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Asteroseismology of M67: Seismic Characterisation Across Evolutionary Phases

Ensemble asteroseismology has revolutionized our ability to empirically link observed seismic properties to fundamental stellar parameters. Star clusters, with their uniform metallicity, distance, and age, provide an ideal environment for refining these relations.

In this study, we present the first asteroseismic analysis of M67 that spans its full evolutionary sequence, from subgiants to core helium-burning red giants, including a yellow straggler.

M67 stars examples — Examples of the characteristic "near Gaussian"-shaped excess power observed in the power spectra of M67 stars from near the tip of the red-giant branch (EPIC 211376143) to the subgiant branch (EPIC 211414203). The yellow curves represent the smoothed power spectrum, the white area represents the portion of the spectrum where the excess power is located, and the red vertical lines indicate the frequency of maximum power, 𝜈_max.

Using Kepler/K2 data, we measure seismic surface gravity, examine the potential role of core magnetic fields, and derive an empirical expression for the seismic surface term. We also extend the analysis of the asymptotic phase term ɛ to evolutionary stages previously unexplored in detail.

Phase term — In all panels the black solid curve shows phase-term and Δ𝜈 from models along the isochrone, and the dashed line shows the same, but after applying the correction given by the power-law fit derived in this work.

A key focus of our work was the calibration of seismic scaling relations for mass and radius. We quantified the systematic errors introduced when surface term corrections are omitted from state-of-the-art stellar models, demonstrating how significantly the scaling relations are affected by missing corrections. Our mass estimates indicated minimal mass-loss during the RGB.

Using isochrone models tailored to M67, we compared individual mode frequencies with theoretical predictions. We found that seismic masses for subgiants and red giant branch stars aligned well with isochrone expectation.

Model grid — Observed stars (white circles) in the GAIA Colour-Absolute magnitude diagram, and their best fitted seismic models (red circles). The grid of over 2,000,000 individual models shows as a continuous grey band in the diagram. The small panels show the χ² distribution of models with Δν near each star's observed value.

A traditional χ²-based seismic fit provides an estimated cluster age of 3.95 ± 0.35 Gyr.

📖 Read the full study here

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