At the Interface: Laboratory and Radio Astronomical Spectroscopy of Exotic Interstellar Molecules

Observations of molecules in the interstellar medium (ISM) have shown that the chemistry in space is full of surprises, with the appearance of many unusual, “non-terrestrial” species. Using high resolution rotational spectroscopy in the laboratory, combined with millimeter-wave astronomy, we have pursued the identification of these exotic molecules in the ISM, in particular those containing metals (in the chemists’ sense) or phosphorus. The laboratory studies have been conducted with millimeter/sub-mm/THz direct absorption spectrometers, as well as a pulsed, Fourier transform microwave/mm-wave instrument, all designed and built in the Ziurys group. Critical to the laboratory work has been the development of synthetic techniques to create the desired molecules, which are typically free radicals. These lab studies have enabled us to identify new magnesium, aluminum, iron and even vanadium-bearing molecules, such an MgCN, AlNC, FeCN, AlOH, and, most recently, VO, typically found in circumstellar envelopes of evolved stars, along with phosphorus-containing species such as CCP and SiP. A key to success in this work has been the synergy between the lab studies and observations. Many of the detections were made at the telescopes of the Arizona Radio Observatory (ARO) and required sensitive, stable instrumentation, that we implemented. Also, such new molecular identifications often required observations of atypical sources. For example, our pioneering observations of the oxygen-rich envelope of hypergiant star VY Canis Majoris (VY CMa) led to the first discoveries of circumstellar metal oxides and hydroxides. Our recent observations have shown also that many planetary nebulae (PNe) have a rich molecular content, containing species as complex as CH₃CN, H₂CNH, and CH₃CCH; such objects could be new molecular hunting grounds. Moreover, PNe appear to be “seeding” the diffuse interstellar medium with gas-phase molecules. Our studies of Galactic Edge Clouds have resulted in the detection of PO and PN at 23 kpc from the Galactic Center, extending the Galactic Habitable Zone. We have additionally teamed with solid-state spectroscopists in the investigation of the origin of interstellar fullerenes. Many “presolar” grains extracted from meteorites are SiC in its 3C-cubic polytype, believed to be formed in circumstellar envelopes. Our experiments this area show that thermal decomposition of analog SiC grains is a facile route to fullerenes, in particular C₆₀, and even carbon nanotubes.