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We have conducted the first comprehensive mm-wave molecular emission linesurveys of the evolved circumstellar disks orbiting the nearby T Tauri stars TWHya and V4046 Sgr AB. Both disks are known to retain significant residualgaseous components, despite the advanced ages of their host stars. Our unbiasedbroad-band radio spectral surveys of the TW Hya and V4046 Sgr disks wereperformed with the Atacama Pathfinder Experiment (APEX) 12 meter telescope andare intended to yield a complete census of bright molecular emission lines inthe 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 range, constitute thestrongest molecular emission from both disks in the spectral region surveyed.The molecule C2H is detected here for the first time in both disks, as is CS inthe TW Hya disk. The survey results also include the first measurements of thefull suite of hyperfine transitions of CN N=3-2 and C2H N=4-3 in both disks.Modeling of these CN and C2H hyperfine complexes in the spectrum of TW Hyaindicates that the emission from both species is optically thick and mayoriginate from very cold disk regions. It furthermore appears that thefractional abundances of CN and C2H are significantly enhanced in these evolvedprotoplanetary disks relative to the fractional abundances of the samemolecules in the environments of deeply embedded protostars.

Unbiased mm-wave Line Surveys of TW Hya and V4046 Sgr: The Enhanced C2H and CN Abundances of Evolved Protoplanetary Disks