The nature of dark matter remains mysterious, with luminous material accounting for at most ∼25 per cent of the baryons in the Universe. We accordingly undertook a survey looking for the microlensing ...of stars in the Large Magellanic Cloud (LMC) to determine the fraction of Galactic dark matter contained in massive compact halo objects (MACHOs). The presence of the dark matter would be revealed by gravitational lensing of the light from an LMC star as the foreground dark matter moves across the line of sight. The duration of the lensing event is the key observable parameter, but gives non-unique solutions when attempting to estimate the mass, distance and transverse velocity of the lens. The survey results to date indicate that between 8 and 50 per cent of the baryonic mass of the Galactic halo is in the form of MACHOs (ref. 3), but removing the degeneracy by identifying a lensing object would tighten the constraints on the mass in MACHOs. Here we report a direct image of a microlens, revealing it to be a nearby low-mass star in the disk of the Milky Way. This is consistent with the expected frequency of nearby stars acting as lenses, and demonstrates a direct determination of a lens mass from a microlensing event. Complete solutions such as this for halo microlensing events will probe directly the nature of the MACHOs.
A study is presented to determine the technical, regulatory, and economic feasibility of a coastal research vessel (named the “Zero-V”) powered solely by hydrogen fuel cells. Feasibility is examined ...in the context of science mission profiles of particular significance in coastal oceanography that require a modern, capable, general-purpose coastal research vessel. These missions translate into the primary Zero-V vessel requirements against which technical feasibility is assessed. Hull form analysis indicated a trimaran design enables a vessel that can meet all of the space and volume requirements as well as for fitment of the machinery, service, and control spaces necessary for operation of the vessel. To meet speed and range (endurance) requirements, the vessel needs to be constructed of aluminum to reduce weight. The beam (56 feet), length (170 feet) and draft (12 feet) of the vessel enable it to dock at all primary ports of call. With 1800 kW of installed proton exchange membrane (PEM) fuel-cell power for primary propulsion, the cruising speed is 10 knots. With 10,900 kg of consumable LH2 stored in two LH2 tanks, the range of the vessel is 2400 nautical miles, with an endurance of 15 days. Both the greenhouse gas (GHG) and criteria pollutant (NOx, HC, PM10) emissions were estimated based on a complete “well-to-waves” (WTW) analysis. Using renewable hydrogen, the annual WTW GHG emissions predicted for the Zero-V are 91.4% less than those from an equivalent vessel running on conventional diesel fuel. Analysis of the WTW criteria pollutant emissions show that hydrogen PEM fuel-cell technology can reduce these emissions below stringent U.S. Environmental Protection Agency (EPA) Tier 4 marine diesel engine emissions limits regardless of whether the hydrogen is made using natural gas or renewable methods.
The capital construction cost is estimated to be ~ $79 M, not unreasonable when compared to other modern diesel-fueled research vessels of similar size and capabilities. The operations and maintenance (O&M) costs of the Zero-V were estimated to be 7.7% higher than operating the equivalent diesel-fueled vessel at the assumed fuel prices for LH2 made from steam reforming of fossil natural gas. The approach to safety for the Zero-V is described in terms of the arrangement of hazardous areas on the vessel. Regulatory review of the vessel design by the United States Coast Guard (USCG) and the Class society DNV GL found no fundamental or “show-stopping” design concerns that would prevent eventual deployment of the Zero-V. Overall, this study found it feasible from technical, regulatory, and economic perspectives to design, build and operate a coastal research vessel powered solely by hydrogen fuel cells.
Photoelectron velocity map imaging is combined with one- and two-photon ionization to study the near threshold photoionization of the 2-butyne molecule. In this region, the photoabsorption and ...photoionization cross sections display a very intense broad feature that is assigned to an l = 4, π(g) shape resonance. The effect of this shape resonance on the vibrational branching ratios and photoelectron angular distributions is explored. Theoretical calculations of the photoionization cross section and photoelectron angular distributions are in good agreement with the experiments. The results for 2-butyne are compared with those of acetylene, propyne, and 1-butyne, none of which show such significant enhancements near threshold, and the differences are rationalized in terms of the symmetries and orbital angular momenta of the highest occupied orbitals and the corresponding shape resonances. Expectations for larger alkynes and alkynyl radicals are also discussed. A preliminary measurement of the ionization energy of the 2-butyne dimer is also presented.
We discuss some aspects of a simple expression for the low-energy dissociative recombination cross section that applies when the recombination process is dominated by the indirect mechanism. In most ...previous applications, this expression has been applied to capture into vibrationally excited Rydberg states with the assumption that capture is always followed by prompt dissociation. Here we consider the dissociative recombination of larger polyatomic ions and electrons. More specifically, we consider capture into electronically core-excited Rydberg states, and begin to assess its potential importance for larger systems.
Plant breeding programs primarily focus on improving a crop's environmental adaptability and biotic stress tolerance in order to increase yield. Crop improvements made since the 1950s - coupled with ...inexpensive agronomic inputs, such as fertilizers, pesticides, and water - have allowed agricultural production to keep pace with human population growth. Plant breeders, particularly those at public institutions, have an interest in reducing agriculture's negative impacts and improving the natural environment to provide or maintain ecosystem services (eg clean soil, water, and air; carbon sequestration), and in creating new agricultural paradigms (eg perennial polycultures). Here, we discuss recent developments in, as well as the goals of, plant breeding, and explain how these may be connected to the specific interests of ecologists and naturalists. Plant breeding can be a powerful tool to bring "harmony" between agriculture and the environment, but partnerships between plant breeders, ecologists, urban planners, and policy makers are needed to make this a reality.
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