Program: GS-2007A-C-3

Disks with optically thin inner regions and optically thick outer regions around young stars (i.e. transitional disks) are often thought to indicate the presence of a planet. While such an inference is exciting, it is important to ensure that this dust morphology is not due to some other process. In this proposal we describe the diverse effects that can give rise to a transitional disk and discuss how high-resolution spectra of ro-vibrational CO emission can be used to distinguish among these scenarios. For example, the presence of a 5M$_{J}$ planet would result in a transitional disk with no gas in the optically thin region, however, a transitional disk caused by gas-grain interactions would result in a gas-rich optically thin region. We propose to use Phoenix on Gemini South to obtain high-resolution spectra of transitional disks in the southern hemisphere. Since Phoenix is being removed from Gemini after March 2007 and its status on other telescopes remains unclear, this may be the last opportunity to obtain such data for the foreseeable future.