A&A 489, 1337-1343 (2008)
DOI: 10.1051/0004-6361:200809922

Photometry and polarimetry of the nucleus of comet 2P/Encke

H. Boehnhardt¹, G. P. Tozzi², S. Bagnulo³, K. Muinonen⁴, A. Nathues¹, and L. Kolokolova<sup>5

1</sup>  Max-Planck-Institute for Solar System Research, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
    e-mail: boehnhardt@mps.mpg.de
²  INAF Arcetri Observatory, Florence, Italy
³  Armagh Observatory, Armagh, Northern Ireland, UK
⁴  University Observatory, Helsinki, Finland
⁵  University of Maryland, College Park, Maryland, USA

Received 7 April 2008 / Accepted 27 June 2008

Abstract
Context. Comet 2P/Encke, a short-period comet, is a favorable target for studies of light scattering by bare cometary nuclei. These studies enable assessment of the nucleus size, the material albedo, and the light-scattering properties of the cometary surface. Observations of 2P/Encke were completed between October to December 2006 when the comet approached the Sun between 2.7 and 2.1 AU.
Aims. We characterize the activity of the cometary coma and the physical properties of the nucleus of 2P/Encke such as its size, albedo, colors, spectral slope, surface roughness, porosity, and single-particle properties.
Methods. Broadband imaging photometry and broadband and narrowband linear polarimetry is measured for the nucleus of 2P/Encke over the phase-angle range 4-28 deg.
Results. An analysis of the point spread function of the comet in polarimetric images reveal only weak coma activity in 2P/Encke, corresponding to dust production of the order of 0.05 kg/s. Over the measured phase-angle range the nucleus displays a color independent photometric phase function of almost linear slope ( = 0.050  0.004 mag/deg). The absolute R filter magnitude at zero phase angle is 15.05  0.05 mag and corresponds to an equivalent radius for the nucleus of 2.43  0.06 km (adopted albedo of 0.047). The nucleus color V-R was measured to be 0.47  0.07, suggesting a spectral slope S' of 11  8%/100 nm. The phase function of linear polarimetry in the V and R filter shows a widely color independent linear increase with phase angle (0.12  0.02%/deg). The test of the empirical albedo-polarization relationship for asteroids reveals unreasonably high albedo values for the nucleus. We find discrepancies in the photometric and polarimetric parameters between 2P/Encke and other minor bodies in the solar system, which may indicate significant differences in the surface material properties and light-scattering behavior of the bodies.
Conclusions. The linear polarimetric phase function of 2P/Encke presented here is the first ever measured for a cometary nucleus. It provides encouragement to future studies of cometary nuclei in order to characterize the light-scattering behavior of comets on firm empirical grounds and provide suitable input to a comprehensive modeling of the light scattering by cometary surfaces.

Key words: comets: general -- comets: individual: 2P/Encke -- minor planets, asteroids -- Kuiper Belt

© ESO 2008

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