Low-Threshold Electrically Pumps Vertical-Cavity Surface-Emitting Microlasers

Vertical-cavity electrically driven lasers with three GaInAs quantum wells and diameters of several μm exhibit room-temperature pulsed current thresholds as low as 1.3mA with 958 nm output wavelength

Molecular Line Emission from Gravitationally Unstable Protoplanetary Disks

In the era of high resolution submillimeter interferometers, it will soon be possible to observe the neutral circumstellar medium directly involved in gas giant planet (GGP) formation at physical scales previously unattainable. In order to explore possible signatures of gas giant planet formation via disk instabilities, we have combined a 3D, non-local thermodynamic equilibrium (LTE) radiative transfer code with a 3D, finite differences hydrodynamical code to model molecular emission lines from the vicinity of a 1.4 M_J self-gravitating proto-GGP. Here, we explore the properties of rotational transitions of the commonly observed dense gas tracer, HCO+. Our main results are the following: 1. Very high lying HCO+ transitions (e.g. HCO+ J=7-6) can trace dense planet forming clumps around circumstellar disks. Depending on the molecular abundance, the proto-GGP may be directly imageable by the Atacama Large Millimeter Array (ALMA). 2. HCO+ emission lines are heavily self-absorbed through the proto-GGP's dense molecular core. This signature is nearly ubiquitous, and only weakly dependent on assumed HCO+ abundances. The self-absorption features are most pronounced at higher angular resolutions. Dense clumps that are not self-gravitating only show minor self-absorption features. 3. Line temperatures are highest through the proto-GGP at all assumed abundances and inclination angles. Conversely, due to self-absorption in the line, the velocity-integrated intensity may not be. High angular resolution interferometers such as the Submillimeter Array (SMA) and ALMA may be able to differentiate between competing theories of gas giant planet formation.Comment: 10 pages, 13 figures; Accepted by Ap

Analysing and modelling train driver performance

Arguments for the importance of contextual factors in understanding human performance have been made extremely persuasive in the context of the process control industries. This paper puts these arguments into the context of the train driving task, drawing on an extensive analysis of driver performance with the Automatic Warning System (AWS). The paper summarises a number of constructs from applied psychological research which are thought to be important in understanding train driver performance. A “Situational Model” is offered as a framework for investigating driver performance. The model emphasises the importance of understanding the state of driver cognition at a specific time (“Now”) in a specific situation and a specific context

Multiple Stellar Populations in the Globular Cluster omega Centauri as Tracers of a Merger Event

The discovery of the Sagittarius dwarf galaxy, which is being tidally disrupted by and merging with the Milky Way, supports the view that the halo of the Galaxy has been built up at least partially by the accretion of similar dwarf systems. The Sagittarius dwarf contains several distinct populations of stars, and includes M54 as its nucleus, which is the second most massive globular cluster associated with the Milky Way. The most massive globular cluster is omega Centauri, and here we report that omega Centauri also has several distinct stellar populations, as traced by red-giant-branch stars. The most metal-rich red-giant-branch stars are about 2 Gyr younger than the dominant metal-poor component, indicating that omega Centauri was enriched over this timescale. The presence of more than one epoch of star formation in a globular cluster is quite surprising, and suggests that omega Centauri was once part of a more massive system that merged with the Milky Way, as the Sagittarius dwarf galaxy is in the process of doing now. Mergers probably were much more frequent in the early history of the Galaxy and omega Centauri appears to be a relict of this era.Comment: 7 pages, 3 figures, Latex+nature.sty (included), To appear in November 4th issue of Natur

Method for remotely powering a device such as a lunar rover

A method of supplying power to a device such as a lunar rover located on a planetary surface is provided. At least one, and preferably three, laser satellites are set in orbit around the planet. Each satellite contains a nuclear reactor for generating electrical power. This electrical power is converted into a laser beam which is passed through an amplifying array and directed toward the device such as a lunar rover. The received laser beam is then converted into electrical power for use by the device

How much do PCB toxic equivalents account for PHAH toxicity in predatory birds?

