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High-Precision C^(17)O, C^(18)O, and C^(16)O Measurements in Young Stellar Objects: Analogues for CO Self-shielding in the Early Solar System

Smith, Rachel L. and Pontoppidan, Klaus M. and Young, Edward D. and Morris, Mark R. and van Dishoeck, Ewine F. (2009) High-Precision C^(17)O, C^(18)O, and C^(16)O Measurements in Young Stellar Objects: Analogues for CO Self-shielding in the Early Solar System. Astrophysical Journal, 701 (1). pp. 163-175. ISSN 0004-637X.

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Using very high resolution (λ/Δλ ≈ 95 000) 4.7 μm fundamental and 2.3 μm overtone rovibrational CO absorption spectra obtained with the Cryogenic Infrared Echelle Spectrograph infrared spectrometer on the Very Large Telescope (VLT), we report detections of four CO isotopologues—C^(16)O, ^(13)CO, C^(18)O, and the rare species, C^(17)O—in the circumstellar environment of two young protostars: VV CrA, a binary T Tauri star in the Corona Australis molecular cloud, and Reipurth 50, an intermediate-mass FU Ori star in the Orion Molecular Cloud. We argue that the observed CO absorption lines probe a protoplanetary disk in VV CrA, and a protostellar envelope in Reipurth 50. All CO line profiles are spectrally resolved, with intrinsic line widths of ≈3-4 km s^(–1) (FWHM), permitting direct calculation of CO oxygen isotopologue ratios with 5%-10% accuracy. The rovibrational level populations for all species can be reproduced by assuming that CO absorption arises in two temperature regimes. In the higher temperature regime, in which the column densities are best determined, the derived oxygen isotope ratios in VV CrA are: [C^(16)O]/[C^(18)O] =690 ± 30; [C^(16)O]/[C^(17)O] =2800 ± 300, and [C^(18)O]/[C^(17)O]=4.1 ± 0.4. For Reipurth 50, we find [C^(16)O]/[C^(18)O] =490 ± 30; [C^(16)O]/[C^(17)O] =2200 ± 150, [C^(18)O]/[C^(17)O] = 4.4 ± 0.2. For both objects, ^(12)C/^(13)C are on the order of 100, nearly twice the expected interstellar medium (ISM) ratio. The derived oxygen abundance ratios for the VV CrA disk show a significant mass-independent deficit of C^(17)O and C^(18)O relative to C^(16)O compared to ISM baseline abundances. The Reipurth 50 envelope shows no clear differences in oxygen CO isotopologue ratios compared with the local ISM. A mass-independent fractionation can be interpreted as being due to selective photodissociation of CO in the disk surface due to self-shielding. The deficits in C^(17)O and C^(18)O in the VV CrA protoplanetary disk are consistent with an analogous origin of the ^(16)O variability in the solar system by isotope selective photodissociation, confirmation of which may be obtained via study of additional sources. The higher fractionation observed for the VV CrA disk compared with the Reipurth 50 envelope is likely due to a combination of disk geometry, grain growth, and vertical mixing processes.

Item Type:Article
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URLURL TypeDescription
Pontoppidan, Klaus M.0000-0001-7552-1562
Morris, Mark R.0000-0002-6753-2066
van Dishoeck, Ewine F.0000-0001-7591-1907
Additional Information:© 2009. The American Astronomical Society. Received 2008 December 12; accepted 2009 June 8; published 2009 July 21. Print publication: Issue 1 (2009 August 10). This work is based on observations collected at the European Southern Observatory Very Large Telescope under program ID 179.C-0151. We thank the anonymous reviewer for constructive and valuable comments that improved this paper. Support for K.M.P. was provided by NASA through Hubble Fellowship grant #01201.01 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. Astrochemistry in Leiden is supported by a Spinoza grant of the Netherlands Organization for Scientific Research (NWO). This work was supported in part by a grant from the NASA Origins program (E.D.Y., M.R.M.) and a grant from the NASA Astrobiology Institute (UCLA lead team). R.L.S. was supported by these NASA Origins and Astrobiology grants.
Funding AgencyGrant Number
European Southern Observatory179.C-0151
NASA Hubble Fellowship01201.01
NASANAS 5-26555
Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)UNSPECIFIED
Space Telescope Science InstituteUNSPECIFIED
Subject Keywords:astrochemistry; circumstellar matter; ISM: abundances; planetary systems: protoplanetary disks; stars: individual (VV CrA, Reipurth 50)
Issue or Number:1
Record Number:CaltechAUTHORS:20090808-142505378
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Official Citation:High-Precision C17O, C18O, and C16O Measurements in Young Stellar Objects: Analogues for Co Self-shielding in the Early Solar System Rachel L. Smith, Klaus M. Pontoppidan, Edward D. Young, Mark R. Morris, and Ewine F. van Dishoeck 2009 ApJ 701 163-175 doi: 10.1088/0004-637X/701/1/163
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:14907
Deposited By: George Porter
Deposited On:02 Sep 2009 21:12
Last Modified:04 Nov 2019 21:54

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