Observational Properties of the Orion Nebula Proplyds

Hubble Space Telescope Planetary Camera images have been used to quantitatively determine key parameters characterizing emission-line proplyds in the Orion Nebula. A previous conclusion that the objects are radiation-bounded ionization fronts is confirmed and found to be in excellent agreement with the observations. Only a very loose correlation of proplyd size with distance from the ionizing star theta^1 Ori C was found. The brightness distribution around the bright cusp facing theta^1 Ori C is found to not be in agreement with simple spherical models. The brightness distribution along radial lines passing through the center of the proplyds was subjected to careful analysis, including the finite resolution of the camera. It was found that the proplyd atmospheres are more compact than the r^-3 relation expected for a freely expanding gas; rather, the best fit is to a surface brightness decreasing exponentially with a scale height of 7 x 10^14 cm. This observation argues that the proplyds are not rapidly losing material and hence may not be short-lived objects, a conclusion also reached through consideration of their frequency and location. This also requires a constraining force that is isotropic, since the scale height does not depend upon the orientation with respect to theta^1 Ori C. Arguments are presented that this constraining force is provided by radiation pressure of nebular Lyalpha photons acting upon the dust mixed with the proplyd gas. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.