The intriguing shape of Bienor

(54 598) Bienor is one of the largest centaurs known to date, with a diameter of ~ 200 km (obtained from Herschel Space Observatory measurements; Duffard et al. 2014b), similar to that of Chiron and Chariklo. Bienor also shares its ellipsoidal shape and a watery surface composition with these two objects (e.g., Dotto et al. 2003, Rabinowitz et al. 2007, Fernández-Valenzuela et al. 2017), although its rotation period of ~ 9.14 h, without being very slow is slightly over the TNO and centaur population’s average (Thirouin et al. 2014). Bienor presents a long-term variation on its absolute magnitude that is considerably larger than what would be expected based on a hydrostatic equilibrium shape (Fernandez-Valenzuela et al. 2017). All of this makes Bienor a quite interesting centaur for which many physical properties remain unknown.

Within our program of physical characterization of TNOs and Centaurs, we predicted a stellar occultation by Bienor to occur on January 11, 2019, with good observability potential. We carried out high accuracy astrometry runs to refine the prediction, and as a result, we derived a shadow path favorable for the south of Europe. This encouraged us to carry out an occultation observation campaign that resulted in 5 positive detections from 4 observing sites (figure 1). The combination of the different chords from each detection provides the instantaneous limb fit of the body at the moment of the stellar occultation.

Additionally, we carried out specif campaigns to obtain the rotational light-curve of Bienor (figure 2) shortly after the occultation event. This is crucial in order to know the rotational phase of the object at the moment of the occultation, thus providing the three-dimensional shape of the body. As can be seen, the rotational light-curve presents two minima with very different depths, indicating that the shape of the body is highly irregular.

In this talk, we will present the combined results from Bienor’s stellar occultation and rotational lightcurve, with which we deduce the shape of the body, its size, albedo, and density.