The metallicity dependence of envelope inflation in massive stars

Recently it has been found that models of massive stars reach the Eddington limit in their interior, which leads to dilute extended envelopes. We perform a comparative study of the envelope properties of massive stars at different metallicities, with the aim to establish the impact of the stellar metallicity on the effect of envelope inflation. We analyse published grids of core-hydrogen burning massive star models computed with metallicities appropriate for massive stars in the Milky Way, the LMC and the SMC, the very metal poor dwarf galaxy I Zwicky 18, and for metal-free chemical composition. Stellar models of all the investigated metallicities reach and exceed the Eddington limit in their interior, aided by the opacity peaks of iron, helium and hydrogen, and consequently develop inflated envelopes. Envelope inflation leads to a redward bending of the zero-age main sequence and a broadening of the main sequence band in the upper part of the Hertzsprung-Russell diagram. We derive the limiting L/M-values as function of the stellar surface temperature above which inflation occurs, and find them to be larger for lower metallicity. While Galactic models show inflation above ~29 Msun, the corresponding mass limit for Population III stars is ~150 Msun. While the masses of the inflated envelopes are generally small, we find that they can reach 1-100 Msun in models with effective temperatures below ~8000 K, with higher masses reached by models of lower metallicity. Envelope inflation is expected to occur in sufficiently massive stars at all metallicities, and is expected to lead to rapidly growing pulsations, high macroturbulent velocities, and might well be related to the unexplained variability observed in Luminous Blue Variables like S Doradus and Eta Carina.Comment: 16 pages (with Appendix), accepted in A&

Characterisation and neutralisation of Aeromonas hydrophila enterotoxin in the rabbit ileal-loop model

Cell-free culture filtrates and crude enterotoxin preparations from six strains of Aeromonas hydrophiza caused the accumulation of fluid in rabbit ileal loops. This activity was due to a non-dialysable, heat and acid-labile antigenic protein and was lost when culture filtrates and crude enterotoxin preparations were heated at 60°C for 20 min. or 56°C for 30 min. respectively. Maximum activity was observed at pH 8.0-10.0; there was a gradual loss at lower pH and activity was abolished in culture filtrates held at pH 3.0 and crude enterotoxin preparations held at pH 4.0. Titration of the crude enterotoxin preparations in rabbit ileal loops showed that the ED50 (equivalent to 1 unit of toxin) was contained in 25 μg of protein; a logarithmic plot of the neutralisation coefficients against antiserum concentrations showed that one unit of antitoxin was contained in 42×10−4 ml of the antiserum

Enterotoxicity of Aeromonas hydrophila

Live cells and cell-free culture supernates of 50 strains of Aeromonas hydro-phila isolated from diarrhoeic and healthy human faeces, drinking water, sewage, the river Ganges and faeces from domestic animals caused accumulation of fluid in ligated ileal loops of adult rabbits. The amount of fluid produced was comparable to that of a toxigenic strain of Vibrio cholerae. Three of the strains gave positive reactions only after two passages in ileal loops of rabbits. Inocula of about 103 viable cells and 0-25 ml of culture supernate caused fluid accumulation in the loops. The enterotoxic factor was inactivated at 60°C for 20 min. and 65°C for 10 min., was precipitated with ammonium sulphate and was non-dialysable; these results indicate the protein nature of the enterotoxin. An inoculum of 40 μg of crude toxin caused as much fluid accumulation as larger inocula. The only histopathological change in the loops was depletion of mucus from the goblet cells

Low-metallicity massive single stars with rotation. Evolutionary models applicable to I Zwicky 18

Massive rotating single stars with an initial metal composition appropriate for the dwarf galaxy I Zw 18 ([Fe/H]=$-$1.7) are modelled during hydrogen burning for initial masses of 9-300 M$_{\odot}$ and rotational velocities of 0-900 km s$^{-1}$. Internal mixing processes in these models were calibrated based on an observed sample of OB-type stars in the Magellanic Clouds. Even moderately fast rotators, which may be abundant at this metallicity, are found to undergo efficient mixing induced by rotation resulting in quasi chemically-homogeneous evolution. These homogeneously-evolving models reach effective temperatures of up to 90 kK during core hydrogen burning. This, together with their moderate mass-loss rates, make them Transparent Wind Ultraviolet INtense stars (TWUIN star), and their expected numbers might explain the observed HeII ionizing photon flux in I Zw 18 and other low-metallicity HeII galaxies. Our slowly rotating stars above $\sim$80 M$_{\odot}$ evolve into late B- to M-type supergiants during core hydrogen burning, with visual magnitudes up to 19$^{\mathrm{m}}$ at the distance of I Zw 18. Both types of stars, TWUIN stars and luminous late-type supergiants, are only predicted at low metallicity. Massive star evolution at low metallicity is shown to differ qualitatively from that in metal-rich environments. Our grid can be used to interpret observations of local star-forming dwarf galaxies and high-redshift galaxies, as well as the metal-poor components of our Milky Way and its globular clusters.Comment: accepted for publication in A\&

Production of haemolysis and its correlation with enterotoxicity in Aeromonas spp.

