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1. Cold Deuterium Fractionation in the Nearest Planet-forming Disk

   https://doi.org/10.3847/1538-4357/aca765  

   Muñoz-Romero, Carlos E. ; Öberg, Karin I. ; Law, Charles J. ; Teague, Richard ; Aikawa, Yuri ; Bergner, Jennifer B. ; Wilner, David J. ; Huang, Jane ; Guzmán, Viviana V. ; Cleeves, L. Ilsedore ( January 2023 , The Astrophysical Journal)

   Deuterium fractionation provides a window into the thermal history of volatiles in the solar system and protoplanetary disks. While evidence of active molecular deuteration has been observed toward a handful of disks, it remains unclear whether this chemistry affects the composition of forming planetesimals due to limited observational constraints on the radial and vertical distribution of deuterated molecules. To shed light on this question, we introduce new Atacama Large Millimeter/submillimeter Array observations of DCO⁺and DCNJ= 2–1 at an angular resolution of 0.″5 (30 au) and combine them with archival data of higher energy transitions toward the protoplanetary disk around TW Hya. We carry out a radial excitation analysis assuming both LTE and non-LTE to localize the physical conditions traced by DCO⁺and DCN emission in the disk, thus assessing deuterium fractionation efficiencies and pathways at different disk locations. We find similar disk-averaged column densities of 1.9 × 10¹²and 9.8 × 10¹¹cm⁻²for DCO⁺and DCN, with typical kinetic temperatures for both molecules of 20–30 K, indicating a common origin near the comet- and planet-forming midplane. The observed DCO⁺/DCN abundance ratio, combined with recent modeling results, provide tentative evidence of a gas-phase C/O enhancement within <40 au. Observations of DCO⁺and DCN in othermore »disks, as well as HCN and HCO⁺, will be necessary to place the trends exhibited by TW Hya in context, and fully constrain the main deuteration mechanisms in disks.

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2. Molecules with ALMA at Planet-forming Scales (MAPS). XI. CN and HCN as Tracers of Photochemistry in Disks

   https://doi.org/10.3847/1538-4365/ac143a  

   Bergner, Jennifer B. ; Öberg, Karin I. ; Guzmán, Viviana V. ; Law, Charles J. ; Loomis, Ryan A. ; Cataldi, Gianni ; Bosman, Arthur D. ; Aikawa, Yuri ; Andrews, Sean M. ; Bergin, Edwin A. ; et al ( November 2021 , The Astrophysical Journal Supplement Series)

   Abstract UV photochemistry in the surface layers of protoplanetary disks dramatically alters their composition relative to previous stages of star formation. The abundance ratio CN/HCN has long been proposed to trace the UV field in various astrophysical objects; however, to date the relationship between CN, HCN, and the UV field in disks remains ambiguous. As part of the ALMA Large Program MAPS (Molecules with ALMA at Planet-forming Scales), we present observations of CN N = 1–0 transitions at 0.″3 resolution toward five disk systems. All disks show bright CN emission within ∼50–150 au, along with a diffuse emission shelf extending up to 600 au. In all sources we find that the CN/HCN column density ratio increases with disk radius from about unity to 100, likely tracing increased UV penetration that enhances selective HCN photodissociation in the outer disk. Additionally, multiple millimeter dust gaps and rings coincide with peaks and troughs, respectively, in the CN/HCN ratio, implying that some millimeter substructures are accompanied by changes to the UV penetration in more elevated disk layers. That the CN/HCN ratio is generally high (>1) points to a robust photochemistry shaping disk chemical compositions and also means that CN is the dominant carrier ofmore »the prebiotically interesting nitrile group at most disk radii. We also find that the local column densities of CN and HCN are positively correlated despite emitting from vertically stratified disk regions, indicating that different disk layers are chemically linked. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.« less
3. The Young Embedded Disk L1527 IRS: Constraints on the Water Snowline and Cosmic-Ray Ionization Rate from HCO+ Observations

   https://doi.org/10.3847/1538-4357/ac63b4  

   van ’t Hoff, Merel L. ; Leemker, Margot ; Tobin, John J. ; Harsono, Daniel ; Jørgensen, Jes K. ; Bergin, Edwin A. ( June 2022 , The Astrophysical Journal)

