Abstract
The solution of a non-radiative, strong, spherical blast wave has been derived from an approximate model of a low-density hot bubble which drives a thin shell (which contains most of the ...displaced environmental material). We show that this model can be extended to the case in which the blast wave is no longer strong, leading to a full analytic solution. We then compare this analytic model with a numerical solution of the full spherically symmetric Euler equations. Finally, we discuss possible applications of the analytic model to supernovae that explode within pre-existing hot bubbles, or inside dense molecular clouds.
We present 3D hydrodynamical simulations of the superbubble M17, also known as the Omega Nebula, carried out with the adaptive grid code yguazú-a, which includes radiative cooling. The superbubble is ...modelled considering the winds of 11 individual stars from the open cluster inside the nebula (NGC 6618), for which there are estimates of the mass-loss rates and terminal velocities based on their spectral types. These stars are located inside a dense interstellar medium, and they are bounded by two dense molecular clouds. We carried out three numerical models of this scenario, considering different line-of-sight positions of the stars (the position in the plane of the sky is known, thus fixed). Synthetic thermal X-ray emission maps are calculated from the numerical models and compared with ROSAT observations of this astrophysical object. Our models successfully reproduce both the observed X-ray morphology and the total X-ray luminosity, without taking into account the thermal conduction effects.
Within rhizobia, two species nodulating chickpea, Mesorhizobium ciceri and Mesorhizobium mediterraneum, are known as good phosphate solubilizers. For this reason, we have analysed the ability to ...solubilize phosphate of a wide number of strains isolated from Cicer arietinum growing in several soils in Spain. The aim of this work was to analyse microbial populations nodulating chickpea, that are able to solubilize phosphates, using molecular techniques. In the present work we analyzed 19 strains isolated from effective nodules of C. arietinum growing in three soils from the North of Spain. Nineteen strains showed ability to solubilize phosphate in YED-P medium. These strains were separated into 4 groups according to the results obtained by 879F-RAPD fingerprinting. The 16S rDNA sequencing of a representative strain from each group allowed the identification of strains as belonging to the genus Mesorhizobium. Strains from groups I and II showed a 99.4% and 99.2% similarity with M. mediterraneum UPM-CA142T, respectively. The strains from group III were related to M. tianshanense USDA 3592T at a 99.4% similarity level. Finally, the strain from group IV was related to M. ciceri USDA 3383T with a 99.3% similarity. The LMW RNA profiles confirmed these results. Strains from groups I and II showed an identical LMW RNA profile to that of M. mediterraneum UPM-CA142T; the profile of strains from group III was identical to that of M. tianshanense USDA 3592T and the profile of strains from group IV was identical to that of M. ciceri USDA 3383T. Different 879F-RAPD patterns were obtained for strains of the group I, group II and the M. mediterraneum type strain (UPM-CA142T). The 879-RAPD patterns obtained for group III also differed from the pattern shown by M. tianshanense USDA 3592T. Finally, the patterns between group IV and M. ciceri USDA 3383T were also different. These results suggest that groups I and II may be subspecies of M. mediterraneum, group III a subspecies of M. tianshanense and group IV a subspecies of M. ciceri. Nevertheless, more studies are needed to establish the taxonomic status of strains isolated in this study.
We report deep long-slit emission-line spectra, the line flux ratios, and Doppler profile shapes of various bright optical lines. The low-ionization lines (primarily NI, OI, SII, and NII) originate ...in shocked knots, as reported by many previous observers. Dust-scattered lines of higher ionization are seen throughout the lobes but do not peak in the knots. Our analysis of these line profiles and the readily discernible stellar continuum shows that (1) the central star is an active symbiotic (whose spectrum resembles the central stars of highly bipolar and young planetary nebulae such as M2-9 and Hen2-437) whose compact companion shows a WC8-type spectrum, (2) extended nebular lines of OIII and He I originate in the heavily obscured nuclear H II region, and (3) the Balmer lines observed throughout the lobes are dominated by reflected H alpha emission from the symbiotic star. Comparing our line ratios with those observed historically shows that (1) the OIII/H beta and He I/H beta ratios have been steadily rising by large amounts throughout the nebula, (2) the H alpha /H beta ratio is steadily decreasing while H gamma /H beta remains nearly constant, and (3) the low-ionization line ratios formed in the shocked knots have been in decline in different ways at various locations. We show that the first two of these results might be expected if the symbiotic central star has been active and if its bright H alpha line has faded significantly in the past 20 years.
