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

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 (β = 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±0.06km (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%/100nm. 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/Enck