Abstract.A commercially available dry chemiluminescence (CI) instrument for fast and precise measurement of ozone (O 3 ) is specified. The sensitivity is ∼9000 counts s −1 per ppbv of ozone. Its precision is entirely determined by the number of photons reaching the detector (being a photomultiplier), i.e. is quantum-noise limited. The relative precision . At typical O 3 mixing ratios between 10 and 100 ppbv (and 1 bar), the precision is 0.3-1.0 % at f = 10 Hz. The maximum measurement frequency is 50 Hz. The mechanical and electronic setup as well as the instrument performance is described. Recommendations on the adequate inlet tube configuration (inlet tube length, sampling flow) and on the way of calibration at stationary ground-based platforms and onboard aircraft are given.
The Milky Way's halo contains clouds of neutral hydrogen with high radial velocities which do not follow the general rotational motion of the Galaxy. Few distances to these high-velocity clouds are known, so even gross properties such as total mass are hard to determine. As a consequence, there is no generally accepted theory regarding their origin. One idea is that they result from gas that has cooled after being ejected from the Galaxy through fountain-like flows powered by supernovae; another is that they are composed of gas, poor in heavy elements, which is falling onto the disk of the Milky Way from intergalactic space. The presence of molecular hydrogen, whose formation generally requires the presence of dust (and therefore gas, enriched in heavy elements), could help to distinguish between these possibilities. Here we report the discovery of molecular hydrogen absorption in a high-velocity cloud along the line of sight to the Large Magellanic Cloud. We also derive for the same cloud an iron abundance which is half of the solar value. From these data, we conclude that gas in this cloud originated in the disk of the Milky Way.
Abstract. During the second flight of the ORFEUS-SPAS mission in November/December 1996, the Echelle spectrometer was used extensively by the Principal and Guest Investigator teams as one of the two focal plane instruments of the ORFEUS telescope. We present the inflight performance and the principles of the data reduction for this instrument. The wavelength range is 90 nm to 140 nm, the spectral resolution is significantly better than λ/∆λ = 10 000, where ∆λ is measured as FWHM of the instrumental profile. The effective area peaks at 1.3 cm 2 near 110 nm. The background is dominated by straylight from the Echelle grating and is about 15% in an extracted spectrum for spectra with a rather flat continuum. The internal accuracy of the wavelength calibration is better than ± 0.005 nm.
A commercially available dry chemiluminescence (CI) instrument for fast and precise measurement of ozone (O<sub>3</sub>) is specified. The sensitivity is ~9000 counts s<sup>−1</sup> per ppbv of ozone. Its precision is entirely determined by the number of photons reaching the detector (being a photomultiplier), i.e. is quantum-noise limited. The relative precision (ΔO<sub>3</sub>/O<sub>3</sub> in %) thus follows Poisson statistics and scales with the square root of the measurement frequency <i>f</i> and with the inverse O<sub>3</sub> mixing ratio: ΔO<sub>3</sub>/O<sub>3</sub> ∝ <i>f</i><sup>0.5</sup> · O<sub>3</sub><sup>−0.5</sup>. At typical O<sub>3</sub> mixing ratios between 10 and 100 ppbv, the precision is 0.3–1.0% at <i>f</i> = 10 Hz. The maximum measurement frequency is 50 Hz. The mechanical and electronic set-up as well as the instrument performance is described. Recommendations on the adequate inlet tube configuration (inlet tube length, sampling flow) and on the way of calibration at stationary ground-based platforms and onboard aircraft are given
Far UV high resolution spectra of 3 LMC and SMC stars were obtained with the Echelle spectrograph during the second ORFEUS mission in Dec. 1996. We present the first results from observations of the LMC star HDE 269546. We find definitely components of very hot gas identified as OVI and SVI absorption in the galactic halo of the Milky Way and in the LMC. Additionally, more than 30 ions of the most abundant elements in different stages of ionization can be identified in both our galaxy and the LMC. For the first time we can identify a significant absorption component of molecular hydrogen in the ORFEUS II Echelle spectrum with a redshift of 200 km s−1, doubtlessly to be attributed to the LMC.
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