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Radiative Transfer Models of Mid-Infrared H_2O Lines in the Planet-Forming Region of Circumstellar Disks

Meijerink, R. and Pontoppidan, K. M. and Blake, G. A. and Poelman, D. R. and Dullemond, C. P. (2009) Radiative Transfer Models of Mid-Infrared H_2O Lines in the Planet-Forming Region of Circumstellar Disks. Astrophysical Journal, 704 (2). pp. 1471-1481. ISSN 0004-637X.

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The study of warm molecular gas in the inner regions of protoplanetary disks is of key importance for the study of planet formation and especially for the transport of H_2O and organic molecules to the surfaces of rocky planets/satellites. Recent Spitzer observations have shown that the mid-infrared spectra of protoplanetary disks are covered in emission lines due to water and other molecules. Here, we present a non-local thermodynamic equilibrium (LTE) two-dimensional radiative transfer model of water lines in the 10-36 μm range that can be used to constrain the abundance structure of water vapor, given an observed spectrum, and show that an assumption of LTE does not accurately estimate the physical conditions of the water vapor emission zones, including temperatures and abundance structures. By applying the model to published Spitzer spectra we find that: (1) most water lines are subthermally excited, (2) the gas-to-dust ratio must be as much as 1-2 orders of magnitude higher than the canonical interstellar medium ratio of 100-200, (3) the gas temperature must be significantly higher than the dust temperature, in agreement with detailed heating/cooling models, and (4) the water vapor abundance in the disk surface must be significantly truncated beyond ~1 AU. A low efficiency of water formation below T ~ 300 K may naturally result in a lower water abundance beyond a certain radius. However, we find that chemistry, although not necessarily ruled out, may not be sufficient to produce a sharp abundance drop of many orders of magnitude and speculate that the depletion may also be caused by vertical turbulent diffusion of water vapor from the superheated surface to regions below the snow line, where the water can freeze out and be transported to the midplane as part of the general dust settling. Such a vertical cold finger effect is likely to be efficient due to the lack of a replenishment mechanism of large, water-ice coated dust grains to the disk surface.

Item Type:Article
Related URLs:
URLURL TypeDescription
Pontoppidan, K. M.0000-0001-7552-1562
Blake, G. A.0000-0003-0787-1610
Additional Information:© 2009 The American Astronomical Society. Received 2009 July 2, accepted for publication 2009 September 8 Published 2009 October 2. R.M. has been supported by NSF grant AST-0708922 to Caltech. 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. D.R.P. is supported by the Marie Curie research Training Network “Constellation” under grant No. MRTN-CT-2006-035890. The authors thank M. Spaans, A.G.G.M. Tielens, A. Glassgold, P. Woitke, and I. Kamp for valuable discussions on the topic.
Funding AgencyGrant Number
NASA Hubble Fellowship01201.01
Association of Universities for Research in AstronomyNAS 5-26555
Marie Curie FellowshipMRTN-CT-2006-035890
Subject Keywords:astrochemistry; line: formation; planetary systems: protoplanetary disks; radiative transfer
Issue or Number:2
Record Number:CaltechAUTHORS:20091110-194402286
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Official Citation:Radiative Transfer Models of Mid-Infrared H2O Lines in the Planet-Forming Region of Circumstellar Disks R. Meijerink, K. M. Pontoppidan, G. A. Blake, D. R. Poelman, and C. P. Dullemond 2009 ApJ 704 1471-1481 doi: 10.1088/0004-637X/704/2/1471
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:16655
Deposited By: Joy Painter
Deposited On:13 Nov 2009 21:42
Last Modified:03 Oct 2019 01:15

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