CO/H2 ABUNDANCE RATIO ≈ 10-4 IN A PROTOPLANETARY DISK

The relative abundances of atomic and molecular species in planet-forming disks around young stars provide important constraints on photochemical disk models and provide a baseline for calculating disk masses from measurements of trace species. A knowledge of absolute abundances, those relative to molecular hydrogen (H-2), are challenging because of the weak rovibrational transition ladder of H-2 and the inability to spatially resolve different emission components within the circumstellar environment. To address both of these issues, we present new contemporaneous measurements of CO and H-2 absorption through the warm molecular layer of the protoplanetary disk around the Classical T Tauri Star RW Aurigae A. We use a newly commissioned observing mode of the Hubble Space Telescope Cosmic Origins Spectrograph to detect warm H-2 absorption in this region for the first time. An analysis of the emission and absorption spectrum of RW Aur shows components from the accretion region near the stellar photosphere, the molecular disk, and several outflow components. The warm H-2 and CO absorption lines are consistent with a disk origin. We model the 1092-1117 angstrom spectrum of RW Aur to derive log(10) N(H-2) = 19.90(-0.22)(+0.33) cm(-2) at T-rot(H-2) = 440 +/- 39 K. The CO A - X bands observed from 1410 to 1520 angstrom are best fit by log10 N(CO) = 16.1(-0.5)(+0.3) cm(-2) at T-rot(CO) = 200(-125)(+650) K. Combining direct measurements of the Hi, H-2, and CO column densities, we find a molecular fraction in the warm disk surface of f(H2) >= 0.47 and derive a molecular abundance ratio of CO/H-2 = 1.6(-1.3)(+4.7) x 10(-4), both consistent with canonical interstellar dense cloud values.