Susan Diane Kern

Evidence for two populations of classical transneptunian objects: The strong inclination dependence of classical binaries

Abstract We have searched 101 Classical transneptunian objects for companions with the Hubble Space Telescope. Of these, at least 21 are binary. The heliocentric inclinations of the objects we observed range from 0.6°–34°. We find a very strong anticorrelation of binaries with inclination. Of the 58 targets that have inclinations of less than 5.5°, 17 are binary, a binary fraction of 29 ± 7 6 % . All 17 are similar-brightness systems. On the contrary, only 4 of the 42 objects with inclinations greater than 5.5° have satellites and only 1 of these is a similar-brightness binary. This striking dichotomy appears to agree with other indications that the low eccentricity, non-resonant Classical transneptunian objects include two overlapping populations with significantly different physical properties and dynamical histories.

The orbit, mass, size, albedo, and density of (65489) Ceto/Phorcys: A tidally-evolved binary Centaur

Abstract Hubble Space Telescope observations of Uranus- and Neptune-crossing object (65489) Ceto/Phorcys (provisionally designated 2003 FX128) reveal it to be a close binary system. The mutual orbit has a period of 9.554 ± 0.011 days and a semimajor axis of 1840 ± 48 km . These values enable computation of a system mass of ( 5.41 ± 0.42 ) × 10 18 kg . Spitzer Space Telescope observations of thermal emission at 24 and 70 μm are combined with visible photometry to constrain the systems effective radius ( 109 −11 +10 km ) and geometric albedo ( 0.084 −0.014 +0.021 ) . We estimate the average bulk density to be 1.37 −0.32 +0.66 g cm −3 , consistent with ice plus rocky and/or carbonaceous materials. This density contrasts with lower densities recently measured with the same technique for three other comparably-sized outer Solar System binaries (617) Patroclus, (26308) 1998 SM165, and (47171) 1999 TC36, and is closer to the density of the saturnian irregular satellite Phoebe. The mutual orbit of Ceto and Phorcys is nearly circular, with an eccentricity ⩽0.015. This observation is consistent with calculations suggesting that the system should tidally evolve on a timescale shorter than the age of the Solar System.

(42355) Typhon–Echidna: Scheduling observations for binary orbit determination

Abstract We describe a strategy for scheduling astrometric observations to minimize the number required to determine the mutual orbits of binary transneptunian systems. The method is illustrated by application to Hubble Space Telescope observations of (42355) Typhon–Echidna, revealing that Typhon and Echidna orbit one another with a period of 18.971 ± 0.006 days and a semimajor axis of 1628 ± 29 km , implying a system mass of ( 9.49 ± 0.52 ) × 10 17 kg . The eccentricity of the orbit is 0.526 ± 0.015 . Combined with a radiometric size determined from Spitzer Space Telescope data and the assumption that Typhon and Echidna both have the same albedo, we estimate that their radii are 76 −16 +14 and 42 −9 +8 km , respectively. These numbers give an average bulk density of only 0.44 −0.17 +0.44 g cm −3 , consistent with very low bulk densities recently reported for two other small transneptunian binaries.

PHYSICAL CHARACTERIZATION OF THE BINARY EDGEWORTH-KUIPER BELT OBJECT 2001 QT297

Following our discovery of 2001 QT297 as the second known binary Edgeworth—Kuiper Belt Object (EKBO) in October of 2001 [IAUC 7733], we have carried out additional high spatial resolution ground based imaging in October and November of 2001 and July, August, and September of 2002. Using the Raymond and Beverly Sackler Magellan Instant Camera (MagIC) on the Baade and Clay 6.5 m telescopes at Las Campanas Observatory in Chile, we have obtained accurate astro-metric and photometric measurements in the Sloan r′,i′, and g′ filters. Superb seeing conditions and PSF fitting allow an accurate determination of the binary component separation and position angle over time as well as a detailed study of color and temporal variability of the individual components. Here we present a physical characterization of the individual components of 2001 QT297 based on these astrometric, color and variability measurements. We find the primary to exhibit colors about 0.3 magnitudes redder than solar with no evidence for variability. The secondary component, however, exhibits strong variability (~0.6 magnitudes) with a best fit period of 4.7526 ± 0.0007 h for a single peak lightcurve or 9.505 ± 0.001 h for a dual peaked lightcurve. The colors measured for the secondary also suggest variability. Based on a preliminary orbit fit for the pair using observations spanning a one year arc, we are able to estimate a system mass of ~ 3.2 × 1018 kg and provide constraints to the surface albedo of 9–14% for assumed densities between 1 and 2 g/cm3.