Crowded Star Fields Research Articles

Search for Dust Emission from (24) Themis Using the Gemini-North Observatory

We report the results of a search for a dust trail aligned with the orbit plane of the large main-belt asteroid (24) Themis, which has been reported to have water ice frost on its surface. Observations were obtained with the GMOS instrument on the Gemini-North Observatory in imaging mode, where we used a chip gap to block much of the light from the asteroid, allowing us to take long exposures while avoiding saturation by the object. No dust trail is detected within 2′ of Themis to a 3-σ limiting surface brightness magnitude of Σlim ∼ 29.7 mag arcsec−2, as measured along the expected direction of the dust trail. Detailed consideration of dust ejection physics indicates that particles large enough to form a detectable dust trail were unlikely to be ejected as a result of sublimation from an object as large as Themis. We nonetheless demonstrate that our observations would have been capable of detecting faint dust emission as close as 20″ from the object, even in a crowded star field. This approach could be used to conduct future searches for sublimation-generated dust emission from Themis or other large asteroids closer to perihelion than was done in this work. It would also be useful for deep imaging of collisionally generated dust emission from large asteroids at times when the visibility of dust features are expected to be maximized, such as during orbit plane crossings, during close approaches to the Earth, or following detected impact events.

New algorithm for high-precision photometry in crowded star fields

An increasing number of astronomical problems require accurate measurements of the brightness of stars situated in crowded fields. These include the search for extrasolar planets by the transit method, which looks for decreases in stellar brightness that happen when a planet passes in front of a star. In this case, the low probability that a planet will pass in front of its parent star implies that a large number of targets have to be monitored at the same time, hence the high degree of crowding. Other applications include dating the oldest stellar clusters and measuring the light curves of bright variable stars in external galaxies (the standard candles used to determine cosmological distances). When the fields are so crowded that the images of neighboring stars overlap, separating the stellar images requires a knowledge of the shape of point sources, the so-called point spread function (PSF). Most methods rely on images of sufficiently isolated stars to construct the PSF. However, in very crowded fields, no such isolated objects can generally be found and an accurate PSF cannot be constructed, which undermines the quality of the brightness measurements.

Constraining the Adaptive Optics Point‐Spread Function in Crowded Fields: Measuring Photometric Aperture Corrections

The point-spread function (PSF) of an adaptive optics (AO) system is often poorly known. This ignorance can lead to significant systematic errors in photometry. Since the degree of AO correction is sensitive to the observing conditions: seeing, wind speed, brightness of the wavefront reference, etc., it would be desirable to estimate the PSF from the data themselves. We have developed a method to estimate the PSF delivered by an AO system in the case where the scene consists of a crowded star field. We model the modulation transfer function (MTF) of several key components of the imaging system. The power spectrum of the image, even a dense star field, can then be used to constrain our model, which in turn can be used to reconstruct the PSF. We show that the method yields reasonable fit parameters and a useful approximation to the PSF when applied to data from laser guide star (LGS) AO system at the Keck Observatory. Comparison of Keck LGS/AO and HST/NICMOS data shows that photometric accuracy of a few percent can be achieved for data with Strehl ratios as low as 4%.

A morphological Study of the Galactic Inner Bulge

A sizable region of the Inner Galaxy was observed during the ISOGAL survey using ISOCAM narrow band imaging at 7 and 15 μm with 3 arcsec resolution. Due to very low, though non-negligible, extinction at mid-infrared wavelengths, this window is very suitable for exploring the morphological features in the inner Galaxy. The images from this survey display spectacular mid-IR emission and absorption features and highly crowded star fields. The starforming regions of Sgr B1 and C stand-out at mid-IR wavelengths. Also very obvious are quiescent molecular clouds which are seen as dark regions in the mid-infrared images. The diffuse mid-IR emission is from ionized gas, warm dust (mainly at 15 μm) and PAH (7 μm). The emission from point sources is mainly due to circumstellar dust, but at 7 μm stellar photospheres also contribute to it. As a first step to understand this complex ensemble of sources, we compare these images with previously known information at other wavelengths particularly the VLA 90 cm map. A good correlation between the mid-IR starforming regions and their thermal counterparts in the 90 cm VLA map is seen.

Globular Cluster Photometry With the Hubble Space Telescope. V. WFPC Study of M15's Central density Cusp

We describe images of the center of the dense globular cluster M15 (NGC 7078) obtained with the Hubble Space Telescope Wide Field and Planetary Camera 2 (WFPC2). Data taken in the F336W, F439W, and F555W filters (approximately U, B, and V) are used to study the surface density distribution of the ∼3×10<SUP>4</SUP> stars detected in a 5 arcmin<SUP>2</SUP> region within r&lt;2' (6.7 pc) of the cluster center. Realistic simulated images have been used to estimate photometric errors and incompleteness in the star counts, which are strong functions of stellar brightness and radius. We have used a combination of point-spread-function fitting and aperture photometry, a technique that yields more accurate photometry than either method alone on the undersampled WFPC2 images of crowded star fields. The error in photometry is 1σ∼0.05 mag for stars with V&lt;18; this increases to 1σ∼0.2 mag at V=20.5, which is 1.5 mag fainter than the main-sequence turnoff. The surface density of stars in M15 (after correction for the effects of incompleteness and photometric bias/scatter) is well represented by a power law in radius: N(r)∼r<SUP>-0.82±0.12,</SUP>, over the radial range 0:3 (0.017 pc) to 6". The observed power law is remarkably similar to what is expected if the center of the cluster harbors a massive black hole. Nonparametric estimates of the density profile show a monotonic rise with decreasing radius all the way in to r=0".3, the smallest radius at which the density can be reliably measured; there is no indication that the profile flattens at smaller radii. Any flat core of radius larger than 2" (0.11 pc) in the stellar distribution is ruled out at the ≳9S% significance level. The star count profile is consistent with that expected from core-collapse models or with the predicted distribution around a massive (few times 10<SUP>3</SUP> M<SUB>sun</SUB>) black hole. The close triplet of bright stars, AC 214, is within 0".5 (1.5σ) of the cluster centroid position. The projected density distribution of stars within the central 15" of M15 departs from circular symmetry at the 95% level, with an ellipticity e=0.05±0.04 (90% confidence limits) at a position angle of +60°±25°, consistent with the rotation measured by Gebhardt et al. (1995) in this region of the cluster.

Differential CCD photometry of faint asteroids in crowded star fields and nonphotometric sky conditions

It is demonstrated that the use of an imaging CCD detector array allows photometric observations of comparatively faint asteroids moving rapidly through a complex background of star images. The shortage of observing time is alleviated by taking advantage of the presence of comparison stars that are observed simultaneously on the CCD imaging area and at small angular separations from the asteroid. These reference stars allow the effects of thin cirrus clouds or haze to be compensated to a degree adequate for the acquisition of scientifically original data.