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The envelopes of Red Supergiants (RSGs) have a unique chemical environment not seen in other types of stars. They foster an oxygen-rich synthesis but are tempered by sporadic and chaotic mass loss, which distorts the envelope and creates complex outflow sub-structures consisting of knots, clumps, and arcs. Near the stellar photosphere, molecules and grains form under approximate LTE conditions, as predicted by chemical models. However, the complicated outflows appear to have distinct chemistries generated by shocks and dust destruction. Various RSG envelopes have been probed for their molecular content, mostly by radio and millimeter observations; however, VY Canis Majoris (VY CMa) and NML Cygni (NML Cyg) display the highest chemical complexity, and also the most complicated envelope structure. Thus far, over 29 different molecules have been identified in the envelopes of RSGs. Some molecules are common for circumstellar gas, including CO, SiO, HCN and H2O, which have abundances of ∼10−6–10−4, relative to H2. More exotic oxides have additionally been discovered, such as AlO, AlOH, PO, TiO2, and VO, with abundances of ∼10−9–10−7. RSG shells support intricate maser emission in OH, H2O and SiO, as well. Studies of isotope ratios in molecules suggest dredge-up at least into the H-burning shell, but further exploration is needed. 

New computational and experimental studies have been carried out for the MgCCH radical in its X2Σ+ state. Coupled cluster theory with single, double, and perturbative triples, CCSD(T), was used in conjunction with post-CCSD(T) and scalar relativistic additive corrections to compute vibrational quartic force fields for this molecule. From the quartic force fields, higher-order spectroscopic properties, including rotational constants, were obtained. In tandem, the five lowest energy rotational transitions for MgCCH, N = 1→0 through N = 5→4, were measured for the first time using Fourier transform microwave/millimeter wave methods in the frequency range 9 -50 GHz. The radical was created in the Discharge Assisted Laser Ablation Source (DALAS) developed in the Ziurys group. A combined fit of these data with previous millimeter direct absorption measurements have yielded the most accurate rotational constants for MgCCH to date. The computed principle rotational constant lies within 1.51-1.65 MHz of the experimental one, validating the computational approach. High-level theory was then applied to produce accurate rovibrational spectroscopic constants for MgCCH+, including a rotational constant of B0 = 5354.5–5359.5 MHz.. These new predictions will further the experimental study of MgCCH+, and aid in the low-temperature characterization of MgCCH, detected towards the circumstellar shell of IRC+10216, a carbon-rich star. 

The detection of the fullerenes C60 and C70 in the interstellar medium (ISM) has transformed our understanding of chemical complexity in space. These discoveries also raise the possibility for the presence of even larger molecules in astrophysical environments. Here we report in situ heating of analog silicon carbide (SiC) presolar grains using transmission electron microscopy (TEM). These heating experiments are designed to simulate the temperature conditions occurring in post-AGB stellar envelopes. Our experimental findings reveal that heating the analog SiC grains to the point of decomposition initially yields hemispherical C60-sized nanostructures, with five- and six-membered rings, which transform into multiwalled carbon nanotubes (MWCNTs) if held isothermally >2 min. These MWCNTs are certainly larger than any of the currently observed interstellar fullerene species, both in overall size and number of C atoms. These experimental simulations suggest that such MWCNTs are likely to form in post-AGB circumstellar material, where the structures, along with the smaller fullerenes, are subsequently injected into the ISM.