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Figure 3. CO fundamental emission from the transitional T Tauri stars V836 Tau (top) and TW Hya (bottom), systems that are developing optically thin inner disks. The spectra show v = 1-0 emission from high-J P-branch lines at 4.9 ¡m (top) and low-J R-branch lines at 4.6 ¡m (bottom). Regions of strong telluric absorption have been excised from the plot. In the top panel, the vertical lines mark the approximate CO line centers at the velocity of the star. The velocity widths of the lines in the V836 Tau spectrum indicate that the emitting gas is located within a few AU of the star. For TW Hya, which is nearly face-on and therefore displays spectrally unresolved line emission, the excitation of the emitting gas suggests a similar interpretation. For V836 Tau, the relative strengths of the lines suggest optically thick emission. Thus, a large reservoir of gas may be present in the inner disk despite the weak infrared excess from this portion of the disk.

where we find that a synthetic stellar spectrum provides a good match to many of the weaker absorption features, but predicts stronger CO absorption in the low vibrational transitions than is seen in the measured spectrum. This indicates that the CO absorption in the stellar photosphere is "veiled" by CO emission from the circumstellar disk. If we were to correct for the CO absorption in the stellar photosphere, in addition to finding stronger emission in the CO v = 1-0 lines that are apparent in Figure 3, we would also uncover previously undetected emission in the higher vibrational transitions of CO. This

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