CCD imaging and aperture polarimetry of comet 2P/Encke: are there two polarimetric classes of comets?

¹ Max–Planck–Institut für Sonnensystemforschung, Max-Planck-Strasse 2, 37191 Katlenburg-Lindau, Germany e-mail: jockers@mps.mpg.de
² Institute of Astronomy, Karazin Kharkiv National University, Sumskaya Str. 35, 61022 Kharkiv, Ukraine e-mail: kiselev@astron.kharkov.ua
³ Institute of Astronomy, 1784 Sofia 72, Tsarigradsko Shose Blvd., Bulgaria e-mail: tbonev@astro.bas.bg
⁴ Main Astronomical Observatory, NAS of Ukraine, Zabolotnoho Str. 27, 03680 Kyiv, Ukraine e-mail: rosevera@mao.kiev.de
⁵ Crimean Astrophysical Observatory, 98409 Nauchny, Crimea, Ukraine
⁶ Astronomical Observatory of Odessa National University, T.G. Shevchenko Park, 65014 Odessa, Ukraine

Received: 2 May 2005
Accepted: 9 June 2005

Abstract

We present results of imaging and aperture polarimetry of the dust of comet 2P/Encke at phase angles 91–105°, obtained during the 2003 apparition. We investigate how strongly molecular emissions transmitted by the filters used in the observations can affect the resulting polarization of cometary dust. This problem is of particular importance for so-called gas-rich comets like comet 2P/Encke which has particularly strong molecular emission as compared to its dust continuum. Aperture polarimetry in the wide-band UBVR filters was performed at the 2.6-m Shain telescope and 1.25-m telescope of the Crimean Astrophysical Observatory on November 17–24. From these measurements a dust polarization of ≈% is derived, which puts the comet in the class of comets with low polarization. The imaging observations of comet 2P/Encke were carried out at the 2-m telescope of the Bulgarian National Astronomical Observatory on November 20–22, 2003. Narrow-band filters centered on the 0–7–0 transition of the Ã^2A_1 – ^2B_1 electronic band system of NH₂ (662 nm) and on an adjacent red continuum at 642 nm were employed. The polarization of NH₂ averaged over the 0–7–0 vibronic transition amounts to ≈% at phase angles close 90°, similar to the polarization of the two-atomic molecules CN and C₂. The dust polarization however, when corrected for the effect of molecular emissions, is larger than 30%. We conclude that the division of comets into two polarimetric classes with one class having in the visual wavelength range a maximum polarization less than 20% is caused by ignoring the contribution of molecular emission and therefore is an artifact. Whether the comet displays a strong silicate feature (i.e. its dust grains are small) or not, the dust polarization is high.