On the role of shear in cosmological averaging

Using the spherically symmetric inhomogeneous Lemaitre-Tolman-Bondi dust solution, we study how the shear and the backreaction depend on the sharpness of the spatial transition between voids and walls and on the size of the voids. The voids considered here are regions with matter density Omega ~ 0 and expansion rate Ht ~ 1, while the walls are regions with matter density Omega ~ 1 and expansion rate Ht ~ 2/3. The results indicate that both the volume-average shear and the variance of the expansion rate grow proportional to the sharpness of the transition and diverge in the limit of a step function, but, for realistic-sized voids, are virtually independent of the size of the void. However, the backreaction, given by the difference of the variance and the shear, has a finite value in the step-function limit. By comparing the exact result for the backreaction to the case where the shear is neglected by treating the voids and walls as separate Friedmann-Robertson-Walker models, we find that the shear suppresses the backreaction by a factor of (r_0/t_0)^2, the squared ratio of the void size to the horizon size. This exemplifies the importance of using the exact solution for the interface between the regions of different expansion rates and densities. The suppression is justified to hold also for a network of compensated voids, but may not hold if the universe is dominated by uncompensated voids.Comment: 17 pages, 3 figure

On the role of shear in cosmological averaging II: large voids, non-empty voids and a network of different voids

We study the effect of shear on the cosmological backreaction in the context of matching voids and walls together using the exact inhomogeneous Lemaitre-Tolman-Bondi solution. Generalizing JCAP 1010 (2010) 021, we allow the size of the voids to be arbitrary and the densities of the voids and walls to vary in the range 0 < Omega_v < Omega_w < 1. We derive the exact analytic result for the backreaction and consider its series expansion in powers of the ratio of the void size to the horizon size, r_0/t_0. In addition, we deduce a very simple fitting formula for the backreaction with error less than 1% for voids up to sizes r_0 = t_0. We also construct an exact solution for a network of voids with different sizes and densities, leading to a non-zero relative variance of the expansion rate between the voids. While the leading order term of the backreaction for a single void-wall pair is of order (r_0/t_0)^2, the relative variance between the different voids in the network is found to be of order (r_0/t_0)^4 and thus very small for voids of the observed size. Furthermore, we show that even for very large voids, the backreaction is suppressed by an order of magnitude relative to the estimate obtained by treating the walls and voids as disjoint Friedmann solutions. Whether the suppression of the backreaction due to the shear is just a consequence of the restrictions of the used exact models, or a generic feature, has to be addressed with more sophisticated solutions.Comment: 19 pages, 5 figure

Modelling dust processing and the evolution of grain sizes in the ISM using the method of moments

Interstellar dust grains do not have a single well-defined origin. Stars are demonstrably dust producers, but also efficient destroyers of cosmic dust. Dust destruction in the ISM is believed to be the result of SN shocks hitting the ambient ISM gas (and dust) and lead to an increased rate of ion sputtering, which reduces the dust mass. Grains located in cold molecular clouds can on the other hand grow by condensation, thus providing a replenishment mechanism or even a dominant channel of dust formation. In dense environments grains may coagulate and form large composite grains and aggregates and if grains collide with large enough energies they may be shattered, forming a range of smaller debris grains. The present paper presents a statistical modelling approach using the method of moments, which is computationally very inexpensive and may therefore be an attractive option when combining dust processing with, e.g., detailed simulations of interstellar gas dynamics. A solar-neighbourhood-like toy model of interstellar dust evolution is presented as an example.Comment: 21 pages, 8 figures and 2 tables. To appear in the special issue in P&SS on cosmic dust ("Cosmic Dust VIII"

The sudden appearance of dust in the early Universe

Observations suggest that high-redshift galaxies are either very dusty or essentially dust free. The evolution from one regime to the other must also be very fast, since evolved and dusty galaxies show up at redshifts corresponding to a Universe which is only about 500 Myr old. In the present paper models which predicts the existence of an apparent dichotomy between dusty and dust-free galaxies at high redshift are considered. Galaxies become dusty as soon as they reach an evolved state and the transition is very rapid. A special case suggests that while stellar dust production is overall relatively insignificant -- contrary to what has been argued recently -- it can at the same time be consistent with efficient dust production in supernovae in the local Universe. Special attention will be given to the recent discovery of a dusty normal galaxy (A1689-zD1) at a very high redshift z = 7.5 +/- 0.2.Comment: 5 pages, 1 figure. Revised version. Submitted as a MNRAS Letter on March 26, 201

