TNOs are Cool: A survey of the trans-Neptunian region II. The thermal lightcurve of (136108) Haumea

Thermal emission from Kuiper belt object (136108) Haumea was measured with Herschel-PACS at 100 mu m and 160 mu m for almost a full rotation period. Observations clearly indicate a 100 mu m thermal lightcurve with an amplitude of a factor of similar to 2, which is positively correlated with the optical lightcurve. This confirms that both are primarily due to shape effects. A 160 mu m lightcurve is marginally detected. Radiometric fits of the mean Herschel- and Spitzer- fluxes indicate an equivalent diameter D similar to 1300 km and a geometric albedo p(v) similar to 0.70-0.75. These values agree with inferences from the optical lightcurve, supporting the hydrostatic equilibrium hypothesis. The large amplitude of the 100 mu m lightcurve suggests that the object has a high projected a/b axis ratio (similar to 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.