Direct N-body Simulations of Rubble Pile Collisions

There is increasing evidence that many km-sized bodies in the Solar System are piles of rubble bound together by gravity. We present results from a project to map the parameter space of collisions between km-sized spherical rubble piles. The results will assist in parameterization of collision outcomes for Solar System formation models and give insight into fragmentation scaling laws. We use a direct numerical method to evolve the positions and velocities of the rubble pile particles under the constraints of gravity and physical collisions. We test the dependence of the collision outcomes on impact parameter and speed, impactor spin, mass ratio, and coefficient of restitution. Speeds are kept low (< 10 m/s, appropriate for dynamically cool systems such as the primordial disk during early planet formation) so that the maximum strain on the component material does not exceed the crushing strength. We compare our results with analytic estimates and hydrocode simulations. Off-axis collisions can result in fast-spinning elongated remnants or contact binaries while fast collisions result in smaller fragments overall. Clumping of debris escaping from the remnant can occur, leading to the formation of smaller rubble piles. In the cases we tested, less than 2% of the system mass ends up orbiting the remnant. Initial spin can reduce or enhance collision outcomes, depending on the relative orientation of the spin and orbital angular momenta. We derive a relationship between impact speed and angle for critical dispersal of mass in the system. We find that our rubble piles are relatively easy to disperse, even at low impact speed, suggesting that greater dissipation is required if rubble piles are the true progenitors of protoplanets.Comment: 30 pages including 4 tables, 8 figures. Revised version to be published in Icarus

Fos co-operation with PTEN loss elicits keratoacanthoma not carcinoma due to p53/p21^WAF-induced differentiation triggered by GSK3b inactivation and reduced AKT activity

To investigate gene synergism in multistage skin carcinogenesis, the RU486-inducible cre/lox system was employed to ablate PTEN function [K14.cre/D5PTENflx] in mouse epidermis expressing activated v-fos [HK1.fos]. RU486-treated HK1.fos/D5PTENflx mice exhibited hyperplasia, hyperkeratosis and tumours that progressed to highly differentiated keratoacanthomas rather than carcinomas, due to re-expression of high p53 and p21WAF levels. Despite elevated MAP kinase activity, cyclin D1/E2 over expression and increased AKT activity forming areas of highly proliferative, papillomatous keratinocytes, increasing levels of GSK3b inactivation exceeded a threshold that induced p53/p21WAF expression to halt proliferation and accelerate differentiation, giving the hallmark keratosis of keratoacanthomas. A pivotal facet to this GSK3b-triggered mechanism centred on increasing p53 expression in basal layer keratinocytes. This reduced activated AKT expression and released inhibition of p21WAF, which accelerated keratinocyte differentiation, as indicated by unique basal layer expression of differentiation-specific keratin K1, alongside premature filaggrin and loricrin expression. Thus, fos synergism with PTEN loss elicited a benign tumour context where GSK3b-induced, p53/p21WAF expression continually switched AKT-associated proliferation into one of differentiation, preventing further progression. This putative compensatory mechanism required the critical availability of normal p53 and/or p21WAF otherwise deregulated fos, Akt and GSK3b associate with malignant progression

Exploring the Relationship Between Online Social Network Site Usage and the Impact on Quality of Life for Older and Younger Users: An Interaction Analysis

This is an Open Access journal. ©Darren Quinn, Liming Chen, Maurice D Mulvenna, Raymond Bond. Originally published in the Journal of Medical Internet Research (http://www.jmir.org), 29.09.2016. This is an open-access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work, first published in the Journal of Medical Internet Research, is properly cited

PATENTS, R&D AND LAG EFFECTS: EVIDENCE FROM FLEXIBLE METHODS FOR COUNT PANEL DATA ON MANUFACTURING FIRMS

Hausman, Hall and Griliches (1984) and Hall, Griliches and Hausman (1986) investigated whether there was a lag in the patent-R&D relationship for the U.S. manufacturing sector using 1970¿s data. They found that there was little evidence of anything but contemporaneous movement of patents and R&D. We reexamine this important issue employing new longitudinal patent data at the firm level for the U.S. manufacturing sector from 1982 to 1992. To address unique features of the data, we estimate various distributed lag and dynamic multiplicative panel count data models. The paper also develops a new class of count panel data models based on series expansion of the distribution of individual effects. The empirical analyses show that, although results are somewhat sensitive to different estimation methods, the contemporaneous relationship between patenting and R&D expenditures continues to be rather strong, accounting for over 60% of the total R&D elasticity. Regarding the lag structure of the patents-R&D relationship, we do find a significant lag in all empirical specifications. Moreover, the estimated lag effects are higher than have previously been found, suggesting that the contribution of R&D history to current patenting has increased from the 1970¿s to the 1980¿s.Innovative activity, Patents and R&D, Individual effects, count panel data methods.

Advanced analog television study final report, 4 nov. - 19 dec. 1963

Information bandwidth reduction for analog television signals - Description of multiple interlace syste

Exchange and Correlation Corrections to the Response Functions of a Spin-Polarized Electron Gas

We analyze the spin and charge responses induced in a spin-polarized electron gas by a weak electromagnetic field. The coupled spin-charge response is derived from the equation of motion of the particle distribution function in the presence of the perturbation. To obtain the correct frequency and the wave-vector dependence we introduce the spin-dependent local-field factors, G±σ=Gxσ±Gcσ, which give the exchange (x) and correlation (c) corrections to the random phase approximation. For an arbitrary degree of polarization of the electron gas, we derive the exact analytical expressions for G±σ(q→,ω) in the limit of high frequency or large wave vectors. The results for q→→∞ are expressed in terms of the two-particle correlation function, g(r→) at r=0

Collective Excitations in an Asymmetrically Spin-Polarized Quantum Well

We present a phenomenological picture for the many-body excitations of a two-dimensional electron gas in a quantum well spin polarized by a dc magnetic field at an angle ␪ with the axis of the well. In the framework of the Landau theory of charged Fermi liquids, we determine the frequencies of collective modes by solving a transport equation for quasiparticles in the local electromagnetic field associated with the charge- and spin-density fluctuations. In the long-wavelength limit, analytic solutions for ␻(qជ) are obtained as functions of the degree of spin polarization and of the angle ␪

Coupling of Spin Waves with Charge- and Spin Density Excitations in Spin-Polarized Quantum Wells

The coupling of spin waves with charge- and spin-density waves is shown to be induced by a spin-dependent interaction in a quantum well, which is spin polarized by a dc magnetic field at an angle ␪ to the symmetry axis. The mixing of the plasmonic and magnonic modes, which occurs for both intra- and intersubband transitions, depends on the coupling constant of the spin-spin interaction, the tilt angle ␪, and the initial spin polarization ␨. ͓S0163-1829͑99͒08247-8