CCD imaging polarimetry of Comet 2P/Encke

We present results and analysis of imaging polarimetric observations of Comet 2P/Encke. The observations were carried out at the 2-m RCC telescope of the Bulgarian National Astronomical Observatory on December 13, 1993 and on January 14, 1994, at phase angles 51.1° and 80.5°, respectively. A wide-band red filter 6940/790 Å was used. This filter is transparent for the continuum and the weak emission bands of NH₂ and H₂O⁺. There is a sunward dust fan with well-defined polarization, which peaks at≈13% in the image obtained on January 14, 1994. Along the sunward fan the degree of polarization decreases progressively. Outside of the fan the coma displays a low polarization of ≈3%. We suggest that this low polarization is caused by the NH₂ emission in the pass-band of the red wide-band filter. Assuming a spherically symmetric NH₂ coma we are able to correct the observed polarization for this effect. The correction leads to an increase of the observed polarization by 1 to 4% at distances 10,000 and 1500 km from the nucleus. A rough estimate shows that the polarization in the near nucleus region of Comet Encke is similar to that for the dusty comets. Even after correction the polarization of Comet Encke's dust fan is significantly less that the polarization observed in dusty comets. The reasons influencing the distribution of dust polarization in the coma are discussed. More polarimetric and colorimetric observations of the dust in Comet Encke on its return in 2003 are needed.     

Near-Infrared Photometric And Polarimetric Observations Of Comet Hale-Bopp

Near-infrared photometric and polarimetric observations of comet Hale-Bopp (1995 O1) using KONIC (Kiso Observatory Near-Infrared Camera) are reported. Observations were carried out on March 18 UT and April 26 UT 1997, when the heliocentric distances of the comet were 0.94 and 1.02 AU, and the phase angles were 48.5 deg and 32.9 deg, respectively. In the J, H, and K′ bands, we obtained linear polarization of the near-nucleus region of 16.4 ± 1.2, 18.8 ± 1.3, and 15.1 ± 0.9 percent on March 18UT and 7.1 ± 1.1, 8.9 ± 1.0, and 6.9 ± 0.6 percent on April 26, respectively. These values were higher than those observed for 1P/Halley. The maximum polarization was found at H band on both dates. Polarization maps showed higher polarization regions toward the anti-solar direction in the J and H bands. No distinct correlation was found between high polarization regions and bright regions. The projected expansion velocity of the arc structure of the dust jet was 375 ± 35.7 m/s on 17–19 March. The periodicity was found to be 11.1 ± 2.8 hours. This revised version was published online in July 2006 with corrections to the Cover Date.     

X-Ray Emission From Comet Hale–Bopp

The discovery of X-ray emission from comets has created a number of questions about the physical mechanism producing the radiation. There are now a variety of explanations for the emission, from thermal bremsstrahlung of electrons off neutrals or dust, to charge exchange induced emission from solar wind ions, to scattering of solar X-rays from attogram dust, to reconnection of solar magnetic field lines. In an effort to understand this new phenomenon, we observed but failed to detect in the X-ray the very dusty and active comet C/Hale-Bopp 1995 O1 over a two year period, September 1996 to December 1997, using the ROSAT HRI imaging photometer at 0.1–2.0 keV and the ASCA SIS imaging spectrometer at 0.5–10.0 keV. The results of our Hale-Bopp non-detections, when combined with spectroscopic imaging 0.08–1.0 keV observations of the comet by EUVE and BeppoSAX, show that the emission has the same spectral shape and strong variability seen in other comets. Comparison of the ROSAT photometry of the comet to our ROSAT database of 8 comets strongly suggests that the overall X-ray faintness of the comet was due to an emission mechanism coupled to gas, and not dust, in the comet’s coma. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Infrared Spectral Energy Distribution And Polarization Of Comet C/1995 O1 (Hale–Bopp) During 1997

Comets, such as C/1995 O1 (Hale-Bopp), are important to studies of the origins of the solar system because they are believed to be frozen reservoirs of the most primitive pre-solar dust grains and ices. Here, we report 1.2–18.5 μm infrared (IR) spectrophotometric and polarimetric observations of comet Hale-Bopp. Our measurements of the spectral energy distribution (SED) and IR polarization near perhelion passage suggest that emission from the coma was dominated by scattering and thermal emission from sub-micron sized dust grains. Hale-Bopp's surprising brightness may have been largely a result of the properties of its coma grains rather than the size of its nucleus. The thermal emission continuum from the grains had a superheat of S = T_color/T_BB ≥ 1.7, the peak of the 10 μm silicate emission feature was 1.7 mags above the carbon grain continuum, and the albedo (reflectivity) of the grains was ≥ 0.4 at a scattering angles, θ ≥ 135° This revised version was published online in July 2006 with corrections to the Cover Date.     

