Externally Illuminated Young Stellar Objects in the Orion Nebula

The circumstellar material surrounding most low-mass young stars in the Orion Nebula Cluster is rendered visible by the ionizing radiation of the hot Trapezium stars and the background light of M42. I discuss Hubble Space Telescope observations of these lower mass young stellar objects and their environments. Photoionization of circumstellar material by 1Ori C produces a bright cusp-shaped ionization front on the side facing the star and tails with ionized skins pointing away. Inside their ionized envelopes, many externally illuminated young stellar objects are surrounded by dark regions seen in silhouette in HÞ and in the transitions of ionized species. In several objects however, the dark regions are bright in the 6300Å [OI] and 2.1221⁄4m H2 lines. Radio observations indicate that the total mass of the circumstellar material is at most 10 2 Mþ. The bright Orion proplyds are evaporating and losing mass at rates of order 3 ð 10 7 to 3 ð 10 8 Mþ yr 1. About 50% of the objects lying within a projected distance of about 30 of 1Ori C are surrounded by arcs of HÞ and [OIII] emission that are roughly concentric with the forward facing ionization fronts. These high excitation arcs tend to lie about 0.5–3 ahead of the ionization fronts and may trace the interface between the 1Ori C stellar wind and the photo-ablation flow from the proplyds. The upper mass limits on the neutral mass content of the proplyd circumstellar environments implies that the photoionization age of the Orion Nebula is less than 105 years and possibly as short as 104 years. This is about an order of magnitude less than the estimated median age of the low-mass stars in the Trapezium Cluster.