Comet 2P/Encke in apparition of 2017: II. Polarization and color (original) (raw)
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Spatial variations of brightness, colour and polarization of dust in comet 67P/Churyumov–Gerasimenko
Monthly Notices of the Royal Astronomical Society, 2017
We present post-perihelion photometric and polarimetric observations of comet 67P/Churyumov-Gerasimenko performed at the 6-m telescope of the SAO RAS in the gsdss (465/65 nm), r-sdss (620/60 nm) and R filters. Observations in November and December 2015 and April 2016 covered the range of heliocentric distance 1.62-2.72 au and phase angle 33.2 •-10.4 •. The comet was very active. Two persistent jets and long dust tail were observed during the whole observing period; one more jet was detected only in December. The radial profiles of surface brightness, colour and polarization significantly differed for the coma, jets and tail, and changed with increasing heliocentric distance. The dust production Afρ decreased from 162 cm at r = 1.62 au to 51 cm at r = 2.72 au. The dust colour (g-r) gradually changed from 0.8 mag in the innermost coma to about 0.4 mag in the outer coma. The spectral slope was 8.2 ± 1.7 per cent/100 nm in the 465 to 620 nm wavelength domain. In November and December, the polarization in the near-nucleus area was about 8 per cent, dropped sharply to 2 per cent at the distance above 5000 km and then gradually increased with distance from the nucleus, reaching ∼8 per cent at 40 000 km. In April, at a phase angle 10.4 • , the polarization varied between-0.6 per cent in the near-nucleus area and-4 per cent in the outer coma. Circular polarization was not detected in the comet. The spatial variations of brightness, colour and polarization in different structural features suggest some evolution of particle properties, most likely decreasing the size of dust particles.
Astronomy & Astrophysics, 2005
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 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.
Icarus, 2001
Efforts to apply a single-scattering polarized radiative transfer code to interpret photopolarimetric measurements of coma dust optical properties in Comet Hale-Bopp corroborate previous photometrically derived conclusions concerning the predominance of small sized particles in Hale-Bopp's coma. Calculations of the degree of linear polarization (DP) as a function of observation phase angle (α) produced by prolate spheroidal crystalline olivine particles with effective radii (a = 0.216 µm) are compatible with the comet's measured polarization in standard filters at λ = 0.4845 and 0.684 µm. Our rudimentary "trade-off" studies highlight the extreme sensitivity of DP to dust particle size and shape. A combination of viewing geometry effects in association with enhanced multiple scattering might provide a quantitative explanation of the negative polarization for 0 • ≤ α ≤ 20 • seen in Hale-Bopp and other comets.
Polarimetry, photometry, and spectroscopy of comet C/2009 P1 (Garradd)
Icarus, 2017
We present the results of photometry, linear spectropolarimetry, and imaging circular polarimetry of comet C/2009 P1 (Garradd) performed at the 6m telescope BTA of the Special Astrophysical Observatory (Russia) equipped by the multi-mode focal reducer SCORPIO-2. The comet was observed at two epochs post-perihelion: on February 2-14, 2012 at r h ∼1.6 au and α∼36° ; and on April 14-21, 2012 at r h ∼2.2 au and α∼27° The spatial maps of the relative intensity and circular polarization as well as the spectral distribution of linear polarization are presented. There were two features (dust and gas tails) oriented in the solar and antisolar directions on February 2 and 14 that allowed us to determine rotation period of the nucleus as 11.1 ± 0.8 hours. We detected emissions of C 2 , C 3 , CN, CH, NH 2 molecules as well as CO + and H 2 O + ions, along with a high level of the dust continuum. On February 2, the degree of linear polarization in the continuum, within the wavelength range of 0.67-0.68 μm, was about 5 ± 0.2% in the near-nucleus region up to ∼60 0 0 km and decreased to about 3 ± 0.2% at ∼40,0 0 0 km. After correction for the continuum contamination, the inherent degree of polarization in the emission band C 2 (ν= 0) is about 3.3%. We detected a small increase of linear polarization with the wavelength with the spectral gradient P/ λ=+ 4 ± 0.8%/μm and P/ λ=+ 6.2 ± 1.3%/μm, respectively, on February 2 and April 14. Linear polarization indicates that this dust-rich comet can be attributed to the high-P max comets. The left-handed (negative) circular polarization at the level approximately from −0.06 ± 0.02% to −0.4 ± 0.02%was observed at the distances up to 3 ×10 4 km from the nucleus on February 14 and April 21, respectively.
Imaging polarimetry of cometary dust: different comets and phase angles
Journal of Quantitative Spectroscopy and Radiative Transfer, 2003
Polarimetric observations of the light scattered by dust in cometary comae have been carried out at Pic-du-Midi Observatory and at Haute-Provence Observatory (France) between 1990 and 2001. The whole coma polarization can easily be inferred from integration of the uxes on the aperture. Maps are obtained for seven comets by CCD imaging polarimetry, which allow to observe three main regions in a cometary coma: the background coma, a circumnuclear halo and bright structures (jets or arcs) which correspond to di erent values of the polarization. The polarization maps of short period comets are compared to the maps obtained for comet Hale-Bopp, for phase angles smaller than 50 • (smaller than 15 • with negative polarization, near the inversion angle of approximately 20 • , between 30 • and 50 • on the linear part of the phase curve). At ∼ = 60 • the polarization map of comet C/2000 WM1 is presented. Between 109 • and 121 • the evolution of brightness images and polarization maps of comet C/1999 S4 is followed during the complete disruption of its nucleus. The polarimetric phase curve for the circumnuclear halo presents highly negative values for ¡ 20 •. On the opposite for the same phase angles range, the polarization in jets is positive for the four studied comets. The di erences between the regions are discussed in terms of physical properties of the particles (size distribution, packing density, mixtures of materials) by comparison to other observational techniques and laboratory measurements.
