Non-rotating Clusters Research Articles

The spin, expansion, and contraction of open star clusters

Empirical constraints on the internal dynamics of open clusters are important for understanding their evolution and evaporation. High-precision astrometry from Gaia DR3 is thus useful to observe aspects of the cluster dynamics. This work aims to identify dynamically peculiar clusters such as spinning and expanding clusters. We also quantify the spin frequency and expansion rate and compare them with $N$-body models to identify the origins of the peculiarities. We used the latest Gaia DR3 and archival spectroscopic surveys to analyse the radial velocities and proper motions of the cluster members in 1379 open clusters. A systematic analysis of synthetic clusters was performed to demonstrate the observability of the cluster spin along with effects of observational uncertainties. $N$-body simulations were used to understand the evolution of cluster spin and expansion for initially non-rotating clusters. We identified spin signatures in ten clusters (and 16 candidates). Additionally, we detected expansion in 18 clusters and contraction in three clusters. The expansion rate is compatible with previous theoretical estimates based on the expulsion of residual gas. The orientation of the spin axis is independent of the orbital angular momentum. The spin frequencies are much larger than what was expected from simulated, initially non-rotating clusters. This indicates that $>1$<!PCT!> of the clusters are born rotating and/or they have undergone strong interactions. Higher precision observations are required to increase the sample of such dynamically peculiar clusters and to characterise them.

Regimes of Convective Self-Aggregation in Convection-Permitting Beta-Plane Simulations

Abstract The spontaneous self-aggregation (SA) of convection in idealized model experiments highlights the importance of interactions between tropical convection and the surrounding environment. The authors have shown that SA fundamentally changes with the background rotation in previous f-plane simulations, in terms of both the resulting forms of organized convection and the relative roles of the physical feedbacks driving them. This study considers the dependence of SA on rotation in one large domain on the β plane, introducing an additional layer of complexity. Simulations are performed with uniform thermal forcing and explicit convection. Focuses include statistical and structural analysis of the convective modes, process-oriented diagnostics of how they develop, and resulting mean states. Two regimes of SA emerge within the first 15 days, separated by a critical zone where f is analogous to 10°–15° latitude. Organized convection at near-equatorial values of f primarily consists of convectively coupled Kelvin waves. Wind speed–surface enthalpy flux feedbacks are the dominant process driving moisture variability early on, then clear-sky shortwave radiative feedbacks are strongest in wave maintenance. In contrast, at higher f, numerous tropical cyclones develop and coexist, dominated by surface flux and longwave processes. Tropical cyclogenesis is most pronounced at intermediate f (analogous to 25°–40°), but are longer-lived at higher f. The resulting modes of SA at low f differ between these β-plane simulations (convectively coupled waves) and prior f-plane simulations (weak tropical cyclones or nonrotating clusters). Otherwise, these results provide further evidence for the changing roles of radiative, surface flux, and advective processes in influencing SA as f changes, as found in our previous study. Significance Statement In model simulations, convection often self-organizes due to interactions with its surrounding environment. These interactions are relevant in the real-world organization of rainfall and clouds, and may thus be useful to understand for improved prediction of tropical weather and climate. Previous work using a set of simple model experiments with constant Coriolis force showed that at different latitudes, different processes dominate, and different types of organized convection result. This study verifies that finding using a more complex and realistic model, where the Coriolis force varies within the domain to resemble different latitudes. Specifically, the convection here self-organizes into atmospheric waves (periodic disturbances) at low latitudes, and tropical cyclones at high latitudes.

