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Unlocking CO Depletion in Protoplanetary Disks II. Primordial C/H Predictions Inside the CO Snowline

Schwarz, Kamber R. and Bergin, Edwin A. and Cleeves, L. Ilsedore and Zhang, Ke and Öberg, Karin I. and Blake, Geoffrey A. and Anderson, Dana E. (2019) Unlocking CO Depletion in Protoplanetary Disks II. Primordial C/H Predictions Inside the CO Snowline. Astrophysical Journal, 877 (2). Art. No. 131. ISSN 1538-4357. doi:10.3847/1538-4357/ab1c5e.

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CO is thought to be the main reservoir of volatile carbon in protoplanetary disks, and thus the primary initial source of carbon in the atmospheres of forming giant planets. However, recent observations of protoplanetary disks point toward low volatile carbon abundances in many systems, including at radii interior to the CO snowline. One potential explanation is that gas phase carbon is chemically reprocessed into less volatile species, which are frozen on dust grain surfaces as ice. This mechanism has the potential to change the primordial C/H ratio in the gas. However, current observations primarily probe the upper layers of the disk. It is not clear if the low volatile carbon abundances extend to the midplane, where planets form. We have run a grid of 198 chemical models, exploring how the chemical reprocessing of CO depends on disk mass, dust grain size distribution, temperature, cosmic-ray and X-ray ionization rate, and initial water abundance. Building on our previous work focusing on the warm molecular layer, here we analyze the results for our grid of models in the disk midplane at 12 au. We find that either an ISM level cosmic-ray ionization rate or the presence of UV photons due to a low dust surface density are needed to chemically reduce the midplane CO gas abundance by at least an order of magnitude within 1 Myr. In the majority of our models CO does not undergo substantial reprocessing by in situ chemistry and there is little change in the gas phase C/H and C/O ratios over the lifetime of the typical disk. However, in the small subset of disks where the disk midplane is subject to a source of ionization or photolysis, the gas phase C/O ratio increases by up to nearly 9 orders of magnitude due to conversion of CO into volatile hydrocarbons.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Schwarz, Kamber R.0000-0002-6429-9457
Bergin, Edwin A.0000-0003-4179-6394
Cleeves, L. Ilsedore0000-0003-2076-8001
Zhang, Ke0000-0002-0661-7517
Öberg, Karin I.0000-0001-8798-1347
Blake, Geoffrey A.0000-0003-0787-1610
Anderson, Dana E.0000-0002-8310-0554
Additional Information:© 2019 The American Astronomical Society. Received 2019 February 4; revised 2019 April 7; accepted 2019 April 23; published 2019 June 4. This work was supported by funding from NSF grant AST-1514670 and NASA NNX16AB48G. K.S. and K.Z. acknowledge the support of NASA through Hubble Fellowship Program grants HST-HF2-51419.001 and HST-HF2-51401.001, awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. Software: GNU Parallel (Tange 2011), IDL, matplotlib (Hunter 2007), numpy (van der Walt et al. 2011), pandas (McKinney et al. 2010), scipy (Jones et al. 2001), TORUS (Harries 2000).
Group:Astronomy Department
Funding AgencyGrant Number
NASA Hubble FellowshipHST-HF2-51419.001
NASA Hubble FellowshipHST-HF2-51401.001
Subject Keywords:astrochemistry – circumstellar matter – ISM: abundances – ISM: molecules – protoplanetary disks
Issue or Number:2
Record Number:CaltechAUTHORS:20190502-091609548
Persistent URL:
Official Citation:Kamber R. Schwarz et al 2019 ApJ 877 131
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:95165
Deposited By: Tony Diaz
Deposited On:02 May 2019 16:28
Last Modified:16 Nov 2021 17:10

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