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We combine our previous optical spectroscopic and photometric analysis of D1600 stars located in the Orion Nebula Cluster (ONC) with our own and published near-infrared photometric surveys of the region in order to investigate the evidence for and properties of circumstellar disks. We use the nearinfrared continuum excess as our primary disk diagnostic, although we also study sources with Ca II triplet emission and those designated as "" proplyds.ÏÏ The measured near-infrared excess is inÑuenced by (1) the presence or absence of a circumstellar disk, (2) the relative importance of disk accretion and inner disk holes, (3) the relative contrast between photospheric and disk emission, and (4) system inclination. After attempting to understand the e †ects of these inÑuences, we estimate the frequency of circumstellar disks and discuss the evidence for trends in the disk frequency with stellar mass (over the mass range \0.1È50 stellar age (over the age range \0.1È2 Myr), and projected cluster radius (over the radial M _), range 0È3 pc). We Ðnd 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 mass and stellar age represented in the ONC and over the entire area of our survey. We Ðnd no trend in the disk fraction with stellar age, at least not over the limited age range of the cluster. We Ðnd that more massive stars are less likely to have disks, consistent with a scenario in which the evolutionary timescales are more rapid for disks surrounding more massive stars than for disks surrounding less massive stars. We also Ðnd that the disk frequency begins to decrease toward the lowest masses, although objects of all masses (including those that appear to be substellar) can have disks. We Ðnd that the disk frequency increases toward the cluster center. We then argue, using several lines of evidence, that a large fraction of the disks associated with stars 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 properties. From the magnitude of the near-infrared excess above that expected from pure irradiation disks, we Ðnd an accretion disk fraction among the stars identiÐed as having disks of 61%È88%. In addition, approximately 20% of the stars in our optical spectroscopic sample show broad (several hundred km s~1 FWHM) Ca II emission lines, which are features often associated with accretion disk/wind phenomena ; another 50% of the sample have Ca II lines that (at our spectral resolution) are "" Ðlled in,ÏÏ indicating an independently derived accretion disk frequency of D70%. Finally, we discuss the near-infrared and optical emission-line properties of that portion of our sample identiÐed from Hubble Space T elescope imaging as having a dark silhouette or an externally ionized structure. This sample, proposed in the literature to have accretion ...

We present an optical and near-infrared study of a 45 0 Â 45 0 field in NGC 2264, which includes both S Mon and the Cone Nebula. We report photometry at optical (UBVR C I C ) and near-infrared (JHK) wavelengths for $5600 stars and spectroscopic classifications for $400 of these stars. We identify circumstellar disk candidates using three techniques: excess ultraviolet (UÀV ) emission, excess near-IR (IÀK and HÀK) emission, and H emission-line equivalent widths for those stars with spectra. We find generally good correlation between disk indicators thought to originate from different physical processes. We find little if any evolution of disk fraction with stellar age or mass. However, when we derive mass accretion rates ( _ M M) from the excess emission at U, we find that _ M M decreases with age over the age range spanned by our data, $0.1-5 Myr, and increases with mass over the range $0.25-1 M . These findings are comparable to results found previously by us in the Orion Nebula cluster flanking fields.

We describe the symmetries present in the point-spread function (PSF) of an optical system either located in space or corrected by an adaptive optics (AO) system to Strehl ratios of about 70% and higher. We present a formalism for expanding the PSF to arbitrary order in terms of powers of the Fourier transform of the residual phase error, over an arbitrarily shaped and apodized entrance aperture. For traditional unapodized apertures at high Strehl ratios, bright speckles pinned to the bright Airy rings are part of an antisymmetric perturbation of the perfect PSF, arising from the term that is first order in the residual phase error. There are two symmetric second degree terms. One is negative at the center, and, like the first order term, is modulated by the perfect image's field strength -it reduces to the Maréchal approximation at the center of the PSF. The other is non-negative everywhere, zero at the image center, and can be responsible for 1 Michelson Graduate Fellow.

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We simulate the actions of a coronagraph matched to diffraction-limited adaptive optics (AO) systems on the Calypso 1.2m, Palomar Hale 5m and Gemini 8.lm telescopes, and identify useful parameter ranges for AO coronagraphy on these systems. We model the action of adaptive wavefront correction with a tapered, high-pass filter in spatial frequency rather than a hard low frequency cutoff, and estimate the minimum number of AO channels required to produce sufficient image quality for coronagraphic suppression within a few diffraction widths of a central bright object (as is relevant to e.g., brown dwarf searches near late-type dwarf stars). We explore the effect of varying the occulting image-plane stop size and shape, and examine the trade-off between throughput and suppression of the image halo and Airy rings. We discuss our simulations in the context of results from the 241-channel Palomar Hale AO coronagraph system, and suggest approaches for future AO coronagraphic instruments on large telescopes.

The major obstacle to the direct detection of companions to nearby stars is the overwhelming brightness of the host star. Current instruments employing the combination of adaptive optics (AO) and coronagraphy can typically detect objects within 2 ′′ of the star that are ∼ 10 4−5 times fainter. Correlated speckle noise is one of the biggest obstacles limiting such high-contrast imaging. We have obtained a series of 284 8 s, AO-corrected, coronagraphically occulted H-band images of the star Vega at the 3.63 m AEOS telescope located on Haleakala, Hawaii. This dataset is unique for studying the temporal behavior of speckle noise, and represents the first time such a study on highly corrected coronagraphic AO images has been carried out in a quantitative way. We find the speckle pattern to be highly stable in both position and time in our data. This is due to the fact that the AO system corrects disturbances to the stellar wave front at the level where the instrumental wave front errors dominate. Because of this, we find that our detection limit is not significantly improved simply with increased exposure time alone. However, we are able to improve our dynamic range by 1.5-2 magnitudes through subtraction of static/quasi-static speckles in two rotating frames: the telescope pupil frame and the deformable mirror frame. The highly stable nature of speckles will exist for any program using a combination of coronagraphy and high-order AO, and underscores the importance of calibration of non-common path errors between the wave front sensor and the image plane. Such calibration is critical for high-contrast AO systems and we demonstrate this using empirical data. From our data, we are able to constrain the mass of any purported companion to Vega to be less than ∼ 45M J at 8 AU and less than ∼ 30M J at 16 AU, radii not previously probed at these sensitivities.

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