CIRCUMSTELLAR DISKS IN THE ORION-NEBULA-CLUSTER

We combine our previous optical spectroscopic and photometric analysis of similar to 1600 stars located in the Orion Nebula Cluster (ONC) wi th our own and published near-infrared photometric surveys of the regi on in order to investigate the evidence for and properties of circumst ellar disks. We use the near-infrared continuum excess as our primary disk diagnostic, although we also study sources with Ca II triplet emi ssion and those designated as ''proplyds.'' The measured near-infrared excess is influenced by (1) the presence or absence of a circumstella r disk, (2) the relative importance of disk accretion and inner disk h oles, (3) the relative contrast between photospheric and disk emission , and (4) system inclination. After attempting to understand the effec ts of these influences, we estimate the frequency of circumstellar dis ks and discuss the evidence for trends in the disk frequency with stel lar mass (over the mass range < 0.1-50 M-.), stellar age (over the age range < 0.1-2 Myr), and projected cluster radius (over the radial ran ge 0-3 pc). We find that the fraction of stars retaining their inner ( < 0.1 AU) circumstellar disks to the present time is at least 55% and probably no more than 90%, averaged over the entire range in stellar m ass and stellar age represented in the ONC and over the entire area of our survey. We find no trend in the disk fraction with stellar age, a t least not over the limited age range of the cluster. We find that mo re massive stars are less likely to have disks, consistent with a scen ario in which the evolutionary timescales are more rapid for disks sur rounding more massive stars than for disks surrounding less massive st ars. We also find that the disk frequency begins to decrease toward th e lowest masses, although objects of all masses (including those that appear to be substellar) can have disks. We find that the disk frequen cy increases toward the cluster center. We then argue, using several l ines of evidence, that a large fraction of the disks associated with s tars in the ONC are accretion disks. The observed trends with stellar age, stellar mass, and projected cluster radius in the disk frequency may, in fact, be driven primarily by trends in the disk accretion prop erties. From the magnitude of the near-infrared excess above that expe cted from pure irradiation disks, we find an accretion disk fraction a mong the stars identified as having disks of 61%-88%. In addition, app roximately 20% of the stars in our optical spectroscopic sample show b road (several hundred km s(-1) FWHM) Ca II emission lines, which are f eatures often associated with accretion disk/wind phenomena; another 5 0% of the sample have Ca II lines that (at our spectral resolution) ar e ''filled in,'' indicating an independently derived accretion disk fr equency of similar to 70%. Finally, we discuss the near-infrared and o ptical emission-line properties of that portion of our sample identifi ed from Hubble Space Telescope imaging as having a dark silhouette or an externally ionized structure. This sample, proposed in the literatu re to have accretion disks, appears to be no different in terms of its stellar or circumstellar properties from the rest of the ONC populati on. The only feature distinguishing these objects from their ONC sibli ngs thus may be their current (but short-lived) proximity to the massi ve stars near the cluster center.