2008ApJ...688..408B

We present HST NICMOS Paschen α (Paα) images and low- and high-resolution IRS spectra of photoevaporating disk-tail systems originally detected at 24 µm near O stars. We find no Paα emission in any of the systems. The resulting upper limits correspond to about (2-3)x10^–6 M_☉ of mass in hydrogen in the tails, suggesting that the gas is severely depleted. The IRAC data and the low-resolution 5-12 µm IRS spectra provide evidence for an inner disk, while high-resolution long-wavelength (14-30 µm) IRS spectra confirm the presence of a gas-free tail that consists of ∼0.01 to ∼1 µm dust grains originating in the outer parts of the circumstellar disks. Overall, our observations support theoretical predictions in which photoevaporation removes the gas relatively quickly (≤10⁵ yr) from the outer region of a protoplanetary disk, but leaves an inner, more robust, and possibly gas-rich disk component of radius 5-10 AU. With the gas gone, larger solid bodies in the outer disk can experience a high rate of collisions and produce elevated amounts of dust. This dust is being stripped from the system by the photon pressure of the O star to form a gas-free dusty tail.