87 research outputs found
Numerical simulations of dynamical mass transfer in binaries
We present results from investigations of mass transfer instability in close binary star systems. By unstable mass transfer we mean the exchange of material where the response of the binary to the initial Roche lobe overflow causes the donor to loose even more material. Our work is guided by approximate arguments that dictate the stability boundaries for binary star systems. To proceed further one must explicitly treat extended mass and velocity distributions that are both nitially, and through their subsequent evolution in time, self-consistent. In this dissertation, we present the first three-dimensional, fully self-consistent treatment of mass transfer in close binary systems. To perform these calculations we have developed and tested a set of tools including a Self-Consistent Field code for generating polytropic binaries executing synchronous rotation upon circular orbits and a parallel, gravitational hydrodynamics code for evolving the binaries in time. We describe, in detail, these tools and their application to the evolution of binary star systems. We present extended simulations of two detached binaries that have been used to examine the accuracy of our computational techniques in addition to the simulations of interacting binaries
Simulated Versus Observed Cluster Eccentricity Evolution
The rate of galaxy cluster eccentricity evolution is useful in understanding
large scale structure. Rapid evolution for 0.13 has been found in two
different observed cluster samples. We present an analysis of projections of 41
clusters produced in hydrodynamic simulations augmented with radiative cooling
and 43 clusters from adiabatic simulations. This new, larger set of simulated
clusters strengthens the claims of previous eccentricity studies. We find very
slow evolution in simulated clusters, significantly different from the reported
rates of observational eccentricity evolution. We estimate the rate of change
of eccentricity with redshift and compare the rates between simulated and
observed clusters. We also use a variable aperture radius to compute the
eccentricity, r. This method is much more robust than the fixed
aperture radius used in previous studies. Apparently radiative cooling does not
change cluster morphology on scales large enough to alter eccentricity. The
discrepancy between simulated and observed cluster eccentricity remains.
Observational bias or incomplete physics in simulations must be present to
produce halos that evolve so differently.Comment: ApJ, in press, minor revision
Morphology and Evolution of Simulated and Optical Clusters: A Comparative Analysis
We have made a comparative study of morphological evolution in simulated DM
halos and X-ray brightness distribution, and in optical clusters. Samples of
simulated clusters include star formation with supernovae feedback, radiative
cooling, and simulation in the adiabatic limit at three different redshifts, z
= 0.0, 0.10, and 0.25. The optical sample contains 208 ACO clusters within
redshift, . Cluster morphology, within 0.5 and 1.0 h Mpc
from cluster center, is quantified by multiplicity and ellipticity.
We find that the distribution of the dark matter halos in the adiabatic
simulation appear to be more elongated than the galaxy clusters. Radiative
cooling brings halo shapes in excellent agreement with observed clusters,
however, cooling along with feedback mechanism make the halos more flattened.
Our results indicate relatively stronger structural evolution and more clumpy
distributions in observed clusters than in the structure of simulated clusters,
and slower increase in simulated cluster shapes compared to those in the
observed one.
Within , we notice an interesting agreement in the shapes of
clusters obtained from the cooling simulations and observation. We also notice
that the different samples of observed clusters differ significantly in
morphological evolution with redshift. We highlight a few possibilities
responsible for the discrepancy in morphological evolution of simulated and
observed clusters.Comment: Accepted for publication in MNRAS, 2006; 15 pages, 13 postscript
figure
A Numerical Method for Generating Rapidly Rotating Bipolytropic Structures in Equilibrium
We demonstrate that rapidly rotating bipolytropic (composite polytropic)
stars and toroidal disks can be obtained using Hachisu's self consistent field
technique. The core and the envelope in such a structure can have different
polytropic indices and also different average molecular weights. The models
converge for high cases, where T is the kinetic energy and W is the
gravitational energy of the system. The agreement between our numerical
solutions with known analytical as well as previously calculated numerical
results is excellent. We show that the uniform rotation lowers the maximum core
mass fraction or the Schnberg-Chandrasekhar limit for a
bipolytropic sequence. We also discuss the applications of this method to
magnetic braking in low mass stars with convective envelopes
Numerical Simulations of Mass Transfer in Binaries with Bipolytropic Components
We present the first self-consistent, three dimensional study of hydrodynamic
simulations of mass transfer in binary systems with bipolytropic (composite
polytropic) components. In certain systems, such as contact binaries or during
the common envelope phase, the core-envelope structure of the stars plays an
important role in binary interactions. In this paper, we compare mass transfer
simulations of bipolytropic binary systems in order to test the suitability of
our numerical tools for investigating the dynamical behaviour of such systems.
The initial, equilibrium binary models possess a core-envelope structure and
are obtained using the bipolytropic self-consistent field technique. We conduct
mass transfer simulations using two independent, fully three-dimensional,
Eulerian codes - Flow-ER and Octo-tiger. These hydrodynamic codes are compared
across binary systems undergoing unstable as well as stable mass transfer, and
the former at two resolutions. The initial conditions for each simulation and
for each code are chosen to match closely so that the simulations can be used
as benchmarks. Although there are some key differences, the detailed comparison
of the simulations suggests that there is remarkable agreement between the
results obtained using the two codes. This study puts our numerical tools on a
secure footing, and enables us to reliably simulate specific mass transfer
scenarios of binary systems involving components with a core-envelope
structure
Cluster Structure in Cosmological Simulations I: Correlation to Observables, Mass Estimates, and Evolution
We use Enzo, a hybrid Eulerian AMR/N-body code including non-gravitational
heating and cooling, to explore the morphology of the X-ray gas in clusters of
galaxies and its evolution in current generation cosmological simulations. We
employ and compare two observationally motivated structure measures: power
ratios and centroid shift. Overall, the structure of our simulated clusters
compares remarkably well to low-redshift observations, although some
differences remain that may point to incomplete gas physics. We find no
dependence on cluster structure in the mass-observable scaling relations, T_X-M
and Y_X-M, when using the true cluster masses. However, estimates of the total
mass based on the assumption of hydrostatic equilibrium, as assumed in
observational studies, are systematically low. We show that the hydrostatic
mass bias strongly correlates with cluster structure and, more weakly, with
cluster mass. When the hydrostatic masses are used, the mass-observable scaling
relations and gas mass fractions depend significantly on cluster morphology,
and the true relations are not recovered even if the most relaxed clusters are
used. We show that cluster structure, via the power ratios, can be used to
effectively correct the hydrostatic mass estimates and mass-scaling relations,
suggesting that we can calibrate for this systematic effect in cosmological
studies. Similar to observational studies, we find that cluster structure,
particularly centroid shift, evolves with redshift. This evolution is mild but
will lead to additional errors at high redshift. Projection along the line of
sight leads to significant uncertainty in the structure of individual clusters:
less than 50% of clusters which appear relaxed in projection based on our
structure measures are truly relaxed.Comment: 57 pages, 18 figures, accepted to ApJ, updated definition of T_X and
M_gas but results unchanged, for version with full resolution figures, see
http://www.ociw.edu/~tesla/sims.ps.g
- …