I. Collisional evolution and reddening of asteroid surfaces: The problem of conflicting timescales and the role of size-dependent effects

Space weathering is the generic term used for processes that modify the optical properties of surfaces of atmosphereless rocky bodies under exposure to the space environment. The general agreement about the relevance of the effects of space weathering on the spectral properties of S-complex asteroids fails when some basic quantitative estimates are attempted. In particular, there is severe disagreement regarding the typical timescales for significant spectral reddening to occur, ranging from 1 Myr to 1 Gyr. Generally speaking, the spectral reddening of an individual object can be considered as the sum of three terms, one (which is relevant for statistical analyses) depending on the exposure of the object to space weathering during its lifetime, a second one due to the original surface composition, and a third one (a "noise" term) due to the combination of poorly constrained effects (e.g., structure and texture of the surface). The surface of an asteroid is usually covered by regolith, and its presence and properties presumably play a critical role in the weathering processes. In this paper we discuss the role played by collisional evolution in affecting the spectral properties of asteroids and refreshing the surfaces due to the formation of ejecta, and the necessity of a simultaneous modeling of collisions and weathering processes. We introduce a new idea, based on the possibility of a sort of saturation of the refreshing process whenever a massive reaccumulation of the impact ejecta takes place. In this case, a dependence of the overall reddening on the asteroid size should naturally come out. We show that this conclusion is indeed supported by available main belt asteroid spectroscopic data.Comment: Accepted by MNRA

Slowing of pulses to c/10 with subwatt power levels and low latency using Brillouin amplification in a bismuth-oxide optical fiber

We report the generation of slow light using Brillouin amplification in a short length of highly nonlinear bismuth-oxide fiber. By using just 2m of fiber, we demonstrate a five-fold reduction in group velocity for ~200ns pulses, which we believe to be a record for a slow-light propagation in an optical fiber. Moreover, by virtue of the high nonlinearity per unit length of this fiber, we achieve this at a very modest pump power level of just ~400mW and with a low inherent device latency of 14ns. These results highlight both the merits and practicality of using high nonlinearity nonsilica fibers for slow-light devices

Asymptotic solution of a model for bilayer organic diodes and solar cells

The current voltage characteristics of an organic semiconductor diode made by placing together two materials with dissimilar electron affinities and ionisation potentials is analysed using asymptotic methods. An intricate boundary layer structure is examined. We find that there are three regimes for the total current passing through the diode. For reverse bias and moderate forward bias the dependency of the voltage on the current is similar to the behaviour of conventional inorganic semiconductor diodes predicted by the Shockley equation and are governed by recombination at the interface of the materials. There is then a narrow range of currents where the behaviour undergoes a transition. Finally for large forward bias the behaviour is different with the current being linear in voltage and is primarily controlled by drift of charges in the organic layers. The size of the interfacial recombination rate is critical in determining the small range of current where there is rapid transition between the two main regimes. The extension of the theory to organic solar cells is discussed and the analogous current voltage curves derived in the regime of interest

All-optical pulse reshaping and retiming systems incorporating pulse shaping fiber Bragg grating

This paper demonstrates two optical pulse retiming and reshaping systems incorporating superstructured fiber Bragg gratings (SSFBGs) as pulse shaping elements. A rectangular switching window is implemented to avoid conversion of the timing jitter on the original data pulses into pulse amplitude noise at the output of a nonlinear optical switch. In a first configuration, the rectangular pulse generator is used at the (low power) data input to a nonlinear optical loop mirror (NOLM) to perform retiming of an incident noisy data signal using a clean local clock signal to control the switch. In a second configuration, the authors further amplify the data signal and use it to switch a (low power) clean local clock signal. The S-shaped nonlinear characteristic of the NOLM results in this instance in a reduction of both timing and amplitude jitter on the data signal. The underlying technologies required for the implementation of this technique are such that an upgrade of the scheme for the regeneration of ultrahigh bit rate signals at data rates in excess of 320 Gb/s should be achievable

Application of the Fernandez-Terrazo et al. one-step chemistry model for partially premixed combustion to n-heptane

A set of model parameters has been derived for the use of the model with n-heptane following the procedure set out by Fernandez et al

Use of a transcendental approximation in transient conduction analysis

Variation method applied to transient thermal conductivity in semiinfinite slab with nonlinear boundary conditio

Transient radiation and conduction in a slotted slab and a hollow cylinder

Transient radiation and conduction in slotted slab and hollow cylinde

Use of a transcendental approximation in transient conduction analysis

Boundary condition solution and transcendental approximation for transient heat conduction proble

Building Cross Cultural Competencies.

The Building Cross Cultural Competencies project was developed with the aim of equipping undergraduate students at the University of York with skills to work in the globalised world, while at the same time assisting with the induction and orientation of international students, new to the institution and to study in the UK. The inspiration for the programme dates back to 2006, when one of the authors visited three Universities in New Zealand and Australia. These Universities were perceived to be further down the route to internationalisation (as defined by Knight 2003) than was the norm in the UK at the time. Innovations observed at Massey and Waikato Universities in New Zealand and the University of Sydney, Australia, were redesigned and redeveloped for use at the University of York, with the agreement of the staff involved at those institutions. In particular, a cross cultural communication module and two distinctive peer mentor schemes provided the nucleus of the idea for a new initiative at York that would span the employability and internationalisation agendas This paper identifies how the project redesigned and developed ideas taken from Australian and New Zealand Universities for use in a UK context. It makes links to the literature on student adjustment and institutional adaptation; peer teaching and cross cultural communication skills. It will also consider the problems and difficulties experienced as the project progressed