How do different protoplanetary disks cook H2CO?

Abstract

Complex organic molecules in protoplanetary disksProtoplanetary disks are considered a material reservoir for planetary assembly. Hence, resolving the spatial distribution of complex organic molecules (COMs) in protoplanetary disks is crucial in estimating the potential habitability of other worlds. Unfortunately, under typical cold disk conditions, a significant portion of the complex organic inventory is thought to be locked on the surface of dust grains. Even methanol (CH3OH), one of the simplest COMs and a cornerstone for making more complex O-bearing molecules, has been barely glimpsed in disks with few detections associated with warm conditions generated by dust cavities or stellar outbursts. Nevertheless, smaller but brighter tracers like formaldehyde (H2CO) could be used in the typical colder disks. Formaldehyde: a potential alternative for tracing the frozen organic content in disksH2CO is a small organic molecule considered a precursor of CH3OH on the icy surfaces of dust grains, with the difference that the former can be more easily released into the gas phase than the latter due to lower desorption energies. However, gas-phase reactions can also form efficiently H2CO. Since the specific contribution of each mechanism is poorly constrained, determining the dominant formation pathway of H2CO is necessary for setting up its potential as a tracer of the cold reservoir in protoplanetary disks. Within this context, we present spectrally and spatially resolved ALMA observations of several H2CO lines toward the bright Herbig Ae protoplanetary disk HD 163296. We determined the excitation conditions of H2CO as a function of disk radius and put constraints on the height of H2CO emission to get clues about its formation. We discuss our findings in the context of previous results in T Tauri and Herbig Ae/Be sources, emphasizing the case of the closest protoplanetary disk, TW Hya, where H2CO was studied with a similar level of analysis that we did for HD 163296.Disentangling H2CO origins in disks: gas-phase or dust-grain surface chemistry?We find that H2CO likely forms from a combination of gas-phase and grain-surface chemistry in HD 163296. This is in contrast to the case of TW Hya, the only T Tauri source with known CH3OH detections and where the cold gas-phase chemistry is expected to predominate in the formation of H2CO. To understand these differences, we compare our observational results with predictions from the physicochemical code DALI by using fiducial models of the physical structure of each disk previously reported in the literature. However, we note that HD163296 and TW Hya do not represent the entire population of protoplanetary disks. To better understand how H2CO forms in more typical disks, we are currently analyzing observations from the DECO ALMA large program. DECO consists of a survey of 80 protoplanetary disks around low-mass stars (the most common exoplanet hosts), targeting multiple molecular lines of several species, including multiple transitions of H2CO.