A major asymmetric ice trap in a planet-forming disk - II. Prominent SO and SO₂ pointing to C/O < 1

Gas-phase sulphur-bearing volatiles appear to be severely depleted in protoplanetary disks. The detection of CS and the non-detections of SO and SO2 in many disks have shown that the gas in the warm molecular layer, where giant planets accrete their atmospheres, has a high C/O ratio. In this Letter, we report the detection of SO and SO2 in the Oph-IRS 48 disk using ALMA. This is the first case of prominent SO2 emission detected from a protoplanetary disk. The molecular emissions of both molecules is spatially correlated with the asymmetric dust trap. We propose that this is due to the sublimation of ices at the edge of the dust cavity and that the bulk of the ice reservoir is coincident with the millimetre-sized dust grains. Depending on the partition of elemental sulphur between refractory and volatile materials, the observed molecules can account for 15–100% of the total volatile sulphur budget in the disk. In stark contrast to previous results, we constrain the C/O ratio from the CS/SO ratio to be < 1 and potentially solar. This has important implications for the elemental composition of planets forming within the cavities of warm transition disks