Melting as a String-Mediated Phase Transition

We present a theory of the melting of elemental solids as a dislocation-mediated phase transition. We model dislocations near melt as non-interacting closed strings on a lattice. In this framework we derive simple expressions for the melting temperature and latent heat of fusion that depend on the dislocation density at melt. We use experimental data for more than half the elements in the Periodic Table to determine the dislocation density from both relations. Melting temperatures yield a dislocation density of (0.61\pm 0.20) b^{-2}, in good agreement with the density obtained from latent heats, (0.66\pm 0.11) b^{-2}, where b is the length of the smallest perfect-dislocation Burgers vector. Melting corresponds to the situation where, on average, half of the atoms are within a dislocation core.Comment: 18 pages, LaTeX, 3 eps figures, to appear in Phys. Rev.

The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets

This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics

Whole-genome sequencing reveals host factors underlying critical COVID-19

Critical COVID-19 is caused by immune-mediated inflammatory lung injury. Host genetic variation influences the development of illness requiring critical care1 or hospitalization2,3,4 after infection with SARS-CoV-2. The GenOMICC (Genetics of Mortality in Critical Care) study enables the comparison of genomes from individuals who are critically ill with those of population controls to find underlying disease mechanisms. Here we use whole-genome sequencing in 7,491 critically ill individuals compared with 48,400 controls to discover and replicate 23 independent variants that significantly predispose to critical COVID-19. We identify 16 new independent associations, including variants within genes that are involved in interferon signalling (IL10RB and PLSCR1), leucocyte differentiation (BCL11A) and blood-type antigen secretor status (FUT2). Using transcriptome-wide association and colocalization to infer the effect of gene expression on disease severity, we find evidence that implicates multiple genes—including reduced expression of a membrane flippase (ATP11A), and increased expression of a mucin (MUC1)—in critical disease. Mendelian randomization provides evidence in support of causal roles for myeloid cell adhesion molecules (SELE, ICAM5 and CD209) and the coagulation factor F8, all of which are potentially druggable targets. Our results are broadly consistent with a multi-component model of COVID-19 pathophysiology, in which at least two distinct mechanisms can predispose to life-threatening disease: failure to control viral replication; or an enhanced tendency towards pulmonary inflammation and intravascular coagulation. We show that comparison between cases of critical illness and population controls is highly efficient for the detection of therapeutically relevant mechanisms of disease

The changing flora of Bedfordshire 1949-2004: Dony's habitat studies revisited

The new Flora of Bedfordshire has now been published, following years of recording by members of the Society. Grass-of-Parnassus painted by Caroline Gaye in 1844 County Floras have been and still are an important feature of Britain’s botanical scene. The Flora of Bedfordshire will provide an up-to-date account of the status and distribution of the flowering plants and mosses of the county, and place their occurrence in the context of over 400 years of study of the local flora. For Bedfordshire it follows in a long line of such works, from Charles Abbot’s Flora Bedfordiensis in 1798, to James Saunders’ Field Flowers of Bedfordshire of 1911 and, more recently John Dony’s masterly Flora of Bedfordshire of 1953 and Bedfordshire Plant Atlas of 1976. The authors, Chris Boon for the flowering plants, and Alan Outen for the mosses, have each spent many years studying their fields of natural history, and are not only recorders for the county, but are also involved with their respective national societies. The flora is the culmination of more than 20 years of a comprehensive programme of recording in the county by the BNHS. There are introductory chapters on many aspects pertaining to the flora of the county including geology, soils, conservation of the countryside and changes in the flora over the last 50 years. Also given are accounts of the history of botany in Bedfordshire, and an overview of the botanical hotspots. Significant records are listed, and the distribution maps for each of the 2,250 species that have ever been recorded in the county show their presence in areas of 2x2km. A unique feature is the indication on the maps of the distribution data from an earlier survey in the 1970s by John Dony, the distinguished former county botanist, showing clearly the changes that have taken place

Impacts of nitrogen deposition on vascular plants in Britain: an analysis of two national observation networks

Large areas of Great Britain currently have nitrogen (N) deposition at rates which exceed the thresholds above which there is risk of damage to sensitive components of the ecosystem (critical loads). Previous studies have focussed primarily on the relationship of species richness to nitrogen, whereas here we look at individual species. We used data from two national observation networks over Great Britain to examine the response of individual vascular plant species to N in acid grasslands, calcareous grasslands and heathlands. Presence absence records of individual species, along with mean Ellenberg N scores, within 10 km hectads were modelled against N deposition whilst at the same time controlling for the effects of climate, land use and sulphur deposition using generalised additive models. Ellenberg N showed a significant increase with increasing N deposition in almost all habitats across both surveys indicating increased fertility. Many individual species showed strong relationships with N deposition and clear negative trends in species prevalence to increasing nitrogen were found in all habitats. A number of these species were either habitat dominants or possessed traits known to be influential in controlling ecosystem function. Many community dominants showing significant negative relationships with N deposition highlight a potentially significant loss of function. Some species that showed negative relationships to N showed signs of decline at low levels, far below the current critical load levels. Some species also showed continuous changes as N deposition levels rose above the current critical load values. This work contributes to the growing evidence base suggesting species level impacts at low N deposition values

Collation of evidence of nitrogen impacts on vegetation in relation to UK biodiversity objectives.

This report presents the largest analysis of national vegetation data sets in relation to nitrogen (N) deposition carried out to date. The study statistically analysed eight independent national vegetation surveillance datasets using a consistent approach, to identify evidence of N deposition impacts in four habitat types; acidic and calcareous grassland, heathland and bogs. This new analysis was the largest of its kind, using data from the Vascular Plant Database (1930-1969 and 1987-1999), BSBI Local Change Survey (1987-1988 and 2003-2004), British Bryological Society, British Lichen Society, Plantlife Common Plant Survey, and three country grassland datasets. The project took four widespread habitats, calcareous and acid grasslands, heathlands, and bogs, and for each, looked for evidence of vegetation change caused by nitrogen deposition, after accounting for other factors that could have an impact. Spatial analysis, and where possible temporal analysis, was conducted to analyse the relationship between N deposition and summary variables (Ellenberg scores, canopy height, specific leaf area), species diversity (species richness, Shannon-Wiener diversity index) and individual species occurrence. The study clearly demonstrated significant changes in the abundance of individual species and variables describing habitat structure and function as a result of N deposition. This, together with previous evidence from surveys and experiments, provides clear evidence of species-level impacts of N deposition in all four of the habitats investigated. Results are presented individually for each of the datasets examined. The discussion section draws the results together examining them for each habitat. Key results are provided in the results section of the report. A complete set of all significant results are presented in the Electronic Appendix. The analysis presented in Report 447 represents the first part of a large study of nitrogen impacts on vegetation. In a second part of the project, reported in JNCC Report 449, the results of this new analysis, together with other sources of independent evidence, are interpreted in respect of implications for “conservation commitments”. A project summary, presenting the key messages and summarising the impacts, is also provided