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Register a new userCatching Galactic open clusters in advanced stages of dynamical evolution
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During their dynamical evolution, Galactic open clusters (OCs) gradually lose their stellar content mainly because of internal relaxation and tidal forces. In this context, the study of dynamically evolved OCs is necessary to properly understand such processes. We present a comprehensive Washington $CT_1$ photometric analysis of six sparse OCs, namely: ESO 518-3, Ruprecht 121, ESO 134-12, NGC 6573, ESO 260-7 and ESO 065-7. We employed Markov chain Monte-Carlo simulations to robustly determine the central coordinates and the structural parameters and $T_1times(C-T_1)$ colour-magnitude diagrams (CMDs) cleaned from field contamination were used to derive the fundamental parameters. ESO 518-03, Ruprecht 121, ESO 134-12 and NGC 6573 resulted to be of nearly the same young age (8.2 $leqtextrm{log}(t textrm{yr}^{-1})leq$ 8.3); ESO 260-7 and ESO065-7 are of intermediate age (9.2 $leqtextrm{log}(t textrm{yr}^{-1})leq$ 9.4). All studied OCs are located at similar Galactocentric distances (R$_{G}sim6-6.9 $kpc), considering uncertainties, except for ESO 260-7 ($R_{G}=8.9 $kpc). These OCs are in a tidally filled regime and are dynamically evolved, since they are much older than their half-mass relaxation times ($t/t_{rh}gtrsim30$) and present signals of low-mass star depletion. We distinguished two groups: those dynamically evolving towards final disruptions and those in an advanced dynamical evolutionary stage. Although we do not rule out that the Milky Way potential could have made differentially faster their dynamical evolutions, we speculate here with the possibility that they have been mainly driven by initial formation conditions.
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Base on Gaia Second Data Release and the combination of nonparametric bivariate density estimation with the least square ellipse fitting, we derive the shape parameters of the sample clusters. By analyzing the dislocation of the sample clusters, the dislocation $d$ is related to the X-axis pointing toward the Galactic center, Y-axis pointing in the direction of Galactic rotation, and the Z-axis (log(|H|/pc)) that is positive toward the Galactic north pole. This finding underlines the important role of the dislocation of clusters in tracking the external environment of the Milky Way. The orientation ($q_{pm}$) of the clusters with $e_{pm}$ $geq$ 0.4 presents an aggregate distribution in the range of -45$degr$ to 45$degr$, about 74% of them. This probably suggests that these clusters tend to deform heavily in the direction of the Galactic plane. NGC 752 is in a slight stage of expansion in the two-dimensional space and will deform itself morphology along the direction perpendicular to the original stretching direction in the future if no other events occur. The relative degree of deformation of the sample clusters in the short-axis direction decreases as their ages increase. On average, the severely distorted sample clusters in each group account for about 26% $pm$ 9%. This possibly implies a uniform external environment in the range of $|$H$|$ $leq$ 300 pc if the sample completeness of each group is not taken into account.
The study of dynamical properties of Galactic open clusters is a fundamental prerequisite for the comprehension of their dissolution processes. In this work, we characterized 12 open clusters, namely: Collinder 258, NGC 6756, Czernik 37, NGC 5381, Ruprecht 111, Ruprecht 102, NGC 6249, Basel 5, Ruprecht 97, Trumpler 25, ESO 129-SC32 and BH 150, projected against dense stellar fields. In order to do that, we employed Washington $CT_{1}$ photometry and GAIA DR2 astrometry, combined with a decontamination algorithm applied to the three-dimensional astrometric space of proper motions and parallaxes. From the derived membership likelihoods, we built decontaminated colour-magnitude diagrams, while structural parameters were obtained from King profiles fitting. Our analysis revealed that they are relatively young open clusters (log($t$ yr$^{-1}$) $sim7.3-8.6$), placed along the Sagittarius spiral arm, and at different internal dynamical stages. We found that the half-light radius to Jacobi radius ratio, the concentration parameter and the age to relaxation time ratio describe satisfactorily their different stages of dynamical evolution. Those relative more dynamically evolved open clusters have apparently experienced more important low-mass star loss.
