A SPITZER SURVEY OF MID-INFRARED MOLECULAR EMISSION FROM PROTOPLANETARY DISKS. II. CORRELATIONS AND LOCAL THERMAL EQUILIBRIUM MODELS
- McDonald Observatory, University of Texas at Austin, 1 University Station, C1402, Austin, TX 78712 (United States)
- Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States)
- Division of Geological and Planetary Sciences, Mail Stop 150-21, California Institute of Technology, Pasadena, CA 91125 (United States)
- National Optical Astronomy Observatory, 950 N. Cherry Ave., Tucson, AZ 85719 (United States)
- Naval Research Laboratory, Code 7211, Washington, DC 20375 (United States)
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.
- OSTI ID:
- 21574680
- Journal Information:
- Astrophysical Journal, Vol. 731, Issue 2; Other Information: DOI: 10.1088/0004-637X/731/2/130; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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