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Scientific paper
Feb 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999phdt........35b&link_type=abstract
PhD Thesis, I. Physikalisches Institut, Universität zu Köln, Germany, 1999.
Other
Scientific paper
Stars form in the denser regions of molecular clouds. The details of the star formation process, however, are not fully understood. The physical processes responsible for the observed complex and filamentary molecular cloud structure are not known in detail although it is widely accepted that turbulence plays an important role. The quantitative treatment of turbulence is notoriously difficult and for the interstellar medium it is complicated by the high compressibility of the gas and the presence of magnetic fields. A quantitative deduction of the spatial and velocity structure from first principles is currently not possible. Therefore, a different approach is used to identify the relevant physical processes for molecular clouds: the structure in observed molecular cloud images is quantified, which allows a comparison to simulations of structure formation by individual processes, such as magneto-hydrodynamic turbulence. A systematic analysis for different molecular clouds (e.g. quiescent, star-forming clouds) then in principle allows to trace the evolution of molecular clouds from molecular cloud to star formation and to deduce key parameters such as the star formation rate and efficiency, and the initial mass function of the newly formed stars. This thesis studies the structure of nearby (d~150 pc) quiescent molecular clouds which have not (yet) given birth to new stars. Observations are made with the KOSMA 3 m telescope in the rotational transitions of CO and its isotopomers toward MCLD 123.5+24.9 (a translucent cloud in the Polaris Flare), L1512 and L134A. They complement observations made at high angular resolution (11'' and 22'') with the 30 m telescope (IRAM key-project) and existing large-scale surveys. The resulting data set allows to study the cloud structure on more than three decades in linear scale, from ~10 pc down to 1800 AU. The accurate intensity calibration of the observations is critical to a later structure analysis. Residual errors significantly affect the results obtained, and potentially dominate them in extreme cases. In observed maps, point-to-point variations of the intensity calibrations typically result from receiver gain drifts or the imperfect correction for the atmospheric attenuation. In this thesis, methods for a self-consistent correction of these systematic errors are studied and successfully applied to the observed spectral line maps. The influence of the error beam (stray radiation) pick-up in single dish observations is studied and correction tools are developed and compared with respect to their accuracy and limitations. The correction of the IRAM key-project data shows that up to 50% of the intensity in each map is due to error beam pick-up. In addition, the line profiles are significantly modified. The corrected maps show a significantly enhanced contrast. The quantitative characterization of the molecular cloud structure is done using the Δ-variance. Introduced by Stutzki et al. (1998), the Δ-variance allows to study the spatial drift behavior of scalar functions, such as the intensity distribution of molecular cloud tracers. For images with a power law power spectrum and random phases, known as fractional Brownian motion (fBm) structures, the Δ-variance analysis allows to determine the spectral index β of the power spectrum. A detailed study of the influence of the finite image size (edge effects), white noise and beam smearing is done. Compared to the power spectrum itself, the Δ-variance is more robust with respect to edge-effects and allows a better discrimination of the structure against white noise. The Δ-variance is applied to the observed spectral line maps and the Bell Labs 13CO(1-0) molecular cloud surveys. For the velocity integrated maps, the spatial structure of the emission is well characterized by a power law power spectrum. The spectral index is remarkably uniform for different molecular clouds and linear scales larger than ~0.5 pc (2.5 <= β <= 2.8). Significantly larger indices (β) are found for observations made at higher spatial resolution toward MCLD 123.5+24.9, suggesting that the structure is smoother at smaller scales. http://chandra.ph1.uni-koeln.de/bensch/astro/publications.html
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