100 research outputs found
Integrated spectrum of the planetary nebula NGC 7027
We present deep optical spectra of the archetypal young planetary nebula (PN)
NGC 7027, covering a wavelength range from 3310 to 9160 A. The observations
were carried out by uniformly scanning a long slit across the entire nebular
surface, thus yielding average optical spectra for the whole nebula. A total of
937 emission features are detected. The extensive line list presented here
should prove valuable for future spectroscopic analyses of emission line
nebulae. The optical data, together with the archival IUE and ISO spectra, are
used to probe the temperature and density structures and to determine the
elemental abundances from lines produced by different excitation mechanisms.
The C++/H+, N++/H+, O++/H+ and Ne++/H+ ionic abundance ratios derived from
optical recombination lines (ORLs) are found to be only slightly higher than
those derived from collisionally excited lines (CELs). We conclude that
whatever mechanism is causing the BJ/CEL temperature discrepanies and the
ORL/CEL abundance discrepancies that have been observed in many PNe, it has an
insignificant effect on this bright young compact PN. The properties of the
central star are also discussed. Based on the integrated spectrum and using the
energy-balance method, we have derived an effective temperature of 219 000 K
for the ionizing star. Finally, we report the first detection in the spectrum
of this bright young PN of Raman-scattered O VI features at 6830 and 7088 A,
pointing to the existence of abundant neutral hydrogen around the ionized
regions. (abridged)Comment: 55 pages, 7 figures, accepted for publication in A&
Homology between a genetic locus ( mdoA ) involved in the osmoregulated biosynthesis of periplasmic glucans in Escherichia coli and a genetic locus ( hrpM ) controlling pathogenicity of Pseudomonas syringae
International audienceMembrane-derived oligosaccharides (MDO) of Escherichia coli are representative members of a family of glucans found in the periplasmic space of Gram-negative bacteria. The two genes forming the mdoGH operon are necessary for the synthesis of MDO. The nucleotide sequence (4759 bp) and the transcriptional start of this operon were determined. Both gene products were further characterized by gene fusion analysis. MdoG is a 56 kDa periplasmic protein whose function remains to be determined. MdoH, whose presence was shown to be necessary for normal glucosyl transferase activity, is a 97 kDa protein spanning the cytoplasmic membrane. To our surprise, these proteins are not homologous to the periplasmic gtucan biosynthetic enzymes previously characterized in the Rhizobiaceae family. However, a considerable homology (69% identical nucleotides out of 2816) was discovered between mdoGH and the two genes present at the hrpM locus of the phytopathogenic bacterium Pseudomonas syringae pv. syringae. Functions of these genes remain mysterious but they are known to be required for both the expression of disease symptoms on host plants and the development of the hypersensitive reaction on non-host plants (Mills and Mukhopadhyay, 1990). These results confirm the importance of periplasmic glucans for the physiological ecology of Gram-negative bacteria
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