Orbital and Physical Properties of the Pleiades Binary 27 Tau (Atlas)

We report new spectroscopic and interferometric observations of the Pleiades binary star Atlas, which played an important role nearly three decades ago in settling the debate over the distance to the cluster from ground-based and space-based determinations. We use the new measurements, together with other published and archival astrometric observations, to improve the determination of the 291-day orbit and the distance to Atlas ($136.2 \pm 1.4$ pc). We also derive the main properties of the components, including their absolute masses ($5.04 \pm 0.17 M_{\odot}$ and $3.64 \pm 0.12 M_{\odot}$), sizes, effective temperatures, projected rotational velocities, and chemical composition. We find that the more evolved primary star is rotationally distorted, and are able to estimate its oblateness and the approximate orientation of its spin axis from the interferometric observations. The spin axis may well be aligned with the orbital axis. Models of stellar evolution from MESA that account for rotation provide a good match to all of the primary's global properties, and point to an initial angular rotation rate on the zero-age main sequence of about 55% of the breakup velocity. The current location of the star in the H-R diagram is near the very end of the hydrogen-burning main sequence, at an age of about 105 Myr, according to these models. Our spectroscopic analysis of the more slowly-rotating secondary indicates that it is a helium-weak star, with other chemical anomalies.