The Dynamics, Structure, and Fate of a Young Cluster During Gas
  Dispersal: Hectoschelle, Chandra, Spitzer, and Gaia Observations of CepOB3b
  We present a study of the kinematics and structure of the Cep OB3b cluster
based on new spectra obtained with the Hectoschelle spectrograph on the MMT and
data from Spitzer, Chandr}, and Gaia. At a distance of 819+/-16 pc, Cep OB3b is
one of the closest examples of a young (~3 - 5 Myr), large (~3000 total
members) cluster at the late stages of gas dispersal. The cluster is broken
into two sub-clusters surrounded by a lower density halo. We fit the empirical
density law of King (1962) to each sub-cluster to constrain their sizes and
structure. The richer eastern sub-cluster has circular symmetry, a modest
central density, and lacks molecular gas toward its core suggesting it has
undergone expansion due to gas dispersal. In contrast, the western sub-cluster
deviates from circular symmetry, has a smaller core size, and contains
significant molecular gas near its core, suggesting that it is in an earlier
phase of gas dispersal. We present posterior probability distributions for the
velocity dispersions from the Hectoschelle spectra. The east will continue to
expand and likely form a bound cluster with ~35% of stars remaining. The west
is undergoing slower gas dispersal and will potentially form a bound cluster
with ~75% of stars remaining. If the halo dissipates, this will leave two
independent clusters with ~300 members; proper motions suggest that the two
sub-clusters are not bound to each other.
