Control of star formation by supersonic turbulence

Understanding the formation of stars in galaxies is central to much of modern astrophysics. For several decades it has been thought that stellar birth is primarily controlled by the interplay between gravity and magnetostatic support, modulated by ambipolar diffusion. Recently, however, both observational and numerical work has begun to suggest that support by supersonic turbulence rather than magnetic fields controls star formation. In this review we outline a new theory of star formation relying on the control by turbulence. We demonstrate that although supersonic turbulence can provide global support, it nevertheless produces density enhancements that allow local collapse. Inefficient, isolated star formation is a hallmark of turbulent support, while efficient, clustered star formation occurs in its absence. The consequences of this theory are then explored for both local star formation and galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28 figures, in pres

Sustainable drainage system site assessment method using urban ecosystem services

The United Kingdom's recently updated approach to sustainable drainage enhanced biodiversity and amenity objectives by incorporating the ecosystem approach and the ecosystem services concept. However, cost-effective and reliable methods to appraise the biodiversity and amenity values of potential sustainable drainage system (SuDS)sites and their surrounding areas are still lacking, as is a method to enable designers to distinguish and link the amenity and biodiversity benefits that SuDS schemes can offer. In this paper, therefore, the authors propose two ecosystem services- and disservices-based methods (i.e. vegetation structure cover-abundance examination and cultural ecosystem services and disservices variables appraisal) to aid SuDS designers to distinguish and link amenity and biodiversity benefits, and allow initial site assessments to be performed in a cost-effective and reliable fashion. Forty-nine representative sites within Greater Manchester were selected to test the two methods. Amenity and biodiversity were successfully assessed and habitat for species, carbon sequestration, recreation and education ecosystem services scores were produced,which will support SuDS retrofit design decision-making. Large vegetated SuDS sites with permanent aquatic features were found to be most capable of enhancing biodiversity- and amenity-related ecosystem services. Habitat for species and recreation ecosystem services were also found to be positively linked to each other. Finally, waste bins on site were found to help reduce dog faeces and litter coverage. Overall, the findings presented here enable future SuDS retrofit designs to be more wildlife friendly and socially inclusive

What controls star formation in the central 500 pc of the Galaxy?

The star formation rate (SFR) in the Central Molecular Zone (CMZ, i.e. the central 500 pc) of the Milky Way is lower by a factor of ≥10 than expected for the substantial amount of dense gas it contains, which challenges current star formation theories. In this paper, we quantify which physical mechanisms could be responsible. On scales larger than the disc scaleheight, the low SFR is found to be consistent with episodic star formation due to secular instabilities or possibly variations of the gas inflow along the Galactic bar. The CMZ is marginally Toomre-stable when including gas and stars, but highly Toomre-stable when only accounting for the gas, indicating a low condensation rate of self-gravitating clouds. On small scales, we find that the SFR in the CMZ may be caused by an elevated critical density for star formation due to the high turbulent pressure. The existence of a universal density threshold for star formation is ruled out. The H I–H2 phase transition of hydrogen, the tidal field, a possible underproduction of massive stars due to a bottom-heavy initial mass function, magnetic fields, and cosmic ray or radiation pressure feedback also cannot individually explain the low SFR. We propose a self-consistent cycle of star formation in the CMZ, in which the effects of several different processes combine to inhibit star formation. The rate-limiting factor is the slow evolution of the gas towards collapse – once star formation is initiated it proceeds at a normal rate. The ubiquity of star formation inhibitors suggests that a lowered central SFR should be a common phenomenon in other galaxies. We discuss the implications for galactic-scale star formation and supermassive black hole growth, and relate our results to the star formation conditions in other extreme environments

Integrated Blue and Green Corridor Restoration in Strasbourg: Green Toads, Citizens, and Long-Term Issues

The Ostwaldergraben is an urban stream located in Strasbourg (northeast of France). Mostly fed by groundwater, it was enlarged some forty years ago, which led to a radical alteration of the flow dynamics and a strong siltation. According to the European Water Framework, the stream displayed a bad status with sediments polluted by discharges of former tanneries. Hence, a project of restoration-both of the stream and the adjacent wasteland-was launched by the City of Strasbourg in 2010 to solve these issues of environmental degradation in accordance with the European regulation. The stream bed was redesigned to energize the flows and to create meanders and vegetated benches. To improve the connectivity between two adjacent wetlands, new habitats and a network of ponds have been created. A hybrid type of stormwater treatment system-a pond followed by a constructed wetland-was implemented to complete the restoration project. In this chapter, we propose to study this project from its construction to its current development, through the lens of ecological engineering and a perspective on long-term issues. We aim at illustrating the facts that nature-based solution management can differ from technological management and that the ecosystem services provided by a nature-based solution result from trade-offs, which requires a global analysis of such restoration project. To reach this goal, the project will be studied from ecological, engineering, and sociological perspectives. Our study shows that the restored socio-ecosystem works on a rustic basis and provides several ecosystem services: supporting services (habitat for amphibians), regulating services (water quality enhancement), and cultural services (urban landscape greening)