Triaxial shapes and densities of G!k'un'h`omd'im`a, Haumea, and Varda from stellar occultations

The shapes and densities of mid-sized and large trans-Neptunian objects (TNOs) are pivotal for understanding a variety of important aspects of planet formation. In this work, we present a Bayesian shape modeling method which combines constraints from rotational light curves and satellite orbits to construct three-dimensional shape models of TNOs. We use it to reanalyze three stellar occultations of the TNOs (229762) G!k'un'h`omd'im`a (2007 UK 126), (136108) Haumea, and (174567) Varda. By assuming that their satellites (or ring) orbit in their respective equatorial planes, we are able to derive unique shape models for both G!k'un'h`omd'im`a and Haumea. Our derived shape for G!k'un'h`omd'im`a is spheroidal with a = b = 329+4-3 km and c = 294+11-10 km, with a system density rho = 1007+50-49 kg m-3. For Haumea, we find a = 1061+87-71 km, b = 844+5-7 km, and c = 514+18-19 km, providing rho = 2050+157-152 kg m-3. For Varda, after updating its mutual orbit with its satellite Ilmar"e, we find that currently published data are unable to fully constrain its three-dimensional shape. Intriguingly, Varda's elongated limb appears to point towards its satellite at the time of the occultation. With a about 2% chance of such an alignment happening randomly, this may be suggestive of a frozen-in tidal and/or rotational bulge. Our work emphasizes the importance of how external constraints can improve occultation analyses. With continued observations of rotational light curves, stellar occultations, and satellite orbits, these and other TNOs can have their shapes and densities further refined.