The search for extra-tidal star candidates around Galactic globular clusters NGC 2808, NGC 6266 and NGC 6397 with Gaia DR2 astrometry

R. KUNDU, C. NAVARRETE, J.G. FERNÁNDEZ-TRINCADO, D. MINNITI, H.P. SINGH, L. SBORDONE, A.E. PIATTI, C.REYLÉ

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2021 vol. 645 p. 1 - 10

0004-6361

Context. Extra-tidal stars are those that lie outside the tidal radius of a cluster due to internal processes or external forces acting on 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 extra-tidal stars. We aim to search for possible extra-tidal 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).Methods. The extra-tidal stars for the clusters were selected based on their distance from the cluster center, similar proper motions to the cluster population and position on the color-magnitude diagram of the clusters. Each cluster was explored in an annulus disk from the tidal radius up to 5 times tidal radii. The significance level of the number of selected extra-tidal stars was determined based on the distribution of Milky Way stars according to the Besançon Galaxy model and Gaia data. To understand the observed extra-tidal features, the orbits of the clusters were also determined using GravPot16.Results. 120, 126 and 107 extra-tidal 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 extra-tidal stars found are located outside the Jacobi radius of NGC 6397, NGC 2808 and NGC 6266, respectively. The spatial distribution of the extra-tidal stars belonging to NGC 6397 appears S-like, extending in 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 extra-tidal stars in its eastern and northern sides.Conclusions. Proper motions and color-magnitude diagrams can be used to identify extra-tidal candidate stars around GCs. Nonethe- less, 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 the three clusters have extra-tidal stars outside their tidal radius. For NGC 6397 and NGC 2808 these stars may be a result of a combined effect of the disc shocks and tidal disruptions. For NGC 6266, the distribution of extra-tidal stars is symmetrical around it, most likely indicating that the cluster has an extended stellar envelope.