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1.
We present results of polarimetric and photometric observations of bright comet C/1995 O1 (Hale-Bopp) obtained at the 0.7
m telescope of Kharkov University Observatory from June 18, 1996 to April 24, 1997. The IHW and HB comet filters were used.
The C2 and C3 production rates for Hale-Bopp are more than one order of magnitude larger and the dust production rates are more than two
orders of magnitude larger than the Halley ones at comparable distances. Hence, Hale-Bopp was one of the most dusty comets.
The average UC-BC and BC-RC colours of the dust were −0.02 and 0.13 mag, respectively. The polarization of comet Hale-Bopp
at small phase angles of 4.8–13.0° was in good agreement with the date for comet P1/Halley at the same phase angles in spite
of the fact that the heliocentric distances of comments differed nearly twice. However, at intermediate phase angles of 34–49°
the polarization of comet Hale-Bopp was significantly larger than the polarization of the other dusty comets. It is the first
case of such a large difference found in the continuum polarization of comets. The wavelength dependence of polarization for
Hale-Bopp was steeper than for other dusty comets. The observed degree of polarization for the anti-sunward side of the coma
was permanently higher than that for the sunward shell side. The polarization phase dependence of Hale-Bopp is discussed and
compared with the polarization curves for other dusty comets. The peculiar polarimetric properties of comet Hale-Bopp are
most likely caused by an over-abundance of small or/and absorbing dust particles in the coma.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
2.
U. C. Joshi A. K. Sen M. R. Deshpande J. S. Chauhan 《Journal of Astrophysics and Astronomy》1992,13(3):267-277
Photopolarimetric observations of comet Austin with the IAU/IHW filter system were obtained on the 2.34 m Vainu Bappu Telescope
(VBT) of the Indian Institute of Astrophysics, at Kavalur, India, during pre-perihelion phase on February 20,1990 and on the
1.2 m telescope of the Physical Research Laboratory at Gurusikhar, Mount Abu during postperihelion phase on May 2 and 4, 1990.
The comet appeared bluer than a solar analog during post-perihelion phase on May 2 and 4. The percent polarization shows a
sharp increase towards the red on May 2 and 4. The dominant sizes of the dust particles appear to lie in a narrow range of
0.1 to 0.5 Μm. Regarding the molecular band emission, CN and C2 bands are quite strong; C3 emission was also found to be strong though the observations on May 2 and 4 show significant variation as compared to C2 emission. Molecular band polarization for CN, C3, C2 and H2 O+ have been calculated. It has been found that emission polarization in CN, C2 and C3 is between 1–7% (phase angle between 107.4–109 degrees). For CN and C2 the polarization values are close to the theoretically predicted values, but for C3 the polarization value falls much below the theoretically predicted value. A similar result was found for comet Halley. 相似文献
3.
We present the imaging polarimetry and photometry of Comet C/1996 Q1 (Tabur) obtained on October 10, 1996, with a two-channel focal reducer attached to the 2-m Pik Terskol Observatory telescope through blue (4430/44 Å) and red (6420/26 Å) continuum filters and through a 6620/59 Å filter that isolated the NH2(0.7.0) band. We analyze the 3600–9300 Å long-slit spectrograms of the comet taken on October 5–6, 1996, with the 2.6-m Crimean Astrophysical Observatory telescope. The NH2(0.8.0) 6408 Å emission and an unidentified 6428 Å emission were found to fall within the pass band of the red filter. The blue filter transmits weak unidentified emissions at 4424–4444 Å and partially C2(4360 Å). Correction for the depolarizing effect of molecular emissions resulted in an increase of the dust polarization by 2–4% in the near-nucleus region and by almost a factor of 2 in the outer coma regions. However, the polarization and color differences between different coma regions remained even after correction for the contribution of emissions. We found no dust polarization difference between the gas comet Tabur and the dust comet C/1988 A1 (Liller), which are believed to be fragments of a common parent comet. The NH2coma was found to be elongated perpendicular to the comet radius vector. The causes of the spatial asymmetry in the NH2molecular distribution are yet to be established. We study the evolution of activity and the spatial distribution of dust brightness, polarization, and color in the comet. We consider a taxonomic classification of gas and dust comets according to dust polarization properties. The polarization differences between dust and gas comets at large phase angles are most likely related both to the actual differences in dust and to the effect of molecular emissions, nuclear gas- and dust-production rates and to the evolution of grain properties with distance from the nucleus. 相似文献
4.
