Investigating the Chemistry of Circumstellar Material through Millimeter-wave Observations and Laboratory Spectroscopy

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The goals of this study by Dr. Ziurys and her group are to i) continue to probe the physical and chemical properties of circumstellar envelopes through mm-wave observations, ii) identify new circumstellar refractory molecules through laboratory spectroscopy, and iii) conduct more detailed observations of older PNe to determine the extent of their molecular composition.Mass loss from circumstellar envelopes of evolved stars enriches the interstellar medium (ISM) in gas, dust, and the heavier elements that are created in these objects through nucleosynthesis. The chemical and physical characteristics of circumstellar shells have major impacts on the overall life cycle of dense material in the ISM, as well as Galactic chemical evolution. The chemistry in circumstellar envelopes is quite different from that in molecular clouds, characterized in part by refractory compounds that are likely related to dust grain formation. Studies of circumstellar molecules have already given insight into the stellar mass loss process. The remnant shells of most evolved stars become planetary nebulae (PNe), whose morphological and chemical evolution, and impact on diffuse cloud formation are not yet well understood. About half of circumstellar envelopes are oxygen-rich, including those of supergiants and hypergiants, and early AGB stars, but are not yet that well studied as their carbon-rich counterparts. The oxygen rich objects represent different stages in stellar evolution.The Ziurys' group previously conducted an in-depth investigation of the O-rich envelope of the supergiant star, VY Canis Majoris (VY CMa)at millimeter wavelengths. The work here builds on this first detailed study and will be investigate the O-rich circumstellar shells of the supergiant NML Cygnus and the E-AGB stars TX Cam and R Leo. The envelope of the S-type star chi Cygnus will also be studied to explore chemical changes when carbon and oxygen abundances are about equal. These objects will be investigated via 1 mm broad-band surveys (215 to 285 GHz), using the highly sensitive ALMA-type receiver at the Sub-millimeter Telescope of the Arizona Radio Observatory (ARO), and the results compared with those already obtained for VY CMa and the carbon-rich object IRC+10216. The spectra are analyzed using radiative transfer methods, using a code developed to account for multiple, non-spherical winds, and molecular abundances, distributions, and temperatures will be determined. The surveys of VY CMa and IRC+10216 have revealed hundreds of unidentified lines in these objects, likely arising from refractory molecules. The laboratory work will entail measurements of the pure rotational spectra of potential circumstellar refractory compounds, such as oxides (ScO, TiOH, KO) for O-rich shells, metal dicarbides MC2 (with M = Al, Fe, Na, Mg)which are likely species in IRC+10216, and nitrides (MgN, AlN, and NaN) for the S-type objects. Once established, the inventory of circumstellar gas-phase refractory molecules will be compared with the solid state composition of circumstellar grains found in meteorites, through a collaboration with Arizona's Lunar and Planetary Lab. Observations of a sample of older PNe (Ring, Dumbell, M2-46) in the molecules HCO+, CS, C3H2, and H2CO will be carried out at the ARO SMT and 12 m in order to more fully characterize the molecular content of PNe. The proposed single-dish work is both preparatory for and complementary to ALMA investigations.The Ziurys group has an active program in laboratory spectroscopy centered on such unusual, reactive species, with four working spectrometers: three direct absorption and one Fourier transform microwave system with a laser ablation source for metals. This work directly links observational studies with laboratory astrophysics, and is very interdisciplinary. Graduate student training involves both astronomical and laboratory experimental aspects, and several female students will be participating. Dr. Ziurys is developing interdisciplinary courses in Astrochemistry and a website illustrating basic molecular spectroscopy.