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 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.

As a result of our continuing photometric survey, we report here optical colors for 36 Kuiper Belt objects, increasing our sample size to 91 objects. We find that certain dynamical classes of objects exhibit distinctive colors-21 out of 21 objects on small-inclination and small-eccentricity orbits with perihelion distances larger than 40 AU exhibit red surface colors (BϪ ), while 17 out of 20 objects on large-inclination and large-R 1 1.5 eccentricity orbits with aphelion distances larger than 70 AU exhibit gray surface colors (BϪ ). Our R ! 1.5 observations are consistent with a primordial origin for Kuiper Belt surface colors, if we assume that gray objects formed closer to the Sun than red objects, and as Neptune migrated outward it scattered gray objects onto dynamically hot orbits. By this model, the contrasting dynamically cold and red objects beyond 40 AU remained far enough away from Neptune that they were never perturbed by the planet.

We present deep optical photometry of the afterglow of gamma-ray burst (GRB) 041006 and its associated hypernova obtained over 65 days after detection (55 R-band epochs on 10 different nights). Our early data (t < 4 days) joined with published GCN data indicates a steepening decay, approaching F ν ∝ t −0.6 at early times (t ≪ 1 day) and F ν ∝ t −1.3 at late times. The break at t b = 0.16 ± 0.04 days is the earliest reported jet break among all GRB afterglows. During our first night, we obtained 39 exposures spanning 2.15 hours from 0.62 to 0.71 days after the burst that reveal a smooth afterglow, with an rms deviation of 0.024 mag from the local power-law fit, consistent with photometric errors. After t ∼ 4 days, the decay slows considerably, and the light 1 Based on data from the MMTO 6.5m telescope, the 1.8m Vatican Advanced Technology Telescope, the Magellan 6.5m Baade and Clay telescopes, and the Keck II 10m telescope -2curve remains approximately flat at R ∼ 24 mag for a month before decaying by another magnitude to reach R ∼ 25 mag two months after the burst. This "bump" is well-fitted by a k-corrected light curve of SN1998bw, but only if stretched by ×1.38 in time. In comparison with the other GRB-related SNe bumps, GRB 041006 stakes out new parameter space for GRB/SNe, with a very bright and significantly stretched late-time SN light curve. Within a small sample of fairly well observed GRB/SN bumps, we see a hint of a possible correlation between their peak luminosity and their "stretch factor", broadly similar to the well-studied Phillips relation for the type Ia supernovae.

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We present high signal precision optical reflectance spectra of the large Kuiper belt objects 2005 FY9 and 2003 EL61. The spectrum of 2005 FY9 exhibits strong CH 4 -ice bands. A comparison between the spectrum and a Hapke model indicates 1 Observer at the MMT Observatory. Observations reported here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution. 2 2006-2007 Loyola Chair, Fordham University

Received; accepted 1 Visiting Astronomer, MMT Observatory. Observations reported here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution 2 Visiting Astronomer, Steward Observatory 2.3 m Telescope. ABSTRACTWe present spectra of Eris from the MMT 6.5 meter telescope and Red Channel Spectrograph (5700−9800Å; 5Å pix −1 ) on Mt. Hopkins, AZ, and of Pluto from the Steward Observatory 2.3 meter telescope and Boller and Chivens spectrograph (7100−9400Å; 2Å pix −1 ) on Kitt Peak, AZ. In addition, we present laboratory transmission spectra of methane-nitrogen and methane-argon ice mixtures. By anchoring our analysis in methane and nitrogen solubilities in one another as expressed in the phase diagram of Prokhvatilov & Yantsevich (1983), and comparing methane bands in our Eris and Pluto spectra and methane bands in our laboratory spectra of methane and nitrogen ice mixtures, we find Eris' bulk methane and nitrogen abundances are ∼ 10% and ∼ 90% and Pluto's bulk methane and nitrogen abundances are ∼ 3% and ∼ 97%. Such abundances for Pluto are consistent with values reported in the literature. It appears that the bulk volatile composition of Eris is similar to the bulk volatile composition ofPluto. Both objects appear to be dominated by nitrogen ice. Our analysis also suggests, unlike previous work reported in the literature, that the methane and nitrogen stoichiometry is constant with depth into the surface of Eris. Finally, we point out that our Eris spectrum is also consistent with a laboratory ice mixture consisting of 40% methane and 60% argon. Although we cannot rule out an argon rich surface, it seems more likely that nitrogen is the dominant species on Eris because the nitrogen ice 2.15 µm band is seen in spectra of Pluto and Triton.