Molecular lines observed towards protoplanetary disks carry information about physical and chemical processes associated with planet formation. We present ALMA Band 6 observations of C 2 H, HCN, and C 18 O in a sample of 14 disks spanning a range of ages, stellar luminosities, and stellar masses. Using C 2 H and HCN hyperfine structure fitting and HCN/H 13 CN isotopologue analysis, we extract optical depth, excitation temperature, and column density radial profiles for a subset of disks. C 2 H is marginally optically thick (τ ∼1-5) and HCN is quite optically thick (τ ∼ 5-10) in the inner 200 AU. The extracted temperatures of both molecules are low (10-30K), indicative of either sub-thermal emission from the warm disk atmosphere or substantial beam dilution due to chemical substructure. We explore the origins of C 2 H morphological diversity in our sample using a series of toy disk models, and find that disk-dependent overlap between regions with high UV fluxes and high atomic carbon abundances can explain a wide range of C 2 H emission features (e.g. compact vs. extended and ringed vs. ringless emission). We explore the chemical relationship between C 2 H, HCN, and C 18 O and find a positive correlation between C 2 H and HCN fluxes, but no relationship between C 2 H or HCN with C 18 O fluxes. We also see no evidence that C 2 H and HCN are enhanced with disk age. C 2 H and HCN seem to share a common driver, however more work remains to elucidate the chemical relationship between these molecules and the underlying evolution of C, N, and O chemistries in disks.

The organic content of protoplanetary disks sets the initial compositions of planets and comets, thereby influencing subsequent chemistry that is possible in nascent planetary systems. We present observations of the complex nitrile-bearing species CH3CN and HC3N toward the disks around the T Tauri stars AS 209, IM Lup, LkCa 15, and V4046 Sgr as well as the Herbig Ae stars MWC 480 and HD 163296. HC3N is detected toward all disks except IM Lup, and CH3CN is detected toward V4046 Sgr, MWC 480, and HD 163296. Rotational temperatures derived for disks with multiple detected lines range from 29 to 73 K, indicating emission from the temperate molecular layer of the disk. V4046 Sgr and MWC 480 radial abundance profiles are constrained using a parametric model; the gas-phase CH3CN and HC3N abundances with respect to HCN are a few to tens of percent in the inner 100 au of the disk, signifying a rich nitrile chemistry at planet- and comet-forming disk radii. We find consistent relative abundances of CH3CN, HC3N, and HCN between our disk sample, protostellar envelopes, and solar system comets; this is suggestive of a robust nitrile chemistry with similar outcomes under a wide range of physical conditions.

H 2 CO is one of the most abundant organic molecules in protoplanetary disks and can serve as a precursor to more complex organic chemistry. We present an ALMA survey of H 2 CO towards 15 disks covering a range of stellar spectral types, stellar ages, and dust continuum morphologies. H 2 CO is detected towards 13 disks and tentatively detected towards a 14th. We find both centrally-peaked and centrally-depressed emission morphologies, and half of the disks show ring-like structures at or beyond expected CO snowline locations. Together these morphologies suggest that H 2 CO in disks is commonly produced through both gas-phase and CO-ice-regulated grain-surface chemistry. We extract disk-averaged and azimuthally-averaged H 2 CO excitation temperatures and column densities for four disks with multiple H 2 CO line detections. The temperatures are between 20-50K, with the exception of colder temperatures in the DM Tau disk. These temperatures suggest that H 2 CO emission in disks is generally emerging from the warm molecular layer, with some contributions from the colder midplane. Applying the same H 2 CO excitation temperatures to all disks in the survey, we find that H 2 CO column densities span almost three orders of magnitude (∼ 5×10 11 −5×10 14 cm −2 ). The column densities appear uncorrelated with disk size and stellar age, but Herbig Ae disks may have less H 2 CO compared to T Tauri disks, possibly because of less CO freeze-out. More H 2 CO observations towards Herbig Ae disks are needed to confirm this tentative trend, and to better constrain under which disk conditions H 2 CO and other oxygen-bearing organics efficiently form during planet formation.

The elemental compositions of planets define their chemistry, and could potentially be used as beacons for their formation location if the elemental gas and grain ratios of planet birth environments, i.e. protoplanetary disks, are well understood. In disks, the ratios of volatile elements, such as C/O and N/O, are regulated by the abundance of the main C, N, O carriers, their ice binding environment, and the presence of snowlines of major volatiles at different distances from the central star. We explore the effects of disk dynamical processes, molecular compositions and abundances, and ice compositions on the snowline locations of the main C, O and N carriers, and the C/N/O ratios in gas and dust throughout the disk. The gas-phase N/O ratio enhancement in the outer disk (exterior to the H 2 O snowline) exceeds the C/O ratio enhancement for all reasonable volatile compositions. Ice compositions and disk dynamics individually change the snowline location of N 2 , the main nitrogen carrier, by a factor of 2-3, and when considered together the range of possible N 2 snowline locations is ∼11-∼79 AU in a standard disk model. Observations that anchor snowline locations at different stages of planet formation are therefore key to develop C/N/O ratios as a probe of planet formation zones.