The Physics and Chemistry of Oxygen-rich Circumstellar Envelopes as Traced by Simple Molecules

<p>The physics and chemistry of the circumstellar envelopes (CSEs) of evolved stars are not fully understood despite decades of research. This thesis addresses two issues in the study of the CSEs of oxygen-rich (O-rich) evolved stars. In the first project, the ammonia (NH₃) chemistry of O-rich stars is investigated with multi-wavelength observations; in the second project, the extended atmosphere and inner wind of the archetypal asymptotic giant branch (AGB) star <em>o</em> Ceti (Mira) is studied with high-angular resolution observations.</p> <p>One of the long-standing mysteries in circumstellar chemistry is the perplexing overabundance of the NH₃ molecule. NH₃ in O-rich evolved stars has been found in much higher abundance, by several orders of magnitude, than that expected in equilibrium chemistry. Several mechanisms have been suggested in the literature to explain this high NH₃ abundance, including shocks in the inner wind, photodissociation of nitrogen by interstellar ultraviolet radiation, and nitrogen enrichment in stellar nucleosynthesis; however, none of these suggestions can fully explain the abundances of NH₃ and various other molecular species in the CSEs of O-rich stars.</p> <p>In order to investigate the distribution of NH₃ in O-rich CSEs, observations of the spectral lines of NH₃ from a diverse sample of evolved stars and in different wavelength regimes are necessary. In this thesis, the NH₃ line emission and absorption from four O-rich stars are studied. These targets include the AGB star IK Tauri, the pre-planetary nebula OH 231.8+4.2, the red supergiant VY Canis Majoris, and the yellow hypergiant IRC +10420. The amount of NH₃ observational data has increased drastically thanks to the recent advancement of instrumentation. Observations of NH₃ rotational line emission at submillimetre/far-infrared wavelengths were possible with the <em>Herschel</em> Space Observatory (2009–2013). The new wideband correlator in the upgraded Karl G. Janksy Very Large Array (VLA) provided data of multiple radio inversion lines of NH₃. Furthermore, mid-infrared absorption of NH₃ has been observed by the NASA Infrared Telescope Facility (IRTF) for IK Tau and VY CMa.</p> <p>Full radiative transfer modelling including mid-infrared pumping to the first vibrationally excited state (v₂=1) has been carried out to reproduce the observed emission and absorption spectra and to retrieve the NH₃ abundances in the targets. It is found that the NH₃ emission in the CSEs of the targets arises from localised spatial-kinematic structures in which the gas density may be higher than in the surrounding gas. Circumstellar shocks may contribute to, but cannot fully account for, the formation of the molecule.</p> <p>Besides circumstellar chemistry, our understanding of the dust formation and wind-driving mechanisms of oxygen-rich evolved stars is still incomplete. One of the obstacles in the past was the difficulty in imaging the dust condensation and wind acceleration zones due to the lack of high-angular resolution instruments. Thanks to the Atacama Large Millimeter/submillimeter Array (ALMA), which has the longest baseline of about 15 km, we are now able to produce high-fidelity images at unprecedented angular resolutions of tens of milliarcseconds (mas) in the (sub)millimetre regime. Such angular resolutions, which are comparable to the stellar radii of nearby objects, are necessary to understand the gas dynamics and chemical evolution in the pulsating atmosphere and dust formation zone of nearby AGB stars.</p> <p>The eponymous Mira-type long-period variable, <em>o</em> Cet, was observed as a Science Verification target during the first ALMA Long Baseline Campaign that took place in 2014. The observations produced images of the stellar radio photosphere and the molecular transitions of SiO and H₂O at an angular resolution of about 30 mas near 220 GHz (1.3 mm). The millimetre stellar disc of <em>o</em> Cet was resolved and modelled. More importantly, this is the first time that molecular line absorption against the background stellar continuum has been clearly imaged in the (sub)millimetre wavelength regime.</p> <p>Through radiative transfer modelling of the SiO and H₂O line absorption and emission, it is found that during the ALMA observations, the extended atmosphere of the star exhibited infall motions in general with a shock front of velocity -1 beyond the radio photosphere of <em>o</em> Cet. Gas-phase SiO starts to deplete beyond 4 stellar radii at the temperature of