arXiv subjects

Andreas Burkert

Publications and source records attributed to Andreas Burkert.

244 records · Page 14Linked to original sources

arXiv1999

Formation of Elliptical and S0 Galaxies by Close Encounters

We study a possible formation mechanism for elliptical/S0 galaxies using N-body simulations with GRAPE. A close galaxy encounter which does not lead to a merger can induce a strong bar in an initially axisymmetric disk. This bar is unstable against bending oscillations and loses angular momentum to the dark halo. After 6 Gyrs the remnant is spheroidal with a de Vaucouleurs-type surface profile and has ellipticities and rotation properties comparable to observed elliptical/S0 galaxies: the system rotates fast (v/sigma \approx 1) and has disky isophotes.

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arXiv1999

On the structure and nature of dark matter halos

The structure of dark matter halos as predicted from cosmological models is discussed and compared with observed rotation curves of dark matter-dominated dwarf galaxies. The theoretical models predict that dark matter halos represent a one-parameter family with a universal density profile. Observations of dark matter-dominated rotation curves indeed confirm the universal structure of dark halos. They are even in excellent agreement with the expected mass-radius scaling relations for the currently favoured cosmological model (standard cold dark matter with Omega_0=0.25 and Omega_{Lambda}=0.75). The rotation profiles however disagree with the predicted dark matter density distributions. Secular processes which might affect the inner halo structure do not seem to provide a good solution to this problem. We discuss, as an alternative, the possibility that dark halos consist of two separate components, a dark baryonic and a dark non-baryonic component.

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arXiv1998

Planet Consumption and Stellar Metallicity Enhancements

The evolution of a giant planet within the stellar envelope of a main-sequence star is investigated as a possible mechanism for enhancing the stellar metallicities of the parent stars of extrasolar planetary systems. Three-dimensional hydrodynamical simulations of a planet subject to impacting stellar matter indicate that the envelope of a Jupiter-like giant planet can be completely stripped in the outer stellar convection zone of a solar-mass star. In contrast, Jupiter-like and less massive Saturn-like giant planets are able to survive through the base of the convection zone of a 1.22 solar-mass star. Although strongly dependent on details of planetary interior models, partial or total dissolution of giant planets can result in significant enhancements in the metallicity of host stars with masses between about 1.0 and 1.3 solar masses. The implications of these results with regard to planetary orbital migration are briefly discussed.

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arXiv1998

Fragmentation of Molecular Clouds: The Initial Phase of a Stellar Cluster

The isothermal gravitational collapse and fragmentation of a region within a molecular cloud 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 interactions is described by a log-normal distribution with a width that is in excellent agreement with observations of multiple stellar systems.

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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

Double Core Evolution X. Through the Envelope Ejection Phase

The evolution of binary systems consisting of an asymptotic giant branch star of mass equal to 3 M_sun or 5 M_sun, and a main sequence star of mass equal to 0.4 M_sun or 0.6 M_sun with orbital periods > 200 days has been followed from the onset through the late stages of the common envelope phase. Using a nested grid technique, the three-dimensional hydrodynamical simulations of an asymptotic giant branch star with radii approximately 1 A.U. indicate that a significant fraction of the envelope gas is unbound (about 31% and 23% for binaries of 3 M_sun and 0.4 M_sun, and 5 M_sun and 0.6 M_sun respectively) by the ends of the simulations, and that the efficiency of the mass ejection process is about 40%. While the original volume of the giant is virtually evacuated in the late stages, most of the envelope gas remains marginally bound on the grid. At the ends of our simulations, when the orbital decay timescale exceeds about 5 years, the giant core and companion orbit one another with a period of about 1 day (2.4 days for a binary involving a more evolved giant), although this is an upper limit to the final orbital period. For a binary of 5 M_sun and 0.4 M_sun, the common envelope may not be completely ejected.

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arXiv1997

Kinetic Energy Decay Rates of Supersonic and Super-Alfvenic Turbulence in Star-Forming Clouds

We present numerical studies of compressible, decaying turbulence, with and without magnetic fields, with initial rms Alfven and Mach numbers ranging up to five, and apply the results to the question of the support of star-forming interstellar clouds of molecular gas. We find that, in 1D, magnetized turbulence actually decays faster than unmagnetized turbulence. In all the regimes that we have studied 3D turbulence-super-Alfvenic, supersonic, sub-Alfvenic, and subsonic-the kinetic energy decays as (t-t0)^(-x), with 0.85 < x < 1.2. We compared results from two entirely different algorithms in the unmagnetized case, and have performed extensive resolution studies in all cases, reaching resolutions of 256^3 zones or 350,000 particles. We conclude that the observed long lifetimes and supersonic motions in molecular clouds must be due to external driving, as undriven turbulence decays far too fast to explain the observations.

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arXiv1997

Fragmentation of Molecular Clouds: The Initial Phase of a Stellar Cluster

Using smoothed particle hydrodynamics in combination with the special-purpose hardware device GRAPE, we follow the fragmentation process and dynamical evolution in the interior of molecular clouds until most of the gas is converted to form a cluster of protostellar cores. The properties of this protostellar cluster are compared to observational data from star forming regions.

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arXiv1997

The structure of dark matter halos. Observation versus theory

The rotation curves of dark matter dominated dwarf galaxies are analysed. The observations show that dark matter halos represent a one-parameter family with self similar density profiles. The global halo parameters, like total mass and scale length are coupled by simple scaling relations. The inner halo regions resemble non-singular, isothermal spheres with constant density cores. The observations are compared with dark matter halos, resulting from cosmological cold dark matter simulations. The theoretical models predict that dark matter halos represent a one-parameter family in agreement with the observations. However, in contradiction to the observations, the calculations lead to dark matter halos with $r^{-1}$ density cusps in the center and non-isothermal velocity dispersion profiles. Processes which might affect the inner halo structure, resulting in isothermal, constant density cores are discussed.

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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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