CONICET | Buscador de Institutos y Recursos Humanos

We obtained high precision relative astrometry and photometry of Orcus system with respect to background stars. The Right Ascension residuals of an orbital fit to the astrometric data revealed a period of 9.7 ± 0.3 days, which is what one would expect to be induced by the known Orcus companion (Vanth). The residuals also are correlated with the theoretical positions of the satellite relative to the primary. The oscillation in the residuals is due to the photocenter motion of the combined Orcus plus satellite system around the barycenter along an orbital revolution of the satellite. We show here that detecting some binaries through a carefully devised astrometric technique might be feasible with telescopes of moderate size. We have also analyzed the system's mid-term photometry in order to determine whether the rotation could be tidally locked to the satellite's orbital period. We found that a photometric variability of 9.7 ± 0.3 days is clear in our data, nearly coincident with the orbital period of the satellite. We believe this variability might be induced by the satellite's rotation. In our photometry there is also a slight hint for an additional very small variability in the 10 hr range that was already reported in the literature. Such short-term variability would indicate that the primary is not tidally locked and therefore the system would not have reached a double synchronous state. From angular momentum considerations we suspect that Orcus satellite might have formed from a rotational fission. This requires that the mass of the satellite would be around 0.09 times that of the primary, close to the value that one derives by using an albedo of 0.12 for the satellite and assuming equal densities for both the primary and secondary.