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

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.

12 pages, 8 figures, accepted for publication inThe Astrophysical Journal

References in corpus (15)

• A spin-down clock for cool stars from observations of a 2.5-billion-year-old cluster
• A double main sequence turn-off in the rich star cluster NGC 1846 in the Large Magellanic Cloud
• Multiple stellar populations in three rich Large Magellanic Cloud star clusters
• Extended Main Sequence Turnoffs in Intermediate-Age Star Clusters: A Correlation Between Turnoff Width and Early Escape Velocity
• Populations of rotating stars III. SYCLIST, the new Geneva Population Synthesis code
• Grids of stellar models with rotation IV. Models from 1.7 to 120 Msun at a metallicity Z = 0.0004
• Multiple stellar populations in Magellanic Cloud clusters. III. The first evidence of an extended main sequence turn-off in a young cluster: NGC1856
• Stars caught in the braking stage in young Magellanic Clouds clusters
• How stellar rotation shapes the colour magnitude diagram of the massive intermediate-age star cluster NGC 1846
• Extended Main Sequence Turnoffs in Intermediate-Age Star Clusters: Stellar Rotation diminishes, but does not eliminate, Age Spreads
• Discovery of Extended Main Sequence Turn-offs in Four Young Massive Clusters in the Magellanic Clouds
• Stellar rotation and the extended main sequence turnoff in the open cluster NGC 5822
• Extended main-sequence turnoffs in the double cluster $h$ and $\chi$ Persei: The complex role of stellar rotation
• Spectroscopic Abundances in the Open Cluster, NGC 6819
• Disappearance of the extended main sequence turn-off in intermediate age clusters as a consequence of magnetic braking

Cited by in corpus (3)

• New insights into the structure of open clusters in the Gaia era
• On the origin of UV-dim stars: a population of rapidly rotating shell stars?
• Extended Main Sequences in Star Clusters