  As part of a program to determine dynamical masses of Cepheids, we have
imaged the nearest and brightest Cepheid, Polaris, with the Hubble Space
Telescope Wide Field Planetary Camera 2 and Wide Field Camera 3. Observations
were obtained at three epochs between 2007 and 2014. In these images, as in HST
frames obtained in 2005 and 2006, which we discussed in a 2008 paper, we
resolve the close companion Polaris Ab from the Cepheid Polaris Aa. Because of
the small separation and large magnitude difference between Polaris Aa and Ab,
we used PSF deconvolution techniques to carry out astrometry of the binary.
Based on these new measurements, we have updated the elements for the 29.59 yr
orbit. Adopting the distance to the system from the recent Gaia Data Release 2,
we find a dynamical mass for the Cepheid of 3.45 +/- 0.75 Msun, although this
is preliminary, and will be improved by CHARA measurements covering periastron.
As is the case for the recently determined dynamical mass for the Cepheid V1334
Cyg, the mass of Polaris is significantly lower than the "evolutionary mass"
predicted by fitting to evolutionary tracks in the HR diagram. We discuss
several questions and implications raised by these measurements, including the
pulsation mode, which instability-strip crossing the stars are in, and possible
complications such as rotation, mass loss, and binary mergers. The distant
third star in the system, Polaris B, appears to be older than the Cepheid,
based on isochrone fitting. This may indicate that the Cepheid Polaris is
relatively old and is the result of a binary merger, rather than being a young
single star.
