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

Abstract

Abstract. We present results of imaging and aperture polarimetry of the dust of comet 2P/Encke at phase angles [91][92][93][94][95][96][97][98][99][100][101][102][103][104][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 gasrich 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 ≈8% 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 à 2 A 1 -X 2 B 1 electronic band system of NH 2 (662 nm) and on an adjacent red continuum at 642 nm were employed. The polarization of NH 2 averaged over the 0-7-0 vibronic transition amounts to ≈7% at phase angles close 90• , similar to the polarization of the two-atomic molecules CN and C 2 . 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.

Imaging polarimetry of comet 73P/Schwassmann–Wachmann 3 main fragments during its 2006 apparition

“…Jewitt (2004). For the same comet, a decreasing P was later observed by Jockers et al (2005) meaning that P in the inner region varies a lot, depending e.g. on the nucleus rotation or on seasonal effects.…”

Circular polarization in comets: Observations of Comet C/1999 S4 (LINEAR) and tentative interpretation

“…From linear polarization measurements it is well known that the presence of gas emission in the passband of the filter reduces the measured polarization. A dilution of the linear polarization in continuum due to the molecular emission contribution is of particular importance for so-called gas-rich comets (Jockers et al, 2005). Comet S4 belongs to the class of dusty comets (Farnham et al, 2001).…”

Linear and circular polarization of comet C/2009 P1 (Garradd)

“…Thus, a gaseous contamination causes depolarization of the observed continuum. The contamination is small for dusty comets, but can be significant for gassy comets (i.e., comets with a low dustto-gas ratio), such as comet Encke (Jockers et al, 2005). Moreover, the dust-to-gas ratio for comets is larger in the near-nucleus region of the coma.…”