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. 相似文献
5.
Ignacio Ferrín 《Icarus》2008,197(1):169-182
We present the secular light curve of Comet 2P/Encke in two phase spaces, the log plot, and the time plot. The main conclusions of this work are: (a) The comet shows activity at perihelion and aphelion, caused by two different active areas: Source 1, close to the south pole, active at perihelion, and Source 2, at the north pole, centered at aphelion. (b) More than 18 physical parameters are measured from the secular light curves, many of them new, and are listed in the individual plots of the comet. Specifically we find for Source 1 the location of the turn on and turn off points of activity, RON=−1.63±0.03 AU, ROFF=+1.49±0.20 AU, TON=−87±5 d, TOFF=+94±15 d, the time lag, LAG(q)=6±1 d, the total active time, TACTIVITY=181±16 d, and the amplitude of the secular light curve, ASEC(1,1)=4.8±0.1 mag. (c) From this information the photometric age and the time-age defined in Ferrín [2005a. Icarus 178, 493-516; 2006. Icarus 185, 523-543], can be calculated, and we find P-AGE = 97 ± 8 comet years and T-AGE = 103 ± 9 comet years (cy). Thus Comet 2P/Encke is an old comet entering the methuselah stage (100 cy < age). (d) The activity at aphelion (Source 2), extends for TACTIVITY=815±30 d and the amplitude of the secular light curve is ASEC(1,Q)=3.0±0.2 mag. (e) From a new phase diagram an absolute magnitude and phase coefficient for the nucleus are determined, and we find RNUC(1,1,0)=15.05±0.14, and β=0.066±0.003. From this data we find a nucleus effective diameter DEFFE=5.12(+2.5;−1.7) km. These values are not much different from previous determinations but exhibit smaller errors. (f) The activity of Source 1 is due to H2O sublimation because it shows curvature. The activity of Source 2 might also be due to H2O due to the circumstantial situation that the poles point to the Sun at perihelion and aphelion. (g) We found a photometric anomaly at aphelion, with minimum brightness between +393 and +413 days after perihelion that may be an indication of topography. (h) We have re-reduced the 1858 secular light curve of Kamel [1991. Icarus 93, 226-245]. There are secular changes in 7 physical parameters, and we achieve for the first time, an absolute age calibration. We find that the comet entered the inner Solar System and began sublimating in 1645±40 AD. (i) It is concluded that the secular light curve can place constraints on the pole orientation of the nucleus of some comets, and we measure the ecliptic longitude of the south pole of 2P/Encke equal to 213.2±4.5°, in excellent agreement with other determinations of this parameter, but with smaller error. (j) Using the observed absolute magnitude of 1858 and 2003 and a suitable theoretical model, the extinction date of the comet is determined. We obtain ED=2056±3 AD, implying that the comet's lifetime is 125±12 revolutions about the Sun after entering the inner Solar System. 相似文献
6.
This work was carried out with the PROGRA2 experiment developed to measure the angular dependence of the polarization of light scattered by dust particles. The dust samples are fluffy aggregates (size range 0.01-1 mm) with constituent grains of about 10 nm. Various setups were used: samples deposited on surfaces, the same samples lifted under the effect of a draft, and particles levitating in microgravity conditions on board the CNES dedicated aircraft. For deposited particles, the maximum value of polarization (Pmax) follows the Umov law. For a cloud of particles (Pmax) near 100° phase angle decreases when: (i) multiple scattering between the particles—or between the grains inside the particles—increases, or (ii) the real part of the refractive index of the materials increases, or (iii) the size parameter of the constituent grains increases between 0.05 and 0.5. A negative branch in the polarization phase curve is found for deposited samples. For levitating particles made of a single material and a single size distribution, a positive increase of polarization appears at phase angles smaller than 20°; for mixtures of these materials the polarization is negative at the same phase angles. These results are compared to modeling results as well as to polarimetric observations of comets. 相似文献
7.
