2014ApJ...793...55K

We have conducted the first comprehensive millimeter-wave molecular emission line surveys of the evolved circumstellar disks orbiting the nearby, roughly solar-mass, pre-main-sequence (T Tauri) stars, TW Hya (D = 54 pc) and V4046 Sgr AB (D = 73 pc). Both disks are known to retain significant residual gaseous components despite the advanced ages of their host stars (∼8 Myr and ∼21 Myr, respectively). Our unbiased broadband radio spectral surveys of the TW Hya and V4046 Sgr disks were performed with the Atacama Pathfinder Experiment 12 m telescope, and are intended to yield a complete census of the bright molecular emission lines in the range 275-357 GHz (1.1-0.85 mm). We find that lines of ¹²CO, ¹³CO, HCN, CN, and C₂H, all of which lie in the higher frequency (>330 GHz) range, constitute the strongest molecular emission from both disks in the spectral region surveyed. The molecule C₂ H is detected here for the first time in both disks, as is CS in the TW Hya disk. The survey results also include the first measurements of the full suite of the hyperfine transitions of CN N = 3 ⟶ 2 and C₂ H N = 4 ⟶ 3 in both disks. Modeling of these CN and C₂H hyperfine complexes in the spectrum of TW Hya indicates that the emission from both species is optically thick and may originate from very cold ( ≲ 10 K) disk regions. The latter result, if confirmed, would suggest the efficient production of CN and C₂H in the outer disk and/or near the disk midplane. It furthermore appears that the fractional abundances of CN and C₂H are significantly enhanced in these evolved protoplanetary disks, relative to the fractional abundances of the same molecules in the environments of deeply embedded protostars. These results, combined with previous determinations of the enhanced abundances of other species (such as HCO⁺) in T Tauri star disks, underscore the importance of properly accounting for high-energy (FUV and X-ray) radiation from the central T Tauri star when modeling protoplanetary disk gas chemistry and physical conditions.