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Register a new userThe Influence of Mass-Loss from a Star Cluster on its Dynamical Friction -- I. Clusters without Internal Evolution
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Many Local Group dwarf spheroidal galaxies are found in the Galactic halo along great circles in the sky. Some of these stellar systems are thought to be the fragments of larger parent galaxies which have once intruded into and were torn apart by the tide of the Galaxy. Supporting evidences for tidal disruption are found in the form of stellar tidal bridges and tails along the orbits of some dwarf galaxies and globular clusters. In this study, we investigate the influence of mass-loss from star clusters or dwarf galaxies on the rate of their orbit decay due to the effect of dynamical friction. Using a series of numerical N-body simulations, we show that stars, which become unbound from their host-systems, but remain in their vicinity and share their orbits, still contribute to the mass responsible for the dynamical friction. As a rule-by-thumb, the magnitude of dynamical friction at any instance can be approximated by the bound mass plus half of the mass which has already become unbound during the proceeding Galactic orbit. Based on these results, we suggest the tidal disruption of relatively massive satellite stellar systems may be more abrupt than previously estimated.
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We describe the interplay between stellar evolution and dynamical mass loss of evolving star clusters, based on the principles of stellar evolution and cluster dynamics and on a grid of N-body simulations of cluster models. The cluster models have different initial masses, different orbits, including elliptical ones, and different initial density profiles. We use two sets of cluster models: initially Roche-lobe filling and Roche-lobe underfilling. We identify four distinct mass loss effects: (1) mass loss by stellar evolution, (2) loss of stars induced by stellar evolution and (3) relaxation-driven mass loss before and (4) after core collapse. Both the evolution-induced loss of stars and the relaxation-driven mass loss need time to build up. This is described by a delay-function of a few crossing times for Roche-lobe filling clusters and a few half mass relaxation times for Roche-lobe underfilling clusters. The relaxation-driven mass loss can be described by a simple power law dependence of the mass dM/dt =-M^{1-gamma}/t0, (with M in Msun) where t0 depends on the orbit and environment of the cluster. Gamma is 0.65 for clusters with a King-parameter W0=5 and 0.80 for more concentrated clusters with W0=7. For initially Roche-lobe underfilling clusters the dissolution is described by the same gamma=0.80. The values of the constant t0 are described by simple formulae that depend on the orbit of the cluster. The mass loss rate increases by about a factor two at core collapse and the mass dependence of the relaxation-driven mass loss changes to gamma=0.70 after core collapse. We also present a simple recipe for predicting the mass evolution of individual star clusters with various metallicities and in different environments, with an accuracy of a few percent in most cases. This can be used to predict the mass evolution of cluster systems.
We discuss the observational properties of a special class of objects (the so-called Blue Straggler Stars, BSSs) in the framework of using this stellar population as probe of the dynamical processes occurring in high-density stellar systems. Indeed, the shape of the BSS radial distribution and their level of central concentration are powerful tracers of the stage of dynamical evolution reached by the host cluster since formation. Hence, they can be used as empirical chronometers able to measure the dynamical age of stellar systems. In addition, the presence of a double BSS sequence in the color-magnitude diagram is likely the signature of the most extreme dynamical process occurring in globular cluster life: the core collapse event. Such a feature can therefore be used to reveal the occurrence of this process and, for the first time, even date it.
We investigate dynamical self-friction, the process by which material that is stripped from a subhalo torques its remaining bound remnant, which causes it to lose orbital angular momentum. By running idealized simulations of a subhalo orbiting within an analytical host halo potential, we isolate the effect of self-friction from traditional dynamical friction due to the host halo. While at some points in a subhalos orbit the torque of the stripped material can boost the orbital angular momentum of the remnant, the net effect over the long term is orbital decay regardless of the initial orbital parameters or subhalo mass. In order to quantify the strength of self-friction, we run a suite of simulations spanning typical host-to-subhalo mass ratios and orbital parameters. We find that the time-scale for self-friction, defined as the exponential decay time of the subhalos orbital angular momentum, scales with mass ratio and orbital circularity similar to standard dynamical friction. The decay time due to self-friction is roughly an order of magnitude longer, suggesting that self-friction only contributes at the 10 percent level. However, along more radial orbits, self-friction can occasionally dominate over dynamical friction close to pericentric passage, where mass stripping is intense. This is also the epoch at which the self-friction torque undergoes large and rapid changes in both magnitude and direction, indicating that self-friction is an important process to consider when modeling pericentric passages of subhaloes and their associated satellite galaxies.
The development and progress of the studies of winds and mass loss from hot stars, from about 1965 up to now, is discussed in a personal historical perspective. The present state of knowledge about stellar winds, based on papers presented at this workshop, is described. About ten years ago the mechanisms of the winds were reasonably well understood, the mass loss rates were known, and the predictions of stellar evolution theory with mass loss agreed with observations. However, recent studies especially those based on FUSE observations, have resulted in a significant reduction of the mass loss rates, that disagrees with predictions from radiation driven wind models. The situation is discussed and future studies that can clarify the situation are suggested. I also discuss what is known about the dissolution of star clusters in different environments. The dissolution time can be derived from the mass and age distributions of cluster samples. The resulting dissolution times of clusters in the solar neighborhood (SN) and in interacting galaxies are shorter than predicted by two-body relaxation of clusters in a tidal field. Encounters with giant molecular clouds can explain the fate of clusters in the SN and are the most likely cause of the short lifetime of clusters in interacting galaxies.
Internal friction in oxide thin films imposes a critical limitation to the sensitivity and stability of ultra-high finesse optical cavities for gravitational wave detectors. Strategies like doping or creating nanolaminates are sought to introduce structural modifications that reduce internal friction. This work describes an investigation of the morphological changes SiO2/Ta2O5 and TiO2/Ta2O5 nanolaminates undergo with annealing and their impact on room temperature internal friction. It is demonstrated that thermal treatment results in a reduction of internal friction in both nanolaminates, but through different pathways. In the SiO2/Ta2O5 nanolaminate, which layers remain intact after annealing, the total reduction in internal friction follows the reduction in the composing SiO2 and Ta2O5 layers. Instead, interdiffusion initiated by annealing at the interface of the TiO2/Ta2O5 nanolaminate and the formation of a mixed phase dictate a more significant reduction in internal friction to ~ 2.6 * 10-4, a value lower than any other Ta2O5 mixture coating with similar cation concentration.
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