We present the results of photometric measurements of the inner jovian satellites Thebe, Amalthea and Metis based on extensive optical observations taken from October 1999 to January 2002. The observations were made in the phase angle range from 8.1° to 0.3°. The Two-Channel Focal Reducer of the Max-Planck Institute for Aeronomy attached to the 2-m RCC telescope at Terskol Observatory (Pik Terskol, Northern Caucasus) was used in coronagraph mode. The observations were performed at a wavelength of 0.887 μm. Mean observational uncertainties corresponding to 1σ rms errors were 3% for the leading and trailing sides of Amalthea, 7 and 9% for the leading and trailing sides of Thebe and 9% for the leading side of Metis after taking into account the longitude brightness variations. Photometric data calibrated on an absolute scale were used to evaluate the near-opposition behavior of satellite brightness. All three satellites exhibit significant opposition brightening, but the strength of this effect, measured as the ratios of intensities at α1=1.6° and α2=6.7° does not vary significantly among these satellites. In order to measure the opposition surge parameters the empirical law proposed by Karkoschka and Hapke's model were used. The parameters of the satellite opposition effects are presented and discussed. The values of geometric albedos calculated with best-fit Hapke parameters are 0.096, 0.157, and 0.24 for Thebe, Amalthea, and Metis respectively. We found that the average leading/trailing ratios of surface reflectance at the measured phase angles are 1.53±0.05, 1.25±0.04, 1.04±0.08 for Amalthea, Thebe, and Metis. 相似文献
8.
We present the characteristics of the dust comae of two comets, 126P/IRAS, a member of the Halley family (a near-isotropic comet), and 2P/Encke, an ecliptic comet. We have primarily used mid- and far-infrared data obtained by the ISOPHOT instrument aboard the Infrared Space Observatory (ISO) in 1996 and 1997, and mid-infrared data obtained by the SPIRIT III instrument aboard the Midcourse Space Experiment (MSX) in 1996. We find that the dust grains emitted by the two comets have markedly different thermal and physical properties. P/IRAS's dust grain size distribution appears to be similar to that of fellow family member 1P/Halley, with grains smaller than 5 microns dominating by surface area, whereas P/Encke emits a much higher fraction of big (20 μm and higher) grains, with the grain mass distribution being similar to that which is inferred for the interplanetary dust population. P/Encke's dearth of micron-scale grains accounts for its visible-wavelength classification as a “gassy” comet. These conclusions are based on analyses of both imaging and spectrophotometry of the two comets; this combination provides a powerful way to constrain cometary dust properties. Specifically, P/IRAS was observed preperihelion while 1.71 AU from the Sun, and seen to have a 15-arcmin long mid-infrared dust tail pointing in the antisolar direction. No sunward spike was seen despite the vantage point being nearly in the comet's orbital plane. The tail's total mass at the time was about 8×109 kg. The spectral energy distribution (SED) is best fit by a modified greybody with temperature T=265±15 K and emissivity ε proportional to a steep power law in wavelength λ: ε∝λ−α, where α=0.50±0.20(2σ). This temperature is elevated with respect to the expected equilibrium temperature for this heliocentric distance. The dust mass loss rate was between 150-600 kg/s (95% confidence), the dust-to-gas mass loss ratio was about 3.3, and the albedo of the dust was 0.15±0.03. Carbonaceous material is depleted in the comet's dust by a factor of 2-3, paralleling the C2 depletion in P/IRAS's gas coma. P/Encke, on the other hand, observed while 1.17 AU from the Sun, had an SED that is best fit by a Planck function with T=270±15 K and no emissivity falloff. The dust mass loss rate was 70-280 kg/s (95% confidence), the dust-to-gas mass loss ratio was about 2.3, and the albedo of the dust was about 0.06±0.02. These conclusions are consistent with the strongly curved dust tail and bright dust trail seen by Reach et al. (2000; Icarus 148, 80) in their ISO 12-μm imaging of P/Encke. The observed differences in the P/IRAS and P/Encke dust are most likely due to the less evolved and insolated state of the P/IRAS nuclear surface. If the dust emission behavior of P/Encke is typical of other ecliptic comets, then comets are the major supplier of the interplanetary dust cloud. 相似文献
9.
Comet NEAT C/2001 Q4 was observed for linear polarization using the optical polarimeter mounted at the 1.2 m 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 BVR 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. 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. 相似文献
10.
