The Chemical Diversity of Giant-planet Nurseries as Revealed by ALMA

Abstract With the giant exoplanet occurrence rate peaking around stars of 1.5–2.0 solar masses ( M ⊙ ), 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 13 C 18 O, CS, SO, and H 2 CO in all six systems, as well as 10 additional molecules in a subset of disks, including detections of H 2 S, 33 SO, and CH 3 OCH 3 . 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, C 2 H) and disk mass, as traced by C 18 O line flux. In contrast, tracers of C/O < 1 environments (e.g., SO, CH 3 OH) 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 protoplanets 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 tens of au scales in Herbig disks.