arXiv subjects

Ralf Klessen

Publications and source records attributed to Ralf Klessen.

64 records · Page 4Linked to original sources

arXiv2001

The mean surface density of companions in a stellar-dynamical context

Applying the mean surface density of companions, Sigma(r), to the dynamical evolution of star clusters is an interesting approach to quantifying structural changes in a cluster. It has the advantage that the entire density structure, ranging from the closest binary separations, over the core-halo structure through to the density distribution in moving groups that originate from clusters, can be analysed coherently as one function of the stellar separations r. This contribution assesses the evolution of Sigma(r) for clusters with different initial densities and binary populations. The changes in the binary, cluster and halo branches as the clusters evolve are documented using direct N-body calculations, and are correlated with the cluster core and half-mass radius. The location of breaks in the slope of Sigma(r) and the possible occurrence of a binary gap can be used to infer dynamical cluster properties.

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arXiv2001

The Formation of Stellar Clusters: Time Varying Protostellar Accretion Rates

Identifying the processes that determine strength, duration and variability of protostellar mass growth is a fundamental ingredient of any theory of star formation. I discuss protostellar mass accretion rates dM/dt from numerical models which follow molecular cloud evolution from turbulent fragmentation towards the formation of stellar clusters. In a dense cluster environment, dM/dt is strongly time varying and influenced by the mutual interaction of protostellar cores and their competition for accretion from the common cluster gas reservoir. Even for protostars with similar final mass, the accretion histories may differ dramatically. High-mass stars build up in the central parts of dense, cluster-forming cloud regions. They begin to form in the early phases of cluster evolution, and continue to grow at a high rate until the available gas is exhausted or expelled by feedback. Lower-mass stars tend to form at later phases, and dM/dt declines rapidly after a short initial phase of strong growth. I present a simple fit formula for the time evolution of the average dM/dt for protostars of different masses in a dense cluster environment.

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arXiv2000

Star Formation from Turbulent Fragmentation

Star formation is intimately linked to the dynamical evolution of molecular clouds. Turbulent fragmentation determines where and when protostellar cores form, and how they contract and grow in mass via accretion from the surrounding cloud material. Efficiency, spatial distribution and timescale of star formation in turbulent clouds are estimated by comparing numerical models of self-gravitating isothermal gas where turbulence is assumed to have decayed or is driven at supersonic speed on various wavelengths. Turbulence that is not continuously replenished or that is driven on large scales leads to a rapid formation of stars in a clustered mode, whereas interstellar turbulence that carries most energy on small scales results in isolated star formation with low efficiency. Protostellar accretion rates strongly vary in time and peak at a few 10^-5 M_sun/year. The clump mass spectrum for models of pure hydrodynamic turbulence is steeper than the observed one, but becomes comparable when including gravity. The mass spectrum of dense cores, on the other hand, is log-normal for decaying and large-wavelength turbulence, similar to the IMF, but is to flat in the case of small-scale turbulence.

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arXiv2000

The Formation of Stellar Clusters: Gaussian Initial Conditions II

Using hydrodynamic simulations we investigate the time evolution and fragmentation of regions within molecular clouds which have lost their turbulent support leading to gravitational contraction. The initial density distributions are described by random Gaussian fluctuations with varying slopes x of the power spectrum P(k) ~ k^-x, covering the range from flat (x=0) to very steep spectra (x=3). We consider molecular cloud volumes containing different masses relative to the average Jeans mass M_J, from 1 M_J to 222 M_J. This parameter study extends the detailed analysis of systems with initially P(k) ~ k^-2 and mass 222 M_J presented by Klessen & Burkert (2000). The dynamical evolution of the simulated molecular cloud regions is insensitive to the slope of the initial density fluctuation spectrum. The system evolves into a complicated network of intersecting filaments and collapsing clumps leading to the formation of a compact cluster of accreting and interacting embedded protostellar cores. The cluster builds up as bound entity, but dissolves later due to collisional effects. In all simulations, the mass spectrum of collapsed cores is very broad, has approximately log-normal shape and peaks roughly at the average Jeans mass. This supports the hypothesis that the average Jeans mass is the main parameter determining the peak in the stellar spectrum, and suggests that the interplay between self-gravity on the one side and thermal and turbulent pressure on the other side is the dominant process that regulates the formation of stellar clusters.

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arXiv1999

MHD Turbulence in Star-Forming Clouds

We review how supersonic turbulence can both prevent and promote the collapse of molecular clouds into stars. First we show that decaying turbulence cannot significantly delay collapse under conditions typical of molecular clouds, regardless of magnetic field strength so long as the fields are not supporting the cloud magnetohydrostatically. Then we review possible drivers and examine simulations of driven supersonic and trans Alfvénic turbulence, finally including the effects of self-gravity. Our preliminary results show that, although turbulence can support regions against gravitational collapse, the strong compressions associated with the required velocities will tend to promote collapse of local condensations.