V. K. Rosenbush F. P. Velichko N. N. Kiselev S. F. Velichko N. M. Shakhovskoy Yu. S. Efimov K. A. Antonyuk S. V. Kolesnikov D. N. Shakhovskoy 《Solar System Research》2006,40(3):230-238
We present the results of polarimetric and photometric observations of dynamically new comet C/2002 T7 (LINEAR) at phase angles from 6° to 26°. During the observations, the comet was at a distance of 2.7–1.3 AU from the Sun and 1.7–2.0 AU from the Earth. The aperture polarimetry was made with the 2.6-m Shain telescope and the 1.25-m AZT-11 telescope of the Crimean Astrophysical Observatory and with the 0.7-m telescope of the Astronomical Institute of the Kharkiv National University during the period from November 21, 2003, to February 21, 2004. The wideband UBVRI and WRC (λ7228/1142 Å) filters and the narrowband GC (λ5260/56 Å) filter were used. The photometric observations of the comet were carried out on February 21, 2004, with narrowband filters isolated the BC (λ4845/65 Å) and RC (λ6840/90 Å) continuum and the C2 emission (λ5140/90 Å). The phase-angle dependence of linear polarization of the comet has been obtained, and its parameters, such as the minimal polarization P min = ?1.63%, the phase angle of the minimal polarization αmin = 10.6°, the inversion angle αinv = 22.7°, and the slope of the phase curve at the inversion angle h = 0.24% per degree, were found. From the photometric observations, the following quantities have been obtained: the column density of molecules C2 in the line of sight logN (C2) = ?9.15 mol/cm2 and their production rate log Q (C2) = 27.11 mol/s, the spectral gradient of reflectivity for the dust S′(BC, RC) ≈ 3%/1000 Å, and the dust production parameter Afρ equal to 371 and 273 cm for the blue and red continuum ranges, respectively. According to these results, the physical parameters of comet C/2002 T7 are close to the average characteristics of typical dusty comets. 相似文献
11.
The results of V-band polarimetric observations of the potentially hazardous near-Earth Asteroid (23187) 2000 PN9 at large phase angles are presented as well as its photometric observations in BVRI bands. Observations were made in March-April 2006 during its close approach to the Earth using the 1.82-m Asiago telescope (Italy) and the 0.7-m telescope at the Chuguevskaya Observational Station (Ukraine). We obtained polarimetric measurements at the phase angle of 115°, the largest phase angle ever observed in asteroid polarimetry. Our data show that the maximum value of the polarization phase curve reached 7.7% and occurred in the phase angle range of 90-115°. The measured values of linear polarization degree, BVRI colors and magnitude-phase dependence correspond to the S-type composition of this asteroid. Based on our observations the following characteristics of the Asteroid (23187) 2000 PN9 were obtained: a rotation period of 2.5325±0.0004 h, a lightcurve amplitude of 0.13 mag, an albedo of 0.24±0.06 and a diameter of 1.6±0.3 km. 相似文献
12.
The motion of dust structures in the circumnuclear region of comet Hale–Bopp is studied. About 270 envelope images were obtained with the AZT-8 reflector (D = 0.7 m, F = 28 m) and the Filin-3 image intensifier. We carried out our observations at the observational station of the Astronomical Observatory of Shevchenko Kiev State University in the village of Lesniki (near Kiev). The recording from the image-intensifier screen was made on Foto-100 film during 23 nights from March 24 to May 10, 1997. The circumnuclear region was imaged both in white light (without filters) and with IHW CO+ (ef = 426 nm), C3 (ef = 496 nm), C2 (ef = 514 nm), and RC (red continuum, ef = 684 nm) narrow-band interference filters. Based on our measurements of the radial expansion of dust structures, we determined the velocities, 0.61–1.99 km s–1; accelerations, from –18.3 × 10–3 to 4.0 × 10–3 m s–2; and rotation period of the cometary nucleus, 111.41h ± 0.05h. 相似文献
13.
Hitoshi Hasegawa Takashi Ichikawa Shinsuke Abe Shiomo Hamamura Koji Ohnishi Jun-Ich Watanabe 《Earth, Moon, and Planets》1997,78(1-3):353-358
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. 相似文献
14.
We made polarimetric observations of comet Hale–Bopp covering awide phase angle range, from 18.8 to 47°. At certain phase angles the heliocentricdistance of the comet was less than 1 AU during its pre and post perihelion passages. Oneof the important findings, based on the data in the visual bands, is the higher polarizationwith stronger wavelength dependence compared to comet Halley, indicating the presenceof much finer grains in comet Hale–Bopp. It may also be noted that comet Hale–Bopphas shown highest degree of polarization known so far for any comet and hence fallsin the class of high polarization comets. Polarimetric observations were made of cometC/2000 WM1 (LINEAR)using narrow band (IHW) filters 4845 Å and 7000 Åand broad bands filters BVR during November 23–26, 2001 when the phase angle rangedfrom 15 to 22°. Some of the results based on these observations are presented anddiscussed. 相似文献
15.
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. 相似文献
16.
