Evolution of dust grain size distribution by shattering in the interstellar medium: robustness and uncertainty

Shattering of dust grains in the interstellar medium is a viable mechanism of small grain production in galaxies. We examine the robustness or uncertainty in the theoretical predictions of shattering. We identify $P_1$ (the critical pressure above which the deformation destroys the original lattice structures) as the most important quantity in determining the timescale of small grain production, and confirm that the same $P_1/t$ ($t$ is the duration of shattering) gives the same grain size distribution [$n(a)$, where $a$ is the grain radius] after shattering within a factor of 3. The uncertainty in the fraction of shocked material that is eventually ejected as fragments causes uncertainties in $n(a)$ by a factor of 1.3 and 1.6 for silicate and carbonaceous dust, respectively. The size distribution of shattered fragments have minor effects as long as the power index of the fragment size distribution is less than ~ 3.5, since the slope of grain size distribution $n(a)$ continuously change by shattering and becomes consistent with $n(a)\propto a^{-3.5}$. The grain velocities as a function of grain radius can have an imprint in the grain size distribution especially for carbonaceous dust. We also show that the formulation of shattering can be simplified without losing sufficient precision.Comment: 12 pages, 7 figures, Accepted for publication in Earth, Planets, and Space (Special Issue: Cosmic Dust V

"Publicly Listed Parent/Subsidiary Pairs: Benchmarking to TOPIX and Market Distortion"

This paper explores the impact of publicly listed parent/subsidiary pairs on the pricing and volatility of companies' shares. We construct a noisy rational expectations equilibrium model in which a parent and its subsidiary company are both publicly listed. Two classes of traders participate in the market: institutional investors who have private information on the fundamentals of listed companies, and individual investors who have no private information. A key feature of the model is that institutional investors attempt to optimize the risk-return tradeoff relative to TOPIX, the capitalization-weighted index of the stock market. Individual investors are assumed to act without reference to any performance benchmark. Within this framework we first establish the rather obvious result that the market portfolio of all outstanding shares is not an efficient portfolio. This result implies that benchmarking to TOPIX, which is the surrogate of the market portfolio without any adjustment for double-counting of parent/subsidiary pairs, generates excessive demand for shares of the subsidiary company. We analyze the equilibrium of our market model and show that (1)the price of the subsidiary companyfs share is pushed up to a level higher than that implied by its fundamentals, (2) the share price of other companies who are highly correlated with the subsidiary company receive similar effect, (3)the subsidiary companyfs shares become more volatile and (4)tend to respond more to good news than to bad news. The results of this paper suggest that using TOPIX as the performance benchmark, which is the prevailing practice in evaluating pension fund managers and other institutional investors, may be causing distortion in share prices and volatilities of subsidiary companies. A new index which corrects for the double counting is worth a serious consideration.

Natural inflation with and without modulations in type IIB string theory

We propose a mechanism for the natural inflation with and without modulation in the framework of type IIB string theory on toroidal orientifold or orbifold. We explicitly construct the stabilization potential of complex structure, dilaton and K\"ahler moduli, where one of the imaginary component of complex structure moduli becomes light which is identified as the inflaton. The inflaton potential is generated by the gaugino-condensation term which receives the one-loop threshold corrections determined by the field value of complex structure moduli and the axion decay constant of inflaton is enhanced by the inverse of one-loop factor. We also find the threshold corrections can also induce the modulations to the original scalar potential for the natural inflation. Depending on these modulations, we can predict several sizes of tensor-to-scalar ratio as well as the other cosmological observables reported by WMAP, Planck and/or BICEP2 collaborations.Comment: 18 pages, 4 figures, Typos corrected, references and comments adde

Evolution of linear cosmological perturbations and its observational implications in Galileon-type modified gravity

A scalar-tensor theory of gravity can be made not only to account for the current cosmic acceleration, but also to satisfy solar-system and laboratory constraints, by introducing a non-linear derivative interaction for the scalar field. Such an additional scalar degree of freedom is called "Galileon". The basic idea is inspired by the DGP braneworld, but one can construct a ghost-free model that admits a self-accelerating solution. We perform a fully relativistic analysis of linear perturbations in Galileon cosmology. Although the Galileon model can mimic the background evolution of standard $\Lambda$CDM cosmology, the behavior of perturbation is quite different. It is shown that there exists a super-horizon growing mode in the metric and Galileon perturbations at early times, suggesting that the background is unstable. A fine-tuning of the initial condition for the Galileon fluctuation is thus required in order to promote a desirable evolution of perturbations at early times. Assuming the safe initial condition, we then compute the late-time evolution of perturbations and discuss observational implications in Galileon cosmology. In particular, we find anticorrelations in the cross-correlation of the integrated Sachs-Wolfe effect and large scale structure, similar to the normal branch of the DGP model.Comment: 15 pages, 11 figures; v2: References added, typos correcte

Magnetized orbifold models

We study (4+2n)-dimensional N=1 super Yang-Mills theory on the orbifold background with non-vanishing magnetic flux. In particular, we study zero-modes of spinor fields. The flavor structure of our models is different from one in magnetized torus models, and would be interesting in realistic model building.Comment: 26 page