Optical polarimetry of Comet NEAT C/2001 Q4

📝 Abstract

Comet NEAT C/2001 Q4 was observed for linear polarization using the optical polarimeter mounted at the 1.2m telescope at Mt. Abu Observatory, during the months of May and June 2004. Observations were conducted through the International Halley Watch narrow band (continuum) and B,V,R broad band filters. During the observing run the phase angle ranged from 85.6 deg in May to 55 deg in June. As expected, polarization increases with wavelength in this phase angle range. Polarization colour in the narrow bands changes at different epochs, perhaps related to cometary activity or molecular emission contamination. The polarization was also measured in the cometary coma at different locations along a line, in the direction of the tail. As expected, we notice minor decrease in the polarization as photocenter (nucleus) is traversed while brightness decreases sharply away from it. Based on these polarization observations we infer that the comet NEAT C/2001 Q4 has high polarization and a typical grain composition- mixture of silicates and organics.

💡 Analysis

Comet NEAT C/2001 Q4 was observed for linear polarization using the optical polarimeter mounted at the 1.2m telescope at Mt. Abu Observatory, during the months of May and June 2004. Observations were conducted through the International Halley Watch narrow band (continuum) and B,V,R broad band filters. During the observing run the phase angle ranged from 85.6 deg in May to 55 deg in June. As expected, polarization increases with wavelength in this phase angle range. Polarization colour in the narrow bands changes at different epochs, perhaps related to cometary activity or molecular emission contamination. The polarization was also measured in the cometary coma at different locations along a line, in the direction of the tail. As expected, we notice minor decrease in the polarization as photocenter (nucleus) is traversed while brightness decreases sharply away from it. Based on these polarization observations we infer that the comet NEAT C/2001 Q4 has high polarization and a typical grain composition- mixture of silicates and organics.

📄 Content

arXiv:0901.2789v1 [astro-ph.EP] 19 Jan 2009 Optical polarimetry of Comet NEAT C/2001 Q4 S. Ganesh, U. C. Joshi, K. S. Baliyan ∗ Astronomy & Astrophysics Division, Physical Research Laboratory, Navrangpura, Ahmedabad 380009, India Abstract Comet NEAT C/2001 Q4 was observed for linear polarization using the optical polarimeter mounted at the 1.2m telescope at Mt. Abu Observatory, during the months of May and June 2004. Observations were conducted through the Inter- national Halley Watch narrow band (continuum) and B, V, R broad band filters. During the observing run the phase angle ranged from 85.6◦in May to 55◦in June. As expected, polarization increases with wavelength in this phase angle range. Po- larization colour in the narrow bands changes at different epochs, perhaps related to cometary activity or molecular emission contamination. The polarization was also measured in the cometary coma at different locations along a line, in the direction of the tail. As expected, we notice minor decrease in the polarization as photocen- ter (nucleus) is traversed while brightness decreases sharply away from it. Based on these polarization observations we infer that the comet NEAT C/2001 Q4 has high polarization and a typical grain composition- mixture of silicates and organics. Key words: Comets, polarimetry, dust scattering, comets - individual, Comet NEAT C/2001 Q4 ∗Corresponding author Email address: baliyan@prl.res.in (K. S. Baliyan). Preprint submitted to Elsevier 29 October 2018 1 Introduction Comets are considered to be primordial, icy bodies which are expected to re- tain imprints of physical conditions prevailing in the proto-solar system. There- fore, study of comets has proved very useful in our understanding of the origin of the solar system. Sun light is scattered by the cometary grains and in this process it gets polarized. The degree of polarization (P %) mainly depends on: the size distribution of the grains and their refractive index (n−ιk); wavelength of the light and the phase angle (sun-comet-earth angle, α). Linear polarization measurements have been made on many comets to help understand their grain properties. The first major efforts for detailed polarization observations were made for Comet P/Halley by many groups (Bastien et al., 1986; Brooke et al., 1987; Dollfus and Suchail, 1987; Kikuchi et al., 1987; Lamy et al., 1987; Le Borgne et al., 1987; Metz and Haefner, 1987; Sen et al., 1988). There were other bright comets after P/Halley such as Comet C/1989 X1 (Austin), C/1996 B2 (Hyakutake), and C/1995 O1 (Hale-Bopp) for which polarization observations were reported (e.g., Sen et al. (1991); Joshi et al. (1997); Ganesh et al. (1998); Manset and Bastien (2000); Hadamcik and Levasseur-Regourd (2003)). A database of comet po- larimetry has been created by Kiselev et al. (2005) which also includes some unpublished data. The study of the dust grains in comets has been an active area of investi- gation for quite some time but the nature and composition of the cometary grains are still not well understood. A study of the variation of polarization with phase angle (α) and the wavelength dependence of polarization helps to characterize the dust grains. Based on the data obtained by various re- searchers, Dollfus et al. (1988) established a typical polarization-phase curve describing the variation of the degree of linear polarization with the phase angle : small negative values of polarization for phase angle α < 22◦, increas- ing nearly linearly in the range 30 < α < 70◦ and reaching a maximum value of 15-35 % in the phase angle range 90-110◦. Dollfus (1989) pointed out the possibility of the grains giving rise to the polarization being large, rough and dark, resembling fluffy aggregates such as IDPs. In-situ measure- ments (Kissel et al., 1986; Mazets et al., 1986; Levasseur-Regourd et al., 1986, 1999) by space missions to Comet Halley and corresponding numerical simula- tions by Fulle et al. (2000) contributed important information on the nature of grains in that comet. Return of the dust particles by the Stardust Space Mis- sion (Brownlee et al., 2006), has been a wonderful achievement in the study of cometary material. The preliminary studies suggest that most of the particles larger than a micron are anhydrous (dry) silicates or sulfides. They include forsterite (Mg2SiO4), an olivine end-member that is one of the first conden- sates from the solar nebula. Apart from their amorphous nature, some such silicate minerals are crystalline solids as also inferred from IR studies in comet 2 Hale-Bopp (Wooden et al., 1999) and Deep Impact mission comet 9P/Tempel 1 (Lisse et al., 2006). The range of iron/magnesium ratios exhibited by the particles indicates that comet Wild 2 is unequilibrated. The comet contains an abundance of silicate grains that are much larger than the predictions of interstellar grain models, and many of these are high-temperature minerals that appear to have formed in the inner regions of th

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