Large Size and Slow Rotation of the Trans-Neptunian Object (225088) 2007 OR₁₀ Discovered from Herschel and K2 Observations

Abstract

We present the first comprehensive thermal and rotational analysis of the second most distant trans-Neptunian object (TNOs) (225088) 2007 OR₁₀. We combined optical light curves provided by the Kepler Space Telescope-K2 extended mission and thermal infrared data provided by the Herschel Space Observatory. We found that (225088) 2007 OR₁₀ is likely to be larger and darker than derived by earlier studies: we obtained a diameter of d={1535}_-225⁺⁷⁵ {{km}} which places (225088) 2007 OR₁₀ in the biggest top three TNOs. The corresponding visual geometric albedo is {p}_V={0.089}_-0.009^+0.031. The light-curve analysis revealed a slow rotation rate of P_rot = 44.81 ± 0.37 hr, superseded by very few objects. The most likely light-curve solution is double-peaked with a slight asymmetry; however, we cannot safely rule out the possibility of having a rotation period of P_rot = 22.40 ± 0.18 hr, which corresponds to a single-peaked solution. Due to the size and slow rotation, the shape of the object should be a MacLaurin ellipsoid, so the light variation should be caused by surface inhomogeneities. Its newly derived larger diameter also implies larger surface gravity and a more likely retention of volatiles—CH₄, CO, and N₂—on the surface.