Issue A&A Volume 491, Number 2, November IV 2008 Page(s) L33 - L36 Section Letters DOI http://dx.doi.org/10.1051/0004-6361:200810938 Published online 27 October 2008

A&A 491, L33-L36 (2008)
DOI: 10.1051/0004-6361:200810938

Letter

Discovery of two distinct polarimetric behaviours of trans-Neptunian objects

S. Bagnulo¹, I. Belskaya², K. Muinonen³, G. P. Tozzi⁴, M. A. Barucci⁵, L. Kolokolova⁶, and S. Fornasier<sup>5

1</sup>  Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland, UK
    e-mail: sba@arm.ac.uk
²  Astronomical Observatory of Kharkiv National University, 35 Sumska str., 61022 Kharkiv, Ukraine
    e-mail: irina@astron.kharkov.ua
³  Observatory, PO Box 14, 00014 University of Helsinki, Finland
    e-mail: muinonen@cc.helsinki.fi
⁴  INAF - Oss. Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
    e-mail: tozzi@arcetri.astro.it
⁵  LESIA, Observatoire de Paris, 5 pl. Jules Janssen, 92195 Meudon Cedex, France
    e-mail: [antonella.barucci;sonia.fornasier]@obspm.fr
⁶  University of Maryland, College Park, MD, USA
    e-mail: ludmilla@astro.umd.edu

Received 8 September 2008 / Accepted 10 October 2008

Abstract
Context. Trans-Neptunian objects (TNOs) contain the most primitive and thermally least-processed materials from the early accretional phase of the solar system. They allow us to study interrelations between various classes of small bodies, their origin and evolution.
Aims. We exploit the use of polarimetric techniques as a remote-sensing tool to characterize the surface of TNOs.
Methods. Using FORS1 of the ESO VLT, we have obtained linear-polarization measurements in the Bessell R filter for five TNOs at different values of their phase angle (i.e., the angle between the Sun, the object, and the Earth). Due to the large distance of the targets (30 AU), the observed range of phase angles is limited to about –.
Results. We have analyzed our new observations of five TNOs, and those of another four TNOs obtained in previous works, and discovered that there exist two classes of objects that exhibit different polarimetric behaviour. Objects with a diameter >1000 km, such as, e.g., Pluto and Eris, show a small polarization in the scattering plane (~0.5%) which slowly changes in the observed phase angle range. In smaller objects such as, e.g., Ixion and Varuna, linear polarization changes rapidly with the phase angle, and reaches ~1% (in the scattering plane) at phase angle 1°. The larger objects have a higher albedo than the smaller ones, and have the capability of retaining volatiles such as CO, N₂ and CH₄. Both of these facts can be linked to their different polarimetric behaviour compared to smaller objects.
Conclusions. In spite of the very limited range of observable phase angles, ground-based polarimetric observations are a powerful tool to identify different properties of the surfaces of TNOs. We suggest that a single polarimetric observation at phase angle ~1 allows one to determine whether the target albedo is low or high.

Key words: Kuiper belt -- polarization -- scattering

© ESO 2008

What is OpenURL?