A mid-term astrometric and photometric study of Trans-Neptunian Object (90482) Orcus

Abstract

From CCD observations of a fixed and large star field that contained the binary TNO Orcus, we have been able to derive high-precision relative astrometry and photometry of the Orcus system with respect to background stars. The RA residuals of an orbital fit to the astrometric data revealed a periodicity of 9.7+-0.3 days, which is what one would expect to be induced by the known Orcus companion. The residuals are also correlated with the theoretical positions of the satellite with regard to the primary. We therefore have revealed the presence of Orcus' satellite in our astrometric measurements. The photocenter motion is much larger than the motion of Orcus around the barycenter, and we show here that detecting some binaries through a carefully devised astrometric technique might be feasible with telescopes of moderate size. We also analyzed the system's mid-term photometry 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, and is nearly coincident with the orbital period of the satellite. We believe this variability might be induced by the satellite's rotation. There is also a slight hint for an additional small variability in the 10 hr range that was already reported in the literature. This short-term variability would indicate that the primary is not tidally locked and therefore the system would not have 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 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 objects.

arXiv commentin Press at A&A

References in corpus (8)

• The second US Naval Observatory CCD Astrograph Catalog (UCAC2)
• The Diverse Solar Phase Curves of Distant Icy Bodies. Part I: Photometric Observations of 18 Trans-Neptunian Objects, 7 Centaurs, and Nereid
• Short-term variability of a sample of 29 trans-Neptunian objects and Centaurs
• Transneptunian objects and Centaurs from light curves
• Light Curves of Dwarf Plutonian Planets and other Large Kuiper Belt Objects: Their Rotations, Phase Functions and Absolute Magnitudes
• The size, density, and formation of the Orcus-Vanth system in the Kuiper belt
• Near Infrared Surface Properties of the Two Intrinsically Brightest Minor Planets (90377) Sedna and (90482) Orcus
• Lightcurves of 20--100 kilometer Kuiper Belt Objects using the Hubble Space Telescope

Cited by in corpus (9)

• The size, shape, density and ring of the dwarf planet Haumea from a stellar occultation
• Rotational fission of Trans-Neptunian Objects. The case of Haumea
• Rotational properties of the binary and non-binary populations in the Trans-Neptunian belt
• Integral-field spectroscopy of (90482) Orcus-Vanth
• Small Bodies Near and Far (SBNAF): a benchmark study on physical and thermal properties of small bodies in the Solar System
• The large Trans-Neptunian Object 2002 TC${}_{302}$ from combined stellar occultation, photometry and astrometry data
• Photometry of Centaurs and trans-Neptunian objects: 2060 Chiron (1977 UB), 10199 Chariklo (1997 CU26), 38628 Huya (2000 EB173), 28978 Ixion (2001 KX76), and 90482 Orcus (2004 DW)
• The trans-Neptunian object (84922) 2003 VS2 through stellar occultations
• 2008 OG${}_{19}$: A highly elongated Trans-Neptunian Object