Letter to the Editor

A major asymmetric ice trap in a planet-forming disk

II. Prominent SO and SO₂ pointing to C/O < 1

¹ Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands
e-mail: abooth@strw.leidenuniv.nl
² Physics & Astronomy Department, University of Victoria, 3800 Finnerty Road, Victoria, BC V8P 5C2, Canada
³ Max-Planck-Institut für Extraterrestrishe Physik, Gießenbachstrasse 1, 85748 Garching, Germany
⁴ RIKEN Cluster for Pioneering Research, 2-1, Hirosawa, Wako-shi, Saitama 351-0198, Japan

Received: 12 April 2021
Accepted: 7 June 2021

Abstract

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.