## Selected Publications

*Icarus*

*Publ. Astron. Soc. Pac.*

*Astron. J.*

*Icarus*

*Icarus*

*Astron. J.*

BYU Authors: Denise C. Stephens, published in *Icarus*

We have identified a binary companion to (42355) 2002 CR46 in our ongoing deep survey using the Hubble Space Telescope's High Resolution Camera. It is the first companion to be found around an object in a non-resonant orbit that crosses the orbits of giant planets. Objects in orbits of this kind, the Centaurs, have experienced repeated strong scattering with one or more giant planets and therefore the survival of binaries in this transient population has been in question. Monte Carlo simulations suggest, however, that binaries in (42355) 2002 CR46-like heliocentric orbits have a high probability of survival for reasonable estimates of the binary's still-unknown system mass and separation. Because Centaurs are thought to be precursors to short period comets, the question of the existence of binary comets naturally arises; none has yet been definitively identified. The discovery of one binary in a sample of eight observed by HST suggests that binaries in this population may not be uncommon. (c) 2006 Elsevier Inc. All rights reserved.

BYU Authors: D. C. Stephens, published in *Publ. Astron. Soc. Pac.*

We have resolved Kelu-1 into a binary system with a separation of similar to 290 mas using the Laser Guide Star Adaptive Optics system on the Keck II telescope. We have also reanalyzed a 1998 HST observation of Kelu-1 and find that the observed PSF is best fit by a binary object separated by 45 mas. Observations at multiple epochs confirm that the two objects share a common proper motion and clearly demonstrate the first evidence of orbital motion. Kelu-1B is fainter than Kelu-1A by 0.39 +/- 0.01 mag in the K' filter and 0.50 +/- 0.01 mag in the H filter. We derive spectral types of L2 +/- 1 and L3.5 +/- 1 for Kelu-1A and B, respectively. The separation of flux into the two components rectifies Kelu-1's overluminosity problem, which has been known for quite some time. Given the available data, we are able to constrain the inclination of the system to >81 degrees and the orbital period to greater than or similar to 40 yr.

BYU Authors: D. C. Stephens, published in *Astron. J.*

We have analyzed a homogeneous set of observations of 81 trans-Neptunian objects obtained with NIC2 on the Hubble Space Telescope with the goal of identifying partially resolved binaries. Using PSF fitting we have identified six likely binaries in addition to the three new binaries already found in this data set. We find that 11(-2)(+5)% of trans-Neptunian objects are binaries at the separation and brightness limits of NIC2. The identification of these new binaries significantly increases the known lower limit to the binary fraction among trans-Neptunian objects. The origin of such a high fraction of binaries remains to be determined. Most interestingly, detectable binaries appear to be about 4 times more common among the cold classical disk than in the dynamically excited populations.

BYU Authors: D. C. Stephens, published in *Icarus*

Discovery of trans-neptunian object (TNO) satellites and determination of their orbits has recently enabled estimation of the size and albedo of several small TNOs, extending the size range of objects having known size and albedo down into the sub-100 km range. In this paper we compute albedo and size estimates or limits for 20 TNOs, using a consistent method for all binary objects and a consistent method for all objects having reported thermal fluxes. As is true for larger TNOs, the small objects show a remarkable diversity of albedos. Although the sample is limited, there do not yet appear to be any trends relating albedo to other observable properties or to dynamical class, with the possible exception of inclination. The observed albedo diversity of TNOs has important implications for computing the size-frequency distribution, the mass, and other global properties of the Kuiper belt derived from observations of objects' apparent magnitudes and may also point the way toward an improved compositional taxonomy based on albedo in addition to color. (c) 2005 Elsevier Inc. All rights reserved.

BYU Authors: D. C. Stephens, published in *Icarus*

We have observed (66652) 1999 RZ(253) with the Hubble Space Telescope at seven separate epochs and have fit an orbit to the observed relative positions of this binary. Two orbital solutions have been identified that differ primarily in the inclination of the orbit plane. The best fit corresponds to an orbital period, P = 46.263 +/- (0.006)(0.074) days. semimajor axis a = 4660 +/- 170 km and orbital eccentricity e = 0.460 +/- 0.013 corresponding to a system mass in = 3.7 +/- 0.4 x 10(18) kg. For a density of p = 1000 kg m(-3) the albedo at 477 inn is p(477) = 0.12 +/- 0.01 significantly higher than has been commonly assumed for objects in the Kuiper belt. Multicolor, multiepoch photometry shows this pair to have colors typical for the Kuiper belt with a spectral gradient of 0.35 per 100 nm it) the range between 475 and 775 nm. Photometric variations at the four epochs we observed were as large a., 12 3% but the sampling is insufficent to confirm the existence of a lightcurve. (C) 2004 Elsevier Inc. All rights reserved.

BYU Authors: D. C. Stephens, published in *Astron. J.*

We have measured the separations and position angles of the two components of the binary trans-Neptunian object (58534) 1997 CQ(29) at eight epochs. From these data we are able to constrain the orbit and mass of this binary system. The best-fitting orbit has an orbital period of P = 312 +/- 3 days. The orbital eccentricity is e 0.45 +/- 0.03 and the semimajor axis is a = 8010 +/- 80 km. The mass of the system is surprisingly low at (0.42 +/- 0.02) x 10(18) kg. To account for the observed brightness consistent with the low mass and a range of plausible densities, the geometric albedo must be very high; for an assumed density of 1000 kg m(-3), we find a red geometric albedo of p(R) = 0.37, an order of magnitude higher than has been generally assumed for trans-Neptunian objects. If objects with comparably high albedos are common in the Kuiper belt, estimates of the mass of the belt must be significantly reduced. The semimajor axis of (58534) 1997 CQ(29)'s orbit is 2.8% of the Hill radius of the primary, a value strikingly similar to this same ratio in other trans-Neptunian binaries, main-belt binaries, and regular satellite systems.