The orbit, mass, and albedo of transneptunian binary (66652) 1999 RZ₂₅₃

Abstract

We have observed (66652) 1999 RZ₂₅₃ 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, $P=46.263\pm \genfrac{}{}{0ex}{}{0.006}{0.074}$ days, semimajor axis $a=4660\pm 170\text{km}$ and orbital eccentricity $e=0.460\pm 0.013$ corresponding to a system mass $m=3.7\pm 0.4\times {10}^{18}\text{kg}$. For a density of $\rho =1000\text{kg}{\text{m}}^{\mathrm{-3}}$ the albedo at 477 nm is $p_{477}=0.12\pm 0.01$, 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 in the range between 475 and 775 nm. Photometric variations at the four epochs we observed were as large as $12\pm 3\text{\%}$ but the sampling is insufficient to confirm the existence of a lightcurve.

Introduction

(66652) 1999 RZ₂₅₃ was discovered in images taken on September 8, 1999 at the Canada–France–Hawaii telescope by Trujillo et al. (2000). Its heliocentric orbit was determined by subsequent observations by this group and others Veillet et al., 2000. The latest orbital elements reported by the Deep Ecliptic Survey team Millis et al., 2002, $a=44.02\text{AU}$, $e=0.091$, $i=0.564\text{deg}$, put it in a class with other low-inclination, low-eccentricity, non-resonant objects sometimes called “classical” Kuiper belt objects.

The Deep Ecliptic Survey website Millis et al., 2002 has compiled astrometric observations of (66652) 1999 RZ₂₅₃, mostly obtained in the R band, with a median R magnitude of 21.7. More accurate photometric observations find $R=21.5$ Delsanti et al., 2001. The minor planet center, using the standard asteroidal color conversion, derives an absolute magnitude in the V band of $H_V=5.9$, suggesting that (66652) 1999 RZ₂₅₃ could be among the larger of the known transneptunian objects (TNO). (We show below that (66652) 1999 RZ₂₅₃ like many TNOs, is significantly redder than the asteroid standard and, thus, has a fainter $H_V$.)

We identified (66652) 1999 RZ₂₅₃ as a binary in images we took using the Hubble Space Telescope's (HST) Near-Infrared Camera and Multi-Object Spectrograph (NICMOS) on 23 April 2003 Noll and Stephens, 2003. Prompted by this discovery, a series of followup observations were made with the HST Advanced Camera for Surveys (ACS) using its High Resolution Camera (HRC). We resolved the binary at all four epochs observed by the ACS. We have been able to use the relative astrometry available from our HST images to determine the relative orbit of the binary pair. With that comes a determination of the system mass, constraints on albedo and density, and photometry of the individual components that we describe in this paper. Despite its great distance and small size, (66652) 1999 RZ₂₅₃ joins the ranks of well-characterized Solar System objects thanks to its beneficent duplicity.