Gaia DR2 orbital properties for field stars with globular cluster-like
  CN band strengths
  Large spectroscopic surveys of the Milky Way have revealed that a small
population of stars in the halo have light element abundances comparable to
those found in globular clusters. The favoured explanation for the peculiar
abundances of these stars is that they originated inside a globular cluster and
were subsequently lost. Using orbit calculations we assess the likelihood that
an existing sample of 57 field stars with globular cluster-like CN band
strength originated in any of the currently known Milky Way globular clusters.
Using Sloan Digital Sky Survey and Gaia data, we determine orbits and integrals
of motion of our sample of field stars, and use these values and metallicity to
identify likely matches to globular clusters. The pivot hypothesis is that had
these stars been stripped from such objects, they would have remained on very
similar orbits. We find that ~ 70% of the sample of field stars have orbital
properties consistent with the halo of the Milky Way; however, only 20 stars
have likely orbital associations with an existing globular cluster. The
remaining ~ 30% of the sample have orbits that place them in the outer Galactic
disc. No cluster of similar metallicity is known on analogous disc orbits. The
orbital properties of the halo stars seem to be compatible with the globular
cluster escapee scenario. The stars in the outer disc are particularly
surprising and deserve further investigation to establish their nature.