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arXiv1998

Fragmentation in Molecular Clouds: The Formation of a Stellar Cluster

The isothermal gravitational collapse and fragmentation of a molecular cloud region and the subsequent formation of a protostellar cluster is investigated numerically. The clump mass spectrum which forms during the fragmentation phase can be well approximated by a power law distribution dN/dM ~ M^(-1.5). In contrast, the mass spectrum of protostellar cores that form in the centers of Jeans unstable clumps and evolve through accretion and N-body interaction is best described by a log-normal distribution. Assuming a star formation efficiency of ~10%, it is in excellent agreement with the IMF of multiple stellar systems.

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arXiv1998

A Model for Dwarf Spheroidal Satellite Galaxies without Dark Matter

Self-consistent simulations of the dynamical evolution of low-mass satellite galaxies without dark matter on different orbits interacting with an extended Galactic dark halo are described. The calculations proceed for many orbital periods until well after the satellite dissolves. In all cases the dynamical evolution converges to a remnant that contains roughly 1% of the initial satellite mass. The stable remnant results from severe tidal shaping of the initial satellite. To an observer from Earth these remnants look strikingly similar to the Galactic dwarf spheroidal satellite galaxies. Their apparent mass-to-light ratios are very large despite the fact that they contain no dark matter. These computations show that a remnant without dark matter displays larger line-of-sight velocity dispersions for more eccentric orbits, which is a result of projection onto the observational plane. Assuming they are not dark matter dominated, it follows that the Galactic dSph satellites with sigma>6km/s should have orbital eccentricities of e>0.5. Some remnants have sub-structure along the line-of-sight that may be apparent in the morphology of the horizontal branch.

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arXiv1997

Dwarf Spheroidal Satellite Galaxies Without Dark Matter: Results From Two Different Numerical Techniques

Self-consistent simulations of the dynamical evolution of a low-mass satellite galaxy without dark matter are reported. The orbits have eccentricities 0.46 <= e <= 0.96 in a Galactic dark halo resembling our Milky Way. For the simulations, a particle-mesh code with nested sub-grids and a direct-summation N-body code running with the special purpose hardware device GRAPE are used. Initially, the satellite is spherical with an isotropic velocity distribution, and has a mass of 10^7 solar masses. Simulations with 1.3x10^5 up to 2x10^6 satellite particles are performed. The calculations proceed for many orbital periods until well after the satellite disrupts. In all cases the dynamical evolution converges to a remnant that contains roughly 1% of the initial satellite mass. The stable remnant results from severe tidal shaping of the initial satellite. To an observer from Earth these remnants look strikingly similar to the Galactic dwarf spheroidal satellite galaxies. Their apparent mass-to-light ratios are very large despite the fact that they contain no dark matter. These computations show that a remnant without dark matter displays larger line-of-sight velocity dispersions, sigma, for more eccentric orbits, which is a result of projection onto the observational plane. Assuming they are not dark matter dominated, it follows that the Galactic dSph satellites with sigma > 6km/s should have orbital eccentricities of e > 0.5. Some remnants have sub-structure along the line-of-sight that may be apparent in the morphology of the horizontal branch.

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arXiv1997

GRAPESPH with Fully Periodic Boundary Conditions: Fragmentation of Molecular Clouds

A method of adapting smoothed particle hydrodynamics (SPH) with periodic boundary conditions for use with the special purpose device GRAPE is presented. GRAPE (GRAvity PipE) solves the Poisson and force equations for an N-body system by direct summation on a specially designed chip and in addition returns the neighbour list for each particle. Due to its design, GRAPE cannot treat periodic particle distributions directly. This limitation of GRAPESPH can be overcome by computing a correction force for each particle due to periodicity (Ewald correction) on the host computer using a PM-like method. This scheme is applied to study the fragmentation process in giant molecular clouds. Assuming a pure isothermal model, we follow the dynamical evolution in the interior of a molecular cloud starting from an Gaussian initial density distribution to the formation of selfgravitating clumps until most of the gas is consumed in these dense cores. Despite its simplicity, this model can reproduce some fundamental properties of observed molecular clouds, like a clump mass distribution of the form $dN/dm \propto m^n$, with $n ~ -1.5$.

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arXiv1995

Constraints on massive black holes as dark matter candidates using Galactic globular clusters

This work considers the idea of massive black holes being the constituents of the Galactic dark matter halo. It constrains the maximum black hole mass to $\mbh \sil 5 \times 10^4 \msun$ by examining their influence on the population of globular clusters in our Milky Way. In the adopted halo model, globular clusters are exposed to constant bombardment of halo objects on their orbits through the Galaxy and thus will steadily gain internal energy. Depending on the mass of these halo objects and the structural parameters of the globular clusters, they can be disrupted on time scales of a few billion years and below. These disruption time scales are calculated using a modification of the well known (classical) impulsive approximation and compared with direct N-body simulations of such encounter events to ensure the method works correctly. For a set of ten prototypical globular cluster models and black hole masses ranging from $10^3$ to $10^7\:\msun$, Monte-Carlo-simulations of $10\,000$ encounter histories over the period of 10 billion years were calculated each, at three different galactocentric distances \mbox{$R = 5$}, $10$ and $15\, \kpc$. These data were compared with the real globular cluster population in our Galaxy and used to obtain the above constraint of $\mbh \sil 5 \times 10^4 \msun$.

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