Kuiper-belt objects (KBOs) are an ancient reservoir of comets beyond Neptune's orbit. Some of these objects were recently found to have the reddest optical colours in the Solar System, but the number of objects for which accurate colours were available was too small for any correlation to be discerned between colour and physical or dynamical properties, which might shed light on the origin of these objects. Here we report that all nine of the KBOs in our survey on near-circular (low-eccentricity) orbits with perihelion distances larger than 40 AU have extremely red surfaces, thereby connecting an observable property with a dynamical class. Of the objects with orbital eccentricities greater than 0.1, about half are also very red, while the rest have colours similar to the Sun, meaning that reflected sunlight is not strongly modified by the objects' surface properties. In addition, of the 13 'classical' KBOs (those with semimajor axis a approximately 45 AU and eccentricity e < 0.15), the ten that are very red are in orbits with small angles of inclination to the ecliptic, whereas the three with solar colours are all in high-inclination orbits. We suggest that these three 'grey' classical KBOs may be part of a dynamical group that is separate from the 'red' classical KBOs.
The discovery of the first member of the Kuiper belt 1 -a formerly hypothetical ancient reservoir of objects located beyond Neptune's orbit-started a revolution in our understanding of the outer Solar System: there is no longer a sharp edge at Pluto's orbit. About 60 Kuiper-belt objects, intermediate in size between comets and planets, are now known 2 to exist on stable circular orbits around the Sun, and no doubt many more objects await discovery. But owing to the recent discovery and intrinsic faintness of these objects, little has been done to explore their physical and chemical properties 3-8 . Here we report the results of a two-year survey of the broad-band optical colours of about one-quarter of the known Kuiper-belt objects. We find that their colours indicate the presence of two distinct populations: one consists of objects whose surface colours are only slightly redder than the colour of the Sun, while the other consists of the reddest objects known in the Solar System.Our survey uses Harris B (450 nm), V (550 nm) and R (650 nm) 5-inch ϫ 5-inch glass filters in front of a 1;200 ϫ 800 pixel CCD (charge coupled device) camera at the f/9 Cassegrain focus of the Steward Observatory 2.3-m telescope. The CCD has outstanding quantum efficiency, particularly in the blue where it exceeds 95%. This high quantum efficiency greatly aided our blue photometry of these faint objects. We used the CCD in a 2 ϫ 2 binning mode, yielding 600 ϫ 400 pixel images, covering 3 ϫ 2 arcmin on the sky at 0.30 arcsec per pixel. Here we will use ''pixel'' to refer to the binned 0.3-arcsec pixels. The typical full-width at half-maximum of the stellar point-spread function is 1.5 arcsec.Observations of Kuiper-belt objects (KBOs) present a formidable challenge for a photometrist. Their faintness requires us to image small areas of the sky to faint brightness levels; at such levels there are numerous faint stars and galaxies in the images. The Earth's revolution about the Sun results in the motion of KBOs relative to the ''fixed'' background of stars and galaxies in the images (ϳ3 arcsec h −1 ). Hence, KBOs sometimes pass near background stars and galaxies, and the stellar and galactic light sometimes contaminates the light from KBOs. Finally, some KBOs show brightness variations that are probably the result of the rotation and irregular shape of the KBOs or of albedo variations over their surfaces. Observations of these faint, moving, variable objects require great care.We have developed an observational and image processing procedure customized for photometry of KBOs 5-7 . The faintness and motion of these objects pushes the optimum exposure time in opposite directions. On the one hand, the faintness of the objects pushes us towards exposure times that are as long as possible, so that we can maximize the signal-to-noise ratio of the data. On the other hand, the motion of KBOs pushes us towards short exposure times, otherwise the light from a KBO is spread over many pixels, thereby degrading the signal-to-noise ratio. We have found tha...
We use narrow-band images of the [O III] X5007 emission line and adjacent continuum in the well-studied Seyfert galaxies NGC 4151 and NGC 5548 to produce models of the surface-brightness distributions of the narrow-line region and host-galaxy starlight distribution in these galaxies. We use these models to compute the expected magnitude of seeing-induced aperture effects that can lead to systematic errors in broad emission-line and continuum flux measurements made from spectra which are flux calibrated based on the narrow emission lines. We find that small spectrograph apertures (e.g., 2"X 10") are highly undesirable as photometric errors as large as 10%-20% can result. Moreover, photometric corrections based on image modeling are not likely to offer great improvement since the corrections are a sensitive function of the seeing. Use of larger apertures (e.g., 5"X7'.'5) can reduce photometric errors to the few percent range, and in principle seeing corrections based on models of the surface-brightness distributions of the narrow-line region and starlight from the host galaxy can reduce the errors to about the 1% level, at which point miscentering errors probably dominate. The limitation in application of seeing corrections is probably our inability to characterize with sufficient accuracy the point-spread function for the spectroscopic observations, although uncertainties in the surface-brightness models are also likely to play a role.
We present new optical colors for 28 Kuiper Belt objects (KBOs) and 35 Centaur objects measured with the 1.8 m Vatican Advanced Technology Telescope and the 4.3 m Discovery Channel Telescope. By combining these new colors with our previously published colors, we increase the sample size of our survey to 154 objects. Our survey is unique in that the uncertainties in our color measurements are less than half the uncertainties in the color measurements reported by other researchers in the literature. Small uncertainties are essential for discerning between a unimodal and a bimodal distribution of colors for these objects as well as detecting correlations between colors and orbital elements. From our survey, it appears red Centaurs have a broader color distribution than gray Centaurs. We find red Centaurs have a smaller orbital inclination angle distribution than gray Centaurs at the 99.3% confidence level. Furthermore, we find that our entire sample of KBOs and Centaurs exhibits bimodal colors at the confidence level. KBOs and Centaurs with H V > 7.0 have bimodal colors at the 99.96% confidence level and KBOs with H V < 6.0 have bimodal colors at the 96% confidence level.
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