The positive detection of buckminsterfullerene (C₆₀) in circumstellar and interstellar environments has changed paradigms of chemical complexity in the hostile conditions of space. While C₆₀ is a significant sink of interstellar carbon, the formation mechanism is still the subject of speculation. To examine C₆₀ formation, we have conducted shock-heating experiments on synthetic, 3C-SiC grains, the most common polytype generated by stars. Measurements of the heated grains, conducted with transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) techniques, show the formation of graphite and carbon nanobuds on the sample surface. The nanobud diameters are nearly identical to that of C₆₀, indicating that spherical structures may be forming as well. These data suggest that C₆₀ is formed by the shock-heating of common SiC grains at the end of the asymptotic giant branch (AGB) phase. In addition, TEM measurements of actual pre-solar grains extracted from meteorites show a central SiC core, surrounded by graphite, confirming our proposed formation mechanism.

Silicon carbide (SiC) grains comprise a significant fraction of the dust found around carbon-rich AGB stars. Their presence in the interstellar medium is thought to originate from self-assembly of organosilicon building blocks, including previously observed species such as carborundum and cyclic silicon dicarbide (c-SiC₂). However, the actual formation mechanisms of even these simple silicon-bearing organic molecules remains elusive. Here it is proposed that disilyne (Si₂H₂) reacts barrierlessly with abundant acetylene (C₂H₂) on a spin-conserving potential to form C₂Si₂H₄. This species has been shown in experimental and theoretical studiesLutz J.J., Inorganics, submittedo photoisomerize under UV irradiation resulting in the formation of several species, one being a c-SiC₂ precursor and another being a highly polar species capable of supporting a dipole-bound electron. This strongly dipolar C₂Si₂H₄ isomer may represent the missing link supporting the molecular aggregation hypothesis for SiC formation. Importantly, its polarity drives molecular aggregation, and, after subsequent oxidation to C₂Si₂, its heteronuclear linkages are well-prepared for SiC nucleation, presumably initiated by a shock-wave pulsation event. Past theoretical studies by our groupLutz J.J., Duan X.F., et al. J. Chem. Phys. 148, 174309 (2018)Byrd J.N., Lutz J.J., et al. J. Chem. Phys. 145, 024312 (2016) are combined with new results, computed at the DFT and coupled-cluster levels of theory, to support the proposed mechanism.

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Lutz J.J., Inorganics, submittedt Lutz J.J., Duan X.F., et al. J. Chem. Phys. 148, 174309 (2018)

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We have confirmed a new circumstellar molecule, VO, observed in the envelope of the supergiant star, VY Canis Majoris, using the Hubble Space Telescope (HST) with its Imaging Spectrograph (STIS). The molecule, a free radical, was detected in two clumps in the ejecta of this star via its B-X electronic transition in the near-infrared. The spectra are clearly circumstellar as the lines are significantly red-shifted from the stellar velocity, and arise from material 40-50 R* from the star. Wallerstein had identified some of the observed lines previously towards VY CMa; our data prove their spatial location. The VO lines are likely excited by resonant scattering from the star through a hole in the dusty shell.

Much research in planetary atmospheres is focused on Titan, one of Saturn’s moons. This interest is driven by the Cassini-Hyugens mission and the fact that Titan’s atmosphere is considered a potential analog of prebiotic Earth. In order to identify the chemical composition of the moon’s atmosphere a catalog of known spectra is needed for accurate comparisons. The molecules of interest are small hydrocarbons, as they have been observed on Titan and can be generated via photochemistry of methane, a primary component of Titan’s atmosphere. This talk will look at propane, as well as new data that will be recorded for some of the larger possible hydrocarbons, such as neopentane, a molecule for which there is little data in the literature, and n-butane. Absorption cross sections were obtained for pure samples of propane and with hydrogen and helium broadening gases to simulate astronomical environments. The propane spectra were taken in the 3 micron region, at temperatures ranging from 200 to 298 K and at broadening gas pressures from 0 Torr to 300 Torr. Calibration of the cross sections were carried out using data from the Pacific Northwest National Laboratory (PNNL) infrared database.

