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

TL;DR

The paper tackles how the chemical composition of protoplanetary disks governs the initial molecular budgets inherited by forming planets, using high-resolution ALMA observations across Bands 3, 4, 6, and 7 combined with DRive and PEGASIS to interpret disk conditions. The authors constrain disk-averaged and radial column densities and excitation temperatures for ten detected species, exploring variations in the initial C/O ratio. They find a best-fit disk C/O of $1.1$, consistent with prior estimates, and present the highest-resolution $DCO^+$ emission map of this disk, which reveals triple-ring substructures aligned with the dust rings. Upper limits on $NH_{2}CHO$ and $HNCO$ are $<7\times 10^{11}$ cm$^{-2}$ and $<1\times 10^{11}$ cm$^{-2}$, respectively, with both species inferred to form mainly on grain surfaces and with inefficient physico-chemical desorption, making them promising targets for future ALMA observations. The results demonstrate the viability of comprehensive, multi-species disk chemistry analyses to constrain the C/O ratio and inform expectations for complex organics and prebiotic molecules in planet-forming environments.

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 modelling, 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 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, HC3N, H2CO, CH3OH, HNCO, and NH2CHO, 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 NH2CHO and HNCO in this protoplanetary disk, measured at < 7e11 cm-2 and < 1e11 cm-2, respectively. Our chemical models suggest that NH2CHO and HNCO predominantly form on grain surfaces within the disk. However, physico-chemical desorption mechanisms are inefficient at releasing these species into detectable gas-phase abundances, yet they remain promising targets for future ALMA observations.