The Early Evolution of Dense Stellar Systems (original) (raw)
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The Astrophysical Journal, 2011
One model for the origin of typical Galactic star clusters such as the Orion Nebula Cluster (ONC) is that they form via the rapid, efficient collapse of a bound gas clump within a larger, gravitationally unbound giant molecular cloud. However, simulations in support of this scenario have thus far not included the radiation feedback produced by the stars; radiative simulations have been limited to significantly smaller or lower-density regions. Here we use the ORION AMR code to conduct the first ever radiation-hydrodynamic simulations of the global collapse scenario for the formation of an ONC-like cluster. We show that radiative feedback has a dramatic effect on the evolution: once the first ∼10%-20% of the gas mass is incorporated into stars, their radiative feedback raises the gas temperature high enough to suppress any further fragmentation. However, gas continues to accrete onto existing stars, and, as a result, the stellar mass distribution becomes increasingly top-heavy, eventually rendering it incompatible with the observed initial mass function (IMF). Systematic variation in the location of the IMF peak as star formation proceeds is incompatible with the observed invariance of the IMF between star clusters, unless some unknown mechanism synchronizes the IMFs in different clusters by ensuring that star formation is always truncated when the IMF peak reaches a particular value. We therefore conclude that the global collapse scenario, at least in its simplest form, is not compatible with the observed stellar IMF. We speculate that processes that slow down star formation, and thus reduce the accretion luminosity, may be able to resolve the problem.
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.
On the formation of massive stellar clusters
Astronomy & Astrophysics, 2003
Here we model a star forming factory in which the continuous creation of stars results in a highly concentrated, massive (globular cluster-like) stellar system. We show that under very general conditions a large-scale gravitational instability in the ISM, which triggers the collapse of a massive cloud, leads with the aid of a spontaneous first generation of massive stars, to a standing, small-radius, cold and dense shell. Eventually, as more of the collapsing matter is processed and incorporated, the shell becomes gravitationally unstable and begins to fragment, allowing the formation of new stars, while keeping its location. This is due to a detailed balance established between the ram pressure from the collapsing cloud which, together with the gravitational force exerted on the shell by the forming cluster, acts against the mechanical energy deposited by the collection of new stars. We present a full analysis of feedback and show how the standing shell copes with the increasing mechanical energy generated by an increasing star-formation rate. The latter also leads to a rapidly growing number of ionizing photons, and we show that these manage to ionize only the inner skin of the standing star-forming shell. We analyze the mass spectrum of fragments that result from the continuous fragmentation of the standing shell and show that its shape is well approximated at the high mass end by a power law with slope −2.25, very close to the value that fits the universal IMF. Furthermore, it presents a maximum near to one solar mass and a rapid change towards a much flatter slope for smaller fragments. The self-contamination resultant from the continuous generation of stars is shown to lead to a large metal spread in massive (∼10 6 M) clusters, while clusters with a mass similar to 10 5 M or smaller, simply reflect the initial metalicity of the collapsing cloud. This is in good agreement with the data available for globular clusters in the Galaxy. Other observables such as the expected IR luminosity and the H α equivalent width caused by the forming clusters are also calculated.
Monthly Notices of the Royal Astronomical Society
We present new griffin project hydrodynamical simulations that model the formation of galactic star cluster populations in low-metallicity (Z = 0.00021) dwarf galaxies, including radiation, supernova, and stellar wind feedback of individual massive stars. In the simulations, stars are sampled from the stellar initial mass function (IMF) down to the hydrogen-burning limit of 0.08 M⊙. Mass conservation is enforced within a radius of 1 pc for the formation of massive stars. We find that massive stars are preferentially found in star clusters and follow a correlation set at birth between the highest initial stellar mass and the star cluster mass that differs from pure stochastic IMF sampling. With a fully sampled IMF, star clusters lose mass in the galactic tidal field according to mass-loss rates observed in nearby galaxies. Of the released stellar feedback, 60 per cent of the supernova material and up to 35 per cent of the wind material reside either in the hot interstellar medium (IS...
Monthly Notices of the Royal Astronomical Society, 2014
We present a model for the radiative output of star clusters in the process of star formation suitable for use in hydrodynamical simulations of radiative feedback. Gas in a clump, defined as a region whose density exceeds 10 4 cm −3 , is converted to stars via the random sampling of the Chabrier IMF. A star formation efficiency controls the rate of star formation. We have completed a suite of simulations which follow the evolution of accretion-fed clumps with initial masses ranging from 0 to 10 5 M ⊙ and accretion rates ranging from 10 −5 to 10 −1 M ⊙ yr −1. The stellar content is tracked over time which allows the aggregate luminosity, ionizing photon rate, number of stars, and star formation rate (SFR) to be determined. For a fiducial clump of 10 4 M ⊙ , the luminosity is ∼4×10 6 L ⊙ with a SFR of roughly 3×10 −3 M ⊙ yr −1. We identify two regimes in our model. The accretion-dominated regime obtains the majority of its gas through accretion and is characterized by an increasing SFR while the reservoirdominated regime has the majority of its mass present in the initial clump with a decreasing SFR. We show that our model can reproduce the expected number of O stars, which dominate the radiative output of the cluster. We find a nearly linear relationship between SFR and mass as seen in observations. We conclude that our model is an accurate and straightforward way to represent the output of clusters in hydrodynamical simulations with radiative feedback.
The Astrophysical Journal, 2000
We present the analysis of a suite of simulations of a Virgo mass galaxy cluster. Undertaken within the framework of standard cold dark matter cosmology, these simulations were performed at differing resolutions and with increasingly complex physical processes, with the goal of identifying the effects of each on the evolution of the cluster. We focus on the cluster at the present epoch and examine properties including the radial distributions of density, temperature, entropy and velocity. We also map 'observable' projected properties such as the surface mass density, X-ray surface brightness and Sunyaev-Zel'dovich signature.
Dynamical Interactions in Dense Stellar Clusters
This chapter 1 reviews the dynamical processes that occur in young stellar clusters. The formation of a stellar cluster is a complex process whereby hundreds to thousands of stars form near-simultaneously in a bound configuration. We discuss the dynamical processes involved in the formation and early evolution of a stellar cluster.
Star-cluster formation and evolution
Proceedings of the International Astronomical Union, 2006
Star clusters are observed to form in a highly compact state and with low starformation efficiencies, and only 10 per cent of all clusters appear to survive to middle-and old-dynamical age. If the residual gas is expelled on a dynamical time the clusters disrupt. Massive clusters may then feed a hot kinematical stellar component into their host-galaxy's field population thereby thickening galactic disks, a process that theories of galaxy formation and evolution need to accommodate. If the gas-evacuation time-scale depends on cluster mass, then a power-law embedded-cluster mass function may transform within a few dozen Myr to a mass function with a turnover near 10 5 M , thereby possibly explaining this universal empirical feature. Discordant empirical evidence on the mass function of star clusters leads to the insight that the physical processes shaping early cluster evolution remain an issue of cutting-edge research.
The early dynamical evolution of cool, clumpy star clusters
Monthly Notices of …, 2010
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 −1 (c.f. the average escape velocity of 2.5 km s −1). 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.
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.