Dwarf planet Ceres: Ellipsoid dimensions and rotational pole from Keck and VLT adaptive optics images

• Ceres, the second target for the DAWN mission, from adaptive optics images at Keck and the VLT over ten years is found to be an oblate spheroid with diameters of 967X967X892, with an uncertainty of 10 km in each dimension.
• Its rotational pole is found to lie within 3 degrees of [RA Dec] = [287 + 64].
• The soft terminator (‘terminator fading’) is measurable in the high resolution images.

The dwarf planet (1) Ceres, the largest object between Mars and Jupiter, is the target of the NASA Dawn mission, and we seek a comprehensive description of the spin-axis orientation and dimensions of Ceres in order to support the early science operations at the rendezvous in 2015. We have obtained high-angular resolution images using adaptive optics cameras at the W.M. Keck Observatory and the ESO VLT over ten dates between 2001 and 2010, confirming that the shape of Ceres is well described by an oblate spheroid. We derive equatorial and polar diameters of 967 ± 10 km and 892 ± 10 km, respectively, for a model that includes fading of brightness towards the terminator, presumably linked to limb darkening. These dimensions lie between values derived from a previous analysis of a subset of these images obtained at Keck by Carry et al. (Carry et al. [2008]. Astron. Astrophys. 478 (4), 235–244) and a study of Hubble Space Telescope observations (Thomas et al. [2005]. Nature 437, 224–226). Although the dimensions are 1–2% smaller than those found from the HST, the oblateness is similar.We find the spin-vector coordinates of Ceres to lie at (287°, +64°) in equatorial EQJ2000 reference frame (346°, +82° in ecliptic ECJ2000 coordinates), yielding a small obliquity of 3°. While this is in agreement with the aforementioned studies, we have improved the accuracy of the pole determination, which we set at a 3° radius.