Opposition Effects in Brightness,Color, and Polarization of Comet 1P/Halley: Comparison with Atmosphereless Solar System Bodies

Systematic and uniform sets of photometric and polarimetric observations of comet 1P/Halley have been analyzed. The phase dependence of brightness for comet Halley was obtained at phase angles α ranging from 1.4° ≤ α ≤ 65°. The following parameters were determined: the amplitude of the opposition effect Δm = 0.75^m ± 0.06^m; the half-width at a half-maximum of intensity HWHM = 6.4° ± 1.6°; the linear phase coefficient β = 0.0045 ± 0.0001 mag/deg for α from 30° ≤ α ≤ 65°; and the phase angle at which a nonlinear increase in brightness starts, α _opp ≈ 31°. For the first time, the phase-angle dependence was obtained for the color of the dust of comet Halley: the color index BC-RC systematically decreases with increasing phase angle. Such a phase behavior of the dust color can be caused by the decrease in the mean size of dust particles that occurs when the comet approaches the Sun. For comet Halley, the negative polarization branch is almost symmetric; the minimal value of polarization is P _min = −1.54% at a phase angle α_min = 10.5°, and the inversion angle is α_inv = 21.7°. A comparative analysis of the phase functions of brightness and polarization has been performed for the cometary dust and atmosphereless bodies. Among the latter are low-albedo asteroids of the P and C types (102 Miriam and 47 Aglaja, respectively), as well as Deimos; high-albedo objects, such as the E-type asteroid 64 Angelina and the icy satellite of Jupiter Europa; and the Moon with its intermediate albedo. The possibility of a weak depression in the negative polarization branch of comets Halley and 47P/Ashbrook-Jackson at phase angles smaller than 2° is discussed.__________Translated from Astronomicheskii Vestnik, Vol. 39, No. 4, 2005, pp. 353–363.Original Russian Text Copyright © 2005 by Rosenbush.     

CCD Polarimetric Imaging Of Comet Hale-Bopp (C/1995 O1)

Polarimetric observations of the light scattered by dust have been carried out at Pic-du-Midi Observatory with the 2 m telescope in June and September–October 1996, and at Haute-Provence Observatory with the 0.80 m telescope in April 1997. They cover a total number of 11 nights and a large (6.9°–47.7°) phase angle range. The spatial resolution allows to underline structures in the coma, as well in the brightness images as in the polarization maps, with a correlation between the regions of bright structures and the regions of higher polarization. A clear difference appears between the sunward and antisunward side, with higher polarization on the antisunward side. The phase angle coverage allows us to obtain a polarimetric phase curve for the whole coma and to compare it with other cometary phase curves. The degree of polarization is higher for Hale-Bopp than for the comets previously observed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cometary dust properties retrieved from polarization observations: Application to C/1995 O1 Hale-Bopp and 1P/Halley

The analysis of the polarized light scattered by cometary dust particles provides information on the physical properties of the solid component of cometary comae for C/1995 O1 Hale-Bopp and 1P/Halley. A model of light scattering by a size distribution of aggregates of up to 256 submicron-sized grains (spherical or spheroidal) mixed with single spheroidal particles has been developed, with its parameters adjusted to fit the phase angle and wavelength dependence of the polarization observations. The particles are built of two materials: a non-absorbing silicates-type material and a more absorbing organic-type material. The model reproduces accurately the inversion angle and the positive branch of the polarization phase curves from the visible to the near-infrared spectral domains. A negative branch of the polarization phase curves appears in our model, although the negative branch is not deep enough to reproduce accurately the observations. Significant differences are shown between the two comets, with dominance of small grains in the coma of Comet C/1995 O1 Hale-Bopp, well fitted by a distribution of the volume-equivalent diameter, a, following a^−3.0 with a lower cutoff around 0.20 μm and an upper cutoff of at least 40 μm. For 1P/Halley, the size distribution follows a^−2.8 with a lower cutoff around 0.26 μm and an upper cutoff of about 38 μm. The relative amount of organic-type particles is larger for 1P/Halley while the amount of aggregates, significant for both comets, is larger for C/1995 O1 Hale-Bopp.     

