Chemistry in Circumstellar Disks as Probed by High Resolution Millimeter-Wave to Infrared Spectroscopy

Abstract

The role of high spectral and spatial resolution spectroscopy in understanding the evolution of the gaseous component of the circumstellar accretion disks encircling low mass stars is described. Millimeter-wave emission lines from trace constituents such as CO, CN, HCO^+, and HCN can be used to probe the kinematic and physiochemical properties in the near-surface regions of disks beyond 100AU, but, thanks to extensive molecular depletion in the midplane, they are not a reliable proxy for the disk mass. Future observations of the pivotal deuterated species HD and the molecular ion H_2D^+ may provide more robust access to the outer disk midplane. In the critical planet-forming region from 1 to 30 AU, mid-infrared observations of the pure rotational transitions of molecular hydrogen would alleviate many of the concerns raised by the (sub)millimeter spectral line observations, and results from the ISO-SWS instrument on several "transitional" disks are presented. The measurements of these weak lines used beams substantially larger than the disk angular diameter, and so must be verified or refuted by high angular resolution spectroscopy from the ground. Finally, the high-resolution M-band (5 µm) spectroscopy of CO in disks is outlined. Emission lines that are likely optically pumped by hot dust in the inner disk (R ≾ 1 AU) are seen toward inclined systems, while the absorption spectra of edge-on disks clearly reveal the molecular depletion inferred at millimeter wavelengths. A wide variety of molecules can be studied with high-resolution spectroscopy in the near-to mid-infrared, and such observations would permit the first direct comparison of cometary and circumstellar ices.