``TNOs are cool'': A survey of the trans-Neptunian region. II. The thermal lightcurve of (136108) Haumea
Jorda, L.; Fornasier, S.; Groussin, O.; Gutiérrez, P. J.; Thomas, N.; Hartogh, P.; Lellouch, E.; Crovisier, J.; Hestroffer, D.; Dotto, E.; Müller, T. G.; Kiss, C.; Mommert, M.; Stansberry, J.; Protopapa, S.; Ortiz, J. L.; Doressoundiram, A.; Santos-Sanz, P.; Duffard, R.; Moreno, R.; Rengel, M.; Lim, T.; Vilenius, E.; Mueller, M.; Delsanti, A.; Thirouin, A.; Henry, F.; Kidger, M.; Barucci, A.; Lacerda, P.; Lara, L.; Swinyard, B.; Horner, J.; Harris, A. W.; Trilling, D.; Hainaut, O.
France, Hungary, Germany, United Kingdom, Spain, United States, Italy, Switzerland
Abstract
Thermal emission from Kuiper belt object (136108) Haumea was measured with Herschel-PACS at 100 μm and 160 μm for almost a full rotation period. Observations clearly indicate a 100 μm 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 μm lightcurve is marginally detected. Radiometric fits of the mean Herschel- and Spitzer- fluxes indicate an equivalent diameter D ~ 1300 km and a geometric albedo pv ~ 0.70-0.75. These values agree with inferences from the optical lightcurve, supporting the hydrostatic equilibrium hypothesis. The large amplitude of the 100 μm 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.
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.