HST PHOTOMETRY OF THE TRAPEZIUM CLUSTER

We have obtained images of 11 fields in the Trapezium cluster with the Planetary Camera (PC) of the Hubble Space Telescope in order to exten d Herbig and Terndrup's (1986) study of this prototype, dense embedded cluster to fainter magnitudes than is possible from the ground. Using these images, we have identified 319 stars within an area of similar to 12 arcmin(2) corresponding roughly to a volume of similar to 0.065 pc(3) assuming the duster is approximately spherically symmetric. Our completeness limits for star identification in V-band and I-band image s are V similar or equal to 20 and I-C similar or equal to 19 respecti vely, corresponding to a mass limit of approximately 0.15 M(.) if the faintest stars have the same average A, as that estimated for the brig hter stars in the cluster. We have compared the V versus V-I color-mag nitude diagram derived from the HST photometry to new theoretical isoc hrones. Star formation in the Trapezium appears to be remarkably coeva l, with greater than or equal to 80% of the stars having inferred ages less than 1 Myr. Over the somewhat limited mass range of the observat ions, there is no evidence for ''bimodal'' star formation-the high- an d low-mass stars appear to have the same ages. The sharp cores of the HST images and the small angular size of the PC pixels has allowed us to identify 35 new visual binaries in the cluster with separations fro m similar to 0''.06 (-26 AU) to similar to 1''.0 (similar to-440 AU). For the range of binary separations that we are sensitive to, the obse rved binary frequency for the Trapezium is essentially identical to th at estimated for field low-mass stars by Duquennoy and Mayor (1991). T he most straightforward inference from this result is that binaries in this separation are unlikely to be formed by a tidal capture process. We have also identified three stars which have associated compact neb ulosity visible in the HST images. One of these star + nebulosity case s was previously identified by O'Dell, Wen, and Hu (1993)-these object s appear to form a class of objects whose circumstellar matter is bein g ''lit up,'' most likely by theta(1) Ori C, enabling the gas to be ob servable at both optical and radio wavelengths (Felli et al. 1993a, b) . We provide a brief summary of the optical properties of the other ra dio sources which appear in our PC images.