  Carbon Monoxide is a commonly used IR/sub-mm tracer of gas in protoplanetary
disks. We present an analysis of ultraviolet CO emission in {HST}-COS spectra
for 12 Classical T Tauri stars. Several ro-vibrational bands of the CO A^1\Pi -
X^1\Sigma^+ (Fourth Positive) electronic transition system are spectrally
resolved from emission of other atoms and H_2. The CO A^1\Pi v'=14 state is
populated by absorption of Ly\alpha photons, created at the accretion column on
the stellar surface. For targets with strong CO emission, we model the Ly\alpha
radiation field as an input for a simple fluorescence model to estimate CO
rotational excitation temperatures and column densities. Typical column
densities range from N_{CO} = 10^{18} - 10^{19} cm^{-2}. Our measured
excitation temperatures are mostly below T_{CO} = 600 K, cooler than typical
M-band CO emission. These temperatures and the emission line widths imply that
the UV emission originates in a different population of CO than that which is
IR-emitting. We also find a significant correlation between CO emission and the
disk accretion rate M_{acc} and age. Our analysis shows that ultraviolet CO
emission can be a useful diagnostic of CTTS disk gas.
