The Dependence of Star Cluster Formation on the Initial `Turbulent' Power Spectrum
Matthew R. Bate
This was the four large-scale calculation to be performed. The initial conditions were identical to those in the original calculation, except that the `turbulent' velocity field imposed on the cloud had a different power spectrum P(k) \propto k-6 which has more power in large scale gas motions than the other calculations (which used P(k) \propto k-4). The aim was to determine whether changes in the initial velocity field (which is somewhat arbitrary because the initial conditions in molecular clouds are not well understood) had an impact on the star formation or not. It is found that the stellar mass distribution (the so-called initial mass function, IMF) was statistically identical to the IMF from the original calculation showing that the initial power spectrum is unimportant.
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Simulation & visualisation by Matthew Bate, University of Exeter unless stated otherwise.
Notes on formats:
The calculation models the collapse and fragmentation of a 50 solar mass molecular cloud that is 0.375 pc in diameter (approximately 1.2 light-years). At the initial temperature of 10 K with a mean molecular weight of 2.46, this results in an thermal Jeans mass of 1 solar mass. The free-fall time of the cloud is 190,000 years and the simulation covers 266,000 years.
The cloud is given an initial supersonic `turbulent' velocity field in the same manner as Ostriker, Stone & Gammie (2001). We generate a divergence-free random Gaussian velocity field with a power spectrum P(k) \propto k-6, where k is the wave-number. This power spectrum is much steeper than the Kolmogorov spectrum, P(k)\propto k11/3 and the Burgers supersonic turbulence power spectrum.
The calculation was performed using a parallel three-dimensional smoothed particle hydrodynamics (SPH) code with 3.5 million particles on the United Kingdom Astrophysical Fluids Facility (UKAFF). It took approximately 80000 CPU hours running on up to 64 processors. The SPH code was parallelised using OpenMP by M. Bate. The code uses sink particles (Bate, Bonnell & Price 1995) to model condensed objects (i.e. the stars and brown dwarfs). Sink particles are point masses that accrete bound gas that comes within a specified radius of them. This accretion radius is to set 5 AU. Thus, the calculation resolves circumstellar discs with radii down to approximately 10 AU. Binary systems are followed to separations as small as 1 AU.
Refereed Scientific Papers
"The dependence of star formation on initial conditions and molecular cloud structure" Bate, M. R., 2009, MNRAS, 397, 232. ( Local PDF preprint or preprint from astro-ph/0905.3562 )
High Resolution Still Images and Commentary
High resolution, unannotated (1800x1800 pixel) versions that are suitable for publication are available on request by emailing Matthew Bate at: mbate @ astro.ex.ac.uk
Click on the images below to view medium resolution, annotated (600x600 pixel) versions.
Copyright: Matthew Bate, University of Exeter.