Computer Science – Performance
Scientific paper
Oct 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992phdt.......210k&link_type=abstract
PhD Thesis, I. Physikalisches Institut, Universität zu Köln, Deutschland, 1992.
Computer Science
Performance
12
Scientific paper
The thesis is structured into two parts. The first deals with the Orion B cloud. The subjects of the second are 'Improvement of the Telescope Driving Performance', `The Terrestrial Atmosphere in the Mm- and Submm-Region', and `Calibration by Planets'. Orion B The analysis of the large scale structure and excitation conditions of the southern part of the giant molecular cloud Orion B was the aim of our multiline study with the KOSMA 3m radiotelescope located on Gornergrat, Switzerland (Winnewisser et al. 1986, 1990). The analysis is based on J = 2-1 and J = 3-2 12CO and 13CO data gathered during several observing sessions between November 1990 and January 1992. We used two cooled GaAs Schottky heterodyne receivers built at Cologne with noise temperatures (DSB) of about 400 K. Fragmentation The large-scale CO-xobservations of Orion B reveal the fragmented structure on all scales between 0.2 pc (the resolution limit) and the extension of the observed region (8 pc). The overall morphology of the cloud is significantly influenced by the Ib Orion OB association which lies roughly 3 degrees (20 pc) to the west. Its HII-region IC434 lies directly to the west of Orion B. CO-intensities fall off rapidly at the western boundary and the velocity structure is formed by the association. Two dense cores, the southern of which is the famous Horsehead-Nebula, have survived the interaction with the HII-region and protrude out of the cloud into the HII-region. The clumpiness of the cloud shows up in detail in channelmaps of the 13CO(2-1)-isotope. Measurements of ionized atomic carbon in the infrared at 158 microns show a large-scale distribution of [CII] (Jaffe et al. 1993). Obviously the UV-radiation penetrates deeply into the molecular material again revealing the clumpy structure of this material. The total mass of the observed part of Orion B is 3150 solar masses, which can be subdivided into 244 clumps by a clump-fitting procedure as described by Stutzki and Guesten (1990, ApJ, 332). The resulting mass spectrum is well fitted by a power-law dN/dM ~ M-α with an index of α = 1.74 +/- 0.05 similar to spectra found for other giant molecular clouds. 44% of the total mass of Orion B is found in the five most massive clumps with M>200 solar masses. These clumps are associated with the prominent star-forming regions NGC2024 and NGC2023. The 244 clumps are not gravitationally bound, though the more massive clumps are closer to virial-equilibrium than the less massive ones. The average density of the clumps is 4 × 103 per cubiccentimeter, which is significantly lower than the critical excitation density of the CO(3-2) transition, indicating the presence of unresolved substructures. Measurements with higher resolution should reveal cores with higher densities. CO Line Comparison In order to derive the excitation conditions within Orion B we also observed 13CO(3-2) along two stripes at constant right ascension and constant declination. We found the following line ratios, which are rather insensitive to position and also essentially constant across the line profile: 12R3,2 = T(12CO(3-2)) / T(12CO(2-1)) = 1 12R3,2 = T(13CO(3-2)) / T(13CO(2-1)) = 1 12,13R2 = T(12CO(3-2)) / T(13CO(3-2)) = 1.5 - 18 The `standard interpretation' of CO-data assumes (1) constant excitation temperatures Tex along the line of sight, (2) equal Tex for 12CO and 13CO and (3) equal Tex for all transitions (`thermalization', LTE). Applying this `standard interpretation' on our data leads to seemingly contradictory results: (1) 12R3,2 = 1 indicates optically thick and thermalized 12CO transitions, (2) the same is valid for 13CO, (3) according to the standard interpretation a ratio 12,13R2 = 1.5 - 18 means mostly optically thin 13CO emission with optical depths between 1 and less than 0.3 in contrast to the above interpretation. Optical thin 13CO emission would lead to a ratio 12R3,2 of more than 2. These apparently contradictory intensity ratios are not found solely in Orion B: Castets et al. (1990) observed similar ratios of the two lowest CO-transitions in Orion A. They point out that the molecular clouds and their constituents, the clumps, are heated by UV-radiation which leads to temperature gradients within each clump provided they are not heated by internal sources. Gierens et al. (1992) show by theoretical modelling of UV irradiated clumps that the observed line ratios are naturally explained by the temperature and abundance variations in the PDR surface of the clumps. A publication with more details and analysis of the data is in preparation. Improvement of the Telescope Driving Capabilities The telescope slewing and nodding performance, its tracking stability, as well as the overall system stability were improved by exchanging the old command micro against an PC/AT and by implementing a new PI-control loop. Hard- and software were tested in simulation runs in Cologne. Tests with the 3m-telescope show that its driving performance has significantly improved. The Terrestrial Atmosphere Observations at 230 and 345 GHz were used together with atmospherical models to derive opacities and extrapolate them to higher frequencies. It was shown that the Gornergrat offers favourable conditions for observations at submillimeter wavelengths during wintertime (Kramer and Stutzki, 1991). Furtheron corrections for different opacities in the two sidebands at selected frequencies were derived and implemented into the calibration procedure. Calibration by Observation of Planets Formula to calculate half power beamwidths, main beam efficiencies, and aperture efficiencies from observations of planets were presented, explained, and implemented into an easy-to-use program. A surface accuracy of approx. 42 micrometer rms is consistent with measurements between 88 and 492 GHz.
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