W. J. Romanishin

Two distinct populations of Kuiper-belt objects

The discovery of the first member of the Kuiper belt—a formerly hypothetical ancient reservoir of objects located beyond Neptunes orbit—started a revolution in our understanding of the outer Solar System: there is no longer a sharp edge at Plutos orbit. About 60 Kuiper-belt objects, intermediate in size between comets and planets, are now known 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. 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.

Color patterns in the Kuiper Belt: A possible primordial origin

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 (B-R > 1.5), while 17 out of 20 objects on large-inclination and large-eccentricity orbits with aphelion distances larger than 70 AU exhibit gray surface colors (B-R < 1.5). Our 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.

New determination of the size and bulk density of the binary Asteroid 22 Kalliope from observations of mutual eclipses

Abstract In 2007, the M-type binary Asteroid 22 Kalliope reached one of its annual equinoxes. As a consequence, the orbit plane of its small moon, Linus, was aligned closely to the Suns line of sight, giving rise to a mutual eclipse season. A dedicated international campaign of photometric observations, based on amateur–professional collaboration, was organized and coordinated by the IMCCE in order to catch several of these events. The set of the compiled observations is released in this work. We developed a relevant model of these events, including a topographic shape model of Kalliope refined in the present work, the orbit solution of Linus as well as the photometric effect of the shadow of one component falling on the other. By fitting this model to the only two full recorded events, we derived a new estimation of the equivalent diameter of Kalliope of 166.2 ± 2.8 km , 8% smaller than its IRAS diameter. As to the diameter of Linus, considered as purely spherical, it is estimated to 28 ± 2 km . This substantial “shortening” of Kalliope, gives a bulk density of 3.35 ± 0.33 g / cm 3 , significantly higher than past determinations but more consistent with its taxonomic type. Some constraints can be inferred on the composition.

Resolution of the Kuiper belt object color controversy: Two distinct color populations

Abstract Four years ago, we reported that the surface colors of ancient, icy bodies at and beyond the orbit of Neptune—Kuiper belt objects— divide into two distinct color populations. Our report has proven quite controversial. Specifically, every other research group looking with large telescopes at Kuiper belt objects finds a continuous range of colors rather than two distinct populations. Here we report new color measurements of 18 objects, primarily from the Keck I 10-m telescope, that confirm the existence of two populations. We have combined the color measurements of the other groups to create a data set comparable in size to our data set. We have carried out a Monte Carlo statistical analysis and found that both data sets are consistent with two color populations and our data set, which has smaller uncertainties, rules out a continuum of colors. In addition, our new observations and those in the literature confirm our earlier report that classical KBOs with perihelion distances beyond 40 AU exhibit extremely red surface colors. Our results rule out a continuous color distribution for both our complete sample and subsamples with perihelion distances greater than or less than 40 AU. We suspect the color patterns will result in a better understanding of the formation and evolution of the outer Solar System.

Rotation rates of Kuiper-belt objects from their light curves

Very little is known about the physical properties of Kuiper-belt objects, due to their relatively small size and large distance from the Earth. For example, a Kuiper-belt object with a diameter of 300 km at a typical distance of ∼30 AUwould subtend an angle of only 0.014 arcsec. It is therefore possible to investigate their surface markings, shapes and rotational properties only through variations in the light that they reflect (their light curves). Here we report a survey of optical light curves from Kuiper-belt objects. Variations are observed only for the faintest objects in the survey. We can rule out eclipsing binary objects and variations in the surface markings as the origin of these light curves, suggesting that the observed variations are due to the rotation of irregularly shaped objects. Irregular shapes may be limited to the smallest Kuiper-belt objects because the material strength in their inner regions is sufficient to maintain the shape against the weight of the overlying material. If, however, all of the objects in our survey are of essentially the same size, then the intrinsically faintest ones may be composed of a stronger and darker material than the brighter ones.

Evidence of N2-ice on the surface of the icy dwarf Planet 136472 (2005 FY9)

Abstract We present high signal precision optical reflectance spectra of 2005 FY9 taken with the Red Channel Spectrograph and the 6.5-m MMT telescope on 2006 March 4 UT (5000–9500 A; 6.33 A pixel −1 ) and 2007 February 12 UT (6600–8500 A; 1.93 A pixel −1 ). From cross-correlation experiments between the 2006 March 4 spectrum and a pure CH 4 -ice Hapke model, we find the CH 4 -ice bands in the MMT spectrum are blueshifted by 3 ± 4 A relative to bands in the pure CH 4 -ice Hapke spectrum. The higher resolution MMT spectrum of 2007 February 12 UT enabled us to measure shifts of individual CH 4 -ice bands. We find the 7296, 7862, and 7993 A CH 4 -ice bands are blueshifted by 4 ± 2, 4 ± 4, and 6 ± 5 A. From four measurements we report here and one of our previously published measurements, we find the CH 4 -ice bands are shifted by 4 ± 1 A. This small shift is important because it suggest the presence of another ice component on the surface of 2005 FY9. Laboratory experiments show that CH 4 -ice bands in spectra of CH 4 mixed with other ices are blueshifted relative to bands in spectra of pure CH 4 -ice. A likely candidate for the other component is N 2 -ice because its weak 2.15 μm band and blueshifted CH 4 bands are seen in spectra of Triton and Pluto. Assuming the shift is due to the presence of N 2 , spectra taken on two consecutive nights show no difference in CH 4 /N 2 . In addition, we find no measurable difference in CH 4 /N 2 at different depths into the surface of 2005 FY9.