Astronomy & Astrophysics, 2018
Context.Spectropolarimetry is a powerful technique for investigating the physical properties of gas and solid materials in cometary comae without mutual contamination, but only a few spectropolarimetric studies have been conducted to extract each component.Aims.We attempt to derive the continuum (i.e., scattered light from dust coma) polarization degree of comet 2P/Encke, free of the influence of molecular emissions. The target is unique in that its orbit is dynamically decoupled from Jupiter, like the main-belt asteroids, but it ejects gas and dust like ordinary comets.Methods.We observed the comet using the Hiroshima Optical and Near-Infrared Camera attached to the Cassegrain focus of the 150 cm Kanata telescope on UT 2017 February 21 when the comet was at the solar phase angle ofα= 75°.7.Results.We find that the continuum polarization degree with respect to the scattering plane isPcont, r= 33.8 ± 2.7% at the effective wavelength of 0.82μm, which is significantly higher than those...
Photometry, spectroscopy, and polarimetry of distant comet C/2014 A4 (SONEAR)
Astronomy & Astrophysics, 2019
Context. The study of distant comets, which are active at large heliocentric distances, is important for better understanding of their physical properties and mechanisms of long-lasting activity. Aims. We analyze the dust environment of the distant comet C/2014 A4 (SONEAR), with a perihelion distance near 4.1 au, using comprehensive observations obtained by different methods. Methods. We present an analysis of spectroscopy, photometry, and polarimetry of comet C/2014 A4 (SONEAR), which were performed on November 5-7, 2015, when its heliocentric distance was 4.2 au and phase angle was 4.7 •. Long-slit spectra and pho-tometric and linear polarimetric images were obtained using the focal reducer SCORPIO-2 attached to the prime focus of the 6-m telescope BTA (SAO RAS, Russia). We simulated the behavior of color and polarization in the coma presenting the cometary dust as a set of polydisperse polyshapes rough spheroids. Results. No emissions were detected in the 3800-7200 Å wavelength range. The continuum showed a reddening effect with the normalized gradient of reflectivity 21.6±0.2% per 1000 Å within the 4650-6200 Å wavelength region. The fan-like structure in the sunward hemisphere was detected. The radial profiles of surface brightness differ for r-sdss and g-sdss filters, indicating predominance of submicron and micron-sized particles in cometary coma. The dust color (g-r) varies from 0.75±0.05 m to 0.45±0.06 m along the tail. For aperture radius near 20 000 km, the dust productions in various filters were estimated as A f ρ = 680±18 cm (r-sdss) and 887±16 cm (g-sdss). The polarization map showed spatial variations of polarization over the coma from about-3% near the nucleus to-8% at cometocentric distance about 150 000 km. Our simulations show that the dust particles were dominated (or covered) by ice and tholin-like organics. Spatial changes in the color and polarization can be explained by particle fragmentation.
Polarimetric observations of comet 67P/Churyumov-Gerasimenko during its 2008–2009 apparition
Astronomy and Astrophysics, 2010
Context. Remote observations of the light scattered by comet 67P/Churyumov-Gerasimenko dust coma are of major importance for determining the physical properties of the particles and prepare the rendezvous with the ESA/Rosetta spacecraft in 2014. Aims. Light scattering and especially linear polarization observations allow comparison between different coma regions and different comets, including comets that have been studied by space probes. Our aim is to retrieve physical properties of the dust particles and to characterize their evolution around perihelion passage. Methods. Recent imaging polarimetric observations were conducted at the Haute-Provence observatory (France) on 2009 March 17−19 at 35 • phase angle and at IUCAA Girawali observatory (India) on 2008 December 25−27 at 36 • phase angle and on 2009 April 30-May 1 at 29 • phase angle. With the imaging technique, the intensity and linear polarization variations are studied through the various coma regions. These observations are compared to other cometary data (e.g. Jupiter family comets) and to numerical and experimental simulations.
Comparative Study of the Dust Polarimetric Properties in Split and Normal Comets
Cometary Science after Hale-Bopp, 2002
Our polarimetric database contains six comets, C/1975 V1 (West), 16P/Brooks 2, C/1988 A1 (Liller), D/1996 Q1 (Tabur), C/1999 S4 (LINEAR), and C/2001 A2 (LINEAR), which can be related to the group of split comets. Comets West, S4 (LINEAR) and A2 (LINEAR) were observed during splitting. We compare the polarimetric measurements of the dust particles in these comets, sometimes together with available photometric and colorimetric data, with those in normal comets. We conclude that there is no significant evidence for differences of polarization between tidally split comets (e.g., Brooks 2), dissipating comets (e.g., Tabur), non-tidally split comets (e.g., West) and normal comets. The total disintegration of Comet S4 (LINEAR), however, did produce significant changes in the observed properties of dust.