An ALMA View on Molecular Complexity: Water, Methanol, and Methyl Formate in the HD 100546 and IRS 48 Disks

Abstract Water and methanol are the fundamental constituents of interstellar and cometary ices; understanding their abundances in disks is therefore of high importance to reveal how this ice reservoir evolves across the timeline of star and planet formation. We present Atacama Large Millimeter/submillimeter Array (ALMA) Band 5 line emission observations of water and methanol in the warm Herbig disks of HD 100546 and IRS 48. While the IRS 48 disk is methanol-rich, water vapor is not detected, implying a high methanol-to-water column density ratio of >0.2, possibly reflecting a distinct ice composition. Methanol and water are both detected in the HD 100546 disk, leading to a CH 3 OH/H 2 O ratio of ∼0.02, consistent with the median of the cometary distribution and with the lower edge of the distribution of young stellar objects (YSOs). We suggest that the gas-phase methanol-to-water ratio in the HD 100546 disk directly reveals the ice reservoir subsequently sublimated in the warm disk stage. Moreover, we report a methyl formate-to-methanol ratio in the IRS 48 disk of ∼0.2, consistent with the upper edge of the distribution of YSOs. The methyl formate-to-methanol ratio in the HD 100546 disk is ∼2, much higher than in younger and older sources, possibly reflecting a chemical enhancement of gas-phase methyl formate. We additionally constrain non-LTE conditions, optical depths, and emitting regions of the methanol lines in both disks. These results offer a new opportunity to probe the chemical trail of molecular complexity in the intermediate-disk stage, bridging early and late phases of the star and planet formation history.