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Register a new userOn the velocity dispersion of young star clusters: super-virial or binaries?
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M. Gieles
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Many young extra-galactic clusters have a measured velocity dispersion that is too high for the mass derived from their age and total luminosity, which has led to the suggestion that they are not in virial equilibrium. Most of these clusters are confined to a narrow age range centred around 10 Myr because of observational constraints. At this age the cluster light is dominated by luminous evolved stars, such as red supergiants, with initial masses of ~13-22 Msun for which (primordial) binarity is high. In this study we investigate to what extent the observed excess velocity dispersion is the result of the orbital motions of binaries. We demonstrate that estimates for the dynamical mass of young star clusters, derived from the observed velocity dispersion, exceed the photometric mass by up-to a factor of 10 and are consistent with a constant offset in the square of the velocity dispersion. This can be reproduced by models of virialised star clusters hosting a massive star population of which ~25 is in binaries, with typical mass ratios of ~0.6 and periods of ~1000 days. We conclude that binaries play a pivotal role in deriving the dynamical masses of young (~10 Myr) moderately massive and compact (<1e5 Msun; > 1 pc) star clusters.
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The majority of massive stars ($>8$ $rm{M_{odot}}$) in OB associations are found in close binary systems. Nonetheless, the formation mechanism of these close massive binaries is not understood yet. Using literature data, we measured the radial-velocity dispersion ($sigma_mathrm{RV}$) as a proxy for the close binary fraction in ten OB associations in the Galaxy and the Large Magellanic Cloud, spanning an age range from 1 to 6 Myrs. We find a positive trend of this dispersion with the clusters age, which is consistent with binary hardening. Assuming a universal binary fraction of $f_mathrm{bin}$ = 0.7, we converted the $sigma_mathrm{RV}$ behavior to an evolution of the minimum orbital period $P_mathrm{cutoff}$ from $sim$9.5 years at 1 Myr to $sim$1.4 days for the oldest clusters in our sample at $sim$6 Myr. Our results suggest that binaries are formed at larger separations, and they harden in around 1 to 2 Myrs to produce the period distribution observed in few million year-old OB binaries. Such an inward migration may either be driven by an interaction with a remnant accretion disk or with other young stellar objects present in the system. Our findings constitute the first empirical evidence in favor of migration as a scenario for the formation of massive close binaries.
Based on our recent work on tidal tails of star clusters (Kuepper et al. 2009) we investigate star clusters of a few 10^4 Msun by means of velocity dispersion profiles and surface density profiles. We use a comprehensive set of $N$-body computations of star clusters on various orbits within a realistic tidal field to study the evolution of these profiles with time, and ongoing cluster dissolution From the velocity dispersion profiles we find that the population of potential escapers, i.e. energetically unbound stars inside the Jacobi radius, dominates clusters at radii above about 50% of the Jacobi radius. Beyond 70% of the Jacobi radius nearly all stars are energetically unbound. The velocity dispersion therefore significantly deviates from the predictions of simple equilibrium models in this regime. We furthermore argue that for this reason this part of a cluster cannot be used to detect a dark matter halo or deviations from Newtonian gravity. By fitting templates to the about 10^4 computed surface density profiles we estimate the accuracy which can be achieved in reconstructing the Jacobi radius of a cluster in this way. We find that the template of King (1962) works well for extended clusters on nearly circular orbits, but shows significant flaws in the case of eccentric cluster orbits. This we fix by extending this template with 3 more free parameters. Our template can reconstruct the tidal radius over all fitted ranges with an accuracy of about 10%, and is especially useful in the case of cluster data with a wide radial coverage and for clusters showing significant extra-tidal stellar populations. No other template that we have tried can yield comparable results over this range of cluster conditions. All templates fail to reconstruct tidal parameters of concentrated clusters, however. (abridged)
We study the compact binary population in star clusters, focusing on binaries containing black holes, using a self-consistent Monte Carlo treatment of dynamics and full stellar evolution. We find that the black holes experience strong mass segregation and become centrally concentrated. In the core the black holes interact strongly with each other and black hole-black hole binaries are formed very efficiently. The strong interactions, however, also destroy or eject the black hole-black hole binaries. We find no black hole-black hole mergers within our simulations but produce many hard escapers that will merge in the galactic field within a Hubble time. We also find several highly eccentric black hole-black hole binaries that are potential LISA sources, suggesting that star clusters are interesting targets for space-based detectors. We conclude that star clusters must be taken into account when predicting compact binary population statistics.
The early evolution of a dense young star cluster (YSC) depends on the intricate connection between stellar evolution and dynamical processes. Thus, N-body simulations of YSCs must account for both aspects. We discuss N-body simulations of YSCs with three different metallicities (Z=0.01, 0.1 and 1 Zsun), including metallicity-dependent stellar evolution recipes and metallicity-dependent prescriptions for stellar winds and remnant formation. We show that mass-loss by stellar winds influences the reversal of core collapse. In particular, the post-collapse expansion of the core is faster in metal-rich YSCs than in metal-poor YSCs, because the former lose more mass (through stellar winds) than the latter. As a consequence, the half-mass radius expands more in metal-poor YSCs. We also discuss how these findings depend on the total mass and on the virial radius of the YSC. These results give us a clue to understand the early evolution of YSCs with different metallicity.
We have carried out a search for massive white dwarfs (WDs) in the direction of young open star clusters using the Gaia DR2 database. The aim of this survey was to provide robust data for new and previously known high-mass WDs regarding cluster membership, to highlight WDs previously included in the Initial Final Mass Relation (IFMR) that are unlikely members of their respective clusters according to Gaia astrometry and to select an unequivocal WD sample that could then be compared with the host clusters turnoff masses. All promising WD candidates in each cluster CMD were followed up with spectroscopy from Gemini in order to determine whether they were indeed WDs and derive their masses, temperatures and ages. In order to be considered cluster members, white dwarfs were required to have proper motions and parallaxes within 2, 3, or 4-$sigma$ of that of their potential parent cluster based on how contaminated the field was in their region of the sky, have a cooling age that was less than the cluster age and a mass that was broadly consistent with the IFMR. A number of WDs included in curre
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