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Hydrocarbon Emission Rings in Protoplanetary Disks Induced by Dust Evolution

Bergin, Edwin A. and Du, Fujun and Cleeves, L. Ilsedore and Blake, G. A. and Schwarz, K. and Visser, R. and Zhang, K. (2016) Hydrocarbon Emission Rings in Protoplanetary Disks Induced by Dust Evolution. Astrophysical Journal, 831 (1). Art. No. 101. ISSN 0004-637X. doi:10.3847/0004-637X/831/1/101.

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We report observations of resolved C_2H emission rings within the gas-rich protoplanetary disks of TW Hya and DM Tau using the Atacama Large Millimeter Array. In each case the emission ring is found to arise at the edge of the observable disk of millimeter-sized grains (pebbles) traced by submillimeter-wave continuum emission. In addition, we detect a C_3H_2 emission ring with an identical spatial distribution to C_2H in the TW Hya disk. This suggests that these are hydrocarbon rings (i.e., not limited to C_2H). Using a detailed thermo-chemical model we show that reproducing the emission from C_2H requires a strong UV field and C/O > 1 in the upper disk atmosphere and outer disk, beyond the edge of the pebble disk. This naturally arises in a disk where the ice-coated dust mass is spatially stratified due to the combined effects of coagulation, gravitational settling and drift. This stratification causes the disk surface and outer disk to have a greater permeability to UV photons. Furthermore the concentration of ices that transport key volatile carriers of oxygen and carbon in the midplane, along with photochemical erosion of CO, leads to an elemental C/O ratio that exceeds unity in the UV-dominated disk. Thus the motions of the grains, and not the gas, lead to a rich hydrocarbon chemistry in disk surface layers and in the outer disk midplane.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Bergin, Edwin A.0000-0003-4179-6394
Cleeves, L. Ilsedore0000-0003-2076-8001
Blake, G. A.0000-0003-0787-1610
Visser, R.0000-0003-1775-5461
Zhang, K.0000-0002-0661-7517
Additional Information:© 2016. The American Astronomical Society. Received 2016 May 6; revised 2016 September 9; accepted 2016 September 20; published 2016 October 31. This work was supported by funding from the National Science Foundation grant AST-1514670 and AST-1344133 (INSPIRE) along with NASA XRP grant NNX16AB48G. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2013.1.00198.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan) and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
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Subject Keywords:astrochemistry; circumstellar matter; molecular processes; planet–disk interactions ; planetary systems; planets and satellites: atmospheres
Issue or Number:1
Record Number:CaltechAUTHORS:20161031-122538195
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Official Citation:Edwin A. Bergin et al 2016 ApJ 831 101
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:71627
Deposited By: George Porter
Deposited On:31 Oct 2016 22:25
Last Modified:11 Nov 2021 04:48

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