An ALMA molecular inventory of warm Herbig Ae disks: I. Molecular rings, asymmetries and complexity in the HD 100546 disk

TL;DR

This study uses ALMA observations to map the complex molecular composition of the HD 100546 Herbig Ae disk, revealing rings of molecules and chemical signatures linked to planet formation, advancing understanding of disk chemistry.

Contribution

First ALMA line survey of a Herbig Ae disk showing detailed molecular rings and complex organic molecules, highlighting chemical diversity and structure in planet-forming regions.

Findings

Detection of 19 molecules, including NO, SO, and methyl formate.

Molecular rings peak beyond the millimeter continuum at 200 au.

First tentative detections of H213CO and 34SO in disks.

Abstract

Observations of disks with the Atacama Large Millimeter/submillimeter Array (ALMA) allow us to map the chemical makeup of nearby protoplanetary disks with unprecedented spatial resolution and sensitivity. The typical outer Class II disk observed with ALMA is one with an elevated C/O ratio and a lack of oxygen-bearing complex organic molecules, but there are now some interesting exceptions: three transition disks around Herbig Ae stars all show oxygen-rich gas traced via the unique detections of the molecules SO and CH3OH. We present the first results of an ALMA line survey at 337 to 357 GHz of such disks and focus this paper on the first Herbig Ae disk to exhibit this chemical signature - HD 100546. In these data, we detect 19 different molecules including NO, SO and CH3OCHO (methyl formate). We also make the first tentative detections of H213CO and 34SO in protoplanetary disks. Multiple molecular species are detected in rings, which are, surprisingly, all peaking just beyond the underlying millimeter continuum ring at 200 au. This result demonstrates a clear connection between the large dust distribution and the chemistry in this flat outer disk. We discuss the physical and/or chemical origin of these sub-structures in relation to ongoing planet formation in the HD 100546 disk. We also investigate how similar and/or different the molecular make up of this disk is to other chemically well-characterised Herbig Ae disks. The line-rich data we present motivates the need for more ALMA line surveys to probe the observable chemistry in Herbig Ae systems which offer unique insight into the composition of disk ices, including complex organic molecules.