On the robustness of H-deficient post-AGB tracks

We analyze the robustness of H--deficient post--AGB tracks regarding previous evolution of their progenitor stars and the constitutive physics of the remnants. Our motivation is a recent suggestion of Werner & Herwig (2006) that previous evolution should be important in shaping the final post--AGB track and the persisting discrepancy between asteroseismological and spectroscopical mass determinations. This work is thus complementary to our previous work (Miller Bertolami & Althaus 2006) and intends to shed some light on the uncertainty behind the evolutionary tracks presented there. We compute full evolutionary models for PG1159 stars taking into account different extramixing (overshooting) efficiencies and lifetimes on the TP-AGB during the progenitor evolution. We also assess the effect of possible differences in the opacities and equation of state by artificially changing them before the PG1159 stage. Also comparisons are made with the few H-deficient post--AGB tracks available in the literature. Contrary to our expectations, we found that previous evolution is not a main factor in shaping H--deficient post--AGB tracks. Interestingly enough, we find that only an increase of $\sim50$ in the intershell opacities at high effective temperatures may affect the tracks as to reconcile spectroscopic and asteroseismologic mass determinations. This forces us to conclude that our previous tracks (Miller Bertolami & Althaus 2006) are robust enough as to be used for spectroscopic mass determinations, unless opacities in the intershell region are substantially different. Our results, then, call for an analysis of possible systematics in the usually adopted asteroseismological mass determination methods.Comment: Accepted for publication in Astronomy and Astrophysc

“The Battle-Fortune of Marshal Hindenburg is not Bound up with the Possession of a Hill”: The Germans and Vimy Ridge, April 1917

On 9 April 1917 four Canadian divisions and one British division of 170,000 men broke through the “Vimy Group” of German Sixth Army of some 40,000 men. By late afternoon, the Germans had been driven off the Ridge. That day, as Brigadier-General Alexander Ross famously put it, constituted “the birth of a nation.” Rivers on ink have been spilled in the Canadians’ actions that day, but little attention has been paid to “the other side of the hill.” Which German units defended the Ridge? What was the quality of their leadership? Why did the defence collapse so quickly? Why did the German soldiers not break and run? And how were they able to prevent a deeper British-Canadian breakthrough? On the basis of German sources, this article seeks to provide answers to those question

Antimicrobially active microorganisms associated with marine bryozoans

Bryozoans are sessile colonial animals that can be found in various aquatic and mainly in marine environments. Due to their sessile nature, bryozoans compete for surfaces they can colonize but, on the other hand, are confronted with microbial colonizers on their surfaces. Interactions of the bryozoan with its associates, as well as within the microbial community, are mediated chemically. Biofilm formation and composition is mainly influenced by the use of chemical compounds. Studies on the bryozoan-associated microbial diversity are scarce, and surveys on the antimicrobial potential of these associated bacteria are missing. The present study focused on isolating bryozoan-associated bacteria, assessing their antimicrobial properties and classifying them phylogenetically. Various bryozoan specimens were collected in the Baltic (10 specimens) and the Mediterranean Sea (11 specimens). Bacteria were isolated using a variety of nutrient media and tested for their antimicrobial abilities against Gram-positive and Gram-negative indicator strains, as well as against the yeast Candida glabrata. 30% of all isolates displayed activity and were phylogenetically classified on the basis of 16S rDNA gene sequences. Whereas all isolates were active against Gram-positive indicators, four isolates exhibited additional anti-Escherichia coli activity, the phylogenetic analysis revealed affiliation to Gram-negative phyla (Flavobacteria, Alpha- and Gammaproteobacteria). One isolate belonged to the Gram-positive Actinobacteria. Both species- and strain-specific activity patterns were revealed. Furthermore, site-specific distribution patterns of associated bacteria were found. Of these antibiotically active isolates, the strain B390 was described as type strain of the novel species Tenacibaculum adriaticum. Also, specimens of the bryozoan Membranipora membranacea were sampled in the Baltic Sea for the first more detailed analysis on antimicrobially active isolates. Low-nutrient media featuring “artificial” or “natural” ingredients were used for isolation of bacteria. Additionally, the antibiotic test panel was extended to six different production media. The impact of these media on the phylogenetic diversity, as well as on activity patterns was determined. Although bacteria were affiliated with same phyla (Alpha- and Gammaproteobacteria, Actinobacteria, additionally Bacilli), the isolates of this sampling were more diverse as far as genus or phylotype affiliation was concerned. Especially within the Alphaproteobacteria, several probably novel bacterial species were found. Furthermore, the use of six different media for activity testing resulted in a more than twofold higher hit rate of active isolates in comparison to only one single medium