The Thermodynamics of Rotating Black Hole Star Clusters

Abstract Rotating star clusters near supermassive black holes are studied using Touma–Tremaine thermodynamics of gravitationally interacting orbital ellipses. A simple numerical procedure for calculating thermodynamic equilibrium states for an arbitrary distribution of stars over masses and semimajor axes is described. Spontaneous symmetry breaking and breakdown of thermodynamics at low positive temperatures are rigorously proven for nonrotating clusters. Rotation is introduced through a second temperature-like parameter. Both axially symmetric and lopsided rotational equilibria are found; the lopsided equilibria precess with the angular velocity that is given by the ratio of the two temperatures. The eccentric stellar disk in the nucleus of the Andromeda galaxy may be an example of a lopsided thermodynamic equilibrium of a rotating black hole star cluster. Stellar-mass black holes occupy highly eccentric orbits in broken-symmetry star clusters, and form flattened disklike configurations in rotating star clusters. They are attracted to orbits that are stationary in the frame of reference rotating with the angular velocity of the cluster. In spherical clusters, stellar-mass black holes’ orbits are significantly more eccentric than those of the lighter stars if the temperature is negative and more circular if the temperature is positive. Finally, we note that planets, comets, dark matter particles, and other light bodies tend to form a spherically symmetric nonrotating subcluster with maximum-entropy eccentricity distribution , even if their host cluster is rotating and lopsided.

Diffusion- and reaction-limited cluster aggregation revisited.

We simulated irreversible aggregation of non-interacting particles and of particles interacting via repulsive and attractive potentials explicitly implementing the rotational diffusion of aggregating clusters. Our study confirms that the attraction between particles influences neither the aggregation mechanism nor the structure of the aggregates, which are identical to those of non-interacting particles. In contrast, repulsive particles form more compact aggregates and their fractal dimension and aggregation times increase with the decrease of the temperature. A comparison of the fractal dimensions obtained for non-rotating clusters of non-interacting particles and for rotating clusters of repulsive particles provides an explanation for the conformity of the respective values obtained earlier in the well established model of diffusion-limited cluster aggregation neglecting rotational diffusion and in experiments on colloidal particles.

Melting transition in rotating sodium clusters

We investigate the possible influence of a global cluster rotation on the melting-like transition of Na$_{30}$, Na$_{55}$ and Na$_{147}$, by means of orbital-free density-functional molecular dynamics simulations. The results reveal that very large angular momenta L are needed to significantly alter the results of simulations performed on nonrotating clusters, and therefore validate the usual L=0 approximation made in computer simulations of cluster thermal properties. However, a global rotation might have a more profound influence on the thermal properties in the limit of very small cluster sizes, when the ratio of rotational to vibrational degrees of freedom is not negligible. For those clusters which are in a highly excited rotational state, we find enhanced isomerization rates and even depressed melting points.

Dynamical evolution of rotating stellar systems - III. The effect of the mass spectrum

We have studied the dynamical evolution of rotating star clusters with mass spectrum using a Fokker-Planck code. As the simplest multimass model, we first investigated two-component clusters. Rotation is found to accelerate the dynamical evolution through the transfer of angular momentum outward, as well as from high masses to low masses. However, the degree of acceleration depends sensitively on the assumed initial mass function since dynamical friction, which generates mass segregation, also tends to accelerate the evolution, and the combined effect of both is not linear or multiplicative. As long as dynamical friction dominates in the competition with angular momentum exchange, the heavy masses lose random energy and angular momentum and sink towards the centre, but their remaining angular momentum is sufficient to speed them up rotationally. This is gravo-gyro instability. As a consequence, we find that the high-mass stars in the central parts rotate faster than low-mass stars. This leads to the suppression of mass segregation compared with the non-rotating clusters. From the study of multicomponent models, we observe similar trends to the two-component models in almost all aspects. The mass function changes less drastically for clusters with rotation. Unlike non-rotating clusters, the mass function depends on R and z. Our models are the only ones that can predict mass function and other quantities to be compared with new observations.

Rotation/translation interplay in the recoil statistics of cluster evaporation

A new statistical model has been developed in the framework of Phase Space Theory to describe the evaporation process of non-rotating clusters. The novelty of the theoretical approach resides in its ability to easily separate the total kinetic energy released in an evaporation process into the rotational and translational contributions. This new model has been tested by comparing its predictions with the results of Molecular Dynamics (MD) simulations for the unimolecular evaporation of two types of van der Waals clusters: the aniline-(argon)n molecular clusters and the neat argon Arn clusters.