The course of the Orphan Stream in the Northern Galactic Hemisphere
  traced with Gaia DR2
  The Orphan Stream is one of the most prominent tidal streams in the Galactic
halo. Using data on red giants, RR Lyrae, and horizontal branch stars from Gaia
and other surveys, we determine the proper motion of the Orphan Stream over a
path of more than 90 degrees on the sky. We also provide updated tracks for the
sky position, distance, and radial velocity of the stream. Our tracks in these
latter dimensions mostly agree with previous results. However, there are
significant corrections to the earlier distance and latitude tracks as the
stream approaches the Galactic disk. Stream stars selected with
three-dimensional kinematics display a very tight red giant sequence.
Concordantly, we find that applying a proper motion cut removes the most
metal-rich stars from earlier spectroscopic samples of stream stars, though a
significant dispersion remains indicating a dwarf galaxy origin. The
deceleration of the stream towards its leading end suggests a circular velocity
of ~200 km/s at a galactocentric radius ~30 kpc, consistent with other
independent evidence. However, the track of the stream departs significantly
from an orbit; the spatial track does not point along the same direction as the
velocity vector, and it exhibits a lateral wiggle that is unlikely to match any
reasonable orbit. The low metallicity and small dispersion of the stream in the
various coordinates point to a progenitor with a relatively low dynamical mass
~$10^8$ Msun.