Thermal Emission From The Dust Coma Of Comet Hale-Bopp And The Composition Of The Silicate Grains

The dust coma of comet Hale-Bopp was observed in the thermal infrared over a wide range in solar heating (R = 4.9–0.9 AU) and over the full wavelength range from 3 μm to 160 μm. Unusual early activity produced an extensive coma containing small warm refractory grains; already at 4.9 AU, the 10 μm silicate emission feature was strong and the color temperature was 30% above the equilibrium blackbody temperature. Near perihelion the high color temperature, strong silicate feature, and high albedo indicated a smaller mean grain size than in other comets. The 8–13 μm spectra revealed a silicate emission feature similar in shape to that seen in P/Halley and several new and long period comets. Detailed spectral structure in the feature was consistent over time and with different instruments; the main peaks occur at 9.3, 10.0 and 11.2 μm. These peaks can be identified with olivine and pyroxene minerals, linking the comet dust to the anhydrous chondritic aggregate interplanetary dust particles. Spectra at 16–40 μm taken with the ISO SWS displayed pronounced emission peaks due to Mg-rich crystalline olivine, consistent with the 11.2 μm peak. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hale-Bopp: What Makes a Big Comet Different? Coma Dynamics: Observations and Theory

Comet Hale-Bopp was the largest comet by almost any definition, observed at least since the advent of modern observing techniques. In a more typical comet both the chemical and dynamical influences of collisional processes are limited by the short time a parcel of gas sublimated from the nucleus remains in the dense part of the coma. The resulting large size of the collisional coma in comet Hale-Bopp had important consequences on the dynamics of the coma, which in turn has important consequences on how observations are interpreted with standard models. Measured velocities of typical gas species (mostly the observed radicals) as well as dust were larger than normal comets. Conversely, velocities of super thermal atomic hydrogen were smaller than normal because of the samecollisional processes. Furthermore, as a consequence, dust particles, which are dragged by the outflowing gas, were also accelerated to larger velocities. Such larger velocities are not simply an interesting curiosity in their own right, because nearly all observations of dust and gas are interpreted with models of the coma that depend directly on some measurement or assumption with regard to velocity. In this presentation both observations and theory regarding the dynamical conditions in the coma of comet Hale-Bopp are summarized.     

All comets are born equal: infrared emission by dust as a key to comet nucleus composition

The infrared emission of various comets can be matched within the framework that all comets are made of aggregated interstellar dust. This is demonstrated by comparing results on Halley (a periodic comet), Borrelly (a Jupiter family short period comet), Hale-Bopp (a long period comet), and extra-solar comets in the β Pictoris disk. Attempts have been made to generalize the chemical composition of comet nuclei based on the observation of cometary dust and volatiles and the interstellar dust model. Finally, we deduce some of the expected dust and surface properties of comet Wirtanen from the interstellar dust model as applied to other comets.     

Narrow-Band Photometry of Comet C/1995 O1 (Hale-Bopp)

Photometric observations of comet C/1995 O1 (Hale-Bopp) carried out at the Stará Lesná Observatory since February to April 1997 are analyzed and discussed. Emission band fluxes and continuum fluxes are presented, from which the total numbers of molecules in the columns of the coma encircled by diaphragms are calculated. The production rates are estimated from the conventional Haser model. We found that the photometric exponent of dust contribution two months prior perihelion was n = 5.2. The photometric exponent n of the cometary magnitude solely to the C₂ emission alone equals 3.3 and that of CN equals 2.5. These values can be explained by a fact that the maximums of production rates of the gases were reached between March 2and 12 and not at the perihelion as it is valid for dust. These results are compared with the values of 1P/Halley (1986 III) under the similar conditions, obtained with the same method and instrument. C/Hale-Bopp exhibited 4.1 times more molecules radiating the CN-emission than 1P/Halley in the same column of the coma. The continuum flux of C/Hale-Bopp was also very strong. The ratios (to 1P/Halley) are 94:1 (Cont. 484.5) and 74:1 (Cont. 365.0). The cometary colour was the same as that of the Sun. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spectropolarimetry of comets Austin and Churyumov-Gerasimenko

Spectropolarimetric observations from 5000 to 8000 Å have been obtained for comets P/Austin (1982g) and P/Churyumov-Gerasimenko (1982f). The observations were spaced over phase angles of 50–125° for comet Austin and 10–40° for comet Churyumov-Gerasimenko. The use of spectropolarimetry allowed an evaluation of continuum polarization without molecular line contamination. Especially for comet Churyumov-Gerasimenko, the curve of polarization versus phase angle resembles curves for asteroids, where the polarization is negative (electric vector maximum parallel to the scattering plane) for phase angles less than 20° and the most negative polarization is from ?1 to ?2%. The negative polarization at backscattering angles may be due to multiple scattering in agglomerated grains, as assumed for asteroids, or to Mie scattering by small dielectric particles. If multiple scattering is important in comet dust, polarization measurements may imply a low albedo, less than 0.08. The polarization of comet Austin remained steady during a large change in the dust production rate. Both comets increased continuum flux by a factor of 2 near perihelion. The continuum of comet Churyumov-Gerasimenko had the shape of the solar spectrum with derivations less than 5%. The equivalent width of spectral features of C₂, NH₂, and O varied as r^?2.     

