Slowly Rotating Fully Convective Stars at 1 Gyr: Evidence from the Hyades and Praesepe

We report the discovery of 10 stars in the Praesepe and Hyades clusters with unusually slow rotation that are likely not field contamination. Even though our search was general, all slow rotators are in the fully convective domain. We find that the fraction of slow rotators in the clusters is more than 5σ above the distribution expected from field contamination. They provide a potential link between the period distribution of the young open clusters and the field. These stars could be a distinct birth population, or they could be an indication that the onset of strong magnetic breaking is earlier than previously realized in this domain. Photometric variability and X-ray luminosity suggest the later but more data is needed to confirm. Searches for similar populations in younger clusters will help determine when slow rotators first emerge and constrain the timescale and physical mechanisms governing angular-momentum loss in fully convective stars. The discovery of these stars also highlights the importance of long-baseline photometric observations for detecting the long rotation periods of low-mass stars.