The emerging state of open clusters upon their violent relaxation (original) (raw)
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Dynamics in Young Star Clusters: From Planets to Massive Stars
2011
The young star clusters we observe today are the building blocks of a new generation of stars and planets in our Galaxy and beyond. Despite their fundamental role we still lack knowledge about the conditions under which star clusters form and the impact of these often harsh environments on the evolution of their stellar and substellar members. We demonstrate the vital role numerical simulations play to uncover both key issues. Using dynamical models of different star cluster environments we show the variety of effects stellar interactions potentially have. Moreover, our significantly improved measure of mass segregation reveals that it can occur rapidly even for star clusters without substructure. This finding is a critical step to resolve the controversial debate on mass segregation in young star clusters and provides strong constraints on their initial conditions.
Oscillatory relaxation of a merging galaxy cluster
2006
Within the cosmic framework clusters of galaxies are relatively young objects. Many of them have recently experienced major mergers. Here we investigate an equal mass merging event at z ≈ 0.6 resulting in a dark matter haloe of ∼ 2.2 × 10 14 h −1 M ⊙ at z = 0. The merging process is covered by 270 outputs of a high resolution cosmological N-body simulation performed with the ART (adaptive refinement tree) code. Some 2 Gyrs elapse between the first peri-centre passage of the progenitor cores and their final coalescence. During that phase the cores experience six peri-centre passages with minimal distances declining from ∼ 30 to ∼ 2 h −1 kpc. The time intervals between the peri-centre passages continuously decrease from 9 to 1 × 10 8 yrs. We follow the mean density, the velocity dispersion and the entropy of the two progenitors within a set of fixed proper radii (25, 50, 100, 250, 500, 1000 h −1 kpc). During the peri-centre passages we find sharp peaks of the mean densities within these radii, which exceed the sum of the corresponding progenitor densities. In addition to the intermixing of the merging haloes, the densities increase due to contraction caused by the momentary deepening of the potential well. The velocity dispersions also peak during peri-centre passages. Within the fixed proper radii the entropy of the most massive progenitor after the merger settles close to its pre-merger values. At the end of the oscillatory relaxation phase the material originating from the less concentrated of the two equal mass progenitors is deposited at larger radii and shows a slightly more radially anisotropic velocity dispersion compared to the material coming from the more concentrated progenitor. Every peri-centre passage is accompanied by a substantial drop of the central potential well. We briefly discuss the possibility that AGN outbursts are triggered by the periodically changing potential.
2010
In this work, we derive the stellar initial mass function (IMF) from the superposition of mass distributions of dense cores, generated through gravoturbulent fragmentation of unstable clumps in molecular clouds (MCs) and growing through competitive accretion. MCs are formed by the turbulent cascade in the interstellar medium at scales L from 100 down to ~0.1 pc. Their internal turbulence is essentially supersonic and creates clumps with a lognormal distribution of densities n. Our model is based on the assumption of a power-law relationship between clump mass and clump density: n~m^x, where x is a scale-free parameter. Gravitationally unstable clumps are assumed to undergo isothermal fragmentation and produce protostellar cores with a lognormal mass distribution, centred around the clump Jeans mass. Masses of individual cores are then assumed to grow further through competitive accretion until the rest of the gas within the clump is being exhausted. The observed IMF is best reproduced for a choice of x=0.25, for a characteristic star formation timescale of ~5 Myr, and for a low star formation efficiency of ~10 %.
On the Origins of the Stellar Initial Mass Function
2015
In this reading, a new theoretical model of star and cluster formation is posited. This model seeks to set a mathematical framework to understand the origins of the stellar Initial Mass Function and within this framework, explain star and cluster formation from a unified perspective by tieing together into a single garment three important observational facts: (1) that the most massive stars of most observed clusters of stars are preferentially found in their centers; (2) Larson's 1982 empirical observation that the maximum stellar mass is related to the total mass of the parent cloud; (3) that clump masses in giant molecular clouds exhibit a power mass spectrum law akin to that found in star clusters and this behavior is also true for molecular clouds as well. Key to this model is the way the cloud fragments to form cores from which the new stars are born. We show that the recently proposed azimuthally symmetric theory of gravitation has two scale of fragmentation where one is the scale that leads to cloud collapse and the other is the scale on which the cloud fragments. The collapse and fragmentation takes place simultaneously. If the proposed model is anything to go by, then, one can safely posit that the slope of the IMF can be explained from two things: the star formation rate of the cores from which these stars form and the density index describing the density profile. Additionally and more importantly, if the present is anything to by, then, fragmentation of molecular clouds is posited as being a result of them possessing some spin angular momentum.
The evolution of two stellar populations in globular clusters I. The dynamical mixing timescale
2008
Aims. We investigate the long-term dynamical evolution of two distinct stellar populations of low-mass stars in globular clusters in order to study whether the energy equipartition process can explain the high number of stars harbouring abundance anomalies seen in globular clusters. Methods. We analyse N-body models by artificially dividing the low-mass stars (m ≤ 0.9 M) into two populations: a small number of stars (second generation) consistent with an invariant IMF and with low specific energies initially concentrated towards the clustercentre mimic stars with abundance anomalies. These stars form from the slow winds of fast-rotating massive stars. The main part of low-mass (first generation) stars has the pristine composition of the cluster. We study in detail how the two populations evolve under the influence of two-body relaxation and the tidal forces due to the host galaxy. Results. Stars with low specific energy initially concentrated toward the cluster centre need about two relaxation times to achieve a complete homogenisation throughout the cluster. For realistic globular clusters, the number ratio between the two populations increases only by a factor 2.5 due to the preferential evaporation of the population of outlying first generation stars. We also find that the loss of information on the stellar orbital angular momentum occurs on the same timescale as spatial homogenisation. Conclusions. To reproduce the high number of chemically anomalous stars in globular clusters by preserving an invariant IMF, more efficient mechanisms such as primordial gas expulsion are needed to expel the stars in the outer cluster parts on a short timescale.
