"TNOs are cool": A survey of the trans-neptunian region. II. The thermal lightcurve of (136108) Haumea

TL;DR

This study measures the thermal lightcurve of trans-Neptunian object Haumea, revealing shape effects and surface properties through Herschel and Spitzer observations, supporting a high axial ratio and low thermal inertia.

Contribution

First detailed thermal lightcurve analysis of Haumea confirming shape effects and surface properties using Herschel and Spitzer data.

Findings

Thermal lightcurve amplitude of ~2 at 100 micrometers

Equivalent diameter of ~1300 km and high albedo (~0.70-0.75)

Indications of a high axial ratio (~1.3) and low thermal inertia

Abstract

Thermal emission from Kuiper Belt object (136108) Haumea was measured with Herschel-PACS at 100 and 160 micrometers for almost a full rotation period. Observations clearly indicate a 100-micrometer thermal lightcurve with an amplitude of a factor of ~ 2, which is positively correlated with the optical lightcurve. This confirms that both are primarily due to shape effects. A 160-micrometer lightcurve is marginally detected. Radiometric fits of the mean Herschel- and Spitzer- fluxes indicate an equivalent diameter D ~ 1300 km and a geometric albedo p_v ~ 0.70-0.75. These values agree with inferences from the optical lightcurve, supporting the hydrostatic equilibrium hypothesis. The large amplitude of the 100-micrometer lightcurve suggests that the object has a high projected a/b axis ratio (~ 1.3) and a low thermal inertia as well as possible variable infrared beaming. This may point to fine regolith on the surface, with a lunar-type photometric behavior. The quality of the thermal data is not sufficient to clearly detect the effects of a surface dark spot.