The sensitivity and behaviour of the curvature in the échelle diagram of red-giant stars

Abstract

In the convective envelopes of relatively cool stars, oscillations are excited by turbulent convection. In these so-called solar-like oscillators, radial oscillation modes appear at nearly equally spaced frequencies. This spacing is referred to as the `large frequency separation'. Deviations from equally-spaced frequencies are a result of the internal structure of a star being different from a sphere of ideal gas at constant temperature. Hence, these deviations provide information on the internal structure of the star. In this work, we investigate the second-order deviation from uniform spacing, referred to as curvature. We aim to provide homegeneous values for observed red-giant stars, understand differences between the results from observations and predictions from stellar models, and reveal the connection between curvature and stellar structure. We used Kepler data of red-giant stars and computed the curvature for several thousand stars. We compared these to the curvature derived from MESA models. We subsequently investigated the trends and differences between results from observations and models. Finally, we computed sensitivity kernels to identify the stellar region to which the curvature is most sensitive and performed a glitch analysis. We found that the curvature is sensitive to evolutionary phase and mass. The observed values and values from models show discrepancies. The glitch analysis shows that in theory this provides information on the location and strength of the HeI and HI ionisation layers. The curvature provides a probe into the near-surface structure of the star. The deviations between the curvature derived from observations and models call henceforth for improvements in the near-surface layers of stellar models.

arXiv commentAccepted for publication by Astronomy \& Astrophysics. Abstract abridged

References in corpus (42)

• Modules for Experiments in Stellar Astrophysics (MESA)
• Modules for Experiments in Stellar Astrophysics (MESA): Giant Planets, Oscillations, Rotation, and Massive Stars
• Modules for Experiments in Stellar Astrophysics (MESA): Binaries, Pulsations, and Explosions
• Modules for Experiments in Stellar Astrophysics (MESA): Convective Boundaries, Element Diffusion, and Massive Star Explosions
• Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation
• Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars
• Modules for Experiments in Stellar Astrophysics (MESA): Time-Dependent Convection, Energy Conservation, Automatic Differentiation, and Infrastructure
• GYRE: An open-source stellar oscillation code based on a new Magnus Multiple Shooting Scheme
• Fast core rotation in red-giant stars revealed by gravity-dominated mixed modes
• Spin down of the core rotation in red giants
• Nominal values for selected solar and planetary quantities: IAU 2015 Resolution B3
• Correcting stellar oscillation frequencies for near-surface effects
• The Second APOKASC Catalog: The Empirical Approach
• The connection between stellar granulation and oscillation as seen by the Kepler mission
• A new correction of stellar oscillation frequencies for near-surface effects
• Mixed modes in red giants: a window on stellar evolution
• The universal red-giant oscillation pattern; an automated determination with CoRoT data
• Fresh insights on the structure of the solar core
• A prevalence of dynamo-generated magnetic fields in the cores of intermediate-mass stars
• Asymptotic and measured large frequency separations
• Global Seismology of the Sun
• An asteroseismic signature of helium ionization
• Automated preparation of Kepler time series of planet hosts for asteroseismic analysis
• Solar-like oscillations in red giants observed with Kepler: comparison of global oscillation parameters from different methods
• Measurement of acoustic glitches in solar-type stars from oscillation frequencies observed by Kepler
• Evidence for a sharp structure variation inside a red-giant star
• Period-luminosity relations in evolved red giants explained by solar-like oscillations
• The Contour Method: a new approach to finding modes of non-adiabatic stellar pulsations
• Period spacings in red giants; III. Coupling factors of mixed modes
• Prospects for asteroseismic inference on the envelope helium abundance in red giant stars
• Helium signature in red giant oscillation patterns observed by Kepler
• A prescription for the asteroseismic surface correction
• Insights from the APOKASC Determination of the Evolutionary State of Red-Giant Stars by consolidation of different methods
• Surface effects on the red giant branch
• Probing the mid-layer structure of red giants I. Mixed-mode coupling factor as a seismic diagnosis
• Gravity mode offset and properties of the evanescent zone in red-giant stars
• Improved asteroseismic inversions for red-giant surface rotation rates
• Near-Core Acoustic Glitches are Not Oscillatory: Consequences for Asteroseismic Probes of Convective Boundary Mixing
• Asteroseismic Inversions for Internal Sound Speed Profiles of Main-sequence Stars with Radiative Cores
• The robustness of inferred envelope and core rotation rates of red-giant stars from asteroseismology
• Power density spectra morphologies of seismically unresolved red-giant asteroseismic binaries
• Asteroseismic Structure Inversions of Main-Sequence Solar-like Oscillators with Convective Cores

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