Orbital Migration of Interacting Stellar Mass Black Holes in Disks around Supermassive Black Holes

The merger rate of stellar-mass black hole binaries (sBHBs) inferred by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) suggests the need for an efficient source of sBHB formation. Active galactic nucleus (AGN) disks are a promising location for the formation of these sBHBs, as well as binaries of other compact objects, because of powerful torques exerted by the gas disk. These gas torques cause orbiting compact objects to migrate towards regions in the disk where inward and outward torques cancel, known as migration traps. We simulate the migration of stellar mass black holes in an example of a model AGN disk, using an augmented N-body code that includes analytic approximations to migration torques, stochastic gravitational forces exerted by turbulent density fluctuations in the disk, and inclination and eccentricity dampening produced by passages through the gas disk, in addition to the standard gravitational forces between objects. We find that sBHBs form rapidly in our model disk as stellar-mass black holes migrate towards the migration trap. These sBHBs are likely to subsequently merge on short time-scales. The process continues, leading to the build-up of a population of over-massive stellar-mass black holes. The formation of sBHBs in AGN disks could contribute significantly to the sBHB merger rate inferred by LIGO.Comment: 18 pages, 13 figures, Accepted to Ap

The Fifth Stage in Water Management: Policy Lessons for Water Governance

Effective management of water resources is a critical policy issue globally. Using a framework developed by Turton, and a common set of characteristics describing key stages of water demand, we examine the effectiveness of isolated technical (e.g. irrigation upgrades) and allocative (e.g. buyback) efficiency for reducing water demand to sustainable levels. We base our analysis on Australia's water reform context which offers an advanced example of applying these levers to achieve allocative and technical efficiency. The study is motivated by appreciation of the benefits from increased policy flexibility and adaptability in response to: potential transformations toward inflexible production systems; uncertainty associated with impacts of climate change on future water reliability; and the need for increased possible future equity between uses/users (productive/consumptive, environmental, cultural). Our results highlight risk issues that rarely feature in current policy assurance reviews or performance assessments, and enables a clearer representation of uncertainty in future policy choices in many global settings with respect to water demand reduction.A. Loch, D. Adamson, N.P. Dumbrel

Trading water to improve environmental flow outcomes

As consumptive extractions and water scarcity pressures brought about by climate change increase in many world river basins, so do the risks to water-dependent ecological assets. In response, public or not for profit environmental water holders (EWHs) have been established in many areas and bestowed with endowments of water and mandates to manage water for ecological outcomes. Water scarcity has also increasingly spawned water trade arrangements in many river basins, and in many instances, EWHs are now operating in water markets. A number of EWHs, especially in Australia, begin with an endowment of permanent water entitlements purchased from irrigators. Such water entitlements typically have relatively constant interannual supply profiles that often do not match ecological water demand involving flood pulses and periods of drying. This article develops a hydrologic-economic simulation model of the Murrumbidgee catchment within the Murray-Darling Basin to assess the scope of possibilities to improve environmental outcomes through EWH trading on an annual water lease market. We find that there are some modest opportunities for EWHs to improve environmental outcomes through water trade. The best opportunities occur in periods of drought and for ecological outcomes that benefit from moderately large floods. We also assess the extent to which EWH trading in annual water leases may create pecuniary externalities via bidding up or down the water lease prices faced by irrigators. Environmental water trading is found to have relatively small impacts on water market price outcomes. Overall our results suggest that the benefits of developing EWH trading may well justify the costs. © 2013. American Geophysical Union. All Rights Reserved.Jeffery D. Connor, Brad Franklin, Adam Loch, Mac Kirby, Sarah Ann Wheele