Our current understanding of the stellar initial mass function and massive star evolution suggests that young globular clusters may have formed hundreds to thousands of stellar-mass black holes, the remnants of stars with initial masses from $sim 20 - 100, M_odot$. Birth kicks from supernova explosions may eject some black holes from their birth clusters, but most should be retained. Using a Monte Carlo method we investigate the long-term dynamical evolution of globular clusters containing large numbers of stellar black holes. We describe numerical results for 42 models, covering a range of realistic initial conditions, including up to $1.6times10^6$ stars. In almost all models we find that significant numbers of black holes (up to $sim10^3$) are retained all the way to the present. This is in contrast to previous theoretical expectations that most black holes should be ejected dynamically within a few Gyr. The main reason for this difference is that core collapse driven by black holes (through the Spitzer mass segregation instability) is easily reverted through three-body processes, and involves only a small number of the most massive black holes, while lower-mass black holes remain well-mixed with ordinary stars far from the central cusp. Thus the rapid segregation of stellar black holes does not lead to a long-term physical separation of most black holes into a dynamically decoupled inner core, as often assumed previously. Combined with the recent detections of several black hole X-ray binary candidates in Galactic globular clusters, our results suggest that stellar black holes could still be present in large numbers in many globular clusters today, and that they may play a significant role in shaping the long-term dynamical evolution and the present-day dynamical structure of many clusters.
The stellar content of Galactic open clusters is gradually depleted during their evolution as a result of internal relaxation and external interactions. The final residues of the evolution of open clusters are called open cluster remnants, barely distinguishable from the field. We aimed to characterise and compare the dynamical states of a set of 16 such objects. The sample also includes 7 objects that are catalogued as dynamically evolved open clusters. We used photometric data from the 2MASS, astrometric data from the GAIA DR2 and a decontamination algorithm that was applied to the three-dimensional astrometric space of proper motions and parallaxes for stars in the objects areas. The luminosity and mass functions and total masses for most open cluster remnants are derived here for the first time. Our analysis used predictions of N-body simulations to estimate the initial number of stars of the remnants from their dissolution timescales. The investigated open cluster remnants present masses and velocity dispersions within well-defined ranges: M between ~10-40M_Sun and sigma_v between ~1-7km/s. Some objects in the remnant sample have a limiting radius R_lim<~2pc, which means that they are more compact than the investigated open clusters; other remnants have R_lim between ~2-7pc, which is comparable to the open clusters. We suggest that the open cluster NGC2180 is entering a remnant evolutionary stage. In general, our clusters show signals of depletion of low-mass stars. This confirms their dynamically evolved states. We conclude that the open cluster remnants we studied are in fact remnants of initially very populous open clusters (No~10^3-10^4 stars). The outcome of the long-term evolution is to bring the final residues of the open clusters to dynamical states that are similar to each other, thus masking out the memory of the initial formation conditions of star clusters.
Radial migration is an important process in the Galactic disk. A few open clusters show some evidence on this mechanism but there is no systematic study. In this work, we investigate the role of radial migration on the Galactic disk based on a large sample of 146 open clusters with homogeneous metallicity and age from Netopil et al. and kinematics calculated from Gaia DR2. The birth site Rb, guiding radius Rg and other orbital parameters are calculated, and the migration distance |Rg-Rb| is obtained, which is a combination of metallicity, kinematics and age information. It is found that 44% open clusters have |Rg-Rb|< 1 kpc, for which radial migration (churning) is not significant. Among the remaining 56% open clusters with |Rg-Rb|> 1 kpc, young ones with t<1.0 Gyr tend to migrate inward, while older clusters usually migrate outward. Different mechanisms of radial migration between young and old clusters are suggested based on their different migration rates, Galactic locations and orbital parameters. For the old group, we propose a plausible way to estimate migration rate and obtain a reasonable value of 1.5(+-0.5) kpc/Gyr based on ten intermediate-age clusters at the outer disk, where the existence of several special clusters implies its complicate formation history.
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