Isochrone fitting to the open cluster M67 in the era of Gaia and improved model physics

TL;DR

This paper presents new stellar models and an isochrone for the open cluster M67, leveraging Gaia data and improved physics, to refine age estimates and test model accuracy against binary star data.

Contribution

It introduces a precise, tailored isochrone for M67 using advanced stellar physics and Gaia data, avoiding common modeling simplifications.

Findings

Derived cluster age of approximately 3.95 Gyr.

Created a downloadable, tailored isochrone for M67.

Tested model masses against an eclipsing binary, highlighting modeling challenges.

Abstract

The Gaia mission has provided highly accurate observations that have significantly reduced the scatter in the colour-magnitude diagrams of open clusters. As a result of the improved isochrone sequence of the open cluster M67, we have created new stellar models that avoid commonly used simplifications in 1D stellar modelling, such as mass-independent core overshooting and a constant mixing length parameter. This has enabled us to deliver a precise isochrone specifically designed for M67, available for download. We follow a commonly used qualitative approach to adjust the input physics to match the well-defined colour-magnitude sequence, and we test the model-predicted masses against a known eclipsing binary system at the main sequence turnoff of the cluster. Despite using improvements in photometry and stellar physics we cannot match the masses of both binary components with the same theoretical isochrone. A chi-square-based isochrone fitting approach using our preferred input physics results in a cluster age of 3.95+0.16-0.15 Gyrs.