Person: Loomis, Ryan
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Loomis
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Ryan
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Loomis, Ryan
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Publication The comet-like composition of a protoplanetary disk as revealed by complex cyanides(Nature Publishing Group, 2015) Oberg, Karin; Guzmán, Viviana V.; Furuya, Kenji; Qi, Chunhua; Aikawa, Yuri; Andrews, Sean; Loomis, Ryan; Wilner, DavidObservations of comets and asteroids show that the Solar Nebula that spawned our planetary system was rich in water and organic molecules. Bombardment brought these organics to the young Earth's surface, seeding its early chemistry. Unlike asteroids, comets preserve a nearly pristine record of the Solar Nebula composition. The presence of cyanides in comets, including 0.01% of methyl cyanide (CH3CN) with respect to water, is of special interest because of the importance of C-N bonds for abiotic amino acid synthesis. Comet-like compositions of simple and complex volatiles are found in protostars, and can be readily explained by a combination of gas-phase chemistry to form e.g. HCN and an active ice-phase chemistry on grain surfaces that advances complexity[3]. Simple volatiles, including water and HCN, have been detected previously in Solar Nebula analogues - protoplanetary disks around young stars - indicating that they survive disk formation or are reformed in situ. It has been hitherto unclear whether the same holds for more complex organic molecules outside of the Solar Nebula, since recent observations show a dramatic change in the chemistry at the boundary between nascent envelopes and young disks due to accretion shocks[8]. Here we report the detection of CH3CN (and HCN and HC3N) in the protoplanetary disk around the young star MWC 480. We find abundance ratios of these N-bearing organics in the gas-phase similar to comets, which suggests an even higher relative abundance of complex cyanides in the disk ice. This implies that complex organics accompany simpler volatiles in protoplanetary disks, and that the rich organic chemistry of the Solar Nebula was not unique.Publication The distribution and chemistry of H2CO in the DM Tau protoplanetary disk(IOP Publishing, 2015) Loomis, Ryan; Cleeves, Lauren; Oberg, Karin; Guzman, Viviana V.; Andrews, SeanH2CO ice on dust grains is an important precursor of complex organic molecules (COMs). H2CO gas can be readily observed in protoplanetary disks and may be used to trace COM chemistry. However, its utility as a COM probe is currently limited by a lack of constraints on the relative contributions of two different formation pathways: on icy grain surfaces and in the gas phase. We use archival Atacama Large (sub-)Millimeter Array observations of the resolved distribution of H2CO emission in the disk around the young low-mass star DM Tau to assess the relative importance of these formation routes. The observed H2CO emission has a centrally peaked and radially broad brightness profile (extending out to 500 AU). We compare these observations with disk chemistry models with and without grain-surface formation reactions and find that both gas and grain-surface chemistry are necessary to explain the spatial distribution of the emission. Gas-phase H2CO production is responsible for the observed central peak, while grain-surface chemistry is required to reproduce the emission exterior to the CO snow line (where H2CO mainly forms through the hydrogenation of CO ice before being non-thermally desorbed). These observations demonstrate that both gas and grain-surface pathways contribute to the observed H2CO in disks and that their relative contributions depend strongly on distance from the host star.Publication Double DCO+ rings reveal CO ice desorption in the outer disk around IM Lup(IOP Publishing, 2015) Oberg, Karin; Furuya, Kenji; Loomis, Ryan; Aikawa, Yuri; Andrews, Sean; Qi, Chunhua; Dishoeck, Ewine F. van; Wilner, DavidIn a protoplanetary disk, a combination of thermal and non-thermal desorption processes regulate where volatiles are liberated from icy grain mantles into the gas phase. Non-thermal desorption should result in volatile-enriched gas in disk-regions where complete freeze-out is otherwise expected. We present Atacama Large Millimeter/Submillimeter Array observations of the disk around the young star IM Lup in 1.4 mm continuum, C18O 2–1, H13CO+ 3–2 and DCO+ 3–2 emission at ~0farcs5 resolution. The images of these dust and gas tracers are clearly resolved. The DCO+ line exhibits a striking pair of concentric rings of emission that peak at radii of ~0farcs6 and 2'' (~90 and 300 AU, respectively). Based on disk chemistry model comparison, the inner DCO+ ring is associated with the balance of CO freeze-out and thermal desorption due to a radial decrease in disk temperature. The outer DCO+ ring is explained by non-thermal desorption of CO ice in the low-column-density outer disk, repopulating the disk midplane with cold CO gas. The CO gas then reacts with abundant H2D+ to form the observed DCO+ outer ring. These observations demonstrate that spatially resolved DCO+ emission can be used to trace otherwise hidden cold gas reservoirs in the outmost disk regions, opening a new window onto their chemistry and kinematics.Publication Cyanide Photochemistry and Nitrogen Fractionation in the Mwc 480 Disk(IOP Publishing, 2015) Guzmán, V. V.; Oberg, Karin; Loomis, Ryan; Qi, CelinaHCN is a commonly observed molecule in Solar System bodies and in interstellar environments. Its abundance with respect to CN is a proposed tracer of UV exposure. HCN is also frequently used to probe the thermal history of objects, by measuring its degree of nitrogen fractionation. To address the utility of HCN as a probe of disks, we present Atacama Large (sub-) Millimeter Array observations of CN, HCN, H13CN, and HC15N toward the protoplanetary disk around Herbig Ae star MWC 480, and of CN and HCN toward the disk around T Tauri star DM Tau. Emission from all molecules is clearly detected and spatially resolved, including the first detection of HC15N in a disk. Toward MWC 480, CN emission extends radially more than 1'' exterior to the observed cut-off of HCN emission. Quantitative modeling further reveals very different radial abundance profiles for CN and HCN, with best-fit outer cut-off radii of >300 AU and 110 ± 10 AU, respectively. This result is in agreement with model predictions of efficient HCN photodissociation into CN in the outer-part of the disk where the vertical gas and dust column densities are low. No such difference in CN and HCN emission profiles are observed toward DM Tau, suggestive of different photochemical structures in Herbig Ae and T Tauri disks. We use the HCN isotopologue data toward the MWC 480 disk to provide the first measurement of the 14N/15N ratio in a disk. We find a low disk averaged 14N/15N ratio of 200 ± 100, comparable to what is observed in cloud cores and comets, demonstrating interstellar inheritance and/or efficient nitrogen fractionation in this disk.