Photometric investigations of distant comets C/2002 VQ94 (LINEAR) and 29P/Schwassmann-Wachmann-1

We present an analysis of the results of photometric investigations of two distant comets, C/2002 VQ94 (LINEAR) and 29P/Schwassmann-Wachmann-1, obtained with the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. The comets under study demonstrate sufficient activity out of the zone of water ice sublimation (at heliocentric distances longer than 5 AU). In the spectra of the investigated comets, we found the CO⁺ and N₂⁺ emission. The presence of this emission may say that the comets were formed in the outer parts of the Solar System, in a protoplanetary cloud at a temperature ≤25 K. We found that the photometric maximum of the ionosphere (in the CO⁺ filter) of the comet C/2002 VQ94 (LINEAR) is shifted relative to the photometric center of the dust coma by 1.4″ (7.44 × 10³ km) in the direction deflected by 63° from the direction to the Sun. Using special filters to process the images, we picked out active structures (jets) in the dust coma of the 29P/Schwassmann-Wachmann-1 comet.     

Composition of Comets: Observations and Models

We analyze the chemical composition and abundances of comets based on in situ measurements of Comet 1P/Halley and remote sensing observations of several recent bright comets including Hale-Bopp (C/1995 O1) and Hyakutake (C/1996 B2), in light of the elemental abundances of the solar system. Nitrogen is underabundant in comets relative to the solar system because nitrogen tends to be in N₂, which is chemically relatively inert. While many details remain uncertain, some gross features are emerging. The abundance of water : silicates: carbonaceous molecules (CO, CO₂, and hydrocarbons) by mass is approximately 1 : 1 : 1. Furthermore, the mass abundance of ice : dust (silicates and hydrocarbon polycondensates) is about1 : 1. We compare a list of identified comet molecules with molecules detected in the interstellar medium, although a comparison with their relative abundances, particularly in the ice phase, would be more meaningful. However, ice-phase abundances are not yet available. One can expect a variation of the abundances of carbon-bearing molecules in comets to be associated with their place of origin in the solar nebula. However, we also note that comets are heterogeneous. Thus, observed differences may be related to the place of origin, heterogeneity of the nucleus, or acquired through evolution. The molecular and elemental compositions of the coma are most likely not the same as those in the nucleus. This is particularly true for volatile ices and their gases and for the dust-to-ice and dust-to-gas ratios. Analyses must carefully consider the three sources of gas: Water from the surface of the nucleus, gases more volatile than water from the interior of the nucleus, and gases from the sublimation of the dust distributed in the coma. Topography on the surface of the nucleus may cause important evolutionary differences in the dust-to-gas mass ratio. Relatively inactive areas on the surface of the nucleus are probably associated with convex topography. Gas sublimated from convex areas (hills and mountains) diverges more strongly relative to gas sublimated from concave areas, which can entrain dust more efficiently. Thus, the entrainment of dust from convex areas is poor and dust may fall back to the surface of the nucleus creating a dust mantle, which further inhibits outgassing.     

An easy-to-use Model for the Optical Thickness and Ambient Illumination within Cometary Dust Comae

We define a procedure which allows estimation of the optical thickness of a cometarydust coma and the ambient illumination of the nucleus for any given comet, if estimatesof the nucleus radius and the dust activity (Afρ) are available. The calculation isperformed for a singly scattering coma with a cos(ϑ) distribution of dust overits day side. We find that the ambient illumination is of the same order as the incidentsunlight if the optical thickness is of order one. The optical thickness increases, all elseequal, linearly with the nucleus radius. Therefore the effect of the presence of the comamay be neglected for small (≈ 1 km diameter) comets, but is important forcomets such as 1P/Halley and Hale–Bopp.     

