DIRECT IMAGING OF CIRCUMSTELLAR DISKS IN THE ORION NEBULA

Recent surveys of the Orion Nebula with the Hubble Space Telescope hav e revealed a number of stars surrounded by dark silhouettes seen proje cted against the bright background H II region. In this paper, we pres ent a detailed analysis of HST and ground-based observations of the si x most distinct silhouettes. We find a variety of morphologies, all co nsistent with thin circumstellar disks spanning a range of diameters ( 50 to 1000 AU) and inclination angles (0 to >80 degrees). The silhouet te intensity profiles cannot be fit by standard disk models in which t he surface density follows a radial power law with an exponent in the range -0.75 to -1.5. Rather, the data are best fit by opaque inner dis ks with exponential edges, and we discuss possible physical origins of this apparent truncation. Masses in the range 6X10(26)-4X10(30) g (i. e., up to 0.002 M.) are determined for the disks by assuming that the faint light measured from them is background light transmitted through the disk. However, we argue that these are strict lower limits on the true disk masses, as most of this light can be accounted for by PSF b lurring and scattering in the HST optical train, and that the present observations are in fact consistent with completely opaque disks. Cent ral stars are seen directly in five of the silhouettes, while the pres ence of a star is inferred in the sixth, where small reflection nebula e are seen above and below the plane of the near edge-on disk. Optical and near-infrared stellar photometry is consistent with young (simila r to 1 Myr) low-mass (0.3-1.5 M.) stars, with several showing evidence for excess near-infrared emission from the disk inner edge. (C) 1996 American Astronomical Society.