A dynamical dichotomy in large binary asteroids

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Bd de l’Observatoire, CS 34229, 06304 Nice cedex 4, France
² LTE, Observatoire de Paris, Université PSL, Sorbonne Université, Université de Lille, LNE, CNRS, 61 Avenue de l’Observatoire, 75014 Paris, France
³ European Southern Observatory (ESO), Alonso de Córdova 3107, 1900, Casilla Vitacura, Santiago, Chile
⁴ Charles University, Faculty of Mathematics and Physics, Institute of Astronomy, V Holešovičkách 2, 180 00 Prague, Czech Republic
⁵ Southwest Research Institute, 1301 Walnut St. #400, Boulder, CO 80302, USA
⁶ Lowell Observatory, 1400 W. Mars Hill Rd., Flagstaff, AZ 86001, USA
⁷ Observatoire de Genève, 1290 Sauverny, Switzerland
⁸ Space sciences, Technologies & Astrophysics Research (STAR) Institute, University of Liege, Liege, Belgium
⁹ UCLA Department of Physics and Astronomy, Department of Earth, Planetary, and Space Sciences, Los Angeles, California, USA
¹⁰ Florida Space Institute, University of Central Florida, 12354 Research Parkway, Partnership 1 building, Orlando, FL 32828, USA
¹¹ Steward Observatory, N420, Department of Astronomy, University of Arizona, 933 N. Cherry Ave. Tucson, AZ 85721, USA
¹² Large Binocular Telescope Observatory, University of Arizona, Tucson, AZ 85721, USA
¹³ Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁴ Observatoire de Blauvac, 293 Chemin de St Guillaume, 84570 Blauvac, France
¹⁵ Turtle Star Observatory, Friedhofstr. 15, 45478 Mülheim-Ruhr, Germany
¹⁶ The Central Astronomical Observatory of the Russian Academy of Sciences at Pulkovo, Pulkovo Observatory, 65 Pulkovskoye shosse, 196140 St. Petersburg, Russia
¹⁷ UnderOak Observatory, Cedar Knolls, NJ, USA

^★ Corresponding author; kate.minker@oca.eu

Received: 1 April 2025
Accepted: 16 July 2025

Abstract

Context. No less than 15% of large asteroids (with diameters greater than 140 km) have satellites. The commonly accepted mechanism for their formation is post-impact reaccumulation. However, the detailed physical and dynamical properties of these systems are not well understood, and many of them have not been studied in detail.

Aims. We studied the population of large binary asteroid systems, in part through the characterization of (283) Emma and (762) Pulcova. To do so, we compared the gravitational fields predicted from the shape of the primary body with the non-Keplerian gravitational components identified in orbital models of the satellites of each system. We contextualize these systems in the greater population of large binary systems, thus providing clues to asteroid satellite formation.

Methods. We reduced all historical high angular resolution adaptive optics (AO) images from ground-based telescopes to conduct astrometric and photometric measurements of each system’s components. We then determined orbital solutions for each system using the genoid algorithm. We modeled the shapes of the system primaries using light curve-inversion techniques scaled with stellar occultations and AO images, and we developed internal structure models using SHTOOLS. Finally, we compared the distribution of the physical and orbital properties of the known binary asteroid systems.

Results. We find a very low residual orbital solution for Emma with a gravitational quadrupole J₂ value that is significantly lower than expected from the shape model, implying that Emma has a significantly nonhomogeneous internal structure, and an overall bulk density of 0.9 ± 0.3 g cm-3⁻³. The circular co-planar orbit of Pulcova’s satellite leaves substantial ambiguity in the orbital solution. We also find that the differences between these systems reflect an overall dichotomy within the population of large binary systems, with a strong correlation between primary elongation and satellite eccentricity observed in one group.

Conclusions. We determine that there may be two distinct formation pathways influencing the end-state dichotomy in these binary systems, and that (762) Pulcova and (283) Emma belong to the two separate groups.