Organics-glued dust aggregates in Halley's coma and some implications of the snow line inward motion for the formation of comets

In view of the solar nebula models, organics-glued dust aggregates (whose disintegration resulted in the two phenomena found in Halley's coma, the dust boundary and small-scale dust structures) originated due to coagulation of iceless dust particles somewhere within the snow line, and then were incorporated into Halley's nucleus as a consequence of the snow line inward motion. This implies that two types of comets exist: outer comets, formed entirely beyond the snow line, and inner comets, similar to Halley, which are bodies intermediate between outer comets and primitive asteroids. The presence of large iceless dust aggregates in nuclei of inner comets constrains the inward drift velocity of meter-sized dust bodies, which in turn implies that the radial transport of water in the solar nebula was predominantly outward. It is shown that in nuclei of inner comets: both the upper mass limit of iceless dust aggregates and the ice mantle thickness increase with decreasing formation heliocentric distance, while the cumulative mass distribution index decreases; the lower limit of the mass index is ∼0.8, and the upper limit of the ice mantle thickness is ∼10⁻³ cm (∼200 times the interstellar value); the lower limit of the latent heat of organics in organic mantles of submicron particles increases toward small heliocentric distances; the recondensation of organics combined with the growth of dust bodies leads to a fractionation of organics within iceless dust aggregates; last accreted sub-units of an aggregate are always glued by organics with the lowest value of the latent heat, which somewhat exceeds 60 kJ/mol. Based on in situ observations at Halley, the parameters characterizing iceless dust aggregates in that comet are calculated. Finally, feasible observational tests of the conclusions drawn are discussed.     

Dust coma of comet C/2009 P1 (Garradd) by imaging polarimetry

Comet C/2009 P1 (Garradd) was observed by imaging polarimetry for nearly 5 months from October 2011 to March 2012, over an intermediate phase angle range (28°–35°). Two months before perihelion and one month after, dust particles seem to be ejected all around the optocenter and jets extend to distances greater than 40,000 km. An increase of activity is noticed in intensity and polarization after perihelion. Two months before perihelion and one month after, the dust emission seems to be all around the optocenter. Two and three months after perihelion the jets are mainly toward the solar direction with an extension of more than 20,000 km projected on the sky. The values of the aperture polarization are comparable to those of other comets. On the polarization maps in October 2011 and January 2012 the higher polarization zones extend in large regions perpendicularly to the solar direction where jets are also observed. In February and March 2012, the polarization in the jets is larger in the solar direction than in the surrounding coma. By its activity visible on intensity images and polarization maps at large distances from the nucleus, comet Garradd probably belongs to the high‐P_max class of comets.     

Infrared emission from comets

A brief discussion of the infrared observations from 4 to 20 micrometers of seven comets is presented. The observed infrared emission from comets depends primarily on their heliocentric distance. A model based on grain populations composed of a mixture of silicate and amorphous carbon particles in the mass ratio of about 40 to 1, with a power-law size distribution similar to that inferred for comet Halley, is applied to the observations. The model provides a good match to the observed heliocentric variation of both the 10 micrometers feature and the overall thermal emission from comets West and Halley. Matches to the observations of comet IRAS-Araki-Alcock and the antitail of comet Kohoutek require slightly larger grains. While the model does not match the exact profile and position of the 3.4 micrometers feature discovered in comet Halley, it does produce a qualitative fit to the observed variation of the feature's strength as a function of heliocentric distance. The calculations predict that the continuum under the 3.4 micrometers feature is due primarily to thermal emission from the comet dust when the comet is close to the Sun and to scattered solar radiation at large heliocentric distances, as is observed. A brief discussion of the determination of cometary grain temperatures from the observed infrared emission is presented. It is found that the observed shape of the emission curve from about 4 to 8 micrometers provides the best spectral region for estimating the cometary grain temperature distribution.     

Polarimetry and Photometry of Comet C/2004 Q2 (Machholz)

We present results of polarimetry and photometry of comet C/2004 Q2 (Machholz) obtained with the 0.7-m telescope of Institute of Astronomy at the Karazin Kharkiv National University on February 3 and 4, and March 4 and 14, 2005. The observations were carried out with a one-channel photoelectric photometer-polarimeter. The IHW continuum UC (λ3650/80 Å), BC (λ4845/65 Å), RC (λ6840/90 Å) and emission filters CN (λ3871/50 Å), C₃ (λ4060/70 Å), and C2 (λ5140/90 Å) were used. Degree of the comet polarization at phase angles ≈ 52° and ≈44° in the red continuum was close to that for so called dusty comets. The comet had a typical spectral gradient of polarization ΔP/Δλ=0.86% per 1000 Å. In the framework of the Haser model we have found the gas production rates Q of the CN, C₃ and C₂ species and the dust production rates Afρ on February 4 and March 14, 2005. The ratio log[Afρ (BC)/Q(CN)] was compared with data for other comets. The normalized spectral gradient of cometary dust S’(BC,RC) was 8.7%/1000 Å for February 4 and 17.0%/1000 Å for March 14. We conclude that comet C/2004 Q2 (Machholz) in many respects is a typical dusty comet.