Detection of Two Binary Trans-Neptunian Objects, 1997 CQ29 and 2000 CF105, with the Hubble Space Telescope

Images of the trans-Neptunian objects 1997 CQ29 and 2000 CF105 obtained with the Hubble Space Telescope WFPC2 camera show them to be binary. The two components of 1997 CQ29 were separated in our images by 020 ± 003 in 2001 November and by 033 ± 001 in 2002 June/July. The corresponding minimum physical distances are 6100 and 10,200 km. The companion to 2000 CF105 was 078 ± 003 from the primary, at least 23,400 km. Six other objects in the trans-Neptunian region, including Pluto and its moon Charon, are known to be binaries; 1997 CQ29 and 2000 CF105 are the seventh and eighth known pair. Binarity appears to be a not uncommon characteristic in this region of the solar system, with detectable companions present in 4% ± 2% of the objects we have examined.

Digging into the surface of the icy dwarf planet Eris

Abstract We describe optical spectroscopic observations of the icy dwarf planet Eris with the 6.5-m MMT telescope and the Red Channel Spectrograph. We report a correlation, that is at the edge of statistical significance, between blue shift and albedo at maximum absorption for five methane ice bands. We interpret the correlation as an increasing dilution of methane ice with another ice component, probably nitrogen, with increasing depth into the surface. We suggest a mechanism to explain the apparent increase in nitrogen with depth. Specifically, if we are seeing Eris 50 degrees from pole-on [Brown, M.E., Schaller, L., 2008. Science 316, 1585], the pole we are seeing now at aphelion was in winter darkness at perihelion. Near perihelion, sublimation could have built up atmospheric pressure on the sunlit (summer) hemisphere sufficient to drive winds toward the dark (winter) hemisphere, where the winds would condense. Because nitrogen is more volatile and scarcer than methane, it sublimated from the sunlit hemisphere relatively early in the season, so the early summer atmosphere was nitrogen rich, and so was the ice deposited on the winter pole. Later in the season, much of the nitrogen was exhausted from the summer pole, but there was plenty of methane, which continued to sublimate. At this point, the atmosphere was more depleted in nitrogen, as was the ice freezing out on top of the earlier deposited nitrogen rich ice. Our increasing nitrogen abundance with depth apparently contradicts the Licandro et al. [Licandro, J., Grundy, W.M., Pinilla-Alonso, N., Leisy, P., 2006. Astron. Astrophys. 458, L5–L8] result of a decreasing nitrogen abundance with depth. A comparison of observational, data reduction, and analysis techniques between the two works, suggests the difference between the two works is real. If so, we may be witnessing the signature of weather on Eris. The work reported here is intended to trigger further observational effort by the community.

1998 SM165: A large Kuiper belt object with an irregular shape

The recent discovery of an ancient reservoir of icy bodies at and beyond the orbit of Neptune—the Kuiper belt—has opened a new frontier in astronomy. Measurements of the physical and chemical nature of Kuiper belt objects (KBOs) can constrain our ideas of the processes of planet formation and evolution. Our 1.8-m Vatican Advanced Technology Telescope and charge-coupled device camera observations of the KBO 1998 SM165 indicate its brightness periodically varies by 0.56 magnitudes over a 4-h interval. If we assume a uniform albedo of 0.04, which is typical of values found in the literature for a handful of KBOs, and an “equator-on” aspect, we find 1998 SM165 has axes of length 600 × 360 km. If our assumptions are correct, such dimensions put 1998 SM165 among the largest elongated objects known in our solar system. Perhaps long ago, two nearly spherical KBOs of comparable size coalesced to form a compound object, or perhaps 1998 SM165 is the residual core of a catastrophic fragmentation of a larger precursor.

Photometry of the Trans-Neptunian Object 1993 SC

We obtained broadband photometry of the Trans-Neptunian Object 1993 SC with the Steward Observatory 1.5-m telescope near Mt. Bigelow, Arizona and the Lunar and Planetary Laboratory CCD on 8 October 1996. 1993 SC exhibited a constant brightness (V = 22.67) with a 1 sigma scatter about the average of 0.06 magnitudes during a five hour interval. In addition, we obtained observations of 1993 SC with the Steward Observatory 2.3-m telescope on Kitt Peak, Arizona during 24 - 27 November 1995. Once again 1993 SC exhibited a constant brightness (V = 22.73) with a 1 sigma scatter about the average of 0.04 magnitudes. If 1993 SC has a lightcurve, the amplitude must be at the level of 0.12 magnitudes or less. If the obliquity of 1993 SC is near zero degrees, then 1993 SC is spherical with a semi-major to semi-minor axis ratio less than or equal to 1.12. A spherical nature for 1993 SC may be the result of self gravity exceeding the tensile strength of the material in the interior of 1993 SC. If the obliquity of 1993 SC is large, then 1993 SC could have an irregular shape. The steady intrinsic brightness for 1993 SC suggests that the object has a relatively uniform surface albedo. Our photometry and the assumption of a comet-like albedo (0.04) indicates that the diameter of 1993 SC is ~ 240 km.