The distribution of shock waves in driven supersonic turbulence

Supersonic turbulence generates distributions of shock waves. Here, we analyse the shock waves in three-dimensional numerical simulations of uniformly driven supersonic turbulence, with and without magnetohydrodynamics and self-gravity. We can identify the nature of the turbulence by measuring the distribution of the shock strengths. We find that uniformly driven turbulence possesses a power law distribution of fast shocks with the number of shocks inversely proportional to the square root of the shock jump speed. A tail of high speed shocks steeper than Gaussian results from the random superposition of driving waves which decay rapidly. The energy is dissipated by a small range of fast shocks. These results contrast with the exponential distribution and slow shock dissipation associated with decaying turbulence. A strong magnetic field enhances the shock number transverse to the field direction at the expense of parallel shocks. A simulation with self-gravity demonstrates the development of a number of highly dissipative accretion shocks. Finally, we examine the dynamics to demonstrate how the power-law behaviour arises.Comment: accepted to Astron. & Astrophys.; ten page

Economics of Water Quality Protection from Nonpoint Sources: Theory and Practice

Water quality is a major environmental issue. Pollution from nonpoint sources is the single largest remaining source of water quality impairments in the United States. Agriculture is a major source of several nonpoint-source pollutants, including nutrients, sediment, pesticides, and salts. Agricultural nonpoint pollution reduction policies can be designed to induce producers to change their production practices in ways that improve the environmental and related economic consequences of production. The information necessary to design economically efficient pollution control policies is almost always lacking. Instead, policies can be designed to achieve specific environmental or other similarly related goals at least cost, given transaction costs and any other political, legal, or informational constraints that may exist. This report outlines the economic characteristics of five instruments that can be used to reduce agricultural nonpoint source pollution (economic incentives, standards, education, liability, and research) and discusses empirical research related to the use of these instruments.water quality, nonpoint-source pollution, economic incentives, standards, education, liability, research, Environmental Economics and Policy,

Exploring Environmental-Economic Benefits from Agri-Industrial Diversification in the Sugar Industry: An Integrated Land Use and Value Chain Approach

The sugar industry in Queensland (Australia) is confronted with increasing economic pressure and environmental constraints. To explore whether agri-industrial diversification of the sugar industry provides a sustainable development pathway for the region, we develop a spatial environmental-economic approach that integrates a land use and value chain model with a hydrological model. Results indicate that agri-industrial diversification can lead to substantial increases in regional income, while at the same time increasing the resilience of a sugar industry facing decreasing sugar prices. Agri-industrial diversification drives land use diversification, which under current sugar prices does not lead to a reduction in sugarcane production. Water quality benefits from this land use diversification are mixed, and depend on the economic viability and erosion characteristics of the concerned production systems.spatial economics, environmental economics, value chains, agri-industries, water quality, Environmental Economics and Policy, Land Economics/Use, C6, O18, Q13, Q53,

Heterogeneity in 12^{12}CO/13^{13}CO Ratios Toward Solar-Type Young Stellar Objects

This study reports an unusual heterogeneity in [$^{12}$C$^{16}$O]/[$^{13}$C$^{16}$O] abundance ratios of carbon monoxide observed in the gas phase toward seven ~ solar-mass YSOs and three dense foreground clouds in the nearby star-forming regions, Ophiuchus, Corona Australis, Orion, Vela and an isolated core, L43. Robust isotope ratios were derived using infrared absorption spectroscopy of the 4.7 $\mu$m fundamental and 2.3 $\mu$m overtone rovibrational bands of CO at very high resolution ($\lambda$/$\Delta$$\lambda\approx 95,000$), observed with the CRIRES spectrograph on the Very Large Telescope. We find [$^{12}$C$^{16}$O]/[$^{13}$C$^{16}$O] values ranging from ~ 85 to 165, significantly higher than those of the local interstellar medium (~ 65 to 69). These observations are evidence for isotopic heterogeneity in carbon reservoirs in solar-type YSO environments, and encourage the need for refined Galactic chemical evolution models to explain the $^{12}$C/$^{13}$C discrepancy between the solar system and local ISM. The oxygen isotope ratios are consistent with isotopologue-specific photodissociation by CO self-shielding toward the disks, VV CrA N and HL Tau, further substantiating models predicting CO self-shielding on disk surfaces. However, we find that CO self-shielding is an unlikely general explanation for the high [$^{12}$C$^{16}$O]/[$^{13}$C$^{16}$O] ratios observed in this study. Comparison of the solid CO against gas-phase [$^{12}$C$^{16}$O]/[$^{13}$C$^{16}$O] suggests that interactions between CO ice and gas reservoirs need to be further investigated as at least a partial explanation for the unusually high [$^{12}$C$^{16}$O]/[$^{13}$C$^{16}$O] observed.Comment: 16 pages, 14 figures, 7 tables. Accepted for publication in The Astrophysical Journa

Propulsion and Cryogenics Advanced Development (PCAD) Project Propulsion Technologies for the Lunar Lander

