Polarimetry of Centaurs (2060) Chiron, (5145) Pholus and (10199) Chariklo

Abstract

Results of the first polarimetric observations of Centaurs (5145) Pholus and (10199) Chariklo, and new observations of (2060) Chiron are presented together with the estimates of their absolute magnitudes. Observations were carried out at the 8 m ESO Very Large Telescope in 2007–2008. They revealed noticeable negative polarization in the phase-angle range 0.5–4.4° with a minimum varying from −1% to −2.1% in the R band. All three objects show diverse polarization phase-angle behaviour, each distinctly different from that of transneptunian objects. We found evidence of surface heterogeneity for Chariklo while Chiron and Pholus appear to have rather homogeneous surfaces. Polarization phase behaviours of Chiron and Pholus are significantly different from any other Solar System bodies studied so far. A shift of negative polarization minima toward small phase angles seems to be a characteristic feature of polarization properties of Centaurs. Presence of a small amount of water frost on a dark surface is considered as one of the possible ways to explain these properties.

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.