Astrometry and photometry of Orcus from a 33-day campaign

ORTIZ, J. L.; CIKOTA, A.; HESTROFFER, D.; CIKOTA, S.; THIROUIN, A.; MOLINA, A.; MORALES, N.; DUFFARD, R.; GIL-HUTTON, R.; SANTOS-SANZ, P.

Roma

Congreso; European Planetary Science Congress; 2010

Europlanet

The binary trans-Neptunian Object (90482) Orcus(formerly known as 2004DW) was observed while ittraversed a large star field. The CCD images of thelarge and fixed field allowed us to carry out high precisionrelative photometry and relative astrometry ofthis large trans-Neptunian object with respect to numerousbackground stars, and for a longer time spanthan is usual in regular observing runs. We present apreliminary study of the astrometry residuals that wereobtained when the astrometric positions were comparedto the predicted orbital positions of Orcus’ systemaround the Sun. The presence of Orcus’ satelliteis revealed in the residuals but not in the direct imagesbecause they lack the needed spatial resolution.From those results we discuss the prospects for usingthe astrometry technique to find new binary TNOs andto study already known binaries with uncertain orbitalperiods. From the analysis of the time series photometry,a periodicity of 9.7 ± 0.3 day is found in the data,with a peak to peak brightness variation of only 0.07± 0.02 mag, although apparent aliases of very similarspectral power are present in the periodogram, makingthe correct identification of the true period difficult.Besides, other periodicities might exist as well.Therefore we cannot conclusively claim that the systemis in double synchronous state like Pluto. We discussthe implications of this regarding basic physicalproperties of Orcus and its satellite.