To identify several strains of Mesorhizobium amorphae and Mesorhizobium tianshanense nodulating Cicer arietinum in Spain and Portugal, and to study the symbiotic genes carried by these strains. The ...sequences of 16S-23S intergenic spacer (ITS), 16S rRNA gene and symbiotic genes nodC and nifH were analysed. According to their 16S rRNA gene and ITS sequences, the strains from this study were identified as M. amorphae and M. tianshanense. The type strains of these species were isolated in China from Glycyrrhiza pallidiflora and Amorpha fruticosa nodules, respectively, and are not capable of nodulating chickpea. These strains carry symbiotic genes, phylogenetically divergent from those of the chickpea isolates, whose nodC and nifH genes showed more than 99% similarity with respect to those from Mesorhizobium ciceri and Mesorhizobium mediterraneum, the two common chickpea nodulating species in Spain and Portugal. The results from this study showed that different symbiotic genes have been acquired by strains from the same species during their coevolution with different legumes in distinct geographical locations. A new infrasubspecific division named biovar ciceri is proposed within M. amorphae and M. tianshanense to include the strains able to effectively nodulate Cicer arietinum.
We present the analytic and numerical models of the ‘cluster wind’ resulting from the multiple interactions of the winds ejected by the stars of a dense cluster of massive stars. We consider the case ...in which the distribution of stars (i.e. the number of stars per unit volume) within the cluster is spherically symmetric, has a power-law radial dependence, and drops discontinuously to zero at the outer radius of the cluster. We carry out comparisons between an analytic model (in which the stars are considered in terms of a spatially continuous injection of mass and energy) and 3D gasdynamic simulations (in which we include 100 stars with identical winds, located in 3D space by statistically sampling the stellar distribution function). From the analytic model, we find that for stellar distributions with steep enough radial dependencies, the cluster wind flow develops a very high central density and a non-zero central velocity, and for steeper dependencies, it becomes fully supersonic throughout the volume of the cluster (these properties are partially reproduced by the 3D numerical simulations). Therefore, the wind solutions obtained for stratified clusters can differ dramatically from the case of a homogeneous stellar distribution (which produces a cluster wind with zero central velocity, and a fully subsonic flow within the cluster radius). Finally, from our numerical simulations, we compute predictions of X-ray emission maps and luminosities, which can be directly compared with observations of cluster wind flows.
Despite the interest of phyllosphere microbiology, no studies have addressed the bacteria present in bract phyllosphere, an ecosystem that has special characteristics in palm trees because the dry ...bracts remain on the plant until pruning and may contain polymer-degrading bacteria involved in plant degradation. Therefore, the aim of this work was to characterize xylanolytic bacteria isolated from palm bract phyllosphere. Twelve xylanolytic strains were isolated and characterized by phenotypic features and complete sequencing of 16S rRNA gene. The results showed that the isolates were phenotypically and genotypically diverse. Gram-positive isolates were classified into genus Paenibacillus some of them belonging to hitherto undescribed species of this genus. Gram-negative isolates were classified into genera Pseudomonas and Acinetobacter. The results of this work confirm the complexity of the bacterial populations present in phyllospheric ecosystems and suggest that bacteria involved in plant degradation are present at the early degradation steps of this process in dry palm tree bracts. This is the first study on bract phyllospheric bacteria able to hydrolyse vegetal polymers and offers a new perspective in the search of unexplored sources of xylanase-producing strains.
PDF
