3,975 research outputs found
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 (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 ( is the duration of
shattering) gives the same grain size distribution [, where 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 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
continuously change by shattering and becomes consistent with . 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
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
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 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
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