PA & angular separation measurements of 80 Tau

The last one was identified and first measured more than 20yr ago. Here we present dynamical mass measurements for a new binary system in the cluster, 80 Tau, which is also a visual pair with a much longer orbital period of about 170yr.

Although we lack the radial-velocity information that has enabled the individual mass determinations in all of the previous binaries, we show that it is still possible to derive the component masses for 80 Tau using only astrometric observations. This is enabled by the accurate proper motion measurements from the Hipparcos and Gaia missions, which constrain the orbital acceleration in the plane of the sky. Separate proper motion values from Gaia for the primary and secondary provide a direct constraint on the mass ratio.