Near-Infrared Polarimetry and Photometry of Recent Comets

Abstract

We present near-infrared (2.2 m) imaging polarimetry and near-infrared photometry for recent comets. The 2.2 m polarization as a function of phase angle is 1%-2% higher than typical comet visual narrowband red polarization. This implies a general, but weak, trend to higher polarization at wavelengths longer than 0.7 m. As found with visual narrowband red polarimetry, comets appear to divide into a high-and a low-polarization class at 2.2 m as well. The high-polarization class is characterized by moderate to strong mid-infrared silicate emission, while the low-polarization class is characterized by weak to absent silicate emission. In contrast to C/1995 O1 (HaleBopp) and some other dusty comets, no significant grain population radial gradient is seen in any of the polarization maps for the six comets in our sample with near-infrared imaging polarimetry. Our analysis of previously published Hale-Bopp data shows that the polarization at 40 phase angle varies by only about a factor of 1.6 from 0.3 to 2.2 m and is nearly flat from 0.7 to 2.2 m. Any model of comet dust must reproduce this weak wavelength dependence.

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

“…However, at larger phase angles, where phase curves are possibly less constrained, the polarization values seem to be below the high-P max phase curve. In near infrared domain (H and I bands), at about 36°, the data points by Jones et al, (2008) are above the high-P max phase curve in red wavelength as usually observed for comets (Hadamcik and Levasseur-Regourd, 2003b;Kelley et al, 2004) and called 'positive polarimetric spectral gradient', but at larger phase angles than 40° (80°-90°, Bonev et al, 2008;Jones et al, 2008;Kiselev et al, 2008), they are below the synthetic red phase curve (negative polarimetric spectral gradient). The more neutral or slightly negative spectral gradient in the near infrared domains as observed for the two SW3 fragments at large phase angles, was previously observed for different comets e.g.…”

Modeling of the light scattering properties of cometary dust using fractal aggregates

“…It is therefore possible that the trichotomy in polarization is an artefact and that the cometary dust in the coma is characterized in all cases by high values of P max . Kelley et al (2004) have shown that the polarization of comets in the near IR (up to the K band) shows a weak trend to higher values at higher wavelengths, resembling the color effect observed at visible wavelengths. Comets appear to divide into a high and low polarization class as well.…”

Nonazimuthal linear polarization in protoplanetary disks

“…On the other hand, DHS predicts radial polarization for scattering angles < ∼ 70 • (forward scattering) and 140 • −160 • (backward scattering), noting that different values of f max can remove the negative branch for forward scattering angles (Min et al 2005). Radial polarization at large scattering angles (backward scattering) has been observed at optical and NIR wavelengths in a number of comets (Kelley et al 2004;Woodward et al 2011), and dust grains in protoplanetary disks can have similar compositions to those found in comets (Bouwman et al 2003). Experimental measurements in the optical show that ensembles of arbitrarily shaped dust aggregates also produce radial polarization patterns at large scattering angles (Muñoz et al 2000(Muñoz et al , 2006Volten et al 2007).…”

“…Any nonazimuthal polarization is associated with noise and/or calibration errors. While this is valid when single scattering dominates in nearly face-on disks, the assumption may break down for higher disk inclinations and for peculiar dust properties (as for those of comets, e.g., Kelley et al 2004), when grains are aligned (Matsumura & Seki 1996;Wolf et al 2002), and/or when disks are optically thick and multiple scattering occurs (Bastien & Menard 1988). In this letter, we explore some of these deviations and quantify their impact on the use of the radial-Stokes formalism at near-infrared (NIR) wavelengths.…”