Equilibrium Star Cluster Formation
The Astrophysical Journal, 2006
We argue that rich star clusters take at least several local dynamical times to form, and so are quasi-equilibrium structures during their assembly. Observations supporting this conclusion include morphologies of star-forming clumps, momentum flux of protostellar outflows from forming clusters, age spreads of stars in the Orion Nebula Cluster (ONC) and other clusters, and the age of a dynamical ejection event from the ONC. We show that these long formation timescales are consistent with the expected star formation rate in turbulent gas, as recently evaluated by Krumholz & McKee. Finally, we discuss the implications of these timescales for star formation efficiencies, the disruption of gas by stellar feedback, mass segregation of stars, and the longevity of turbulence in molecular clumps.
On the Initial Conditions for Star Formation and the Initial Mass Function
The Astrophysical Journal, 2011
Density probability distribution functions (PDFs) for turbulent self-gravitating clouds should be convolutions of the local log-normal PDF, which depends on the local average density ρ ave and Mach number M, and the PDFs for ρ ave and M, which depend on the overall cloud structure. When self-gravity drives a cloud to increased central density, the total PDF develops an extended tail. If there is a critical density or column density for star formation, then the fraction of the local mass exceeding this threshold becomes higher near the cloud center. These elements of cloud structure should be in place before significant star formation begins. Then the efficiency is high so that bound clusters form rapidly, and the stellar initial mass function (IMF) has an imprint in the gas before destructive radiation from young stars can erase it. The IMF could arise from a power-law distribution of mass for cloud structure. These structures should form stars down to the thermal Jeans mass M J at each density in excess of a threshold. The high-density tail of the PDF, combined with additional fragmentation in each star-forming core, extends the IMF into the brown dwarf regime. The core fragmentation process is distinct from the cloud structuring process and introduces an independent core fragmentation mass function (CFMF). The CFMF would show up primarily below the IMF peak.
Dynamical ejections of massive stars from young star clusters under diverse initial conditions
Astronomy & Astrophysics, 2016
We study the effects that initial conditions of star clusters and their massive star population have on dynamical ejections of massive stars from star clusters up to an age of 3 Myr. We use a large set of direct N-body calculations for moderately massive star clusters (M ecl ≈ 10 3.5 M). We vary the initial conditions of the calculations, such as the initial half-mass radius of the clusters, initial binary populations for massive stars and initial mass segregation. We find that the initial density is the most influential parameter for the ejection fraction of the massive systems. The clusters with an initial half-mass radius r h (0) of 0.1 (0.3) pc can eject up to 50% (30)% of their O-star systems on average, while initially larger (r h (0) = 0.8 pc) clusters, that is, lower density clusters, eject hardly any OB stars (at most ≈4.5%). When the binaries are composed of two stars of similar mass, the ejections are most effective. Most of the models show that the average ejection fraction decreases with decreasing stellar mass. For clusters that are efficient at ejecting O stars, the mass function of the ejected stars is top-heavy compared to the given initial mass function (IMF), while the mass function of stars that remain in the cluster becomes slightly steeper (top-light) than the IMF. The top-light mass functions of stars in 3 Myr old clusters in our N-body models agree well with the mean mass function of young intermediate-mass clusters in M 31, as reported previously. This implies that the IMF of the observed young clusters is the canonical IMF. We show that the multiplicity fraction of the ejected massive stars can be as high as ≈60%, that massive high-order multiple systems can be dynamically ejected, and that high-order multiples become common especially in the cluster. We also discuss binary populations of the ejected massive systems. Clusters that are initially not mass-segregated begin ejecting massive stars after a time delay that is caused by mass segregation. When a large kinematic survey of massive field stars becomes available, for instance through Gaia, our results may be used to constrain the birth configuration of massive stars in star clusters. The results presented here, however, already show that the birth mass-ratio distribution for O-star primaries must be near uniform for mass ratios q 0.1.
Stellar-Dynamics of Young Star Clusters
The stellar-dynamical evolution of bound star clusters during the first few Myr is dominated by binary-binary and binary-star interactions, the rapid sinking of the most massive stars to the centre of the clusters and mass loss from evolving stars. The consequences of these processes for the binary and stellar population in clusters, and for the star clusters as a whole, are studied by following the evolution over 150 Myr of a library of compact cluster models containing up to 10 4 stars.
The Stellar Initial Mass Function
If magnetic fields are frozen in gravitational collapse, the resulting magnetic tension can prevent the outer part of the subcritical envelope of a molecular cloud from falling in with the supercritical core. However, the implied surface magnetic fields much exceed measured values for young stars. Moreover, it is virtually impossible for Keplerian disks to form in these circumstances. Magnetic reconnection can eliminate the long lever arms of the split monopole formed by the gravitational collapse that contibutes to catastrophic magnetic braking. The natural appearance then of a Keplerian disk adjoining a rotating star with an outer convective envelope will lead to an X-wind driven magnetocentrifugally from the inner edge of the disk. This wind can cut off the continued infall from the envelope and build-up of the central stellar mass. We use these ideas and results to calculate the initial mass function and star formation efficiency for the distributed and clustered modes of star formation.