Late-type stars eject large amounts of material into outer space. At the very beginning of this process, i.e. close to the star, atoms form small molecules which finally react further to form larger species like nanoparticles. This very first step of chemical evolution is still not well understood. How do the first molecules form? What is the chemical inventory of the stellar atmosphere? To investigate these processes in the vicinity of stars requires both, high spatial and high frequency resolution. We have performed mid-infrared observations towards the stars VY Canis Majoris and Mira (o-ceti) using the high resolution TEXES instrument at the IRTF observatory. As the identification of molecular species requires high confidence in the transition frequency positions accompanying laboratory measurements have been performed, e.g for Si₂C, Al₂O and TiO around 8 and 10 μm.

We study four lines of sight that probe the transition from diffuse molecular gas to molecular cloud material in Taurus. Measurements of atomic and molecular absorption are used to infer the distribution of species and the physical conditions in the direction to stars behind the Taurus Molecular Cloud. New high-resolution spectra at visible and near infrared wavelengths of interstellar K I, CH, CH⁺, C₂, CN, and CO toward HD 28975 and HD 29647 are combined with published results for HD 27778 and HD 30122. Gas densities and temperatures are inferred from analyses of C₂, CN, and CO excitation. Our results for HD 29647 are noteworthy in that the CO column density is 10¹⁸ cm⁻², our analysis of CO and C₂ excitation reveal a temperature of 10 K and densities of about 1000 cm⁻³, and the CO excitation and radiation temperatures are the same, more like emission-line studies of dark molecular clouds. Similar results arise from our chemical analysis leading to CN through reactions involving the observed species CH and C₂. The other directions are typical of molecule-rich diffuse clouds and can be considered CO-dark gas.

e-PYTHEAS is a multidisciplinary project which combines theoretical and experimental work with exoplanet modelling applications. It sits on the frontier between molecular physics, theoretical chemistry and astrophysics. It aims at enhancing our understanding of the radiative properties of hot gaseous media to allow for improved analysis and interpretation of the large mass of data available on the thousands of exoplanets and exoplanetary systems known to date. Our approach is to use theoretical research validated by laboratory experiments and to then inject it into models of the atmospheres of the giant gaseous planets in the solar system and other planetary systems. This will help to analyse data and address essential questions on the formation and evolution of planetary systems, such as retrieved by ESA's M4 space mission ARIEL. Our consortium of 5 French laboratories and associated partners proposes to improve the existing high-temperature spectroscopy data for several molecular species detected in exoplanets. The provision of infrared (IR) laboratory data of methane, acetylene, ethylene and ethane, between 500 and 2500 K will help to refine thermal profiles and provide information on the gaseous composition, the hazes and their temporal variability. See the project's website: http://e-pytheas.cnrs.fr

The atmosphere of Titan, one of Saturn’s moons, is of great interest to the scientific community. With its primary components of nitrogen and methane, many believe Titan to be an analog of prebiotic Earth. The Cassini-Hyugens mission, launched in 1997, gathered massive amounts of data from both Saturn and Titan that is still being interpreted. Analysis of astronomical spectra is dependent on high-quality laboratory spectra. In the case of Titan this includes small hydrocarbons such as ethane, propane and benzene that are the products of photochemistry of methane. One complication that arises when making these assignments comes from incomplete or absent line-by-line spectroscopic data commonly used to determine molecular abundances. This problem can be avoided by utilizing absorption cross sections, as they are only dependent on the environment of the target molecule, environments that can be replicated in the lab. One of the molecules that potentially exists on Titan is isobutane. This talk focuses on the absorption cross sections of isobutane needed for for Titan and the Giant Planets. Absorption cross sections were obtained for pure samples, and with hydrogen and nitrogen broadening gases. The data were taken between 203 K and 295 K, at broadening gas pressures ranging from 0 Torr to 100 Torr, in the CH stretching region (2500-3280 cm⁻¹) and from 1050 cm⁻¹to 1900 cm⁻¹. Calibration of the cross sections were carried out using data from the Pacific Northwest National Laboratory (PNNL) infrared database. These cross sections were then used to calculate an upper limit for isobutane in Titan’s atmosphere.