The chemistry of metal-containing molecules in the circumstellar envelopes of late-type stars: Millimeter-wave observations and abundance modeling

Abstract

An intensive observational program of Asymptotic Giant Branch (AGB) and post-AGB stars has been conducted in order to investigate the role of metal-containing compounds in the circumstellar envelopes surrounding these objects. The role of metal bearing molecules in the chemical evolution of these envelopes was also examined. These studies have led to the detection of a new interstellar molecule, AlNC, as well as the first identification of metal species in two new sources. Specifically, MgNC, AlF, NaCl, and NaCN were seen for the first time towards CRL 2688 and MgNC was also detected in CRL 618. These detections, and the non-detection of metal carbide or metal nitride species, indicate that metals preferentially form cyanide complexes in circumstellar clouds. The process of formation for these species is believed to be radiative association of a metal ion with a cyanopolyne chain followed by dissociative recombination. It has also been discovered that, as a star evolves, abundances of metal molecules in the inner circumstellar envelope decrease due to photo-destruction processes or adsorption onto dust grains. A new chemical code to study molecular abundances in the inner circumstellar envelope of AGB stars has also been developed. This new chemistry code is unique in that it is the first attempt to take a kinetic approach to the equilibrium problem. A system of reactions is used as a pathway to reach equilibrium at a specified temperature and density. This new technique allows the monitoring of abundance changes over time and can be used as a foundation for non-LTE calculations. The model results were consistent with previous work and older methods. The abundances of metal halides and NaCN in the AGB phase can be explained because of equilibrium processes at temperatures less than 1600 K for moderate to high densities. The model also demonstrated that AlNC could be produced in equilibrium conditions in amounts greater than the observed abundance of this molecule in IRC+10216. Chemical changes with time were monitored and it was shown that equilibrium processes occur on timescales larger than the typical pulsational period of an AGB star. Finally, abundance predictions were made for several potential new circumstellar metal-containing molecules.