The physical and chemical conditions of molecular clouds on large scales

Abstract

Observations of molecular emission lines in the radio and (sub)millimetre wavelength regime provide insights into the interiors of molecular clouds which are inaccessible by other forms of radiation. Examining the physical and chemical conditions within these centres of star formation furthers our understanding of young stellar objects in the earlier stages of stellar evolution and their interplay with the interstellar medium. Technological advancements are now allowing observations of molecules in distant galaxies, but our ability to interpret these data are limited by our knowledge of exactly how and where molecular emission lines originate within our own galaxy. Until recent years, observational biases caused by pre-selection of either certain positions in a molecular cloud or a few molecular lines only allowed very selective studies.<br /> This thesis describes the acquisition and analysis of the first spectroscopically unbiased data set of the northern part of the prominent Orion A molecular cloud in the 1.3 mm atmospheric window. Orion A is a uniquely suited target as it offers a large range of environments but can also be spatially resolved due to its close proximity. The data were obtained with the PI230 receiver at the APEX telescope, which increased the simultaneous frequency coverage in the 1.3 mm window by a factor of eight. The observations cover an area of around 0.8 × 3.8 pc with a resolution of 0.06 pc (∼ 1.2 × 10⁴ AU). We examine how 29 molecular species (55 isotopologues in total) emit from environments with a variety of conditions. We thereby aim to not only advance our understanding of the source itself, but the scope of our study allows placing it into the context of the general interstellar medium of the Milky Way and of extragalactic systems.<br /> We develop templates of distinct regions and contrast their respective emission profiles. This highlights common features but also unique characteristics that enable differentiation. We explore the reliability of commonly used line ratios as a tool to infer physical parameters like temperature or UV irradiation, examine correlations between different molecular species and isotopologues, and calculate line luminosities.<br /> We further report on the surprising first detection of larger scale CF+ emission in a variety of environments. This molecule is expected to form under quite specific conditions and previous detections focused on regions meeting those requirements. We confirm its presence throughout Orion A with additional higher frequency observations obtained with the LASMA receiver at APEX and aim to explain its widespread and fairly uniform distribution. Our data set also enables us to examine the nature of the so-called ‘radical region’ within Orion A and to determine whether or not it is a distinct entity characterised by unusual molecular abundances. Prior investigations of this region were not conclusive and their respective observational biases precluded a definite assessment.<br /> Apart from these scientific questions, we also report on the PI230 receiver’s performance during its commissioning phase and describe and evaluate its characteristics, e.g. changing main beam efficiencies, spurious features, and sideband rejection.