ADDITIONAL CONSTRAINTS ON CIRCUMSTELLAR DISKS IN THE TRAPEZIUM CLUSTER

We discuss new constraints on the population of compact ionized source s in the Trapezium Cluster thought to arise from the ionization by the central OB stars of circumstellar disks around low-mass pre-main sequ ence stars. We present new HST Planetary Camera observations of two of these candidate disk sources, resolving extended nebulosity around th em. One source shows a small-scale (greater than or similar to 100 AU) bow-shock structure, previously seen on larger scales by O'Dell et al . We show that the circumstellar disk model is the most likely one for the majority of sources, although it remains plausible that some of t he larger objects could be equilibrium globules. We combine the most c omplete censuses of compact radio sources and stars in the core region to derive the fraction of the stellar population that may be associat ed with a circumstellar disk. Our estimate of 25%-75% is comparable to that found for PMS stars in the Taurus-Auriga dark clouds, indicating that the dense cluster environment of the Trapezium has not drastical ly reduced the frequency of disks seen around pre-main sequence stars.