Green infrastructure (GI) revitalizes vegetation and soil, restores hydro-ecological processes destroyed by traditional urbanization, and naturally manages stormwater on-site, offering numerous ...sustainability benefits. However, despite being sustainable and despite being the object of unrelenting expert advocacy for more than two decades, GI implementation remains slow. On the other hand, the practice of traditional gray infrastructure, which is known to have significant adverse impacts on the environment, is still ubiquitous in urban areas throughout the world. This relationship between knowledge and practice seems unaccountable, which has not yet received adequate attention from academia, policy makers, or research communities. We deal with this problem in this paper. The specific objective of the paper is to explore the barriers to GI, and suggest policies that can both overcome these barriers and expedite implementation. By surveying the status of implementation in 10 US cities and assessing the relevant city, state and federal policies, we identified 29 barriers and grouped them into 5 categories. The findings show that most of the barriers stem from cognitive limitations and socio-institutional arrangements. Accordingly, we suggest 33 policies, also grouped into 5 categories, which span from conducting public education and awareness programs to changing policies and governance structures.
•Green infrastructure manages urban stormwater on-site and offers numerous sustainability benefits.•Unfortunately, the pace of implementation of the technology is frustratingly slow.•We explore the barriers to implementation of the technology.•We suggest policies that can both overcome the barriers and expedite implementation.
Peristalsis enables transport of the food bolus in the gut. Here, I show by dynamic ex vivo intra-cellular calcium imaging on living embryonic gut explants that the most primitive form of peristalsis ...that occurs in the embryo is the result of inter-cellular, gap-junction-dependent calcium waves that propagate in the circular smooth muscle layer. I show that the embryonic gut is an intrinsically mechanosensitive organ, as the slightest externally applied mechanical stimulus triggers contractile waves. This dynamic response is an embryonic precursor of the ‘law of the intestine’ (peristaltic reflex). I show how characteristic features of early peristalsis such as counter-propagating wave annihilation, mechanosensitivity and nucleation after wounding all result from known properties of calcium waves. I finally demonstrate that inter-cellular mechanical tension does not play a role in the propagation mechanism of gut contractile waves, unlike what has been recently shown for the embryonic heartbeat. Calcium waves are a ubiquitous dynamic signalling mechanism in biology: here I show that they are the foundation of digestive movements in the developing embryo.
This article is part of the Theo Murphy meeting issue on ‘Mechanics of development’.
The gastrointestinal tract transports the food bolus by peristalsis. Gut motility starts at an early age in the developing embryo, well before it is required for nutrition of the organism. We present ...a comprehensive kinematic study of the emergence and physiological development of gut motility in all regions of the lower digestive tract of the chicken embryo from embryonic days E5 through E9. We characterized motility emergence time, propagation patterns, speed, frequency and amplitude of peristalsis waves. We found that the emergence of an uninterrupted circular ring of smooth muscle correlated with the appearance of propagative contractile waves, at E6 in the hindgut and midgut, and at E9 in the caecal appendix. We show that peristalsis at these stages is critically dependent on calcium and is not mediated by neurons as gut motility is insensitive to tetrodotoxin and takes place in the hindgut in the absence of neurons. We further demonstrate that motility also matures in ex-vivo organ culture. We compare our results to existing literature on zebrafish, mouse and human motility development, and discuss their chronological relationship with other major developmental events occurring in the chicken embryonic gut at these stages. Our work sets a baseline for further investigations of motility development in this important animal model.
Over the past decade, long-duration γ-ray bursts (GRBs)-including the subclass of X-ray flashes (XRFs)-have been revealed to be a rare variety of type Ibc supernova. Although all these events result ...from the death of massive stars, the electromagnetic luminosities of GRBs and XRFs exceed those of ordinary type Ibc supernovae by many orders of magnitude. The essential physical process that causes a dying star to produce a GRB or XRF, and not just a supernova, is still unknown. Here we report radio and X-ray observations of XRF 060218 (associated with supernova SN 2006aj), the second-nearest GRB identified until now. We show that this event is a hundred times less energetic but ten times more common than cosmological GRBs. Moreover, it is distinguished from ordinary type Ibc supernovae by the presence of 1048 erg coupled to mildly relativistic ejecta, along with a central engine (an accretion-fed, rapidly rotating compact source) that produces X-rays for weeks after the explosion. This suggests that the production of relativistic ejecta is the key physical distinction between GRBs or XRFs and ordinary supernovae, while the nature of the central engine (black hole or magnetar) may distinguish typical bursts from low-luminosity, spherical events like XRF 060218.
ABSTRACT Due to its proximity, SN 1987A offers a unique opportunity to directly observe the geometry of a stellar explosion as it unfolds. Here we present spectral and imaging observations of SN ...1987A obtained ∼10,000 days after the explosion with HST/STIS and VLT/SINFONI at optical and near-infrared wavelengths. These observations allow us to produce the most detailed 3D map of H to date, the first 3D maps for Ca ii , O i , and Mg ii , as well as new maps for Si i+Fe ii and He i 2.058 m. A comparison with previous observations shows that the Si i+Fe ii flux and morphology have not changed significantly during the past ten years, providing evidence that this line is powered by 44Ti. The time evolution of H shows that it is predominantly powered by X-rays from the ring, in agreement with previous findings. All lines that have sufficient signal show a similar large-scale 3D structure, with a north-south asymmetry that resembles a broken dipole. This structure correlates with early observations of asymmetries, showing that there is a global asymmetry that extends from the inner core to the outer envelope. On smaller scales, the two brightest lines, H and Si i+Fe ii 1.644 m, show substructures at the level of ∼200-1000 km s-1and clear differences in their 3D geometries. We discuss these results in the context of explosion models and the properties of dust in the ejecta.
