The gas at the surfaces of molecular clouds in galaxies is heated and dissociated by photons from young stars both near and far. H i resulting from the dissociation of molecular hydrogen H 2 emits hyperfine line emission at 21 cm, and warmed CO emits dipole rotational lines such as the 2.6 mm line of CO (1-0). We use previously developed models for photodissociation regions (PDRs) to compute the intensities of these H i and CO (1-0) lines as a function of the total volume density n in the cloud and the far-ultraviolet (FUV) flux G 0 incident on it and present the results in units familiar to observers. The intensities of these two lines behave differently with changing physical conditions in the PDR, and taken together, the two lines can provide a ground-based radio astronomy diagnostic for determining n and G 0 separately in distant molecular clouds. This diagnostic is particularly useful in the range G 0 P100, 10 cm À3 P n P10 5 cm À3 , which applies to a large fraction of the volume of the interstellar medium in galaxies. If the molecular cloud is located near discrete sources of far-UV (FUV) emission, the PDR-generated H i and CO (1-0) emission on the cloud surface can be more easily identified, appearing as layered ''blankets'' or ''blisters'' on the side of the cloud nearest the FUV source. As an illustration, we consider the Galactic object G216À2.5, i.e., ''Maddalena's Cloud,'' which has been previously identified as a large PDR in the Galaxy. We determine that this cloud has n % 200 cm À3 and G 0 % 0:8, consistent with other data.
Context. At least a fraction of the atomic hydrogen in spiral galaxies is suspected to be the result of molecular hydrogen which has been dissociated by radiation from massive stars. Aims. In this paper, we extend our earlier set of data from a small region of the Western spiral arm of M 81 with CO observations in order to study the interplay between the radiation field and the molecular and atomic hydrogen. Methods. We report CO(1-0) observations with the Nobeyama 45 m dish and the Owens Valley interferometer array of selected regions in the Western spiral arm of M 81.Results. From our Nobeyama data, we detect CO(1-0) emission at several locations, coinciding spatially with H i features near a far-UV source. The levels and widths of the detected CO profiles are consistent with the CO(1-0) emission that can be expected from several large photo-dissociation regions with typical sizes of some 50 × 150 pc located within our telescope beam. We do not detect emission at other pointings, even though several of those are near far-UV sources and accompanied by bright H i. This non-detection is likely a consequence of the marginal area filling factor of photo-dissociation regions in our observations. We detect no emission in our Owens Valley data, consistent with the low intensity of the CO emission detected in that field by the Nobeyama dish. Conclusions. We explain the lack of CO(1-0) emission at positions farther from far-UV sources as a consequence of insufficient heating and excitation of the molecular gas at these positions, rather than as an absence of molecular hydrogen.
Principal-components analysis of a new set of highly resolved (< 1 nm) fluorescence cross-section spectra excited at 354.7 nm over the 370-646 nm band has been used to demonstrate the potential ability of UV standoff lidars to discriminate among particular biological warfare agents and simulants over short ranges. The remapped spectra produced by this technique from Bacillus globigii (Bg) and Bacillus anthracis (Ba) spores were sufficiently different to allow them to be cleanly separated, and the Ba spectra obtained from Sterne and Ames strain spores were distinguishable. These patterns persisted as the spectral resolution was subsequently degraded in processing from approximately 1 to 34 nm. This is to the author's knowledge the first time that resolved fluorescence spectra from biological warfare agents have been speciated or shown to be distinguishably different from those normally used surrogates by optical spectroscopy.
We present the design and expected performance of a Fourier-transform fiber-optic Raman spectrometer that should be capable of rapidly diagnosing environmental contamination to levels approaching a few parts per thousand. The system design is predicated on fiber arms that are unequal in length initially. A voltage applied to the piezoceramic substrate hosting the shorter fiber strains it through zero path difference to a new length that exceeds the reference arm by its initial length deficiency. This approach permits one to resolve spectral features separated by 0.5 cm(-1) over an electromagnetic bandwidth that exceeds 2000 cm(-1) without using a moving mirror or introducing a second laser to provide the sampling reference. Specific expressions are given for computing the spectral resolution, modulation bandwidth, and modulated signal-to-noise ratio of the device as a function of the system's design parameters.
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