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A Spitzer Survey of Mid-infrared Molecular Emission from Protoplanetary Disks. II. Correlations and Local Thermal Equilibrium Models

Salyk, C. and Pontoppidan, K. M. and Blake, G. A. and Najita, J. R. and Carr, J. S. (2011) A Spitzer Survey of Mid-infrared Molecular Emission from Protoplanetary Disks. II. Correlations and Local Thermal Equilibrium Models. Astrophysical Journal, 731 (2). Art. No. 130. ISSN 0004-637X.

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We present an analysis of Spitzer Infrared Spectrograph observations of H_(2)O, OH, HCN, C_(2)H_2, and CO_2 emission, and Keck-NIRSPEC observations of CO emission, from a diverse sample of T Tauri and Herbig Ae/Be circumstellar disks. We find that detections and strengths of most mid-IR molecular emission features are correlated with each other, suggesting a common origin and similar excitation conditions for this mid-infrared line forest. Aside from the remarkable differences in molecular line strengths between T Tauri, Herbig Ae/Be, and transitional disks discussed in Pontoppidan et al., we note that the line detection efficiency is anti-correlated with the 13/30 μm spectral slope, which is a measure of the degree of grain settling in the disk atmosphere. We also note a correlation between detection efficiency and Hα equivalent width, and tentatively with accretion rate, suggesting that accretional heating contributes to line excitation. If detected, H_(2)O line fluxes are correlated with the mid-IR continuum flux, and other co-varying system parameters, such as L_*. However, significant sample variation, especially in molecular line ratios, remains, and its origin has yet to be explained. Local thermal equilibrium (LTE) models of the H_(2)O emission show that line strength is primarily related to the best-fit emitting area, and this accounts for most source-to-source variation in H_(2)O emitted flux. Best-fit temperatures and column densities cover only a small range of parameter space, near ~10^(18) cm^(–2) and 450 K for all sources, suggesting a high abundance of H_(2)O in many planet-forming regions. Other molecules have a range of excitation temperatures from ~500 to 1500 K, also consistent with an origin in planet-forming regions. We find molecular ratios relative to water of ~10^(–3) for all molecules, with the exception of CO, for which n(CO)/n(H_(2)O) ~ 1. However, LTE fitting caveats and differences in the way thermo-chemical modeling results are reported make comparisons with such models difficult, and highlight the need for additional observations coupled with the use of line-generating radiative transfer codes.

Item Type:Article
Related URLs:
URLURL TypeDescription
Salyk, C.0000-0003-3682-6632
Pontoppidan, K. M.0000-0001-7552-1562
Blake, G. A.0000-0003-0787-1610
Carr, J. S.0000-0002-6695-3977
Additional Information:© 2011 American Astronomical Society. Received 2010 November 10; accepted 2011 February 24; published 2011 April 4. The authors thank Eric Feigelson for useful discussions about the connection between C2H2 and PAHs. This work is based 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. Support for this work was provided by NASA. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. Support for K.M.P. was provided by NASA through Hubble Fellowship grant No. 01201.01 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Re- search in Astronomy, Inc., for NASA, under contract NAS 5-26555. Research support for J.S.C. was also provided by 6.1 base funding at the Naval Research Laboratory.
Funding AgencyGrant Number
NASA Hubble Fellowship01201.01
Naval Research LaboratoryUNSPECIFIED
Subject Keywords:protoplanetary disks
Issue or Number:2
Classification Code:PACS: 95.80.+p; 95.30.Ky; 97.10.Ex; 97.21.+a; 97.10.Fy
Record Number:CaltechAUTHORS:20110518-111144319
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Official Citation:A Spitzer Survey of Mid-infrared Molecular Emission from Protoplanetary Disks. II. Correlations and Local Thermal Equilibrium Models C. Salyk et al. 2011 ApJ 731 130
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:23716
Deposited By: Jason Perez
Deposited On:18 May 2011 20:05
Last Modified:09 Dec 2019 21:58

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