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The TW Hya Rosetta Stone Project. II. Spatially Resolved Emission of Formaldehyde Hints at Low-temperature Gas-phase Formation

Terwisscha van Scheltinga, Jeroen and Hogerheijde, Michiel R. and Cleeves, L. Ilsedore and Loomis, Ryan A. and Walsh, Catherine and Öberg, Karin I. and Bergin, Edwin A. and Bergner, Jennifer B. and Blake, Geoffrey A. and Calahan, Jenny K. and Cazzoletti, Paolo and van Dishoeck, Ewine F. and Guzmán, Viviana V. and Huang, Jane and Kama, Mihkel and Qi, Chunhua and Teague, Richard and Wilner, David J. (2021) The TW Hya Rosetta Stone Project. II. Spatially Resolved Emission of Formaldehyde Hints at Low-temperature Gas-phase Formation. Astrophysical Journal, 906 (2). Art. No. 111. ISSN 1538-4357. doi:10.3847/1538-4357/abc9ba.

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Formaldehyde (H₂CO) is an important precursor to organics like methanol (CH₃OH). It is important to understand the conditions that produce H₂CO and prebiotic molecules during star and planet formation. H₂CO possesses both gas-phase and solid-state formation pathways, involving either UV-produced radical precursors or CO ice and cold ( 20 K) dust grains. To understand which pathway dominates, gaseous H₂CO's ortho-to-para ratio (OPR) has been used as a probe, with a value of 3 indicating "warm" conditions and <3 linked to cold formation in the solid state. We present spatially resolved Atacama Large Millimeter/submillimeter Array observations of multiple ortho- and para-H₂CO transitions in the TW Hya protoplanetary disk to test H₂CO formation theories during planet formation. We find disk-averaged rotational temperatures and column densities of 33 ± 2 K, (1.1 ± 0.1) × 10¹² cm⁻² and 25 ± 2 K, (4.4 ± 0.3) × 10¹¹ cm⁻² for ortho- and para-H₂CO, respectively, and an OPR of 2.49 ± 0.23. A radially resolved analysis shows that the observed H₂CO emits mostly at rotational temperatures of 30–40 K, corresponding to a layer with z/R ≥ 0.25. The OPR is consistent with 3 within 60 au, the extent of the pebble disk, and decreases beyond 60 au to 2.0 ± 0.5. The latter corresponds to a spin temperature of 12 K, well below the rotational temperature. The combination of relatively uniform emitting conditions, a radial gradient in the OPR, and recent laboratory experiments and theory on OPR ratios after sublimation, led us to speculate that gas-phase formation is responsible for the observed H₂CO across the TW Hya disk.

Item Type:Article
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URLURL TypeDescription Paper
Terwisscha van Scheltinga, Jeroen0000-0002-3800-9639
Hogerheijde, Michiel R.0000-0001-5217-537X
Cleeves, L. Ilsedore0000-0003-2076-8001
Loomis, Ryan A.0000-0002-8932-1219
Walsh, Catherine0000-0001-6078-786X
Öberg, Karin I.0000-0001-8798-1347
Bergin, Edwin A.0000-0003-4179-6394
Bergner, Jennifer B.0000-0002-8716-0482
Blake, Geoffrey A.0000-0003-0787-1610
Calahan, Jenny K.0000-0002-0150-0125
Cazzoletti, Paolo0000-0002-1917-7370
van Dishoeck, Ewine F.0000-0001-7591-1907
Guzmán, Viviana V.0000-0003-4784-3040
Huang, Jane0000-0001-6947-6072
Kama, Mihkel0000-0003-0065-7267
Qi, Chunhua0000-0001-8642-1786
Teague, Richard0000-0003-1534-5186
Wilner, David J.0000-0003-1526-7587
Additional Information:© 2021 The American Astronomical Society. Received 2020 August 28; revised 2020 October 23; accepted 2020 November 10; published 2021 January 14. The authors thank the anonymous referee for the constructive feedback on this manuscript. The authors acknowledge the help with the ALMA data processing by Allegro, the European ALMA Regional Center node in the Netherlands; Allegro is funded by NWO, the Netherlands Organisation for Scientific Research. This paper makes use of the following ALMA data: 1. ADS/JAO.ALMA#2013.1.00114.S, 2. ADS/JAO.ALMA#2016.1.00311.S, 3. ADS/JAO.ALMA#2016.1.00464.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST 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. J.T.v.S. and M.R.H. are supported by the Dutch Astrochemistry II program of the Netherlands Organization for Scientific Research (648.000.025). L.I.C. gratefully acknowledges support from the David and Lucille Packard Foundation, the VSGC New Investigators Award, and NASA ATP 80NSSC20K0529. C.W. acknowledges financial support from the University of Leeds and from the Science and Technology Facilities Council (Grant Nos. ST/R000549/1 and ST/T000287/1). J.K.C. acknowledges support from the National Science Foundation Graduate Research Fellowship under Grant No. DGE 1256260 and the National Aeronautics and Space Administration FINESST grant, under Grant No. 80NSSC19K1534. V.V.G. acknowledges support from FONDECYT Iniciación 11180904. J.H. acknowledges support for this work provided by NASA through the NASA Hubble Fellowship Grant No. HST-HF2-51460.001-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. "M.K. gratefully acknowledges funding by the University of Tartu ASTRA project 2014–2020.4.01.16-0029 KOMEET, financed by the EU European Regional Development Fund.
Funding AgencyGrant Number
Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)648.000.025
David and Lucile Packard FoundationUNSPECIFIED
Virginia Space Grant ConsortiumUNSPECIFIED
University of LeedsUNSPECIFIED
Science and Technology Facilities Council (STFC)ST/R000549/1
Science and Technology Facilities Council (STFC)ST/T000287/1
NSF Graduate Research FellowshipDGE-1256260
Fondo Nacional de Desarrollo Científico y Tecnológico (FONDECYT)11180904
NASA Hubble FellowshipHST-HF2-51460.001-A
University of Tartu2014-2020.4.01.16-0029
European Regional Development FundUNSPECIFIED
Subject Keywords:Astrochemistry ; Protoplanetary disks ; Interstellar molecules ; Interstellar abundances
Issue or Number:2
Classification Code:Unified Astronomy Thesaurus concepts: Astrochemistry (75); Protoplanetary disks (1300); Interstellar molecules (849); Interstellar abundances (832)
Record Number:CaltechAUTHORS:20210106-102323115
Persistent URL:
Official Citation:Jeroen Terwisscha van Scheltinga et al 2021 ApJ 906 111
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:107341
Deposited By: George Porter
Deposited On:07 Jan 2021 15:09
Last Modified:16 Nov 2021 19:01

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