Simulated Versus Observed Cluster Eccentricity Evolution

The rate of galaxy cluster eccentricity evolution is useful in understanding large scale structure. Rapid evolution for $z <$ 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$_{200}$. 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, $z \leq 0.25$. Cluster morphology, within 0.5 and 1.0 h$^{-1}$ 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 $z \leq 0.1$, 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

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

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 $T/|W|$ 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 Sch$\ddot{\rm{o}}$nberg-Chandrasekhar limit for a bipolytropic sequence. We also discuss the applications of this method to magnetic braking in low mass stars with convective envelopes

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

Do R Coronae Borealis Stars Form from Double White Dwarf Mergers?

A leading formation scenario for R Coronae Borealis (RCB) stars invokes the merger of degenerate He and CO white dwarfs (WD) in a binary. The observed ratio of 16O/18O for RCB stars is in the range of 0.3-20 much smaller than the solar value of ~500. In this paper, we investigate whether such a low ratio can be obtained in simulations of the merger of a CO and a He white dwarf. We present the results of five 3-dimensional hydrodynamic simulations of the merger of a double white dwarf system where the total mass is 0.9 Mdot and the initial mass ratio (q) varies between 0.5 and 0.99. We identify in simulations with $q\lesssim0.7$ a feature around the merged stars where the temperatures and densities are suitable for forming 18O. However, more 16O is being dredged-up from the C- and O-rich accretor during the merger than the amount of 18O that is produced. Therefore, on a dynamical time scale over which our hydrodynamics simulation runs, a 16O/18O ratio of ~2000 in the "best" case is found. If the conditions found in the hydrodynamic simulations persist for 10^6 seconds the oxygen ratio drops to 16 in one case studied, while in a hundred years it drops to ~4 in another case studied, consistent with the observed values in RCB stars. Therefore, the merger of two white dwarfs remains a strong candidate for the formation of these enigmatic stars.Comment: 42 pages, 19 figures. Accepted for publication in the Astrophysical Journa

The role of dredge-up in double white dwarf mergers

We present the results of an investigation of the dredge-up and mixing during the merger of two white dwarfs with different chemical compositions by conducting hydrodynamic simulations of binary mergers for three representative mass ratios. In all the simulations, the total mass of the two white dwarfs is $\lesssim1.0~{\rm M_\odot}$. Mergers involving a CO and a He white dwarf have been suggested as a possible formation channel for R Coronae Borealis type stars, and we are interested in testing if such mergers lead to conditions and outcomes in agreement with observations. Even if the conditions during the merger and subsequent nucleosynthesis favor the production of $^{18}{\mathrm O}$, the merger must avoid dredging up large amounts of $^{16}{\mathrm O}$, or else it will be difficult to produce sufficient $^{18}{\mathrm O}$ to explain the oxygen ratio observed to be of order unity. We performed a total of 9 simulations using two different grid-based hydrodynamics codes using fixed and adaptive meshes, and one smooth particle hydrodynamics (SPH) code. We find that in most of the simulations, $>10^{-2}~{\rm M_\odot}$ of $^{16}{\mathrm O}$ is indeed dredged up during the merger. However, in SPH simulations where the accretor is a hybrid He/CO white dwarf with a $\sim 0.1~{\rm M_\odot}$ layer of helium on top, we find that no $^{16}{\mathrm O}$ is being dredged up, while in the $q=0.8$ simulation $<10^{-4}~{\rm M_\odot}$ of $^{16}{\mathrm O}$ has been brought up, making a WD binary consisting of a hybrid CO/He WD and a companion He WD an excellent candidate for the progenitor of RCB stars.Comment: Accepted for publication in Ap