The Propulsion and Cryogenics Advanced Development (PCAD) Project in the Exploration Technology Development Program is developing technologies as risk mitigation for Orion and the Lunar Lander. An integrated main and reaction control propulsion system has been identified as a candidate for the Lunar Lander Ascent Module. The propellants used in this integrated system are Liquid Oxygen (LOX)/Liquid Methane (LCH4) propellants. A deep throttle pump fed Liquid Oxygen (LOX)/Liquid Hydrogen (LH2) engine system has been identified for the Lunar Lander Descent Vehicle. The propellant combination and architecture of these propulsion systems are novel and would require risk reduction prior to detailed design and development. The PCAD Project addresses the technology requirements to obtain relevant and necessary test data to further the technology maturity of propulsion hardware utilizing these propellants. This plan and achievements to date will be presented

Technological Change and the Design of Plant Variety Protection Regimes

In this paper, we examine the potential for plant variety protection ( PVP ) regimes—that is, sui generis, industry-specific intellectual property regimes—to become compromised as a result of technological change. In particular, we analyze the shift in plant breeding from phenotypic selection to genotypic selection, and consider the impact of that shift on existing plant variety protection. We also lay out an alternative structure for plant intellectual property protection based on unfair competition, a model that differs radically in some respects from current PVP schemes. We offer our model as a starting point for debate on adaptations that might improve PVP schemes, whether those adaptations be systemic or (more likely) incremental changes to existing rules and practices

Janus Scorpionates: Supramolecular Tectons for the Directed Assembly of Hard−Soft Alkali Metallopolymer Chains

A new scorpionate ligand [HB(mtda)3-] containing mercaptothiadiazolyl (mtda) heterocyclic rings with both hard nitrogen donors and soft sulfur donors has been prepared. This new ligand, the Janus scorpionate, is a hybrid of a tris(pyrazolyl)borate and a tris(mercaptoimidazolyl)borate. The differential hard/soft character of the dissimilar donor groups in this bridging ligand was exploited for the controlled solid-state organization of homometallic and heterometallic alkali metal coordination polymers. Remarkably, in the case of sodium, coordination polymers with both acentric (with NaS3N3H kernels) and centric (with alternating NaN6 and NaS6H2 kernels) chains are found in the same crystal (where the centricity is defined by the relative orientations of the B−H bonds of the ligands along the lattice). For the homometallic potassium congener, the larger cation size, compared to sodium, induced significant distortions and favored a polar arrangement of ligands in the resulting coordination polymer chain. An examination of the solid-state structure of the mixed alkali metal salt system revealed that synergistic binding of smaller sodium cations to the nitrogen portion and of the larger potassium cations to the sulfur portion of the ligand minimizes the ligand distortions relative to the homometallic coordination polymer counterparts, a design feature of the ligand that likely assists in thermodynamically driving the self-assembly of the heterometallic chains. The effect of alkali metal complexation on the solution properties of the ligand was studied by comparing NMR chemical shifts, B−H stretching frequencies, and electrochemical properties with those of the noncoordinating tetrabutylammonium salt of the scorpionate. The similarity of these data regardless of cation indicates that the salts are likely dissociated in solution rather than maintaining their solid-state polymeric structures. This data is augmented by the ESI(±) mass spectral data for a series of mixed alkali metal tris(mercaptothiadiazolyl)borates that also indicate that dissociation occurs in solution

Toward Charge-neutral ‘soft scorpionates’: Coordination Chemistry and Lewis Acid Promoted Isomerization of tris(1-organo-imidazol-2-ylthio)methanes

Two tris(1-organo-imidazol-2-ylthio)methanes, HC(S-timR)3 (R = organo = methyl, tert-butyl), have been prepared by a triphasic reaction between chloroform, the appropriate heterocycle, and saturated aqueous solutions of Na2CO3, in the presence of a phase transfer agent, (NBu4)(Br). These ligands have been characterized both spectroscopically and by single crystal X-ray diffraction. The reaction chemistry of these potentially N,N,N-tripodal ligands with AgBF4 was also explored where simple 1:1 coordination complexes could be isolated from reactions performed in THF solution at room temperature. The derivative {Ag[HC(S-timMe)3]}(BF4) was structurally characterized which showed that the ligand binds in a μ–κ2N,κ1N-mode to give a coordination polymer with an interesting layered supramolecular structure. Surprisingly, heating CH3CN solutions of the silver complexes at reflux resulted in decomposition of the complex and concomitant isomerization of the ligands to give metal-free tris(3-organo-1-imidazole-2-thione)methane, HC(N-imtR)3; the heretofore elusive charge-neutral analogues of the well-studied ‘soft scorpionate’ TmR− anions. The solution isomerization of HC(S-timR)3 to HC(N-imtR)3 was found to be general, occurring in a variety of solvents with any of a host of different Lewis acids [para-toluenesulfonic acid, KPF6, and M(CO)5Br (M = Mn, Re)] but did not occur by heating in the absence of Lewis acid. The compound HC(N-imtMe)3 exhibited unusually low solubility in common organic solvents. Single crystal X-ray diffraction of HC(N-imtMe)3 revealed a remarkable honeycomb supramolecular structure with ca. 5 Å channels filled with solvent. The robust nature of this solid is a result of strong dipolar stacking interactions of molecules into polymer chains bolstered by concerted π–π and CH–π interactions involving the heterocycles, holding the chains together in the remaining two dimensions