The Complex Rotational Light Curve of (385446) Manw\"e-Thorondor, a Multi-Component Eclipsing System in the Kuiper Belt

TL;DR

This study analyzes the complex rotational light curve of the Kuiper Belt system (385446) Manwë-Thorondor, revealing a bi-lobed primary body, long-term variability, and mutual eclipses, advancing understanding of multi-component distant objects.

Contribution

First detailed multi-epoch photometric analysis of Manwë-Thorondor, modeling its bi-lobed shape and complex variability, and providing new physical parameters of the system.

Findings

Manwë is likely bi-lobed with a barbell shape.

The primary's rotation period is approximately 11.88 hours.

Thorondor exhibits long-term brightness variability with a ~300-day period.

Abstract

Kuiper Belt Object (385446) Manw\"e-Thorondor is a multi-object system with mutual events predicted to occur from 2014 to 2019. To detect the events, we observed the system at 4 epochs (UT 2016 Aug 25 and 26, 2017 Jul 22 and 25, 2017 Nov 9, and 2018 Oct 6) in g, r, and VR bands using the 4-m SOAR and the 8.1-m Gemini South telescopes at Cerro Pach\'on, Chile and Lowell Observatory ' s 4.3-m Discovery Channel Telescope at Happy Jack, Arizona. These dates overlap the uncertainty range (+/- 0.5 d) for four inferior events (Thorondor eclipsing Manw\"e). We clearly observe variability for the unresolved system with a double-peaked period 11.88190 +/- 0.00005 h and ~0.5 mag amplitude together with much longer-term variability. Using a multi-component model, we simultaneously fit our observations and earlier photometry measured separately for Manw\"e and Thorondor with the Hubble Space Telescope. Our fit suggests Manw\"e is bi-lobed, close to the barbell shape expected for a strengthless body with density ~0.8 g/cm3 in hydrostatic equilibrium. For Manw\"e, we thereby derive maximum width to length ratio ~0.30, surface area equivalent to a sphere of diameter 190 km, geometric albedo 0.06, mass 1.4x1018 kg, and spin axis oriented ~75 deg from Earth ' s line of sight. Changes in Thorondor ' s brightness by ~0.6 mag with ~300-d period may account for the system ' s long-term variability. Mutual events with unexpectedly shallow depth and short duration may account for residuals to the fit. The system is complex, providing a challenging puzzle for future modeling efforts.