Relaxing the Small Particle Approximation for Dust-grain opacities in Carbon-star Wind Models

We have computed wind models with time-dependent dust formation and grain-size dependent opacities, where (1) the problem is simplified by assuming a fixed dust-grain size, and where (2) the radiation pressure efficiency is approximated using grain sizes based on various means of the actual grain size distribution. It is shown that in critical cases, the effect of grain sizes can be significant. For well-developed winds, however, the effects on the mass-loss rate and the wind speed are small.Comment: 2 pages, 2 figures. To be published in the proceedings of the conference "Why Galaxies Care about AGB Stars II", held in Vienna, August 16-20, 2010; F. Kerschbaum, T. Lebzelter T. and B. Wing eds, ASP Conf. Serie

Improved implementation of dust-driven winds and dust formation in models of AGB evolution: Effects of pulsation and gas-pressure forcing

Mass loss is a crucial component in stellar evolution models, since it largely determines the rate of evolution at the later stages of a star's life. The dust-driven outflows from AGB stars are particularly important in this regard. Including AGB dust formation in a stellar evolution model does also require a model of these outflows. Since AGB stars exhibit large-amplitude pulsation, a model based on time-dependent radiation hydrodynamics (RHD) is needed in order to capture all the important physical aspects of dust formation. However, this cannot be afforded in a stellar evolution model. Here, a mean-flow model is presented, which include corrections to the steady-state model currently being used in AGB evolution models with dust formation.Comment: 4 pages, 1 figure. Conference proceedings, to appear in MEMORIE della Societ\`a Astronomica Italian

Runaway growth of fractal dust grains

Fractal grains have large surface area, which leads to more efficient condensation. The special limit case where the volume-area ratio is constant (corresponding to, e.g., a very rough grain surface or non-compacts aggregates) is particularly interesting, as well as convenient, from a mathematical point of view. If dust grains from AGB stars have `rough surfaces', it may have important implications for our understanding of dust and wind formation in AGB stars.Comment: 2 pages, 1 figure. To appear in the proceedings of "Why Galaxies Care About AGB Stars III", Vienna, July 201

Stable and Accurate Interpolation Operators for High-Order Multi-Block Finite-Difference Methods

Block-to-block interface interpolation operators are constructed for several common high-order finite difference discretizations. In contrast to conventional interpolation operators, these new interpolation operators maintain the strict stability, accuracy and conservation of the base scheme even when nonconforming grids or dissimilar operators are used in adjoining blocks. The stability properties of the new operators are verified using eigenvalue analysis, and the accuracy properties are verified using numerical simulations of the Euler equations in two spatial dimensions

Quantum Monte Carlo applied to solids

We apply diffusion quantum Monte Carlo (DMC) to a broad set of solids, benchmarking the method by comparing bulk structural properties (equilibrium volume and bulk modulus) to experiment and DFT based theories. The test set includes materials with many different types of binding including ionic, metallic, covalent and van der Waals. We show that, on average, the accuracy is comparable to or better than that of density functional theory (DFT) when using the new generation of functionals, including one hybrid functional and two dispersion corrected functionals. The excellent performance of quantum Monte Carlo (QMC) on solids is promising for its application to heterogeneous systems and high-pressure/high density conditions. Important to the results here is the application of a consistent procedure with regards to the several approximations that are made, such as finite-size corrections and pseudo-potential approximations. This test set allows for any improvements in these methods to be judged in a systematic way

Dust driven mass loss from carbon stars as a function of stellar parameters - I. A grid of Solar-metallicity wind models (Corrigendum)

The purpose of this corrigendum is to point out that a handful of models in the original paper were computed with faulty initial structures. Using exactly the same modelling methods we have recomputed the faulty models with new initial structures. The new results slightly changes some of the trends in the wind properties with stellar parameters, but the overall effects are small. The conclusions are not affected.Comment: 1 page, corrigendum to Mattsson et al., 2010, A&A, 509, A1