Observations of sulfur and nitrogen molecules in star-forming regions of the Milky Way

<sec><p indent="0mm">Astrochemistry investigates the radiation and absorption features of molecules in the interstellar medium (ISM) through a multidisciplinary framework encompassing astronomical observations, laboratory experiments, and theoretical modeling. Core methodologies focus on determining elemental and molecular abundances, analyzing reaction networks, and studying molecular formation and destruction processes to decode the structure, dynamics, and evolution of molecular clouds. To date, over 300 molecular species have been identified in interstellar environments, with the detection of complex organic molecules (COMs) representing a major research frontier. While COMs are synthesized from simpler precursors via chemical pathways, even fundamental mechanisms governing basic molecular species remain poorly constrained. We adopt S-bearing and N-bearing molecules as probes to unravel the chemical complexity of the ISM. Sulfur and nitrogen are critical elements for star formation and interstellar chemistry, serving as key tracers of physical and chemical processes. </sec><sec> We synthesize observational and theoretical advancements in understanding S-bearing and N-bearing molecules within star-forming regions. S-bearing species, such as carbon monosulfide (CS), are widely employed to trace high-density molecular clouds. Molecules like hydrogen sulfide (H₂S), sulfur monoxide (SO), and sulfur dioxide (SO₂) act as shock tracers while S-bearing species (e.g., SO, H₂S, CS, and OCS) liberated from ice mantles are pivotal indicators of hot cores in Class 0 protostellar systems of solar-like masses. Despite extensive efforts, sulfur chemistry in star-forming regions remains enigmatic, particularly the dominant reservoirs of sulfur in dense cloud cores. Nitrogen, existing in forms such as ammonia (NH₃) and nitric oxide (NO), is crucial for probing dense gas dynamics and magnetic fields in these regions. </sec><sec> We present a large-sample analysis of S-bearing molecules in late-stage massive star-forming regions. A robust linear correlation was identified among the abundances of H₂S, thioformaldehyde (H₂CS), and thiomethylium (HCS⁺) across 51 sources, with H₂CS and HCS⁺ displaying the strongest relationship. A three-phase gas-dust-ice chemical model was developed to interpret these findings, yet simulations failed to fully replicate observations. The detection of SiO 4-3 emission lines revealed weak correlations between SiO and H₂S/H₂CS/HCS⁺ abundances, suggesting that shock-driven processes must be integrated into sulfur chemistry models to resolve discrepancies. Notably, the current dataset relies on single-point observations; spatially resolved mapping is imperative to advance our understanding of chemical distributions. </sec><sec> Ongoing mapping observations of 22 late-stage massive star-forming regions using the IRAM <sc>30m</sc> millimeter-wave telescope have uncovered spatial misalignments between H¹³CO⁺ and HCS⁺ distributions in select sources. Despite shared chemical traits among S/O-bearing molecules, these species trace distinct spatial zones, implying divergent chemical pathways. For nitrogen chemistry, NO exhibits anomalously high abundances in specific environments compared to other N-bearing species. In low-velocity shock regions, NO directly reflects oxygen molecular abundances, positioning it as a unique tracer of oxygen chemistry. However, observational constraints on NO remain limited, underscoring the need for high spatial-resolution studies (e.g., ALMA, JWST) to elucidate its role in astrochemical networks. Future efforts must combine multi-scale observations with advanced chemical models to unravel the lifecycle of molecular clouds and the origins of chemical complexity in the universe. </sec>