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.

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.

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.

The volatile contents of protoplanetary disks both set the potential for planetary chemistry and provide valuable probes of defining disk system characteristics such as stellar mass, gas mass, ionization, and temperature structure. Current disk molecular inventories are fragmented, however, giving an incomplete picture: unbiased spectral line surveys are needed to assess the volatile content. We present here an overview of such a survey of the protoplanetary disks around the Herbig Ae star MWC 480 and the T Tauri star LkCa 15 in ALMA Band 7, spanning ∼36 GHz from 275-317 GHz and representing an order of magnitude increase in sensitivity over previous single-dish surveys. We detect 14 molecular species (including isotopologues), with 5 species (C 34 S, 13 CS, H 2 CS, DNC, and C 2 D) detected for the first time in protoplanetary disks. Significant differences are observed in the molecular inventories of MWC 480 and LkCa 15, and we discuss how these results may be interpreted in light of the different physical conditions of these two disk systems.

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In this era of Gaia and ALMA, dynamical stellar mass measurements, derived from spatially and spectrally resolved observations of the Keplerian rotation of circumstellar disks, provide benchmarks that are independent of observations of stellar characteristics and their uncertainties. These benchmarks can then be used to validate and improve stellar evolutionary models, the latter of which can lead to both imprecise and inaccurate mass predictions for pre-main-sequence, low-mass (≤0.5 M ⊙) stars. We present the dynamical stellar masses derived from disks around three M stars (FP Tau, J0432+1827, and J1100–7619) using ALMA observations of 12CO (J = 2–1) and 13CO (J = 2–1) emission. These are the first dynamical stellar mass measurements for J0432+1827 and J1100–7619 (0.192 ± 0.005 M ⊙ and 0.461 ± 0.057 M ⊙, respectively) and the most precise measurement for FP Tau (0.395 ± 0.012 M ⊙). Fiducial stellar evolutionary model tracks, which do not include any treatment of magnetic activity, agree with the dynamical stellar mass measurement of J0432+1827 but underpredict the mass by ∼60% for FP Tau and by ∼80% for J1100–7619. Possible explanations for the underpredictions include inaccurate assumptions of stellar effective temperature, undetected binarity for J1100–7619, and that fiducial stellar evolutionary models are not complex enough to represent these stars. In the former case, the stellar effective temperatures would need to be increased by amounts ranging from ∼40 to ∼340 K to reconcile the fiducial stellar evolutionary model predictions with the dynamically measured masses. In the latter case, we show that the dynamical masses can be reproduced using results from stellar evolutionary models with starspots, which incorporate fractional starspot coverage to represent the manifestation of magnetic activity. Folding in low-mass M stars from the literature and assuming that the stellar effective temperatures are imprecise but accurate, we find tentative evidence of a relationship between fractional starspot coverage and observed effective temperature for these young, cool stars.

The nature and abundance of sulfur chemistry in protoplanetary disks (PPDs) may impact the sulfur inventory on young planets and therefore their habitability. PPDs also present an interesting test bed for sulfur chemistry models, since each disk present a diverse set of environments. In this context, we present new sulfur molecule observations in PPDs, and new S-disk chemistry models. With ALMA we observed the CS 5 − 4 rotational transition toward five PPDs (DM Tau, DO Tau, CI Tau, LkCa 15, MWC 480), and the CS 6 − 5 transition toward three PPDs (LkCa 15, MWC 480 and V4046 Sgr). Across this sample, CS displays a range of radial distributions, from centrally peaked, to gaps and rings. We also present the first detection in PPDs of 13 CS 6 − 5 (LkCa 15 and MWC 480), C 34 S 6 − 5 (LkCa 15), and H 2 CS 8 17 − 7 16 , 9 19 − 8 18 and 9 18 − 8 17 (MWC 480) transitions. Using LTE models to constrain column densities and excitation temperatures, we find that either 13 C and 34 S are enhanced in CS, or CS is optically thick despite its relatively low brightness temperature. Additional lines and higher spatial resolution observations are needed to distinguish between these scenarios. Assuming CS is optically thin, CS column density model predictions reproduce the observations within a factor of a few for both MWC 480 and LkCa 15. However, the model underpredicts H 2 CS by 1-2 orders of magnitude. Finally, comparing the H 2 CS/CS ratio observed toward the MWC 480 disk and toward different ISM sources, we find the closest match with prestellar cores. two of the Taurus disks: the Herbig Ae disk MWC 480, and the T Tauri disk LkCa 15. The spectral resolution for these lines was ∼ 975 kHz, i.e. corresponding to a velocity resolution of ∼ 1 km/s. The 12 CS 6 − 5 transition was observed in three execution blocks on January 17, 2016, April 23, 2016 and December 12, 2016. For the first execution block 31 antennas were included and covered baselines from 15 to 331 m. 36 antennas were included for the two other execution blocks, covering baselines from 15 to 463 m and 15 to 650 m, respectively. The first and third blocks used the source J0510+1800 as band-pass and flux calibrators, and the source J0438+3004 as phase calibrator. The second block of execution used the source J0238+1636 as band-pass calibrator, the source J0433+2905 as phase calibrator, and the source J0510+1800 as flux calibrator. The total on-source integration time were 17.6, 12.6 and 20.7 min., respectively.The 13 CS and C 34 S 6−5, and H 2 CS 8 17 −7 16 rotational transitions were observed in a single execution block, on January 17, 2016, with the same calibrator sources as those used for the first and second execution blocks used for 12 CS 6 − 5 (see above) but with 36 antennas and a total on-source integration time of ∼ 19.2 min.