2009ApJ...699..330S

We present high-resolution M-band (∼5 µm) spectra of 14 transitional disks–circumstellar disks with an optically thick outer zone but an inner region significantly depleted of small dust grains–obtained with NIRSPEC on the Keck II telescope. We detect CO emission from nine disks, and show that for the majority of these systems, the emission originates in the depleted inner disk region. We find that the presence of high 5 µm veiling, strong CO emission, and high accretion rates are usually correlated, suggesting that at least two classes of transitional disks exist–those nearly completely cleared, and those only partially depleted, within their transition radius. Cleared inner disks are consistent with the presence of a close stellar companion, or with formation by photoevaporation. Of the cleared transitional disks, at least two (HD 98800 B and CoKu Tau/4) are known to be circumbinary with projected binary separations of several AU or less. Partially depleted inner disks most often have CO that extends to small (≲1 AU) radii, but compared to "classical" disks the CO excitation temperature is lower and the emission radii are larger than that expected for dust sublimation. These disks are consistent with the presence of a giant planet, and inconsistent with having been formed by photoevaporation. Although the inner regions of such disks are vertically optically thin in dust emission, line-of-sight opacities from the star can be large, and the complex physical and chemical processes therein make it difficult to derive a fiducial CO abundance with respect to molecular hydrogen. Thus, CO M-band lines are best suited to providing lower bounds as to the total inner disk gas mass. Amongst the partially depleted sources, veiling measurements and CO emission models demonstrate a great diversity of inner disk gas content and gas/dust ratios, suggesting a variety of planet-forming environments.