Various diffuse polyhalogenated aromatic hydrocarbons (PHAHs) exert common toxicity through the aryl hydrocarbon receptor (AhR). Apex predators spatially and temporally integrate diffuse contamination and simultaneous exposure can cause additive toxicity. We investigated the extent to which PCBs, still amongst the most prevalent PHAHs accumulated by predators, accounted for total PHAH toxicity in raptors and fish eating birds from Britain. We analysed egg or liver extracts from six species and compared chemically determined ΣPCB-TEQs concentrations with total AhR-mediated toxicity determined using the chemical-activated luciferase gene expression bioassay (CALUX-TEQ). Dioxin-like PCB profiles in eggs and livers were dominated by congeners 118, 105 and 167. ΣPCB-TEQ and CALUX-TEQ concentrations were positively associated but not in a 1:1 relationship. ΣPCB-TEQ were broadly similar to CALUX-TEQ concentrations in eggs and livers with CALUX-TEQ concentrations >50–80 and 160–320 pg g−1 lipid respectively, but were lower than CALUX-TEQ concentrations in less contaminated samples

Finite-source and finite-lens effects in astrometric microlensing

The aim of this paper is to study the astrometric trajectory of microlensing events with an extended lens and/or source. We consider not only a dark lens but also a luminous lens as well. We find that the discontinuous finite-lens trajectories given by Takahashi (2003) will become continuous in the finite-source regime. The point lens (source) approximation alone gives an under (over)estimation of the astrometric signal when the size of the lens and source are not negligible. While the finiteness of the source is revealed when the lens transits the surface of the source, the finite-lens signal is most prominent when the lens is very close to the source. Astrometric microlensing towards the Galactic bulge, Small Magellanic Cloud and M31 are discussed, which indicate that the finite-lens effect is beyond the detection limit of current instruments. Nevertheless, it is possible to distinguish between self-lensing and halo lensing through a (non-)detection of the astrometric ellipse. We also consider the case where the lens is luminous itself, as has been observed where a lensing event was followed up with the Hubble Space Telescope. We show that the astrometric signal will be reduced in a luminous-lens scenario. The physical properties of the event, such as the lens-source flux ratio, the size of the lens and source nevertheless can be derived by fitting the astrometric trajectory.Comment: 12 pages, 12 figures, 1 table, published in MNRA

Microwave Conductivity due to Scattering from Extended Linear Defects in d-Wave Superconductors

Recent microwave conductivity measurements of detwinned, high-purity, slightly overdoped YBa$_{2}$Cu$_{3}$O$_{6.993}$ crystals reveal a linear temperature dependence and a near-Drude lineshape for temperatures between 1 and 20 K and frequencies ranging from 1 to 75 GHz. Prior theoretical work has shown that simple models of scattering by point defects (impurities) in d-wave superconductors are inconsistent with these results. It has therefore been suggested that scattering by extended defects such as twin boundary remnants, left over from the detwinning process, may also be important. We calculate the self-energy and microwave conductivity in the self-consistent Born approximation (including vertex corrections) for a d-wave superconductor in the presence of scattering from extended linear defects. We find that in the experimentally relevant limit ($\Omega, 1/\tau \ll T \ll \Delta_{0}$), the resulting microwave conductivity has a linear temperature dependence and a near-Drude frequency dependence that agrees well with experiment.Comment: 13 pages, 7 figure

Oscillations of a solid sphere falling through a wormlike micellar fluid

We present an experimental study of the motion of a solid sphere falling through a wormlike micellar fluid. While smaller or lighter spheres quickly reach a terminal velocity, larger or heavier spheres are found to oscillate in the direction of their falling motion. The onset of this instability correlates with a critical value of the velocity gradient scale $\Gamma_{c}\sim 1$ s$^{-1}$. We relate this condition to the known complex rheology of wormlike micellar fluids, and suggest that the unsteady motion of the sphere is caused by the formation and breaking of flow-induced structures.Comment: 4 pages, 4 figure