CCD imaging and aperture polarimetry of comet 2P/Encke: are there two polarimetric classes of comets? (original) (raw)
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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.
Optical and Near-infrared Polarimetry of Non-periodic Comet C/2013 US10 (Catalina)
The Astronomical Journal, 2017
We present an optical and near-infrared (hereafter NIR) polarimetric study of a comet C/2013 US10 (Catalina) observed on UT 2015 December 17-18 at phase angles of α=52°.1-53°. 1. Additionally, we obtained an optical spectrum and multi-band images to examine the influence of gas emission. We find that the observed optical signals are significantly influenced by gas emission; that is, the gas-to-total intensity ratio varies from 5 to 30% in the R C and 3%-18% in the I C bands, depending on the position in the coma. We derive the "gas-free dust polarization degrees" of 13.8%±1.0% in the R C and 12.5%±1.1% in the I C bands and a gray polarimetric color, i.e., −8.7%±9.9% μm −1 in optical and 1.6%±0.9% μm −1 in NIR. The increments of polarization obtained from the gas correction show that the polarimetric properties of the dust in this low-polarization comet are not different from those in high-polarization comets. In this process, the cometocentric distance dependence of polarization has disappeared. We also find that the R C-band polarization degree of the southeast dust tail, which consists of large dust particles (100 μm-1 mm), is similar to that in the outer coma where small and large ones are mixed. Our study confirms that the dichotomy of cometary polarization does not result from the difference of dust properties, but from depolarizing gas contamination. This conclusion can provide a strong support for similarity in origin of comets.
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.
Experimental phase function and degree of linear polarization of cometary dust analogs
Monthly Notices of the Royal Astronomical Society
We present experimental phase function and degree of linear polarization curves for seven samples of cometary dust analogues namely: ground pieces of Allende, DaG521, FRO95002 and FRO99040 meteorites, Mg-rich olivine and pyroxene, and a sample of organic tholins. The experimental curves have been obtained at the IAA Cosmic Dust Laboratory at a wavelength of 520 nm covering a phase angle range from 3 • to 175 •. We also provide values of the backscattering enhancement (BCE) for our cometary analogue samples. The final goal of this work is to compare our experimental curves with observational data of comets and asteroids to better constrain the nature of cometary and asteroidal dust grains. All measured phase functions present the typical behavior for µm-sized cosmic dust grains. Direct comparison with data provided by the OSIRIS/Rosetta camera for comet 67P Churyumov-Gerasimenko reveals significant differences and supports the idea of a coma dominated by big chunks, larger than one micrometer. The polarization curves are qualitatively similar to ground-based observations of comets and asteroids. The position of the inversion polarization angle seems to be dependent on the composition of the grains. We find opposite dependence of the maximum of the polarization curve for grains sizes in the Rayleigh-resonance and geometric optics domains, respectively.
Experimental phase function and degree of linear polarization of cometary dust analogues
Monthly Notices of the Royal Astronomical Society, 2019
We present experimental phase function and degree of linear polarization curves for seven samples of cometary dust analogues namely: ground pieces of Allende, DaG521, FRO95002, and FRO99040 meteorites, Mg-rich olivine and pyroxene, and a sample of organic tholins. The experimental curves have been obtained at the IAA Cosmic Dust Laboratory at a wavelength of 520 nm covering a phase angle range from 3 • to 175 •. We also provide values of the backscattering enhancement for our cometary analogue samples. The final goal of this work is to compare our experimental curves with observational data of comets and asteroids to better constrain the nature of cometary and asteroidal dust grains. All measured phase functions present the typical behaviour for μm-sized cosmic dust grains. Direct comparison with data provided by the OSIRIS/Rosetta camera for comet 67P/Churyumov-Gerasimenko reveals significant differences and supports the idea of a coma dominated by big chunks, larger than one micrometer. The polarization curves are qualitatively similar to ground-based observations of comets and asteroids. The position of the inversion polarization angle seems to be dependent on the composition of the grains. We find opposite dependence of the maximum of the polarization curve for grains sizes in the Rayleigh-resonance and geometric optics domains, respectively.
Comet 2P/Encke in apparition of 2017: II. Polarization and color
Icarus, 2020
We present results of imaging polarimetry of comet 2P/Encke performed on January 23, 2017 at the heliocentric (1.052 au) and geocentric (1.336 au) distances and phase angle 46.8 � , 46 days before perihelion. Observations were made through the medium-band SED500 (λ5019/246 Å) and broadband r-sdss (λ6200/1200 Å) filters with the multimode focal reducer SCORPIO-2 at the 6-m BTA telescope of the Special Astrophysical Observatory (Russia). Dust in comet 2P/Encke was mainly concentrated in the near-nucleus region of the coma: the maximum dust/gas to leave F em /F cont ratios were 1.5 and 2.9 in the SED500 and the r-sdss filters near the nucleus but dropped sharply to ~0.2 and ~1 at the distance ~2.500 km, respectively. Then these ratios began to increase at distances ~12,000 km from the nucleus, the ratio was ~0.3 (SED500) and ~ 1.3 (r-sdds). There were significant variations of polarization over the coma, which correlated with the variations in the dust color and dust/gas ratio. The maximum degree of polarization, ~8% in the r-sdss filter, was observed in the dust shell which was shifted by ~1.000 km towards the Sun. Polarization sharply dropped to ~4% at the distance ~3.000 km and then gradually increased with wave-like fluctuations with the distance from the nucleus, reaching ~8% at the distance ~12,000 km. A similar change in polarization was observed in the SED500 filter. After correction for gas contamination, using the dust/gas ratios from spectroscopy made on the same night, the values of polarization appeared to be ~4% in the near-nucleus region (~1.000 km), and reached 11-12% at the distance ~12,000 km in both filters. We also found an effect of nucleus polarization on the polarization of the dust coma in comet Encke in the r-sdss filter. The maximum value of the nucleus contamination was ~0.7%. Changes in polarization and color across the 2P/ Encke coma indicate changes in physical properties of the dust particles with the distance from the nucleus. Our Sh-matrix computer simulations of light scattering by Gaussian particles allow us to suggest that the observed trends in color and polarization are mainly result from changing particle size.
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.
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.
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.