CO and H₂ Absorption in the AA Tauri Circumstellar Disk

Abstract

The direct study of molecular gas in inner protoplanetary disks is complicated by uncertainties in the spatial distribution of the gas, the time variability of the source, and the comparison of observations across a wide range of wavelengths. Some of these challenges can be mitigated with far-ultraviolet spectroscopy. Using new observations obtained with the Hubble Space Telescope Cosmic Origins Spectrograph, we measure column densities and rovibrational temperatures for CO and H₂ observed on the line of sight through the AA Tauri circumstellar disk. CO A - X absorption bands are observed against the far-UV continuum. The CO absorption is characterized by log₁₀(N(¹²CO)) = 17.5 ± 0.5 cm^-2 and T _rot(CO) = 500⁺⁵⁰⁰ _-200 K, although this rotational temperature may underestimate the local kinetic temperature of the CO-bearing gas. We also detect ¹³CO in absorption with an isotopic ratio of ~20. We do not observe H₂ absorption against the continuum; however, hot H₂ (v > 0) is detected in absorption against the Lyα emission line. We measure the column densities in eight individual rovibrational states, determining a total log₁₀(N(H₂)) = 17.9^+0.6 _-0.3 cm^-2 with a thermal temperature of T(H₂) = 2500⁺⁸⁰⁰ _-700 K. The high temperature of the molecules, the relatively small H₂ column density, and the high inclination of the AA Tauri disk suggest that the absorbing gas resides in an inner disk atmosphere. If the H₂ and CO are cospatial within a molecular layer ~0.6 AU thick, this region is characterized by \langle n_{H_{2}} \rangle ~ 10⁵ cm^-3 with an observed langCO/H₂rang ratio of ~0.4. We also find evidence for a departure from a purely thermal H₂ distribution, suggesting that excitation by continuum photons and H₂ formation may be altering the level populations in the molecular gas.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.