Asteroseismic Study of Subgiants and Giants of the Open Cluster M67 using Kepler/K2: Expanded Sample and Precise Masses

TL;DR

This study uses Kepler/K2 asteroseismic data to analyze stars in the open cluster M67 across various evolutionary stages, deriving precise stellar masses, calibrating seismic relations, and estimating the cluster's age.

Contribution

It provides the first comprehensive asteroseismic analysis of M67 spanning from subgiants to red giants, including calibration of seismic relations and assessment of systematic errors.

Findings

Seismic masses agree with isochrone predictions for most stars.

The Reimers mass loss parameter is constrained to be less than ~0.23.

Cluster age estimated at approximately 3.95 Gyr.

Abstract

Sparked by the asteroseismic space revolution, ensemble studies have been used to produce empirical relations linking observed seismic properties and fundamental stellar properties. Cluster stars are particularly valuable because they have the same metallicity, distance, and age, thus reducing scatter to reveal smoother relations. We present the first study of a cluster that spans the full evolutionary sequence from subgiants to core helium-burning red giants using asteroseismology to characterise the stars in M67, including a yellow straggler. We use Kepler/K2 data to measure seismic surface gravity, examine the potential influence of core magnetic fields, derive an empirical expression for the seismic surface term, and determine the phase term $\epsilon$ of the asymptotic relation for acoustic modes, extending its analysis to evolutionary states previously unexplored in detail. Additionally, we calibrate seismic scaling relations for stellar mass and radius, and quantify their systematic errors if surface term corrections are not applied to state-of-the-art stellar models. Our masses show that the Reimers mass loss parameter can not be larger than $\eta$ $\sim$ 0.23 at the 2-$\sigma$ level. We use isochrone models designed for M67 and compare their predictions with individual mode frequencies. We find that the seismic masses for subgiants and red giant branch stars align with the isochrone-predicted masses as per their luminosity and colour. However, our results are inconsistent with the mass of one of the stellar components of an eclipsing binary system near the TAMS. We use traditional seismic $\chi^2$ fits to estimate a seismic cluster age of 3.95 $\pm$ 0.35 Gyrs.