Photoevaporation of Protoplanetary Disks

TL;DR

This study uses HST/NICMOS and IRS spectra to investigate photoevaporating protoplanetary disks near O stars, revealing rapid gas loss from outer regions and the presence of dust tails, supporting theoretical models of disk evolution.

Contribution

First observational confirmation of rapid gas depletion in outer disk regions due to photoevaporation, with detailed spectral analysis of dust and gas components.

Findings

Gas in the tails is severely depleted, with upper limits of 0.000002-0.000003 solar masses.

Inner disks are present despite outer gas loss, indicating differential photoevaporation.

Dust tails are composed of 0.01 to 1 micron grains, driven by stellar photon pressure.

Abstract

We present HST/NICMOS Paschen alpha images and low and high resolution IRS spectra of photoevaporating disk-tail systems originally detected at 24 micron near O stars. We find no Paschen alpha emission in any of the systems. The resulting upper limits correspond to about 0.000002-0.000003 solar mass of mass in hydrogen in the tails suggesting that the gas is severely depleted. The IRAC data and the low resolution 5-12 micron IRS spectra provide evidence for an inner disk while high resolution long wavelength (14-30 micron) IRS spectra confirm the presence of a gas free ``tail'' that consists of ~ 0.01 to ~ 1 micron 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 (<= 100000 yrs) 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.