The chemical diversity of giant-planet nurseries as revealed by ALMA

TL;DR

This study addresses how the chemical environments in disks around young intermediate-mass stars set the stage for giant planet formation and exoplanet atmospheres. The authors perform a targeted ALMA survey of six transition disks around F-, A-, and B-type stars, cataloging a broad suite of molecules including ^13C^18O, CS, SO, H_2CO, CH_3OH, and several complex organic species, and they compare molecular line fluxes to bulk disk properties and to literature data. They find robust detections of ^13C^18O, CS, SO, and H_2CO across all systems, with notable inter-disk diversity in other species; high $C/O$ tracers correlate with outer-disk gas mass while low $C/O$ tracers do not, and COMs vary in abundance with disk structure. By integrating their results with exoplanet C/O measurements, the work reveals overlapping chemical regimes between disk gas at planet-forming radii and directly imaged exoplanet atmospheres, implying that low-$C/O$ gas is accessible in the regions where giant planets accrete. The findings emphasize the role of disk mass, sub-structures, and irradiative processing in shaping volatile budgets and prebiotic reservoirs, underscoring the need for high-resolution, multi-line follow-up to connect disk chemistry to planet formation outcomes.

Abstract

With the giant exoplanet occurrence rate peaking around stars of 1.5-2 solar masses, there is strong motivation to characterize the disks that set their formation conditions. Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) allow us to investigate both the availability of different molecules in disks and infer the radial distribution of elemental abundances, enabling us to make connections to exoplanet systems. Here we present a survey of six transition disks around young F-, A-, and B-type stars using ALMA. We find 13C18O, CS, SO, and H2CO in all six systems, as well as ten additional molecules in a subset of disks, including detections of H2S, 33SO, and CH3OCH3. Using these data, and literature data where available, we construct the first comprehensive picture of Herbig disk chemistry. We find clear correlations between molecular tracers of C/O>1 environments (e.g., CS, C2H) and disk mass, as traced by C18O line flux. In contrast, tracers of C/O<1 environments (e.g., SO, CH3OH) do not show significant correlations with disk mass. Interestingly, these molecules are relatively brighter in lower-mass disks, with their presence primarily linked to disks with central cavities and spirals. Finally, we show that the observed chemical diversity seen across Herbig disks leads to varying C/O regimes at the orbital radii of candidate proto-planets identified within these disks. When comparing these inferred disk C/O ratios with those measured for directly imaged exoplanets, we find a notable overlap and show that low C/O gas is common on 10's of au scales in Herbig disks.

Figures (13)

• Figure 1: Top: Disk-integrated fluxes for the molecules detected in at least one disk across the sample. Bottom: Disk-integrated fluxes relative to the ^13C^18O J=3-2 line flux for each disk. The disks listed in order of decreasing dust mass as listed in Table \ref{['table1']}. Both detections and tentative detections are shown as circles whereas the triangles are 3 $\sigma$ upper limits - as listed in Table \ref{['table_images']}. The dashed lines and numbers note the factor difference between the highest flux detection and lowest detection or upper-limit where this difference is greater than one order of magnitude. For most of the lines, the $\pm$1$\sigma$ error bars are smaller than the plot markers.
• Figure 2: Integrated intensity maps of the dust continuum emission (taken from 2018ApJ...860..124D2018AA...619A.161C2019NatAs...3.1109P2019MNRAS.486.4638U2020MNRAS.491.1335R2019AJ....158...15P) and molecular line emissions of ^13C^18O, CS, SO, and SO_2 are presented. Each row of line maps shares a common color bar scale and the contours indicate the 3, 5, 10, and 30 $\sigma$ levels of the integrated line emission. A scale bar in the leftmost column represents 50 au and the position of the central star is marked with a star symbol. Non-detections and tentative detections are noted. Note that the CS J=7-6 data taken from 2025ApJ...984L...6T.
• Figure 3: Same as for Figure \ref{['maps1']} for H_2S in HD 169142 and ^33SO in HD 100453.
• Figure 4: Same as for Figure \ref{['maps1']} for H_2CS, H_2CO, c-C_3H_2, HC_3N and CH_3OH. Note the transitions in each column are not all the same as where possible the brighter detected line was chosen.
• Figure 5: Same as for Figure \ref{['maps1']} for CH_3OH, CH_3CN, CH_3OCH_3 and CH_3OCHO in HD 97048 and HD 100453.