A major asymmetric ice trap in a planet-forming disk. III. First detection of dimethyl ether

The complex organic molecules (COMs) detected in star-forming regions are the precursors of the prebiotic molecules that can lead to the emergence of life. By studying COMs in more evolved protoplanetary disks we can gain a better understanding of how they are incorporated into planets. This paper presents ALMA band 7 observations of the dust and ice trap in the protoplanetary disk around Oph IRS 48. We report the first detection of dimethyl ether (CH₃OCH₃) in a planet-forming disk and a tentative detection of methyl formate (CH₃OCHO). We determined column densities for the detected molecules and upper limits on non-detected species using the CASSIS spectral analysis tool. The inferred column densities of CH₃OCH₃ and CH₃OCHO with respect to methanol (CH₃OH) are of order unity, indicating unusually high abundances of these species compared to other environments. Alternatively, the ¹²CH₃OH emission is optically thick and beam diluted, implying a higher CH₃OH column density and a smaller emitting area than originally thought. The presence of these complex molecules can be explained by thermal ice sublimation, where the dust cavity edge is heated by irradiation and the full volatile ice content is observable in the gas phase. This work confirms the presence of oxygen-bearing molecules more complex than CH₃OH in protoplanetary disks for the first time. It also shows that it is indeed possible to trace the full interstellar journey of COMs across the different evolutionary stages of star, disk, and planet formation.