Statistics of Optical Colors of KBOs and Centaurs W. Romanishin – U. of Oklahoma S. C. Tegler – Northern Arizona U.

Solar System pre inner (rocky) planets – M,V,E,M Main Asteroid belt (rocks: <1 – 1000 km, between Mars and Jupiter) 4 outer (giant – gas/liquid) planets- J,S,U,N Pluto (small – 1/7 mass of Moon!) Comets (long and short period) Oort Cloud (large spherical comet reservoir)

Brief history of Kuiper Belt idea 1930 – Pluto discovered ~1950 Kuiper and Edgeworth- Solar nebula out past Neptune – No major planet formed- What kind of objects out there, if any survive?? Kuiper Belt= remains of solar nebula out past Neptune 1970s – search for objects (besides Pluto) past Neptune- no luck Objects in KB probably icy, much like comets Comets = “dirty snowball” – need a few subtleties of comets to follow Kuiper Belt history

Comet= Icy nucleus + active gas/dust tail when near Sun When inactive, very hard to see

Comets – Oort Cloud vs. Ecliptic Oort Cloud comets- long periods, random inclinations – Originate from Oort cloud reservoir (“deep freeze” for comets) Occasionally get knocked into inner Solar System, are heated by Sun and we see them as comets Ecliptic comets- short orbital periods ( < 200 years), orbital inclinations near plane of Solar System

Oort Cloud- Large “deep freeze” for comets ~spherical = random orbital inclinations

Kuiper Belt – found at last! 1980s- dynamicists said “Ecliptic comets NOT from Oort cloud”= need a flattened comet reservoir Inspired further observational searches with better telescopes and detectors (CCDs) 1992 Jewitt and Luu found first object past Neptune = 1992 QB1, about 44 AU from Sun

Sky motion – KBO and asteroid Jewitt and Luu looked for “slow moving objects” = distant solar system bodies Angular motion primarily reflex motion of Earth going around Sun

Why so hard to find? Over 1200 outer solar system objects now known, some larger than largest asteroid Why so long to find? Largest asteroid can be seen with binoculars! Objects seen (in visible light) only by reflected sunlight Typical KBO a million times fainter than same size MB asteroid

Kuiper belt objects provide an opportunity to study the preserved building blocks of a large outer solar system planet

Classical (e.g QB1) = KBO “main belt” r~ 42-46AU, stable orbits, near-circular orbits, low inclination Plutinos (little Plutos) a~39.5AU, significant eccentricity and inclination For stability, orbits in 3:2 resonance with Neptune- even though Pluto gets closer to Sun than Neptune, Pluto orbit stable Dynamical Zoo of Outer Solar System Objects With ~ 1200 objects discovered (about ½ with good orbits), we see a number of different dynamical classes:

Stability of Pluto’s (and Plutino’s) orbits Neptune orbits Sun 3 times for every 2 times Pluto does This resonance insures Pluto doesn’t come close to Neptune (inclination helps also)

Outer Planets/ Plutinos

Scattered disk objects (SDOs) Objects with large orbits (Q> 60AU), presumably gravitationally scattered outwards from origin radius Eccentric orbits Centaurs Objects presumably scattered inwards towards Sun- have low lifetime (<10**7 year) orbits that cross orbits of giant planets “refugees from the Kuiper Belt” Ecliptic comets Objects close enough to Sun to be active, easily visible comet, perhaps evolving dynamically through Centaur stage

SDO and Classical orbits Note scale of box

Outer Planets / Classical KBOs + Centaurs

Centaurs and Ecliptic comets

Kuiper Belt and related objects Qualitatively, at least, a nice story: Outer solar nebula disk formed into some objects in km size range, but not dense enough for a planet to form These objects formed the Primordial Kuiper Belt (PKB) Classical and Plutinos surviving members of PKB Centaurs, SDOs and Ecliptic comets show continuing loss of objects from PKB by gravitational scattering

Optical colors of KBOs In 1995, we started a program to measure optical colors of KBOs CCD imaging on 2 – 10 meter telescopes through 3 filters, B ( nm), V ( nm), R( nm) Measure brightness in each filter- ratio gives us colors (measure of slope of spectral energy distribution) Why not spectroscopy? Objects very faint, and colors much easier to obtain than spectra Idea is to get colors of significant samples of objects in different dynamical classes to look for patterns (color vs. dynamical class) as possible clues to object’s origin and history To date, we have colors for ~120 objects / ~150 scheduled nights on 2-10 meter telescopes

We began our CCD imaging survey of KBOs in nights on 2 m to 10 m class telescopes 12 nights of NASA Keck time Colors & magnitudes for 120 of ~ 500 objects with good orbits Surface colors, densities, & albedos

Color distributions From our first color measurements, it was clear that objects not all same color. Some had ~solar colors (gray), some were much redder than solar color. (Why? Different surface compositions- water ice vs. methane?? Still trying to figure out!) In 1998, we claimed that KBOs/Centaurs showed a bimodal color distribution Peixinho etal showed this was partially due to our lumping Centaurs with KBOs- we and they agree Centaurs show bimodal color distribution. Peixinho etal claimed Plutinos not bimodal We have sought maximum size sample for different classes by combining our colors with those of other groups

Two distinct color populations (Tegler and Romanishin NATURE 1998) Surface Colors

Our B-R colors vs. Europeans Solid line is 1:1 – Good agreement of colors for objects in common, so combination of samples is valid.

Centaur Color Distribution Diptest shows significant bimodality (99.5% SL for combined sample)

Plutino Color Distribution Diptest does not indicate significant bimodality in any sample

B-R Colors of Classical KBOs vs. Plutinos Nonparametric Wilcoxon rank-sum test shows Classical and Plutino color distribution differ at 99.98% SL

Conclusions Wilcoxon rank-sum test shows classical KBO color distribution different from Plutinos Diptest shows Centaurs have a bimodal B-R color distribution, while Plutinos do not Bottom line: Using nonparametric statistical tests, we HAVE found statistically different color distributions for different dynamical classes of objects. We hope these distributions will help elucidate the origin and history of these objects.