Molecular cloud structure in the star-forming region W43

In the struggle toward an understanding of the process of star formation it is one of the most important tasks to study the initial properties of the cold and dense interstellar medium. Stars are known to form in dense molecular clouds, but it is not yet fully understood how they from from the neutral gas that permeates the Galaxy. Therefore, it is crucial to fully understand the creation process of molecular clouds. Colliding flows are a possible explanation for this process. This class of models considers streams of warm, diffuse, and atomic hydrogen gas that stream into each other. Fluctuations of the gas at the collision area create regions of higher density and lower temperature, where molecules can form. W43 is one of the largest molecular cloud complexes in the Milky Way with a total gas mass of several million Solar masses. Prior investigations have identified it as one of the largest and most luminous star-forming regions in the Galaxy. This is most probably due to its exceptional position, which is assumed to be at the junction point of the Galactic spiral arms and the bar. At this location, gas is piled up, because the elliptical orbits in the bar and the circular orbits in the spiral arms interfere. Therefore, W43 poses an excellent laboratory to study the formation of molecular clouds. This thesis aims at characterizing the W43 star-forming region. The distribution of molecular clouds, their velocity structure, and physical properties are analyzed and the origin of a single filament is studied in detail. The first part of this work describes the project W43 HERO (W43 Hera/EmiR Observations) that has been initiated to observe the large scale distribution of 13CO (2-1) and C18O (2-1) in the W43 complex with the IRAM 30m telescope. These molecular emission lines provide information of the medium dense molecular gas (~10^3 cm^-3) in the complex. While 13CO (2-1) traces the more diffuse, widespread gas in the clouds, C18O (2-1) is used to depict the denser central clumps of the clouds. We analyze the velocity structure of our data and align our findings with velocity models of the spiral arms in the Galaxy. We thus confirm the position of W43 near the tangential point in the Scutum arm, which is situated near the tip of the elongated bar. This point has a distance of 6 kpc from the Sun. We then derive the optical depth, the excitation temperature, and the H2 column density of the gas from our observations. The total mass of medium dense molecular clouds in the W43 complex is found to be 1.9x10^6 Solar masses. An estimation of the shear parameter of the cloud complex provides the insight that it is massive enough to withstand the shear forces generated by the motions of gas streams in the Galaxy. Mass estimations are in agreement with those taken from Herschel dust emission maps. Plots of the probability distribution functions of the H2 column density, derived from Herschel and the IRAM 30m data, are created. Both show a log-normal distribution on lower masses and power-law deviation on the high-mass end of the function. This is commonly associated with the influence of star formation. The slope of the CO power-law tail is less steep than that derived from dust emission. We could have found an efficient tool, that only traces the gas that collapses into a protostar. In the second part of this thesis we pick one single filament in the W43 complex and analyze it in detail. Additional observations have been carried out to complement the IRAM 30m dataset. In particular, there are observations of CO (6-5) from the APEX telescope, [C II] from the Herschel satellite, [C I], observed with NANTEN2, and the CARMA observations of HCN (1-0), HCO+ (1-0), and N2H+ (1-0). We use the radiative transfer code RADEX to estimate the temperature and density of the molecular gas. The best solution is a kinetic temperature around 30 K and a density of ~10^4 cm^-3. We then study the origin of the ionized carbon which we have observed with Herschel. This species is often discussed in the framework of static photon-dominated regions (PDRs). The physical conditions are studied with the program KOSMA-tau, which yields a very high UV-field, necessary for the creation of the observed data. A comparison with typical UV-tracers results in the insight that this radiation strength is unrealistically high. Thus, the creation of C+ by a PDR is implausible and a different mechanism has to be the origin. We find it more likely that the [C II] emission traces the transition zone between the initial atomic gas and the molecular cloud which forms from it. We would thus witness the dynamic formation of a young molecular filament. We conclude that W43 is indeed one of the most massive and important molecular cloud complexes in the Galaxy and that colliding flows are a possible explanation for its formation.