Disentangling the Arcturus stream
  The Arcturus stream is an over-density of stars in velocity space and its
origin has been much debated recently without any clear conclusion. Resolving
the nature of the Arcturus stream can provide clues to the formation history of
the Milky Way and its stellar populations. The space velocities, angular
momenta and actions for a sample of more than 5.8 million stars, composed from
Gaia DR2, are analysed with a wavelet transform. The kinematic characteristics
of each identified group is used to select possible members of the groups from
the GALAH and APOGEE spectroscopic surveys to further study and constrain their
chemical properties. In the velocity and angular momentum spaces the already
known Sirius, Pleiades, Hyades, Hercules, AF06, Arcturus and KFR08 streams are
clearly identified. The Hercules stream appears to be a mixture of thin and
thick disk stars. The Arcturus stream, as well as the AF06 and KFR08 streams,
are low-velocity and low-angular momentum structures with chemical compositions
similar to the thick disk. These three groups extend further from the Galactic
plane compared to the Hercules stream. The detections of all the groups were
spaced by approximately 20-30 km/s in azimuthal velocity. A wide spread of
chemical abundances within the Arcturus stream indicates that the group is not
a dissolved open cluster. Instead the Arcturus stream, together with the AF06
and KFR08 streams, are more likely to be part of a phase-space wave, that could
have been caused by an ancient merger event. This conclusion is based on that
the different structures are detected in steps of 20-30 km/s in azimuthal
velocity, that the kinematic and chemical features are different from what is
expected for bar-originated structures, and that the lower-velocity streams
extend further from the disk than bar-originated structures.
