An ALMA Molecular Inventory of Warm Herbig Ae Disks. I. Molecular Rings, Asymmetries, and Complexity in the HD 100546 Disk

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 ≈337–357 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, SO2, and CH3OCHO (methyl formate). We also make the first tentative detections of H 2 13 CO 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 disk. We discuss the physical and/or chemical origin of these substructures in relation to ongoing planet formation in the HD 100546 disk. We also investigate how similar and/or different this molecular makeup of this disk is to other chemically well-characterized Herbig Ae disks. The line-rich data we present motivate the need for more ALMA line surveys to probe the observable chemistry in Herbig Ae systems, which offer unique insight into the composition of disks ices, including complex organic molecules.

An ALMA Molecular Inventory of Warm Herbig Ae Disks. II. Abundant Complex Organics and Volatile Sulphur in the IRS 48 Disk

“…This filter can be a smooth model, e.g., a Keplerian mask, the FITS output of a line radiative transfer model, or a strong line detection in the disk. In Figure 1 we present the resulting matched filter response over the full data set for the IRS 48 disk with a Keplerian model with an outer radius of 150 au compared to the HD 100546 response (outer radius of 300 au) that is presented in Booth et al (2024). HD 100546 was observed in the same manner as IRS 48, and we find that the IRS 48 disk is more line rich, but there are different molecules detected in each disk.…”

“…These spectral windows are centered at 338.790824, 340.732413, 348.916936, and 350.775389 GHz for setting A and 344.240980, 3459.40999, 354.367095, and 356.067114 GHz for setting B. Further details on the individual execution blocks are provided in Table 4 of the Appendix; for full details of the data reduction, observational setup, and imaging please refer to the companion paper, which also presents data on the HD 100546 system (Booth et al 2024). The self-calibration was performed on the IRS 48 continuum data after flagging the strong lines, which resulted in a continuum signal-to-noise ratio increase from ≈475 to ≈3220.…”

“…In Figure 5 we show the disk-integrated fluxes for molecules detected/nondetected in the IRS 48 disk compared to the HD 100546 disk, which was observed as part of the same ALMA program (Booth et al 2024). For some molecules, we detected multiple transitions but we only report the flux of a representative transition.…”

“…We estimate column densities following the methods outlined in Loomis et al (2018a) and in the same manner as Booth et al (2024). For the molecules where multiple transitions are detected, e.g., CH 3 OH and SO 2 , we pick one representative transition.…”

“…Additionally, we further unravel the volatile sulfur and complex organic reservoir of the disk and discuss the physical/chemical origin of the molecular substructures observed. We particularly make a direct comparison between the molecular inventory of the IRS 48 and HD 100546 disks, where the initial results for the latter are presented in Booth et al (2024), and contextualize the detections of COMs in these systems with protostellar environments.…”