The nearby SN 1987A offers a spatially resolved view of the evolution of a young supernova (SN) remnant. Here we present recent Hubble Space Telescope imaging observations of SN 1987A, which we use ...to study the evolution of the ejecta, the circumstellar equatorial ring (ER), and the increasing emission from material outside the ER. We find that the inner ejecta have been brightening at a gradually slower rate and that the western side has been brighter than the eastern side since ∼7000 days. This is expected given that the X-rays from the ER are most likely powering the ejecta emission. At the same time, the optical emission from the ER continues to fade linearly with time. The ER is expanding at 680 50 km s−1, which reflects the typical velocity of transmitted shocks in the dense hot spots. A dozen spots and a rim of diffuse H emission have appeared outside the ER since 9500 days. The new spots are more than an order of magnitude fainter than the spots in the ER and also fade faster. We show that the spots and diffuse emission outside the ER may be explained by fast ejecta interacting with high-latitude material that extends from the ER toward the outer rings. Further observations of this emission will make it possible to determine the detailed geometry of the high-latitude material and provide insight into the formation of the rings and the mass-loss history of the progenitor.
The final chapter in the long-standing mystery of the gamma-ray bursts (GRBs) centres on the origin of the short-hard class of bursts, which are suspected on theoretical grounds to result from the ...coalescence of neutron-star or black-hole binary systems. Numerous searches for the afterglows of short-hard bursts have been made, galvanized by the revolution in our understanding of long-duration GRBs that followed the discovery in 1997 of their broadband (X-ray, optical and radio) afterglow emission. Here we present the discovery of the X-ray afterglow of a short-hard burst, GRB 050709, whose accurate position allows us to associate it unambiguously with a star-forming galaxy at redshift z = 0.160, and whose optical lightcurve definitively excludes a supernova association. Together with results from three other recent short-hard bursts, this suggests that short-hard bursts release much less energy than the long-duration GRBs. Models requiring young stellar populations, such as magnetars and collapsars, are ruled out, while coalescing degenerate binaries remain the most promising progenitor candidates.
We report extensive radio and X-ray observations of SN 2003bg, whose spectroscopic evolution shows a transition from a broad-lined Type Ic to a hydrogen-rich Type II, and later to a typical ...hydrogen-poor Type Ibc. We show that the extraordinarily luminous radio emission is well described by a self-absorption-dominated synchrotron spectrum, while the observed X-ray emission at t-30 days is adequately fit by inverse Compton scattering of the optical photons off of the synchrotron-emitting electrons. Our radio model implies a subrelativistic ejecta velocity, -0.24c, at t sub(0)-10 days after the explosion, which emphasizes that broad optical absorption lines do not imply relativistic ejecta. We find that the total energy of the radio-emitting region evolves as E-7.3 x 10 super(48)(t/t sub(0)) super(0.4) ergs, assuming equipartition of energy between relativistic electrons and magnetic fields (e sub(e) = e sub(B) = 0.1). The circumstellar density is well described by a stellar wind profile, with modest (factor of 62) episodic density enhancements that produce abrupt achromatic flux variations. We estimate an average mass-loss rate of -3 x 10 super(-4) M sub( )yr super(-1) (assuming a wind velocity of u sub(w) = 10 super(3) km s super(-1)) for the progenitor, consistent with the observed values for Galactic Wolf-Rayet stars. Comparison with other events reveals that 650% of radio supernovae show similar short-timescale flux variations, attributable to circumstellar density irregularities. Specifically, the radio light curves of SN 2003bg are strikingly similar to those of the Type IIb SN 2001ig, suggestive of a common progenitor evolution for these two events. Based on the relative intensity of the inferred density enhancements, we conclude that the progenitors of SNe 2003bg and 2001ig experienced quasi-periodic mass-loss episodes just prior to the SN explosion. Finally, this study emphasizes that abrupt radio light-curve variations cannot be used as a reliable proxy for an engine-driven explosion, including off-axis gamma-ray bursts.
Most massive stars end their lives in core-collapse supernova explosions and enrich the interstellar medium with explosively nucleosynthesized elements. Following core collapse, the explosion is ...subject to instabilities as the shock propagates outward through the progenitor star. Observations of the composition and structure of the innermost regions of a core-collapse supernova provide a direct probe of the instabilities and nucleosynthetic products. SN 1987A in the Large Magellanic Cloud is one of very few supernovae for which the inner ejecta can be spatially resolved but are not yet strongly affected by interaction with the surroundings. Our observations of SN 1987A with the Atacama Large Millimeter/submillimeter Array are of the highest resolution to date and reveal the detailed morphology of cold molecular gas in the innermost regions of the remnant. The 3D distributions of carbon and silicon monoxide (CO and SiO) emission differ, but both have a central deficit, or torus-like distribution, possibly a result of radioactive heating during the first weeks ("nickel heating"). The size scales of the clumpy distribution are compared quantitatively to models, demonstrating how progenitor and explosion physics can be constrained.
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