At what mass are stars braked? The implication from the turnoff morphology of NGC 6819

TL;DR

This study investigates the mass threshold for magnetic braking in stars by analyzing the turnoff morphology of the intermediate-age cluster NGC 6819, finding evidence that stars below approximately 1.54 solar masses have undergone significant magnetic braking.

Contribution

It provides observational constraints on the stellar mass limit for magnetic braking using the turnoff morphology of NGC 6819, linking stellar rotation and cluster age.

Findings

NGC 6819 lacks an extended main-sequence turnoff (eMSTO).

MSTO stars in NGC 6819 have low rotational velocities, indicating magnetic braking.

The critical stellar mass for magnetic braking at solar metallicity is near 1.54 solar masses.

Abstract

Extended main-sequence turnoffs apparent in most young and intermediate-age clusters (younger than ~2 Gyr) are known features caused by fast rotating early-type (earlier than F-type) stars. Late-type stars are not fast rotators because their initial angular momenta have been quickly dispersed due to magnetic braking. However, the mass limit below which stars have been magnetically braked has not been well constrained by observation. In this paper, we present an analysis of the eMSTO of NGC 6819, an open cluster of an intermediate-age (~2.5 Gyr), believed to be comparable to the lifetime of stars near the mass limit for magnetic braking. By comparing the observation with synthetic CMDs, we find that NGC 6819 does not harbor an obvious eMSTO. The morphology of its TO region can be readily explained by a simple stellar population considering the observational uncertainties as well as the differential reddening. In addition, the MSTO stars in NGC 6819 have very small values of average rotational velocity and dispersion, indicating that they have undergone significant magnetic braking. Combining with results in the literature for clusters of younger ages, our current work suggests that the critical age for the disappearance of eMSTO in star clusters must be shorter but very close to the age of NGC 6819, and this, in turn, implies a critical stellar mass for magnetic braking at solar metallicity above but close to 1.54 $M_{\odot}$ based on the PARSEC model. We emphasize that the phenomenon of eMSTO could provide a unique way to constrain the onset mass of magnetic braking.