$ Scuti stars and its impact on their characterisation
• A double-modulation effect detected in a double-mode high-amplitude $\delta$ Scuti star: KIC 10284901
• Frequency analysis of KIC 1573174 : shedding light on the nature of HADS stars
• TESS Observations of Seven Newly Identified High-amplitude $\delta$ Scuti Stars
• Discovery of resonating integration modes in triple-mode high-amplitude $\delta$ Scuti stars: A new evolutionary phase indicator