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Measurements of Water Surface Snow Lines in Classical Protoplanetary Disks

Blevins, Sandra M. and Pontoppidan, Klaus M. and Banzatti, Andrea and Zhang, Ke and Najita, Joan R. and Carr, John S. and Salyk, Colette and Blake, Geoffrey A. (2016) Measurements of Water Surface Snow Lines in Classical Protoplanetary Disks. Astrophysical Journal, 818 (1). Art. No. 22. ISSN 0004-637X.

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We present deep Herschel-PACS spectroscopy of far-infrared water lines from a sample of four protoplanetary disks around solar-mass stars, selected to have strong water emission at mid-infrared wavelengths. By combining the new Herschel spectra with archival Spitzer-IRS spectroscopy, we retrieve a parameterized radial surface water vapor distribution from 0.1 to 100 au using two-dimensional dust and line radiative transfer modeling. The surface water distribution is modeled with a step model composed of a constant inner and outer relative water abundance and a critical radius at which the surface water abundance is allowed to change. We find that the four disks have critical radii of ~3–11 au, at which the surface water abundance decreases by at least 5 orders of magnitude. The measured values for the critical radius are consistently smaller than the location of the surface snow line, as predicted by the observed spectral energy distribution. This suggests that the sharp drop-off of the surface water abundance is not solely due to the local gas-solid balance, but may also be driven by the deactivation of gas-phase chemical pathways to water below 300 K. Assuming a canonical gas-to-dust ratio of 100, as well as coupled gas and dust temperatures T_(gas) = T_(dust), the best-fit inner water abundances become implausibly high (0.01–1.0 H_2^(-1)). Conversely, a model in which the gas and dust temperatures are decoupled leads to canonical inner-disk water abundances of ~10^(-4) H_(2)^(-1), while retaining gas-to-dust ratios of 100. That is, the evidence for gas–dust decoupling in disk surfaces is stronger than for enhanced gas-to-dust ratios.

Item Type:Article
Related URLs:
URLURL TypeDescription
Pontoppidan, Klaus M.0000-0001-7552-1562
Banzatti, Andrea0000-0003-4335-0900
Zhang, Ke0000-0002-0661-7517
Carr, John S.0000-0002-6695-3977
Salyk, Colette0000-0003-3682-6632
Blake, Geoffrey A.0000-0003-0787-1610
Additional Information:© 2016 American Astronomical Society. Received 2015 September 16; accepted 2015 December 21; published 2016 February 3. Support for S.M.B. was provided by the STScI Director's Discretionary Fund (DDRF). K.M.P. and A.B. acknowledge financial support by a NASA Origins of the Solar System grant No. OSS 11-OSS11-0120, a NASA Planetary Geology and Geophysics Program under grant NAG 5-10201. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. This work is based in part on observations made with the Herschel Space Observatory, a European Space Agency Cornerstone Mission with significant participation by NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Space Telescope Science InstituteUNSPECIFIED
NASAOSS 11-OSS11-0120
NASANAG 5-10201
Issue or Number:1
Record Number:CaltechAUTHORS:20160314-095807576
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Official Citation:Sandra M. Blevins et al 2016 ApJ 818 22
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:65328
Deposited By: Ruth Sustaita
Deposited On:14 Mar 2016 18:17
Last Modified:09 Dec 2019 21:54

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