Characterizing CO Fourth Positive Emission in Young Circumstellar Disks
Abstract
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 A1 - X1+ (Fourth Positive) electronic transition system are spectrally resolved from emission of other atoms and H2. The CO A1 v'=14 state is populated by absorption of Lyα photons, created at the accretion column on the stellar surface. For targets with strong CO emission, we model the Lyα radiation field as an input for a simple fluorescence model to estimate CO rotational excitation temperatures and column densities. Typical column densities range from NCO = 1018 - 1019 cm-2. Our measured excitation temperatures are mostly below TCO = 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 Macc and age. Our analysis shows that ultraviolet CO emission can be a useful diagnostic of CTTS disk gas.