CONICET | Buscador de Institutos y Recursos Humanos

From time series CCD observations of a fixed and large star field that contained the binary trans-Neptunian object (90482) Orcus(formerly 2004 DW), taken during a period of 33 days, we have been able to derive high-precision relative astrometry and photometryof the Orcus system with respect to background stars. The right ascension residuals of an orbital fit to the astrometric data revealed aperiodicity of 9.7 ± 0.3 days, which is what one would expect to be induced by the known Orcus companion (Vanth). The residualsare also correlated with the theoretical positions of the satellite with regard to the primary. We therefore have revealed the presenceof Orcus satellite in our astrometric measurements, although the residuals in declination did not show the expected variations. Theoscillation in the residuals is caused by the photocenter motion of the combined Orcus plus satellite system around the barycenter alongan orbital revolution of the satellite. The photocenter motion is much larger than the motion of Orcus around the barycenter, and weshow here that detecting some binaries through a carefully devised astrometric technique might be feasible with telescopes of moderatesize. We discuss the prospects for using the technique to find new binary trans-Neptunian objects (TNOs) and to study already knownbinary systems with uncertain orbital periods. We also analyzed the systems mid-term photometry in order to determine whether therotation could be tidally locked to the satellites orbital period. We found that a photometric variability of 9.7 ± 0.3 days is clear inour data, and is nearly coincident with the orbital period of the satellite. We believe this variability might be induced by the satellitesrotation. In our photometry there is also a slight hint for an additional very small variability in the 10 h range that was already reportedin the literature. This short-term variability would indicate that the primary is not tidally locked and therefore the system would nothave reached a double synchronous state. Implications for the basic physical properties of the primary and its satellite are discussed.From angular momentum considerations we suspect that the Orcus satellite might have formed from a rotational fission. This requiresthat 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 albedoof 0.12 for the satellite and assuming equal densities for both the primary and secondary.