A total of 147 clinical and environmental isolates of Aeromonas that included 14 A. hydrophila, 60 A. sobria and 73 A. caviae strains was tested for haemolysin production and its correlation with enterotoxicity; 108 isolates produced β-haemolysis. For A. hydrophila and A. sobria, titres of haemolysin were 16-128 HU/ml and for A. caviae, 16-64 HU/ml. In the ileal loop test, 82 (55.8%) strains of Aeromonas spp. produced enterotoxin. Of the β-haemolytic strains, 72.7% of A. hydrophila, 58.6% of A. sobria and 68.6% of A. caviae isolates caused fluid accumulation in rabbit ileal loops. One strain each of α-haemolytic A. sobria and A. caviae, one of non-haemolytic A. sobria and nine of non-haemolytic A. caviae also caused a secretory response. The β-haemolytic strains caused significantly more (p<0.05) fluid accumulation than the α, β- and non-haemolytic isolates regardless of their species designation. The remaining 65 (44.2%) isolates belonging to the three species included α-, β- and non-haemolytic strains: they failed to cause fluid accumulation in the initial experiments but did so after one to three consecutive passages through rabbit ileal loops. Two -α and 13 non-haemolytic strains switched to production of β-haemolysis when they showed positive ileal loop reactions. However, on repeated subcultures or on storage in the laboratory, all of them reverted to their original haemolytic character and no longer produced enterotoxic activity

Haemagglutinating activity, serum sensitivity and enterotoxigenicity of Aeromonas spp.

Of 97 isolates of Aeromonas spp. that were examined for haemagglutination (HA) and enterotoxigenicity, 35 were from clinical and 62 from environmental sources; 66 of them were also screened for sensitivity to normal human serum (NHS). HA was caused by 44 isolates (45%); it was unrelated to the source of the strain, but it was caused by a higher proportion of the isolates of A. hydrophila than of A. sobria or A. caviae. Of the haemagglutinating strains, 82% were enterotoxigenic, whereas most of the non-haemagglutinating strains were non-toxigenic when tested initially. All the latter became enterotoxin producers after serial passage through rabbit ileal loops, but without change in HA. Most (64%) of the isolates, including 68% of A. caviae (72% of clinical and 65% of environmental), were resistant to the bactericidal action of NHS. Most (92%) of the serum-sensitive strains were killed by activation of both the classical and alternate pathways of complement, the others only by the alternate pathway. Most (74%) of the serum-resistant strains caused fluid accumulation in the initial tests in ileal loops, regardless of species or source. Haemagglutinating and serum-resistant strains caused significantly more accumulation of fluid (p≪0.05) than non-haemagglutinating and serum-sensitive strains. This study shows partial correlation between HA or serum sensitivity and enterotoxigenicity, but the properties are probably not genetically linked

Enteropathogenicity of Aeromonas hydrophila and Plesiomonas shigelloides

Aeromonas hydrophila was enteropathogenic in ligated ileal loops of rabbits, causing a fluid accumulation of 14-2.0 ml per cm of gut length. Gut reaction could be produced with an inoculum as low as 104 viable bacteria. There was no difference in the nature of the positive reactions given by strains isolated from diarrhoea1 and non-diarrhoea1 children and adults and from water. Plesiomonas shigelloides, on the other hand, did not cause a significant gut reaction. A. hydrophila multiplied in the ileal loop by about 105 whereas P. shigelloides did so at only 102-3. These experiments on an animal model thus indicated the enteropathogenic nature of A. hydrophila, but no definite conclusion could be drawn from this study on P. shigelloides. We are grateful to Professor Hardas Singh, Head, Department of Microbiology, for his valuable suggestions and encouragement. We acknowledge with thanks the technical assistance of Mr Ram Achal Ram and Mr S. N. Pathak rendered during this study

Enteropathogenicity of Plesiomonas Shigelloides

Plesiomonas Shigelloides has been implicated as an aetiological agent in sporadic cases and outbreaks of diarrhoea, and in food poisoning, in various parts of the world. (Schmid, Velaudapillai and Niles, 1954; Osada and Shibata, 1956; Vandepitte et al., 1957; Ueda, Yamasaki and Hori, 1963; Aldova, Rakovsky and Chovanova, 1966; Geizer, Kopecky and Aldova, 1966; Hori et al., 1966; Cooper and Brown, 1968; Pauova and Fukalova, 1968; von Graevenitz and Mensch, 1968; Winton, 1968; Sakazaki et al., 1971; Chatterjee and Neogy, 1972; Sanyal et al., 1972a and b; Zajc-Satler, Dragas and Kumelj, 1972; Bhat, Shantha Kumari and Rajan, 1974; Vandepitte, Makulu and Gatti, 1974; Sanyal, Singh and Sen, 1975; Jandl and Linke, 1976; Tsukamoto et al., 1978). P. shigelloides was isolated in pure culture from stools of patients with diarrhoea of otherwise unexplained origin, and more often from patients than from symptomless persons. Experimental evidence has not, however, been reported in support of its enteropathogenicity, except our preliminary communication on the activity of a few strains in the rabbit ileal-loop model (Saraswathi, Sharma and Sanyal, 1978). The present investigation was undertaken to examine its enterotoxicity and invasiveness

Structure, bonding and magnetism in cobalt clusters

The structural, electronic and magnetic properties of Co$_n$ clusters ($n=2-$20) have been investigated using density functional theory within the pseudopotential plane wave method. An unusual hexagonal growth pattern has been observed in the intermediate size range, $n=15-$20. The cobalt atoms are ferromagnetically ordered and the calculated magnetic moments are found to be higher than that of corresponding hcp bulk value, which are in good agreement with the recent Stern-Gerlach experiments. The average coordination number is found to dominate over the average bond length to determine the effective hybridization and consequently the cluster magnetic moment.Comment: 12 pages and 9 figure