Small Molecules, Big Impact

Abstract

This thesis investigates hydride molecules in the interstellar medium (ISM) and their use as diagnostics of different phases of the ISM. Particular emphasis is given to the central CH radical, a probe of diffuse and translucent molecular clouds, including those not traced by the otherwise common CO. We discuss our searches for the rare isotopologues of CH and address questions regarding the origin of another molecule, CH₂, which despite being chemically associated with the ubiquitous CH, has largely remained elusive. In addition, the molecular ion, ArH⁺, a tracer for diffuse atomic gas is studied. <br /> Whilst the 9 cm ground state radio lines of CH are widely observed, their excitation has been poorly understood as their intensities have long been found to be inconsistent with the assumptions of local thermodynamic equilibrium (LTE). We present the first interferometric observations of these CH lines using the <em>Karl G. Jansky</em> Very Large Array (VLA) and provide constraints on the physical and excitation conditions traced by these lines, through non-LTE radiative transfer models which invoke the effects of far-infrared (FIR) line overlap. Furthermore, by taking advantage of the synergies between the FIR and radio transitions of CH we constrain our models using reliable column densities determined from the FIR lines of CH at 149 µm observed using the high spectral resolution upGREAT receiver on board the Stratospheric Observatory for Infrared Astronomy (SOFIA). Our results reveal that the physical conditions traced by CH are consistent with those found within the warm layers of photodissociation regions (PDRs), which along with astrochemical considerations suggest that CH maybe formed via the dissipation of turbulence, possible even in dark clouds. This analysis establishes the use of the widely observed ground state transitions of CH as a powerful tool to probe the diffuse and translucent ISM at radio wavelengths. <br /> By further exploiting the unique capabilities provided by SOFIA, we discuss our search for, and successful detection, of sup>13</sup>CH in the ISM for the first time. Combined with observations of its main isotopologue we highlight its use as a potentially unbiased tracer of the ¹²C/¹³C isotopic ratio, an important diagnostic tool for probing the nucleosynthesis history of the Galaxy. <br /> Based on early observations of high energy level transitions of CH₂ between 68 and 71 GHz toward the hot cores in Orion and W51, astronomers associated the observed emission as arising from dense, hot environments. The subsequent non-detection of these lines toward the archetypal Orion-KL hot core in high resolution observations have raised questions regarding the molecule’s origin. To address this, we conducted new observations of these transitions toward the Orion region and other star-forming regions. Our data, with the aid of ancillary carbon recombination line data and radiative transfer models, establishes CH₂’s association with warm but dilute PDR gas layers. Additionally, we find that for the physical conditions derived, the observed CH₂ lines show level inversion and acts as a weak maser. <br />Finally, we extend the view of the chemistry of argonium (ArH⁺) using observations carried out with the Atacama Pathfinder EXperiment (APEX) 12 m telescope. Combined with archival observations of OH⁺ and H₂O⁺, we determined the properties of the gas traced by ArH⁺ by analysing the steady-state chemistry. Our results confirm the role of ArH⁺ as a tracer of atomic gas, probing regions with an extremely low average molecular fraction of 8.8×10^-4. Furthermore, by comparing the abundance and molecular fraction traced by ArH⁺ and by atomic gas tracers OH⁺ and H₂O⁺ and molecular gas tracers CH or HF, we study the transition from the diffuse atomic to the diffuse and translucent molecular clouds in the ISM.