Unveiling the Chemical Complexity and C/O Ratio of the HD 163296 Protoplanetary Disk: Constraints from Multiline ALMA Observations of Organics, Nitriles, and Sulfur-bearing and Deuterated Molecules

Abstract The physical and chemical conditions within a protoplanetary disk play a crucial role in determining its chemical composition, which is subsequently inherited by any forming planets. To probe these conditions, high-resolution molecular line observations, coupled with modeling, are essential. In this study, we investigate the chemistry of the nearby, massive, and relatively line-rich protoplanetary disk around HD 163296 using high-resolution observations from the Atacama Large Millimeter/submillimeter Array (ALMA) across Bands 3, 4, 6, and 7. We constrain the disk-averaged and radial distributions of column density and excitation temperature for the detected molecules using the new retrieval code DRive . The disk chemistry is modelled using the astrochemical code PEGASIS , with variations in the initial elemental C/O ratio. Our modelling, informed by molecular observations of HCO + , DCO + , HCN, DCN, CS, HC 3 N, H 2 CO, CH 3 OH, HNCO, and NH 2 CHO, allows us to place strong constraints on the C/O ratio, with a best-fit value of 1.1 that is broadly consistent with previous estimates. We present the highest-resolution DCO + emission map of this disk to date, revealing triple-ringed chemical substructures that closely align with the dust continuum rings. Additionally, our results provide the first and most stringent upper limits on the column densities of NH 2 CHO and HNCO in this protoplanetary disk, measured at