Near-infrared spatially resolved spectroscopy of (136108) Haumea's multiple system

Context. The transneptunian region of the solar system is populated by a wide variety of icy bodies showing great diversity in orbital behavior, size, surface color, and composition. Aims. The dwarf planet (136108) Haumea is among the largest transneptunian objects (TNOs) and is a very fast rotator (similar to 3.9 h). This dwarf planet displays a highly elongated shape and hosts two small moons that are covered with crystalline water ice, similar to their central body. A particular region of interest is the Dark Red Spot (DRS) identified on the surface of Haumea from multiband light-curve analysis (Lacerda et al. 2008). Haumea is also known to be the largest member of the sole TNO family known to date, and an outcome of a catastrophic collision that is likely responsible for the unique characteristics of Haumea. Methods. We report here on the analysis of a new set of near-infrared Laser Guide Star assisted observations of Haumea obtained with the Integral Field Unit (IFU) Spectrograph for INtegral Field Observations in the Near Infrared (SINFONI) at the European Southern Observatory (ESO) Very Large Telescope (VLT) Observatory. Combined with previous data published by Dumas et al. (2011), and using light-curve measurements in the optical and far infrared to associate each spectrum with its corresponding rotational phase, we were able to carry out a rotationally resolved spectroscopic study of the surface of Haumea. Results. We describe the physical characteristics of the crystalline water ice present on the surface of Haumea for both regions, in and out of the DRS, and analyze the differences obtained for each individual spectrum. The presence of crystalline water ice is confirmed over more than half of the surface of Haumea. Our measurements of the average spectral slope (1 : 45 +/- 0 . 82% by 100 nm) confirm the redder characteristic of the spot region. Detailed analysis of the crystalline water-ice absorption bands do not show significant differences between the DRS and the remaining part of the surface. We also present the results of applying Hapke modeling to our data set. The best spectral fit is obtained with a mixture of crystalline water ice (grain sizes smaller than 60 mu m) with a few percent of amorphous carbon. Improvements to the fit are obtained by adding similar to 10% of amorphous water ice. Additionally, we used the IFU-reconstructed images to measure the relative astrometric position of the largest satellite Hi'iaka and determine its orbital elements. An orbital solution was computed with our genetic-based algorithm GENOID and our results are in full agreement with recent results.