The formation of a star cluster: predicting the properties of stars and brown dwarfs

By Matthew R. Bate, Ian A. Bonnell, Volker Bromm. 2002

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We present results from the largest numerical simulation of star formation to resolve the fragmentation process down to the opacity limit. The simulation follows the collapse and fragmentation of a large-scale turbulent molecular cloud to form a stellar cluster and, simultaneously, the formation of circumstellar discs and binary stars. This large range of scales enables us to predict a wide variety of stellar properties for comparison with observations. The calculation clearly demonstrates that star formation is a highly-dynamic and chaotic process. Star formation occurs in localized bursts within the cloud via the fragmentation both of dense molecular cloud cores and of massive circumstellar discs. Star-disc encounters form binaries and truncate discs. Stellar encounters disrupt bound multiple systems. We find that the observed statistical properties of stars are a natural consequence of star formation in such a dynamical environment. The cloud produces roughly equal numbers of stars and brown dwarfs, with masses down to the opacity limit for fragmentation (5 Jupiter masses). The initial mass function is consistent with a Salpeter slope (Γ = -1.35) above 0.5 M O ., a roughly flat distribution (r = 0) in the range 0.006-0.5 M O ., and a sharp cut-off below 0.005 M O .. This is consistent with recent observational surveys. The brown dwarfs form by the dynamical ejection of low-mass fragments from dynamically unstable multiple systems before the fragments have been able to accrete to stellar masses. Close binary systems (with separations ≤10 au) are not formed by fragmentation in situ. Rather, they are produced by hardening of initially wider multiple systems through a combination of dynamical encounters, gas accretion, and/or the interaction with circumbinary and circumtriple discs. Finally, we find that the majority of circumstellar discs have radii less than 20 au due to truncation in dynamical encounters. This is consistent with observations of the Orion Trapezium cluster and implies that most stars and brown dwarfs do not form large planetary systems.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/5575d76767c48a28bf18c425a4427cc826024c46

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Citations in the corpus

A SPITZER SURVEY OF YOUNG STELLAR CLUSTERS WITHIN ONE KILOPARSEC OF THE SUN: CLUSTER CORE EXTRACTION AND BASIC STRUCTURAL ANALYSIS (2009, methodology): “…sophisticated numerical simulations are now being used to examine these processes in detail (e.g., Klessen & Burkert 2000; Klessen et al. 2000; Bate et al. 2003; Bonnell et al. 2003; Gammie et al. 2003; Li et al. 2004); however, the predictions of these simulations are difficult to verify by…”
The dynamical state of stellar structure in star‐forming regions (2011, background): “Current advancements in numerical calculations of turbu- lent fragmentation in star-forming regions enable the study of clustered star formation in increasing detail (e.g. Bonnell et al. 1998; Klessen & Burkert 2000; Bate et al. 2003; Bonnell et al. 2008).”
The T Tauri Phase Down to Nearly Planetary Masses: Echelle Spectra of 82 Very Low Mass Stars and Brown Dwarfs (2005, methodology): “For example, in the simulations by Bate et al. (2003), brown dwarfs and very low mass stars are produced through ejection.”
Interpreting Spectral Energy Distributions from Young Stellar Objects. I. A Grid of 200,000 YSO Model SEDs (2006, background): “…truncation of the outer regions of disks in dense clusters due to stellar encounters or photoevaporation (see e.g. observations by Vicente & Alves 2005 who find that only ∼ 50% of disks in the Trapezium cluster are larger than 50AU; see also simulations of cluster formation by Bate et al. 2003).”
Formation of Globular Clusters in Hierarchical Cosmology (2003, background): “In the models of molecular clouds including thermal support, turbulence, and magnetic fields star formation proceeds rapidly, on one or two dynamical times (Pringle 1989; Ostriker et al. 2001; Bate et al. 2003).”
Cooling, Gravity, and Geometry: Flow-driven Massive Core Formation (2007, background): “While several investigations have adopted various assumed forms of initial and/or driven turbulent motions to produce the necessary small-scale structure (Klessen & Burkert 2000; Padoan & Nordlund 2002; Bate et al. 2002, 2003), it is preferable to have these structures arise naturally.”
New Brown Dwarfs and an Updated Initial Mass Function in Taurus (2004, result): “In a model of the fragmentation of a turbulent cloud, Bate et al. (2003) derived an IMF that was characterized by a power law at high masses and a flattening (in logarithmic units) somewhere below a solar mass into the substellar regime, which is consistent with the observations of star-forming…”
Fragmentation of massive protostellar discs (2006, background): “Moreover, the stellar densities in regions of HMSF are high, suggesting that other cluster stars might be close enough to interfere with disks stretching out to ∼ 100 AU (Bate et al. 2003).”
The hierarchical formation of a stellar cluster (2003, background): “The supersonic turbulence leads to the development of shocks in the gas, producing filamentary structures (Bate et al. 2003).”
Phantom: A Smoothed Particle Hydrodynamics and Magnetohydrodynamics Code for Astrophysics (2017, background): “The initial setup follows Bate et al. (2003) — a uniform density sphere of 0.375 pc in diameter with a mass of 50 M .”
THE SPATIAL STRUCTURE OF MONO-ABUNDANCE SUB-POPULATIONS OF THE MILKY WAY DISK (2011, background): “…flat between 20 and 100percent of the primary’s mass (Raghavan et al. 2010), with numerical simulations indicating that multiple-star systems form preferentially with approximately equal-mass members (Bate 2005), and an overall multiplicity fraction of around 40percent (Bate et al. 2003).”
Global hierarchical collapse in molecular clouds. Towards a comprehensive scenario (2019, background): “This is true even in simulations starting with turbulent fluctuations with kinetic energies comparable or even larger than the gravitational energy (e.g., Bate et al. 2003; Bate 2009b; Dale et al. 2012; Howard et al. 2018; Grudić et al. 2018).”
Scenarios for the Origin of the Orbits of the Trans-Neptunian Objects 2000 CR105 and 2003 VB12 (Sedna) (2004, background): “n. Therefore, the – 18 – encounter with a Solar-mass star at ∼ 800 AU at early times requires that the environment in which that Sun formed was signiﬁcantly denser, such as that of a stellar cluster (Bate et al., 2003). In the future the existence of 2000 CR105, 2003 VB12, and their cohorts my suppl”
Astrophysical smooth particle hydrodynamics (2009, methodology): “…of SPH, the symmetrization is imposed somewhat ad hoc, yet, it works well in practice and it is commonly used throughout astrophysics, see e.g. (Benz, 1990; Steinmetz, 1996; Klessen and Burkert, 2000; Bate et al., 2003; Wadsley et al., 2004; Rosswog and Davies, 2002; Wetzstein et al., 2008).”
Magnetic processes in a collapsing dense core II. Fragmentation. Is there a fragmentation crisis (2007, background): “Indeed, all simulations considering clouds evolving far from equilibrium (Bate et al. 2003, Ballesteros-Paredes et al. 2003, Hennebelle et al. 2006, Peretto et al. 2007) do find dense cores with initially strong perturbations.”