Evidences of tidal distortion and mass loss from the old open cluster NGC 6791
FFormation, evolution, and survival of massive star clustersProceedings IAU Symposium No. 316, 2015C. Charbonel and A. Nota, eds. c (cid:13) Evidences of tidal distortion and mass lossfrom the old open cluster NGC 6791
Giovanni Carraro , and Emanuele Dalessandro ESO, Alondo de Cordova 3107, 19001, Santiago de Chile, Chile Dept. of Physics and Astronomy, University of Padova, Vicolo Osservatorio 3, 35122, Padova,Italyemail: [email protected] Dept. of Physics and Astronomy, University of Bologna, Via Ranzani 1, 40127, Bologna, Italyemail: [email protected]
Abstract.
We present the first evidence of clear signatures of tidal distortions in the densitydistribution of the fascinating open cluster NGC 6791. We find that the 2D density map showsa clear elongation and an irregular distribution starting from ∼ (cid:48)(cid:48) from the cluster center andtwo tails extending in opposite directions beyond the tidal radius. These features are alignedto both the absolute proper motion and to the Galactic centre directions. Accordingly we findthat both the surface brightness and star count density profiles reveal a departure from a Kingmodel starting from ∼ (cid:48)(cid:48) . These observational evidences suggest that NGC 6791 is currentlyundergoing mass-loss likely due to gravitational shocking and interactions with the tidal field ofthe Milky Way. We derive the expected mass-loss due to stellar evolution and tidal interactionsand we estimate the initial cluster mass to be M ini = (1 . − . × M (cid:12) . Keywords.
Open clusters: general - Open clusters: individual: NGC 6791
1. Context and results
NGC 6791 is one of the most massive open cluster (Carraro 2014) in the MW, possiblyharbouring more than a stellar population (Geisler et al. 2012, Bragaglia et al. 2014).Weused deep images obtained with the wide field imager MegaCam mounted at the Canada-France-Hawaii Telescope (CFHT) to cover a 2 o × o area around the cluster (Dalessandroet al. 2015). The upper left panel in Fig 1 shows the obtained ( g (cid:48) , g (cid:48) − r (cid:48) ) CMDs of theinnermost region of NGC 6791 (left) and the external control field (right) including starslocated at a distance r (cid:62) (cid:48)(cid:48) from the cluster center. Large scale 2D colour-codedsurface density map of NGC6791 obtained by using the optical matched filter techniqueis shown in the upper right panel of Fig. 1. The contour levels span from 3 σ to 40 σ withirregular steps. The solid arrow represents the direction of the absolute proper motionwhile the dashed ones mark the direction of the Galactic Center and that perpendicularto the Galactic plane (Z=0).The observed star count density profile is shown in the upperleft panel of Fig. 1 (open grey squares). The dashed line represents the density value of thebackground as obtained in the control field . The black filled dots are densities obtainedafter background subtraction. The best single-mass King model is also over-plotted to theobservations (solid line). For r (cid:62) (cid:48)(cid:48) the density profile clearly deviates from the Kingmodel following a power-law with exponent α ∼ − . t .In particular the red curve (lower right panel in Fig. 1) shows the dependence for thecurrent cluster mass (5000 M (cid:12) ) and its actual age of 8 Gyr.1 a r X i v : . [ a s t r o - ph . GA ] O c t Carraro & Dalessandro
2. Conclusions
NGC 6791 shows clear evidence of tidal features in its star distribution in the form ofirregular but evident elongations and tidal tails. These features are present also in thestar density and surface brightness profile and they represent clear indication of recentmass-loss. By using the simple recipes we derived the initial mass of NGC6791 to be M ini = (1 . − × M (cid:12) , i.e. several tens larger than its present day mass. This findingwould qualitatively explain why the cluster could have survived for such a long timecontrary to the expectations of current estimates of the destruction rate of Galactic openclusters. References
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Figure 1. Upper left : CMD of NGC 6791 and nearby field.
Upper right : star counts .
Lower left : King profile fitting .