On the Chemical Composition of Interstellar Molecular Clouds: A Millimeter and Submillimeter Spectral Line Survey of OMC-1

Citation

Blake, Geoffrey A. (1986) On the Chemical Composition of Interstellar Molecular Clouds: A Millimeter and Submillimeter Spectral Line Survey of OMC-1. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/0F73-PJ76. https://resolver.caltech.edu/CaltechETD:etd-12092003-145807

Abstract

The same basic principles govern the chemical and physical evolution of systems throughout the universe. However, the dissimilar conditions on the Earth and in the interstellar medium lead to remarkably different chemical compositions for these two environments. While less familiar than terrestrial chemistry, the study of the chemical composition of the interstellar medium is important because it bears directly on the understanding of phenomena as diverse as star formation, galactic structure and dynamics, and the cosmological origin of the universe, in addition to providing a unique opportunity to investigate a number of fundamental chemical and physical processes.

We present here results from a millimeter and submillimeter spectral line survey of the core of the Orion molecular cloud (OMC-1). The millimeter-wave survey, conducted at the Owens Valley Radio Observatory (OVRO), covers a 55 GHz interval in the 1.3 mm (230 GHz) atmospheric window and contains emission from 29 molecules. Together with the frequency selective submillimeter observations of H₂D⁺ (372.4 GHz), Cl (492.2 GHz), NH₃ (572.5 GHz), and HCl (625.9 GHz) performed aboard NASA's Kuiper Airborne Observatory, over 800 emission lines have been detected from 33 chemically distinct species during the course of this work. The uniformly calibrated results from the unique and extensive OVRO spectral line survey place significant constraints on models of interstellar chemistry, and have allowed the chemical composition of the various regions in OMC-1 to be definitively characterized.

A global analysis of the observed abundances has shown that the markedly different chemical compositions of the kinematically distinct Orion subsources may be simply interpreted in the framework of an evolving, initially quiescent, gas-phase chemistry influenced by the process of massive star formation. The chemical composition of the extended Orion cloud complex is similar to that found in a number of other objects, but the central regions of OMC-1 have had their chemistry selectively altered by the high velocity outflow from the young star(s) embedded deep within the interior of the molecular cloud. Detailed arguments are presented in this thesis which relate the seemingly disparate chemical compositions of the individual regions to each other and to the expected physical manifestations of the circumstellar mass loss, and which suggest that similar mechanisms may operate in other molecular clouds as well.

By performing supporting laboratory spectroscopy to supplement existing millimeter-wave catalogues only 33 of the over 800 lines remain unidentified, in contradiction to earlier expectations which had predicted that the near millimeter-wave spectrum of molecular clouds would contain hundreds of strong, unidentifiable emission features. It is probable that a number of the unidentified lines left in the OVRO survey are due to transitions between states of either isotopically substituted or highly excited abundant and complex molecules such as CH₃OH, CH₃OCH₃, and HCOOCH₃ whose rotational spectra are poorly known at present. The very small percentage and weak strength of the unidentified lines implies that the dominant chemical constituents visible at millimeter wavelengths have been identified in the Orion molecular cloud.

Also presented are high resolution laboratory millimeter, submillimeter, and far-infrared absorption spectra of the transient molecular species OH, CN, HOC⁺, and HCO, The zero-field pure rotational spectrum of the OH radical was observed, for the first time, with a frequency agile far-infrared laser sideband spectrometer which promises to revolutionize high resolution spectroscopy at submillimeter and far-infrared wavelengths, while the HOC⁺ molecular ion was synthesized in a novel glow discharge cell that increases ion abundances by roughly two orders of magnitude as compared with those produced by previously reported methods. Studies of several ions produced in the new discharge cell have provided a theory of the mechanism responsible for the ion enhancement. Sixty-five transitions of CN in its first four vibrational states have been observed, allowing a detailed examination of vibrational and electronic effects in this astrophysically important free radical. The investigation of HCO is the first extensive zero-field analysis of the formyl radical, and is one of the very few millimeter and submillimeter laboratory studies of a non-linear free radical performed to date.

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