Search

(Abridged) Interacting galaxies are well-known for their high star formation rates and rich star cluster populations, but the rapidly changing tidal field can also efficiently destroy clusters. We use numerical simulations of merging disc galaxies to investigate which mechanism dominates. The simulations include a model for the formation and dynamical disruption of the entire star cluster population. We find that the dynamical heating of clusters by tidal shocks is about an order of magnitude higher in interacting galaxies than in isolated galaxies. This is driven by the increased gas density, and is sufficient to destroy star clusters at a higher rate than new clusters are formed: the total number of clusters in the merger remnant is 2-50% of the amount in the progenitor discs, with low-mass clusters being disrupted preferentially. By adopting observationally motivated selection criteria, we find that the observed surplus of star clusters in nearby merging galaxies is caused by the bias to detect young, massive clusters. We provide a general expression for the survival fraction of clusters, which increases with the gas depletion time-scale. Due to the preferential disruption of low-mass clusters, the mass distribution of the surviving star clusters in a merger remnant develops a peak at a mass of about 10^3 Msun, which evolves to higher masses at a rate of 0.3-0.4 dex per Gyr. The peak mass initially depends weakly on the galactocentric radius, but this correlation disappears as the system ages. We discuss the similarities between the cluster populations of the simulated merger remnants and (young) globular cluster systems. Our results suggest that the combination of cluster formation and destruction should be widespread in the dense star-forming environments at high redshifts, which could provide a natural origin to present-day globular cluster systems., 15 pages, 9 figures, 2 tables; Accepted for publication in MNRAS. A movie of the full time sequence in Figure 1 can be found at http://www.mpa-garching.mpg.de/~diederik/1m11clusters.html

Observations and theory both suggest that star clusters form sub-virial (cool) with highly sub-structured distributions. We perform a large ensemble of N-body simulations of moderate-sized (N=1000) cool, fractal clusters to investigate their early dynamical evolution. We find that cool, clumpy clusters dynamically mass segregate on a short timescale, that Trapezium-like massive higher-order multiples are commonly formed, and that massive stars are often ejected from clusters with velocities > 10 km/s (c.f. the average escape velocity of 2.5 km/s). The properties of clusters also change rapidly on very short timescales. Young clusters may also undergo core collapse events, in which a dense core containing massive stars is hardened due to energy losses to a halo of lower-mass stars. Such events can blow young clusters apart with no need for gas expulsion. The warmer and less substructured a cluster is initially, the less extreme its evolution., Accepted for publication in MNRAS; supplementary material can be downloaded at http://sgoodwin.staff.shef.ac.uk/all_plots.pdf.gz

The relation between the globular cluster luminosity function (GCLF, dN/dlogL) and globular cluster mass function (GCMF, dN/dlogM) is considered. Due to low-mass star depletion, dissolving GCs have mass-to-light (M/L) ratios that are lower than expected from their metallicities. This has been shown to lead to an M/L ratio that increases with GC mass and luminosity. We model the GCLF and GCMF and show that the power law slopes inherently differ (1.0 versus 0.7, respectively) when accounting for the variability of M/L. The observed GCLF is found to be consistent with a Schechter-type initial cluster mass function and a mass-dependent mass-loss rate., 4 pages, 2 figures. To appear in the proceedings of "Galaxy Wars: Stellar Populations and Star Formation in Interacting Galaxies" (Tennessee, July 2009)

We reconstruct the initial two-body relaxation time at the half mass radius for a sample of young $\aplt 300$ Myr star clusters in the large Magellanic cloud. We achieve this by simulating star clusters with 12288 to 131072 stars using direct $N$-body integration. The equations of motion of all stars are calculated with high precision direct $N$-body simulations which include the effects of the evolution of single stars and binaries. We find that the initial relaxation times of the sample of observed clusters in the large Magellanic cloud ranges from about 200 Myr to about 2 Gyr. The reconstructed initial half-mass relaxation times for these clusters has a much narrower distribution than the currently observed distribution, which ranges over more than two orders of magnitude., Comment: To appear in ``Mass loss from stars and the evolution of stellar clusters''. Proc. of a workshop held in honour of H.J.G.L.M. Lamers, Lunteren, The Netherlands. Eds. A. de Koter, L. Smith and R. Waters (San Francisco: ASP)

We present a new model for the Galactic population of close double white dwarfs. The model accounts for the suggestion of the avoidance of a substantial spiral-in during mass transfer between a giant and a main-sequence star of comparable mass and for detailed cooling models. It agrees well with the observations of the local sample of white dwarfs if the initial binary fraction is close to 50% and an ad hoc assumption is made that white dwarfs with mass less than about 0.3 solar mass cool faster than the models suggest. About 1000 white dwarfs brighter than V=15 have to be surveyed for detection of a pair which has total mass greater than the Chandrasekhar mass and will merge within 10 Gyr., 15 pages, 7 figures, to appear in Proc. ``The influence of binaries on stellar population studies'', Brussels, August 2000 (Kluwer, D. Vanbeveren ed.)

