CO/H sub(2) ABUNDANCE RATIO approximately 10 super(-4) IN A PROTOPLANETARY DISK

• Published in: The Astrophysical journal 2014-01, Vol.794 (2), p.1-12
• Main Authors: France, Kevin, Herczeg, Gregory J, McJunkin, Matthew, Penton, Steven V
• Format: Article
• Language: English
• Subjects: Abundance; Accretion disks; Cobalt; Density; Disks; Origins; Planet formation; Protoplanets
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/794/2/160

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 sub(2)), are challenging because of the weak rovibrational transition ladder of H sub(2) and the inability to spatially resolve different emission componentswithin the circumstellar environment. To address both of these issues,we present new contemporaneous measurements of CO and H sub(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 sub(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 sub(2) and CO absorption lines are consistent with a disk origin. We model the 1092-1117A spectrum of RW Aur to derive log sub(10) N(H super(2)) = 19.90 super(+0.33) sub(-0.22) cm super(-2) at T sub(rot)(H sub(2)) = 440 + or - 39 K. The CO A - X bands observed from 1410 to 1520A are best fit by log sub(10) N(CO) = 16.1 super(+0.3) sub(-0.5) cm super(-2) at T sub(rot)(CO) = 200 super(+650) sub(-125) K. Combining direct measurements of the H I, H sub(2), and CO column densities, we find a molecular fraction in the warm disk surface of f sub(H2) [> or =, slanted] 0.47 and derive a molecular abundance ratio of CO/H sub(2) = 1.6 super(+4.7) sub(-1.3) x 10-4 super(-4), both consistent with canonical interstellar dense cloud values.