A 4 Gyr M-dwarf Gyrochrone from CFHT/MegaPrime Monitoring of the Open Cluster M67

TL;DR

This study measures rotation periods of late K- and early M-dwarfs in the 4 Gyr old cluster M67 to test and calibrate stellar spin-down models, finding that a Skumanich-like law best describes their rotational evolution.

Contribution

It provides the first comprehensive rotation period dataset for M67 members and compares observed spin-down with theoretical models, highlighting the adequacy of Skumanich-like laws for certain spectral types.

Findings

Rotation of very cool dwarfs follows a simple solid-body spin-down between 2.7 and 4 Gyr.

Skumanich-like spin-down (P_rot ∝ t^0.62) best matches the observed data.

Spin-down must have resumed before 4 Gyr for M1-M3 stars, challenging some models.

Abstract

We present stellar rotation periods for late K- and early M-dwarf members of the 4 Gyr old open cluster M67 as calibrators for gyrochronology and tests of stellar spin-down models. Using Gaia EDR3 astrometry for cluster membership and Pan-STARRS (PS1) photometry for binary identification, we build this set of rotation periods from a campaign of monitoring M67 with the Canada-France-Hawaii Telescope's MegaPrime wide field imager. We identify 1807 members of M67, of which 294 are candidate single members with significant rotation period detections. Moreover, we fit a polynomial to the period versus color-derived effective temperature sequence observed in our data. We find that the rotation of very cool dwarfs can be explained by a simple solid-body spin-down between 2.7 and 4 Gyr. We compare this rotational sequence to the predictions of gyrochronological models and find that the best match is Skumanich-like spin-down, P_rot \propto t^0.62, applied to the sequence of Ruprecht 147. This suggests that, for spectral types K7-M0 with near-solar metallicity, once a star resumes spinning down, a simple Skumanich-like is sufficient to describe their rotation evolution, at least through the age of M67. Additionally, for stars in the range M1-M3, our data show that spin-down must have resumed prior to the age of M67, in conflict with predictions of the latest spin-down models.