The Origin of Enhanced Activity in the Suns of M67

TL;DR

This study investigates the activity levels of solar-type stars in M67, finding that high activity correlates with increased rotation rates, which has implications for stellar angular momentum evolution and solar activity understanding.

Contribution

It provides the first high-resolution spectroscopic analysis linking enhanced chromospheric activity to faster rotation in M67's solar-type stars, suggesting a different angular momentum evolution pathway.

Findings

Most stars show solar-like rotation velocities.

Two stars exhibit activity levels exceeding solar maximum.

Rapid rotation in one star may result from reduced magnetic braking.

Abstract

We report the results of the analysis of high resolution photospheric line spectra obtained with the UVES instrument on the VLT for a sample of 15 solar-type stars selected from a recent survey of the distribution of H and K chromospheric line strengths in the solar-age open cluster M67. We find upper limits to the projected rotation velocities that are consistent with solar-like rotation (i.e., v sini ~< 2-3 km/s) for objects with Ca II chromospheric activity within the range of the contemporary solar cycle. Two solar-type stars in our sample exhibit chromospheric emission well in excess of even solar maximum values. In one case, Sanders 1452, we measure a minimum rotational velocity of vsini = 4 +/- 0.5 km/s, or over twice the solar equatorial rotational velocity. The other star with enhanced activity, Sanders 747, is a spectroscopic binary. We conclude that high activity in solar-type stars in M67 that exceeds solar levels is likely due to more rapid rotation rather than an excursion in solar-like activity cycles to unusually high levels. We estimate an upper limit of 0.2% for the range of brightness changes occurring as a result of chromospheric activity in solar-type stars and, by inference, in the Sun itself. We discuss possible implications for our understanding of angular momentum evolution in solar-type stars, and we tentatively attribute the rapid rotation in Sanders 1452 to a reduced braking efficiency.