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We present a new measurement of the α-spectroscopic factor (S α ) and the asymptotic normalization coefficient for the 6.356 MeV 1/2 + subthreshold state of 17 O through the 13 C( 11 B, 7 Li) 17 O transfer reaction and we determine the α-width of this state. This is believed to have a strong effect on the rate of the 13 C(α, n) 16 O reaction, the main neutron source for slow neutron captures (the s-process) in asymptotic giant branch (AGB) stars. Based on the new width we derive the astrophysical S-factor and the stellar rate of the 13 C(α, n) 16 O reaction. At a temperature of 100 MK, our rate is roughly two times larger than that by Caughlan & Fowler and two times smaller than that recommended by the NACRE compilation. We use the new rate and different rates available in the literature as input in simulations of AGB stars to study their influence on the abundances of selected s-process elements and isotopic ratios. There are no changes in the final results using the different rates for the 13 C(α, n) 16 O reaction when the 13 C burns completely in radiative conditions. When the 13 C burns in convective conditions, as in stars of initial mass lower than ∼2 M and in post-AGB stars, some changes are to be expected, e.g., of up to 25% for Pb in our models. These variations will have to be carefully analyzed when more accurate stellar mixing models and more precise observational constraints are available.

Time-dependent hybrid density functional theory in combination with polarized continuum model has been applied to study the solvent effects on the geometrical and electronic structures, as well as one- and two-photon absorption processes, of a newly synthesized asymmetrical charge-transfer (CT) two-photon absorption (TPA) organic molecule. The TPA cross section calculated from a generalized two-state model and solvatochromic shift of the CT state are found to be solvent dependent, for which a nonmonotonic behavior with respect to the polarity of the solvents has been observed. The calculated properties are in good agreement with the experimental data available. The character of the CT state is visualized by plotting its charge density difference from ground state, in which an excess of electron density on the donor side of the molecule is found. This implies that the excited molecule is ready to donate its electron to the surroundings. The energetic aspect of the electron donation is discussed by examining the solvent dependence of the molecular ground state oxidation potential. The importance of the electron correlation for describing the two-photon absorption is also demonstrated.

BackgroundAerobic gammaproteobacteria affiliated to the OM60/NOR5 clade are widespread in saline environments and of ecological importance in several marine ecosystems, especially the euphotic zone of coastal areas. Within this group a close relationship between aerobic anoxygenic photoheterotrophs and non-phototrophic members has been found.ResultsSeveral strains of aerobic red-pigmented bacteria affiliated to the OM60/NOR5 clade were obtained from tidal flat sediment samples at the island of Sylt (North Sea, Germany). Two of the novel isolates, Rap1red and Ivo14T, were chosen for an analysis in detail. Strain Rap1red shared a 16S rRNA sequence identity of 99% with the type strain of Congregibacter litoralis and was genome-sequenced to reveal the extent of genetic microheterogeneity among closely related strains within this clade. In addition, a draft genome sequence was obtained from the isolate Ivo14T, which belongs to the environmental important NOR5-1 lineage that contains so far no cultured representative with a comprehensive description. Strain Ivo14T was characterized using a polyphasic approach and compared with other red-pigmented members of the OM60/NOR5 clade, including Congregibacter litoralis DSM 17192T, Haliea rubra DSM 19751T and Chromatocurvus halotolerans DSM 23344T. All analyzed strains contained bacteriochlorophyll a and spirilloxanthin as photosynthetic pigments. Besides a detailed phenotypic characterization including physiological and chemotaxonomic traits, sequence information based on protein-coding genes and a comparison of draft genome data sets were used to identify possible features characteristic for distinct taxa within this clade.ConclusionsComparative sequence analyses of the pufLM genes of genome-sequenced representatives of the OM60/NOR5 clade indicated that the photosynthetic apparatus of these species was derived from a common ancestor and not acquired by multiple horizontal gene transfer from phylogenetically distant species. An affiliation of the characterized bacteriochlorophyll a-containing strains to different genera was indicated by significant phenotypic differences and pufLM nucleotide sequence identity values below 82%. The revealed high genotypic and phenotypic diversity of closely related strains within this phylogenetic group reflects a rapid evolution and frequent niche separation in the OM60/NOR5 clade, which is possibly driven by the necessities of an adaptation to oligotrophic marine habitats.