   Abstract The water snowline in circumstellar disks is a crucial component in planet formation, but direct observational constraints on its location remain sparse owing to the difficulty of observing water in both young embedded and mature protoplanetary disks. Chemical imaging provides an alternative route to locate the snowline, and HCO + isotopologues have been shown to be good tracers in protostellar envelopes and Herbig disks. Here we present ∼0.″5 resolution (∼35 au radius) Atacama Large Millimeter/submillimeter Array (ALMA) observations of HCO + J = 4 − 3 and H 13 CO + J = 3 − 2 toward the young (Class 0/I) disk L1527 IRS. Using a source-specific physical model with the midplane snowline at 3.4 au and a small chemical network, we are able to reproduce the HCO + and H 13 CO + emission, but for HCO + only when the cosmic-ray ionization rate is lowered to 10 −18 s −1 . Even though the observations are not sensitive to the expected HCO + abundance drop across the snowline, the reduction in HCO + above the snow surface and the global temperature structure allow us to constrain a snowline location between 1.8 and 4.1 au. Deep observations aremore »required to eliminate the envelope contribution to the emission and to derive more stringent constraints on the snowline location. Locating the snowline in young disks directly with observations of H 2 O isotopologues may therefore still be an alternative option. With a direct snowline measurement, HCO + will be able to provide constraints on the ionization rate.« less
4. The Central 1000 au of a Prestellar Core Revealed with ALMA. II. Almost Complete Freeze-out

   https://doi.org/10.3847/1538-4357/ac5913  

   Caselli, Paola ; Pineda, Jaime E. ; Sipilä, Olli ; Zhao, Bo ; Redaelli, Elena ; Spezzano, Silvia ; Maureira, Maria José ; Alves, Felipe ; Bizzocchi, Luca ; Bourke, Tyler L. ; et al ( April 2022 , The Astrophysical Journal)

   Abstract Prestellar cores represent the initial conditions in the process of star and planet formation. Their low temperatures (<10 K) allow the formation of thick icy dust mantles, which will be partially preserved in future protoplanetary disks, ultimately affecting the chemical composition of planetary systems. Previous observations have shown that carbon- and oxygen-bearing species, in particular CO, are heavily depleted in prestellar cores due to the efficient molecular freeze-out onto the surface of cold dust grains. However, N-bearing species such as NH 3 and, in particular, its deuterated isotopologues appear to maintain high abundances where CO molecules are mainly in the solid phase. Thanks to ALMA, we present here the first clear observational evidence of NH 2 D freeze-out toward the L1544 prestellar core, suggestive of the presence of a “complete depletion zone” within a ≃1800 au radius, in agreement with astrochemical prestellar core model predictions. Our state-of-the-art chemical model coupled with a non-LTE radiative transfer code demonstrates that NH 2 D becomes mainly incorporated in icy mantles in the central 2000 au and starts freezing out already at ≃7000 au. Radiative transfer effects within the prestellar core cause the NH 2 D(1 11 − 1 01 ) emission tomore »appear centrally concentrated, with a flattened distribution within the central ≃3000 au, unlike the 1.3 mm dust continuum emission, which shows a clear peak within the central ≃1800 au. This prevented NH 2 D freeze-out from being detected in previous observations, where the central 1000 au cannot be spatially resolved.« less
5. Water Shielding in the Terrestrial Planet-forming Zone: Implication for Inner Disk Organics

   https://doi.org/10.3847/2041-8213/ac822b  

   Duval, Sara E. ; Bosman, Arthur D. ; Bergin, Edwin A. ( July 2022 , The Astrophysical Journal Letters)

   Abstract The chemical composition of the inner region of protoplanetary disks can trace the composition of planetary-building material. The exact elemental composition of the inner disk has not yet been measured and tensions between models and observations still exist. Recent advancements have shown UV shielding to be able to increase the emission of organics. Here, we expand on these models and investigate how UV shielding may impact chemical composition in the inner 5 au. In this work, we use the model from Bosman et al. and expand it with a larger chemical network. We focus on the chemical abundances in the upper disk atmosphere where the effects of water UV shielding are most prominent and molecular lines originate. We find rich carbon and nitrogen chemistry with enhanced abundances of C 2 H 2 , CH 4 , HCN, CH 3 CN, and NH 3 by >3 orders of magnitude. This is caused by the self-shielding of H 2 O, which locks oxygen in water. This subsequently results in a suppression of oxygen-containing species like CO and CO 2 . The increase in C 2 H 2 seen in the model with the inclusion of water UV shielding allows us tomore »explain the observed C 2 H 2 abundance without resorting to elevated C/O ratios as water UV shielding induced an effectively oxygen-poor environment in oxygen-rich gas. Thus, water UV shielding is important for reproducing the observed abundances of hydrocarbons and nitriles. From our model result, species like CH 4 , NH 3 , and NO are expected to be observable with the James Webb Space Telescope (JWST).« less