Organosulfur Chemistry in the Birthplaces of Stars and Planets (Suchitra Narayanan, UH/Harvard)

Talk given November 12, 2024. Of the elements critical for life, sulfur is poorly understood due to its 1-2 orders of magnitude depletion in the gas phase of star-forming regions (also known as the “missing sulfur problem”). To reconcile this, sulfur is believed to be locked up in icy grains; however, the sum of the solid sulfur inventory in ices accounts for only ≤ 4% of the cosmic sulfur abundance. The most updated astrochemical gas-grain reaction network predicts that this observed sulfur depletion could be explained if the majority of the sulfur exists in the form of solid organosulfur species. However, due to the limited number of solid-state sulfur experiments, this model, like many others, heavily depends on the theoretical assumption that sulfur and oxygen chemistry proceed comparably. My PhD dissertation fills this gap in literature by characterizing the simplest S-bearing complex organic molecule, methyl mercaptan (CH₃SH), with respect to its well-studied and relatively abundant O-bearing counterpart, methanol (CH₃OH). I present new laboratory experiments on CH₃SH's thermal desorption kinetics, entrapment behavior, and formation/destruction pathways, while contextualizing all results with analogous CH₃OH experiments. This allows us to probe for the first time in the laboratory how, when, and why does S vs. O chemistry proceed (dis)similarly. In most cases, I find that under identical experimental conditions, CH₃SH behaves differently from CH₃OH, and these discrepancies cannot be fully explained with current computational chemistry capabilities. In particular, we find that the physical and chemical properties of a molecule (e.g., size, ability to form allotropes, bonding potential) significantly affects its behavior and stability in astrophysically relevant conditions. This is the first time that such a size effect has been shown to impact solid-state chemistry significantly. By studying how two theoretically similar elements are empirically different, my work serves as a foundational guide for further investigations into more complex molecules, enabling us to better predict their behavior based on whether they exhibit characteristics similar to sulfur or oxygen. I also use key findings from the laboratory to inform my complementary theoretical studies and observational programs with the Atacama Large Millimeter/submillimeter Array and the Submillimeter Array, where I probe sulfur chemistry in the earliest stages of star and planet formation. Overall, these results both emphasize the necessity of dedicated sulfur experiments and highlight the value of comparative chemistry for rationalizing observations and refining the theoretical understanding of sulfur (astro)chemistry.