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Title: A SPITZER SURVEY OF MID-INFRARED MOLECULAR EMISSION FROM PROTOPLANETARY DISKS. II. CORRELATIONS AND LOCAL THERMAL EQUILIBRIUM MODELS

Abstract

We present an analysis of Spitzer Infrared Spectrograph observations of H{sub 2}O, OH, HCN, C{sub 2}H{sub 2}, and CO{sub 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 {mu}m 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 H{alpha} equivalent width, and tentatively with accretion rate, suggesting that accretional heating contributes to line excitation. If detected, H{sub 2}O line fluxes are correlated with the mid-IR continuum flux, and other co-varying system parameters, such as L{sub *}. 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{sub 2}O emission show that line strength ismore » primarily related to the best-fit emitting area, and this accounts for most source-to-source variation in H{sub 2}O emitted flux. Best-fit temperatures and column densities cover only a small range of parameter space, near {approx}10{sup 18} cm{sup -2} and 450 K for all sources, suggesting a high abundance of H{sub 2}O in many planet-forming regions. Other molecules have a range of excitation temperatures from {approx}500to1500 K, also consistent with an origin in planet-forming regions. We find molecular ratios relative to water of {approx}10{sup -3} for all molecules, with the exception of CO, for which n(CO)/n(H{sub 2}O) {approx} 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.« less

Authors:
 [1];  [2];  [3];  [4];  [5]
  1. McDonald Observatory, University of Texas at Austin, 1 University Station, C1402, Austin, TX 78712 (United States)
  2. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States)
  3. Division of Geological and Planetary Sciences, Mail Stop 150-21, California Institute of Technology, Pasadena, CA 91125 (United States)
  4. National Optical Astronomy Observatory, 950 N. Cherry Ave., Tucson, AZ 85719 (United States)
  5. Naval Research Laboratory, Code 7211, Washington, DC 20375 (United States)
Publication Date:
OSTI Identifier:
21574680
Resource Type:
Journal Article
Journal Name:
Astrophysical Journal
Additional Journal Information:
Journal Volume: 731; Journal Issue: 2; Other Information: DOI: 10.1088/0004-637X/731/2/130; Journal ID: ISSN 0004-637X
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; CARBON DIOXIDE; CARBON MONOXIDE; CORRELATIONS; EMISSION; HYDROCYANIC ACID; LTE; PROTOPLANETS; CARBON COMPOUNDS; CARBON OXIDES; CHALCOGENIDES; EQUILIBRIUM; HYDROGEN COMPOUNDS; INORGANIC ACIDS; INORGANIC COMPOUNDS; OXIDES; OXYGEN COMPOUNDS

Citation Formats

Salyk, C, Pontoppidan, K M, Blake, G A, Najita, J R, and Carr, J S. A SPITZER SURVEY OF MID-INFRARED MOLECULAR EMISSION FROM PROTOPLANETARY DISKS. II. CORRELATIONS AND LOCAL THERMAL EQUILIBRIUM MODELS. United States: N. p., 2011. Web. doi:10.1088/0004-637X/731/2/130.
Salyk, C, Pontoppidan, K M, Blake, G A, Najita, J R, & Carr, J S. A SPITZER SURVEY OF MID-INFRARED MOLECULAR EMISSION FROM PROTOPLANETARY DISKS. II. CORRELATIONS AND LOCAL THERMAL EQUILIBRIUM MODELS. United States. https://doi.org/10.1088/0004-637X/731/2/130
Salyk, C, Pontoppidan, K M, Blake, G A, Najita, J R, and Carr, J S. Wed . "A SPITZER SURVEY OF MID-INFRARED MOLECULAR EMISSION FROM PROTOPLANETARY DISKS. II. CORRELATIONS AND LOCAL THERMAL EQUILIBRIUM MODELS". United States. https://doi.org/10.1088/0004-637X/731/2/130.
@article{osti_21574680,
title = {A SPITZER SURVEY OF MID-INFRARED MOLECULAR EMISSION FROM PROTOPLANETARY DISKS. II. CORRELATIONS AND LOCAL THERMAL EQUILIBRIUM MODELS},
author = {Salyk, C and Pontoppidan, K M and Blake, G A and Najita, J R and Carr, J S},
abstractNote = {We present an analysis of Spitzer Infrared Spectrograph observations of H{sub 2}O, OH, HCN, C{sub 2}H{sub 2}, and CO{sub 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 {mu}m 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 H{alpha} equivalent width, and tentatively with accretion rate, suggesting that accretional heating contributes to line excitation. If detected, H{sub 2}O line fluxes are correlated with the mid-IR continuum flux, and other co-varying system parameters, such as L{sub *}. 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{sub 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{sub 2}O emitted flux. Best-fit temperatures and column densities cover only a small range of parameter space, near {approx}10{sup 18} cm{sup -2} and 450 K for all sources, suggesting a high abundance of H{sub 2}O in many planet-forming regions. Other molecules have a range of excitation temperatures from {approx}500to1500 K, also consistent with an origin in planet-forming regions. We find molecular ratios relative to water of {approx}10{sup -3} for all molecules, with the exception of CO, for which n(CO)/n(H{sub 2}O) {approx} 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.},
doi = {10.1088/0004-637X/731/2/130},
url = {https://www.osti.gov/biblio/21574680}, journal = {Astrophysical Journal},
issn = {0004-637X},
number = 2,
volume = 731,
place = {United States},
year = {2011},
month = {4}
}