Molecules and Chemistry in Red Supergiants

The envelopes of Red Supergiants (RSGs) have a unique chemical environmentnot seen in other types of stars. They foster an oxygen-rich synthesis but are temperedby sporadic and chaotic mass loss, which distorts the envelope and creates complex outflowsub-structures consisting of knots, clumps, and arcs. Near the stellar photosphere,molecules and grains form under approximate LTE conditions, as predicted by chemicalmodels. However, the complicated outflows appear to have distinct chemistries generatedby shocks and dust destruction. Various RSG envelopes have been probed for their molecularcontent, mostly by radio and millimeter observations; however, VY Canis Majoris(VY CMa) and NML Cygni (NML Cyg) display the highest chemical complexity, and alsothe most complicated envelope structure. Thus far, over 29 different molecules have beenidentified 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, andVO, 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 intothe H-burning shell, but further exploration is needed.