We study the occurrence of physical collisions between stars in young and compact star cluster. The calculations are performed on the GRAPE-4 with the starlab software environment which include the dynamical evolution and the nuclear evolution of all stars and binaries. The selection of the initial conditions is based on existing and well observed star clusters, such as R136 in the 30 Doradus region in the Large Magellanic Cloud and the Arches and Quintuplet star clusters in the vicinity of the Galactic center. Collisions between stars occurred rather frequently in our models. At any time a single star dominates the collision history of the system. The collision rate of this runaway merger scales with the initial relaxation time of the cluster and is independent on other cluster parameters, such as the initial mass function or the initial density profile of the cluster. Subsequent encounters result in a steady grow in mass of the coagulating star, until it escapes or explodes in a supernova. The collision rate in these models is about 0.00022 collisions per star per Myr for a cluster with an initial relaxation time of 1 Myr., Comment: 15 pages, to be published in 'Stellar Collisions, Mergers, and their Consequences', ASP Conference Series (ed. M. Shara)

We study the evolution and observability of young and compact star clusters near the Galactic center, such as the Arches cluster and the Quintuplet. The star clusters are modeled with a combination of techniques; using direct N-body, integration to calculate the motions of all stars and detailed stellar and binary evolution to follow the evolution of the stars. The modeled star clusters dissolve within 10 to 60 million years in the tidal field of the Galaxy. The projected stellar density in the modeled clusters drops within 5% to 70% of the lifetime to a level comparable to the projected background density towards the Galactic center. And it will be very hard to distinguished these clusters at later age among the background stars. This effect is more severe for clusters at larger distance from the Galactic center but in projection at the same distance. Based on these arguments we conclude that the Galactic center easily hides 10 to 40 clusters with characteristics similar to the Arches and the Quintuplet cluster., Comment: 7 pages, with 2 postscript figures, to appear in Dynamics of Star Clusters and the Milky Way, eds. R. Spurzem et al. (ASP Conference Series)

We discuss the formation and evolution of binaries which contain neutron stars or black holes. It is shown that in a stellar system which for $10^{10}$ yr had star formation rate similar to the current one in the Galactic disc, the rate of neutron star mergers is $\sim 2 \times 10^{-5} yr{-1}$, consistent with the observational estimates. The rate of black hole and neutron star mergers is by an order of magnitude lower. The pairs of black holes form but do not merge because heavy mass loss by their progenitors efficiently moves the binary components apart., 9 pages, 3 figures, LaTex (uses art10.sty, frontier.sty, psfig.sty). To appear in the proceedings of the "Second Workshop on Gravitational Wave Data Analysis", held 13-15 November 1997, Orsay, France

Three-body effects greatly complicate stellar evolution. We model the effects of encounters of binaries with single stars, based on parameters chosen from conditions prevalent in the cores of globular clusters. For our three-body encounters, we start with a population of primordial binaries, and choose incoming stars from the evolving single star populations of Portgies Zwart, Hut and Verbunt (1997). In addition, we study the formation of new binaries through tidal capture among the single stars. In subsequent papers in this series, we will combine stellar evolution with the dynamics of a full N-body system., 16 pages including 11 figures, Latex

The formation and evolution of binaries which contain two neutron stars or a neutron star with a black hole are discussed in detail. The evolution of the distributions in orbital period and eccentricity for neutron star binaries are studied as a function of time. In the model which fits the observations of high mass binary pulsars best the deposition of orbital energy into common envelopes has to be very efficient and a kick velocity distribution has to contain a significant contribution of low velocity kicks. The estimated age of the population has to be between several 100 Myr and 1 Gyr. The birthrate of binary neutron stars is about 3.4 10^{-5} per year (assuming 100% binarity) and their merger rate is about 2 x 10^{-5} per year. The merger rate of neutron star binaries is consistent with the estimated rate of gamma-ray bursts, if the latter are beamed into an opening angle of a few degrees. We argue that PSR B2303+46 is possibly formed in a scenario in which the common envelope is avoided while for the other three known high-mass binary pulsars a common envelope is required to explain their orbital period., Comment: 18 pages including 9 figures, Latex; Accepted for publication in A&A