Mutual Events in the Cold Classical Transneptunian Binary System Sila and Nunam

TL;DR

This paper reports on Hubble observations of the Sila-Nunam binary system, revealing a nearly circular, tidally locked orbit, and discusses the potential to learn more about the system through mutual event observations.

Contribution

It provides the first detailed orbital parameters and physical insights into the Sila-Nunam binary, including evidence of tidal locking and low bulk density, based on a decade of Hubble data.

Findings

Determined the mutual orbit period as approximately 12.51 days.

Estimated the system mass to be about 10.84 x 10^18 kg.

Inferred a low bulk density around 0.72 g/cm^3.

Abstract

Hubble Space Telescope observations between 2001 and 2010 resolved the binary components of the Cold Classical transneptunian object (79360) Sila-Nunam (provisionally designated 1997 CS29). From these observations we have determined the circular, retrograde mutual orbit of Nunam relative to Sila with a period of 12.50995 \pm 0.00036 days and a semimajor axis of 2777 \pm 19 km. A multi-year season of mutual events, in which the two near-equal brightness bodies alternate in passing in front of one another as seen from Earth, is in progress right now, and on 2011 Feb. 1 UT, one such event was observed from two different telescopes. The mutual event season offers a rich opportunity to learn much more about this barely-resolvable binary system, potentially including component sizes, colors, shapes, and albedo patterns. The low eccentricity of the orbit and a photometric lightcurve that appears to coincide with the orbital period are consistent with a system that is tidally locked and synchronized, like the Pluto-Charon system. The orbital period and semimajor axis imply a system mass of (10.84 \pm 0.22) \times 10^18 kg, which can be combined with a size estimate based on Spitzer and Herschel thermal infrared observations to infer an average bulk density of 0.72 +0.37 -0.23 g cm^-3, comparable to the very low bulk densities estimated for small transneptunian binaries of other dynamical classes.