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Unlocking CO Depletion in Protoplanetary Disks. I. The Warm Molecular Layer

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 (2018) Unlocking CO Depletion in Protoplanetary Disks. I. The Warm Molecular Layer. Astrophysical Journal, 856 (1). Art. No. 85. ISSN 1538-4357.

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CO is commonly used as a tracer of the total gas mass in both the interstellar medium and in protoplanetary disks. Recently, there has been much debate about the utility of CO as a mass tracer in disks. Observations of CO in protoplanetary disks reveal a range of CO abundances, with measurements of low CO to dust mass ratios in numerous systems. One possibility is that carbon is removed from CO via chemistry. However, the full range of physical conditions conducive to this chemical reprocessing is not well understood. We perform a systematic survey of the time dependent chemistry in protoplanetary disks for 198 models with a range of physical conditions. We vary dust grain size distribution, temperature, comic-ray and X-ray ionization rates, disk mass, and initial water abundance, detailing what physical conditions are necessary to activate the various CO depletion mechanisms in the warm molecular layer. We focus our analysis on the warm molecular layer in two regions: the outer disk (100 au) well outside the CO snowline and the inner disk (19 au) just inside the midplane CO snowline. After 1 Myr, we find that the majority of models have a CO abundance relative to H_2 less than 10^(−4) in the outer disk, while an abundance less than 10^(−5) requires the presence of cosmic-rays. Inside the CO snowline, significant depletion of CO only occurs in models with a high cosmic-ray rate. If cosmic-rays are not present in young disks, it is difficult to chemically remove carbon from CO. Additionally, removing water prior to CO depletion impedes the chemical processing of CO. Chemical processing alone cannot explain current observations of low CO abundances. Other mechanisms must also be involved.

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, Dana0000-0002-8310-0554
Additional Information:© 2018. The American Astronomical Society. Received 2017 November 8; revised 2018 January 29; accepted 2018 February 6; published 2018 March 27. This work was supported by funding from NSF grants AST-1514670 and AST-1344133 (INSPIRE) as well as NASA NNX16AB48G. L.I.C. acknowledges the support of NASA through Hubble Fellowship grant HST-HF2-51356.001. K.Z. acknowledges the support of NASA through Hubble Fellowship grant HST-HF2-51401.001-A 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.
Errata:The model dust distribution reported in the published article is incorrect. The published article states that dust grains with the same height as the gas, referred to as small grains, have an MRN size distribution of r_d = 0.005–1 μm, while the grains that are more settled than the gas, i.e., large grains, have a size distribution of r_d = 0.005–1000 μm. The correct dust distribution is as follows: there are two populations of dust grains with the same scale height as the gas. Fifteen percent of these grains have an MRN size distribution of r_d = 0.005–1 μm, the remaining 85% have a size distribution of r_d = 0.005–1000 μm. The large-grain population, which is more settled than the gas, has a size distribution of r_d = 10–1000 μm. The results and conclusions remain unchanged.
Group:Astronomy Department
Funding AgencyGrant Number
NASA Hubble FellowshipHST-HF2-51356.001
NASA Hubble FellowshipHST-HF2-51401.001-A
Subject Keywords:astrochemistry; circumstellar matter; ISM: abundances; molecular data; protoplanetary disks
Issue or Number:1
Record Number:CaltechAUTHORS:20180328-133308392
Persistent URL:
Official Citation:Kamber R. Schwarz et al 2018 ApJ 856 85
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:85482
Deposited By: George Porter
Deposited On:29 Mar 2018 14:40
Last Modified:20 Apr 2020 08:47

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