Schematic representation of a proplyd [61], where the young star has a circumstellar disk and a bipolar outflow. The scheme is accompanied by an artistic depiction of the circumstellar disk (upper left-hand side; Subaru Telescope Press Release on 31 August, 2005) and an H image of the proplyd HST1 (lower right-hand side [6]). Stellar extreme-UV (EUV) and far-UV (FUV) photons enter from the right. The FUV photons penetrate the surface of the circumstellar disk around the star, driving a slow neutral flow (~3 ), which for most proplyds [57, 58] accelerates to mildly supersonic velocities before shocking and passing through an ionization front (I-Front) at a distance of a few disk radii. In the I-Front the gas is rapidly accelerated to about 10–20 and continues to accelerate as it expands away from the I-Front and reaches progressively higher stages of ionization due to the EUV photons. The interaction between the photoevaporation flow and the stellar wind can produce a wind shock in front of the proplyd. The neutral flow in the tail is fed by diffuse UV photons, which evaporate the back side of the disk, and possibly also by gas that left the front side of the disk but was redirected into the tail by pressure gradients in the shocked neutral layer. The ionized flow from the tail is induced by diffuse EUV photons, but stellar EUV photons entering from the side also play an important role in maintaining the ionization of the tail flow once it has left the I-Front, especially toward the front of the tail [59].
