The Age and Age Spread of the Praesepe and Hyades Clusters: a Consistent, ~800 Myr Picture from Rotating Stellar Models

Abstract

We fit the upper main sequence of the Praesepe and Hyades open clusters using stellar models with and without rotation. When neglecting rotation, we find that no single isochrone can fit the entire upper main sequence at the clusters' spectroscopic metallicity: more massive stars appear, at high significance, to be younger than less massive stars. This discrepancy is consistent with earlier studies, but vanishes when including stellar rotation. The entire upper main sequence of both clusters is very well-fit by a distribution of 800 Myr-old stars with the spectroscopically measured [Fe/H]=0.12. The increase over the consensus age of ~600-650 Myr is due both to the revised Solar metallicity (from $Z_\odot \approx 0.02$ to $Z_\odot \approx 0.014$) and to the lengthening of main sequence lifetimes and increase in luminosities with rapid rotation. Our results show that rotation can remove the need for large age spreads in intermediate age clusters, and that these clusters may be significantly older than is commonly accepted. A Hyades/Praesepe age of ~800 Myr would also require a recalibration of rotation/activity age indicators.Comment: 6 pages, 4 figures, ApJ accepted. Replaced with accepted version, conclusions unchange