A major asymmetric ice trap in a planet-forming disk: IV. Nitric oxide gas and a lack of CN tracing sublimating ices and a C/O ratio <1

Abstract

Context. Most well-resolved disks observed with the Atacama Large Millimeter/submillimeter Array (ALMA) show signs of dust traps. These dust traps set the chemical composition of the planet-forming material in these disks, as the dust grains with their icy mantles are trapped at specific radii and could deplete the gas and dust at smaller radii of volatiles.Aims. In this work, we analyse the first detection of nitric oxide (NO) in a protoplanetary disk. We aim to constrain the nitrogen chemistry and the gas-phase C/O ratio in the highly asymmetric dust trap in the Oph-IRS 48 disk.Methods. We used ALMA observations of NO, CN, C2H, and related molecules in the Oph-IRS 48 disk. We modeled the effect of the increased dust-to-gas ratio in the dust trap on the physical and chemical structure using a dedicated nitrogen chemistry network in the thermochemical code DALI. Furthermore, we explored how ice sublimation contributes to the observed emission lines. Finally, we used the model to put constraints on the nitrogen-bearing ices.Results. Nitric oxide (NO) is only observed at the location of the dust trap, but CN and C2H are not detected in the Oph-IRS 48 disk. This results in an CN/NO column density ratio of