Millimeter-wave spectroscopy of metal-bearing molecules in the ISM

Over the past thirty years of millimeter astronomy, a great deal has been learned about interstellar molecules and their chemistry. Despite this progress, there are still areas of molecular astrophysics where challenges remain. One of these areas concerns the chemistry and distribution of molecules containing metals. Identifying the carriers of these elements in the interstellar medium, including circumstellar gas, is crucial for the evaluation of dust grain composition, ionization balance, mass loss from evolved stars, and elemental depletions. Over the past decade, the Ziurys group has been actively involved in both the laboratory measurement of the millimeter/sub-millimeter spectra of metal-containing molecules and in radio astronomical searches for these species in the ISM. A wide variety of small, metal-bearing species have been studied in our laboratory in the gas-phase using direct absorption techniques. These compounds include diatomic hydrides, halides, carbides, nitrides, oxides, and sulfides, and the polyatomic amides, hydroxides, acetylides, and cyanides. Some of the species recently studied are CrH (X⁶Σ⁺), CaC (X³Σ^-), AlSH (X¹A′), AlNC (X ¹Σ⁺), and CoCN (X ³_i). We have begun to study metal-bearing molecular ions as well (TiCl⁺: X ³_r; VCl⁺: X⁴Σ^-; FeCO⁺: X⁴Σ^-; for example), using a new millimeter/sub-millimeter velocity modulation system. A new Fourier transform microwave spectrometer has also just been completed and will be used for laser-ablation investigations of such species. Radio astronomical searches have been made for almost all of these molecules, using the Arizona Radio Observatory's facilities and the IRAM 30 m telescope. Interestingly, besides the halides, only metal cyanide and isocyanide species have been detected in astronomical sources, and only in circumstellar envelopes of carbon-rich AGB stars. Among the molecules identified are AlNC, MgCN, and KCN. Metal-bearing species have been generally found in the envelope of the carbon star IRC+10216, but some are also present in other circumstellar shells of such objects as CRL2688 and CRL618. Nucleosynthesis occurs in AGB stars, and may produce metals such as sodium and magnesium. Therefore, the metal-bearing molecules detected in AGB envelopes may reflect enhancements in elemental abundances resulting from such nucleosynthesis, rather than cosmic abundances. Such elements would be mixed into the circumstellar shell in the third dredge-up, which also brings carbon to the stellar surface, creating a C-rich envelope. Additional metal-containing molecules need to be observed in AGB shells to verify this connection. These observations await new laboratory spectra. Metal cyanide/isocyanide compounds are the obvious choices for such laboratory work, in particular FeNC. This species has remained elusive, although the spectrum of CrCN has now been measured.