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We derived upper limits of the circumstellar gas disk masses around the TTauri stars St 34 and RX J0432.8+1735 in order to place constraints on theoriesof planet formation and to explore the evolution of the gas-to-dust ratioduring the epoch of disk dissipation around young sun-like stars. Sincesub-millimeter lines of ^{12}CO trace of the cold, outer regions ofcircumstellar disks, we observed ^{12}CO J=2-1 emission with the 10 mSub-Millimeter Telescope (SMT) for two carefully chosen targets. St 34 is arare classical T Tauri star with an age of 8\pm3 Myr, and RX J0432.8+1735 is arare weak-emission T Tauri star with far-infrared excess. Both exhibit radialspace motion enabling us to distinguish disk emission from ambient cloudmaterial. Assuming a ^{12}CO excitation temperature of 20 K, a ^{12}COline-width of 5 km s^{-1}, and optically-thin emission, we derive 3{\sigma}upper limits on the H_{2} circumstellar disk mass for St 34 and RX J0432.8+1735to be <4.20 M\odot for both disks. Placing these results in the context ofother studies, we discuss their implications on planet formation models.

Limits on the Gas Disk Content of Two "Evolved" T Tauri Stars