The search for extratidal star candidates around Galactic globular
  clusters NGC 2808, NGC 6266, and NGC 6397 with {\textit{Gaia}} DR2 astrometry
  Context. Extratidal stars are stellar bodies that end up outside the tidal
radius of a cluster as a result of internal processes or external forces acting
upon it. The presence and spatial distribution of these stars can give us
insights into the past evolution of a cluster inside our Galaxy. Aims. Previous
works suggest that globular clusters, when explored in detail, show evidence of
extratidal stars. We aim to search for possible extratidal stars in the
Galactic globular clusters NGC 6397, NGC 2808, and NGC 6266 using the
photometry and proper motion measurements from Gaia DR2 database (Gaia
Collaboration et al. 2018). Results. Finally, 120, 126, and 107 extratidal
candidate stars were found lying outside the tidal radius of the globular
clusters NGC 6397, NGC 2808, and NGC 6266, respectively. 70%, 25.4%, and 72.9%
of the extratidal stars found are located outside the Jacobi radius of NGC
6397, NGC 2808, and NGC 6266, respectively. The spatial distribution of the
extratidal stars belonging to NGC 6397 appears S-like, extending along the
curved leading and trailing arms. NGC 2808 has an overdensity of stars in the
trailing part of the cluster and NGC 6266 seems to have overdensities of
extratidal stars in its eastern and northern sides. Conclusions. Proper motions
and color-magnitude diagrams can be used to identify extratidal candidate stars
around GCs. Nonetheless, depending on how different the kinematics and stellar
populations of a cluster are compared to the Milky Way field, the fraction of
contamination can be larger. All three clusters are found to have extratidal
stars outside their tidal radii. For NGC 6397 and NGC 2808, these stars may be
the result of a combined effect of the disc shocks and tidal disruptions. For
NGC 6266, the distribution of extratidal stars is symmetrical around it, most
likely indicating that the cluster has an extended stellar envelope.
