Polarimetry of Centaurs (2060) Chiron, (5145) Pholus and (10199) Chariklo☆
Introduction
Polarimetry is a powerful tool for investigating the physical properties of Solar System bodies. The intricate inverse problem to constrain surface properties from phase-angle-resolved polarimetry and photometry is almost the only way to assess the microscopic properties of the surface from remote observations. The study of polarimetric properties of transneptunian objects (TNOs) and Centaurs is in its early stages. The first polarimetric observations for a transneptunian object (except Pluto) were carried out in 2002 with the FORS1 instrument at the 8 m Very Large Telescope (VLT) by Boehnhardt et al. (2004). These observations have demonstrated both the capability of the instrument to provide good-quality observations of faint objects (∼20 mag), and the capability of the polarimetric technique to study distant objects even if they are observable only at very small phase angles. Successive polarimetric observations were carried out for Centaur (2060) Chiron and a classical TNO (50000) Quaoar by Bagnulo et al. (2006) and for a scattered disk object 29981 (1999 TD10) by Rousselot et al. (2005). The obtained data have shown a noticeable diversity in the behaviour of their polarization-phase dependences.
New polarimetric observations were made within the context of the ESO-VLT Large Program 178.C-0036 in 2006–2008 with the aim to probe surface properties of objects from different dynamical groups and to make conclusions on their similarities and differences. The obtained results on two dwarf planets and three TNOs were published by Belskaya et al., 2008a, Bagnulo et al., 2008. The analysis of these data together with the data on four other TNOs obtained in the previous works led to the conclusion on two different types of polarization phase behaviours for TNOs (Bagnulo et al., 2008). It was found that objects with diameters D > 1000 km show modest negative polarization slowly changing in the phase-angle range observed. Smaller objects display mutually similar phase dependencies with rapid changes in linear polarization reaching −1% at phase angle of 1°. The different types of polarimetric behaviour have been suggested to be related to different albedos and different capability of retaining volatiles for large and small TNOs (Bagnulo et al., 2008).
In this paper, we present and analyze polarimetric observations of three Centaurs and will show their diverse polarization phase-angle behaviour different from that of TNOs.
Section snippets
Observations and results
Polarimetric observations of selected Centaurs were made during an observing period from March 2007 to October 2008 within the ESO-VLT Large Program. Measurements of the linear polarization were carried out with the FORS1 instrument in the Bessell R filter. Taking advantage of the flexibility offered by the VLT service observing mode, we distributed the observations along the observing period to cover the maximum possible range of phase angles and to obtain a good phase angle sampling.
Discussion
Our observations of selected Centaurs have clearly demonstrated a great diversity in their polarization properties. To understand what properties may be responsible for such diversity we have summarized the orbital and physical parameters of these objects in Table 3. It contains values of different parameters compiled from the literature with corresponding references and values of polarization parameters found in the present work.
All three objects have been thoroughly studied by different
Conclusions
Polarimetric observations of Chiron, Chariklo and Pholus and their analysis allow us to make the first conclusions on polarization properties of Centaurs which can be summarized as follows:
- 1.
Observed Centaurs revealed noticeable negative polarization of their surfaces with a minimum varying from −1% to −2.1%. All three objects show diverse polarization phase-angle behaviour, each distinctly different from that of transneptunian objects. This implies noticeable differences in physical properties
Acknowledgments
The research of INB has been supported by a Marie Curie Fellowship of the European Community. We thank Francesca DeMeo for her assistance in improving the manuscript.
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Based on observations made with ESO Very Large Telescope at the La Silla-Paranal Observatory under programme ID 178.C-0036 (PI: A. Barucci).