Evidence of tidal distortions and mass-loss from the old open cluster NGC 6791

Abstract

We present the first evidence of clear signatures of tidal distortions in the density distribution of the fascinating open cluster NGC 6791. We used deep and wide-field data obtained with the Canada-France-Hawaii-Telescope covering a 2 • ×2 • area around the cluster. The two-dimensional density map obtained with the optimal matched filter technique shows a clear elongation and an irregular distribution starting from ∼ 300 ′′ from the cluster center. At larger distances, two tails extending in opposite directions beyond the tidal radius are also visible. These features are aligned to both the absolute proper motion and to the Galactic center directions. Moreover, other overdensities appear to be stretched in a direction perpendicular to the Galactic plane.Accordingly to the behaviour observed in the density map, we find that both the surface brightness and the star count density profiles reveal a departure from a King model starting from ∼ 600 ′′ from the center.These observational evidence suggest that NGC 6791 is currently experiencing mass loss likely due to gravitational shocking and interactions with the tidal field. We use this evidence to argue that NGC 6791 should have lost a significant fraction of its original mass. A larger initial mass would in fact explain why the cluster survived so long. Using available recipes based on analytic studies and N -body simulations, we derived the expected mass loss due to stellar evolution and tidal interactions and estimated the initial cluster mass to be M ini = (1.5-4) × 10 5 M ⊙ .

On the mass of the Galactic star cluster NGC 4337

“…To derive an estimate of the cluster initial mass, several constants characterising the cluster and the environment need to be adopted. To evaluate the equation (1) of Dalessandro et al (2015), we use t = 1.6 ± 0.1 Gyr for the age of the cluster, and the three mass estimates derived above: m 1 = 1820 ± 620M (Sect. 6), m 2 = 1880 ± 820M (Sect.…”

“…To characterise the mass loss due to the stellar evolution (equation (2) in Dalessandro et al (2015)), we use the metallicity Z = 0.02 (the coefficients given in Table 1 of Lamers et al (2005), which is sufficiently close to the value of 0.025 estimated here. The tidal mass loss is characterised by the dissolution timescale t 0 = 3.3 +1.4 −1.0 Myr.…”

NGC 6362: The Least Massive Globular Cluster With Chemically Distinct Multiple Populations*

“…In addition we have found that NGC 6362 is the first cluster where the radial distributions of its MPs show no significant radial difference up to its tidal radius. We interpreted such unexpected evidence as the result of a very advanced dynamical evolution and possibly of a significant mass-loss due to interactions with the Galactic disk and potential well (see Dalessandro et al 2015).…”

Structure and mass segregation in Galactic stellar clusters

“…As clusters age, the expulsion of gas by stellar feedback as well as dynamical interactions between stars and binary systems in the cluster soften its gravitational potential, leading to their expansion and to their partial or total dissolution into the field of their host galaxy (e.g., Spitzer & Harm 1958;Parker & Meyer 2012;Parker & Dale 2013;Dib et al 2011a,b;Dib 2011;Dib et al 2013;Pfalzner & Kaczmarek 2013;Brinkmann et al 2017). Clusters can also be ⋆ E-mail, SD: sami.dib@gmail.com † Royal Society Dorothy Hodgkin Fellow disrupted by close encounters with giant molecular clouds as they orbit the Galactic centre (e.g., Gieles et al 2006) or by being subjected to strong tidal fields (e.g., Dalessandro et al 2015;Zhai et al 2017;Martinez-Medina et al 2017). …”