CCD observations of Mercury were obtained with the large angle spectrometric coronograph (LASCO) on the solar and heliospheric observatory spacecraft, near superior and inferior solar conjunctions. Whole disk photometry was extracted from the orange and blue filter images and transformed to V magnitudes on the UBV system. The LASCO data were combined with ground-based, V-filter photometry acquired at larger elongation angles. The resulting photometric phase function covers the greatest span of angles to date and is the first wide-range function to be obtained since the era of visual observation. We analyzed the data using a polynomial fit and a Hapke function fit, and derived the following photometric results. Mercury's fully lit brightness, adjusted to a distance of 1.0 AU from the Sun and observer, was found to be V=−0.694(±0.030), which is more luminous than previously measured. The corresponding geometric albedo is 0.142(±0.005). The phase integral is 0.478(±0.005) and resulting spherical albedo is 0.068(±0.003). The upper limit of a possible rotational brightness variation is about 0.05 magnitude. Mercury's brightness surges by more than 40% between phase angles 10 and 2°, while the illuminated fraction of the disk increases by less than 1%. A set of coefficients for Hapke's function that fit most of the phase curve includes h=0.065±0.002 indicating that Mercury and the Moon have similar regolith compaction states and particle size distributions, and θ-bar=16°±1° implying a macroscopically smoother surface than the Moon. However, we found other solutions that fit the observations nearly as well with significantly smaller and larger values of h, and with values of θ-bar around 25°. The wide range for θ-bar is due to the inability of the model to fit the photometry obtained at large phase angles. 相似文献
17.
K.O. Mason M. Chester C. Gronwall S. Hunsberger S. Koch P.T. O'Brien P. Roming A. Wells R. Willingale N. Gehrels 《Icarus》2007,187(1):123-131
We report time-resolved imaging UV photometry of Comet 9P/Tempel 1 during the interval 2005 June 29-2005 July 21, including intensive coverage of the collision with the Deep Impact probe and its immediate aftermath. The nuclear flux of the comet begins to rise within minutes of the collision, and peaks about 3 h after impact. There is no evidence for a prompt flash at the time of impact. The comet exhibits a significant re-brightening about 40 h after the initial outburst, consistent with the rotation period of the comet, with evidence for further periodic re-brightenings on subsequent rotations. Modelling of the brightness profile of the coma as a function of time suggests two distinct velocity systems in the ejecta, at de-projected expansion speeds of 190 and 550 m/s, which we suggest are due to dust and gas, respectively. There is a distinct asymmetry in the slower-moving (dust) component as a function of position angle on the sky. This is confirmed by direct imaging analysis, which reveals an expanding plume of material concentrated in the impact hemisphere. The projected expansion velocity of the leading edge of this plume, measured directly from the imaging data, is 190 m/s, consistent with the velocity of the dust component determined from the photometric analysis. From our data we determine that a total of (1.4±0.2)×1032 water molecules were ejected in the impact, together with a total scattering area of dust at 300 nm of 190±20 km2. 相似文献
18.
Jian-Yang Li Michael F. A'Hearn Christopher J. Crockett Carey M. Lisse Lucy A. McFadden Jessica M. Sunshine Joe Veverka 《Icarus》2007,187(1):41-55
The photometric properties of the nucleus of Comet 9P/Tempel 1 are studied from the disk-resolved color images obtained by Deep Impact (DI). Comet Tempel 1 has typical photometric properties for comets and dark asteroids. The disk-integrated spectrum of the nucleus of Tempel 1 between 309 and 950 nm is linear without any features at the spectral resolution of the filtered images. At V-band, the red slope of the nucleus is 12.5±1% per 100 nm at 63° phase angle, translating to B-V=0.84±0.01, V-R=0.50±0.01, and R-I=0.49±0.02. No phase reddening is confirmed. The phase function of the nucleus of Tempel 1 is constructed from DI images and earlier ground-based observations found from the literature. The phase coefficient is determined to be β=0.046±0.007 mag/deg between 4° and 117° phase angle. Hapke's theoretical scattering model was used to model the photometric properties of this comet. Assuming a single Henyey-Greenstein function for the single-particle phase function, the asymmetry factor of Tempel 1 was fitted to be g=−0.49±0.02, and the corresponding single-scattering albedo (SSA) was modeled to be 0.039±0.005 at 550 nm wavelength. The SSA spectrum shows a similar linear slope to that of the disk-integrated spectrum. The roughness parameter is found to be 16°±8°, and independent of wavelength. The Minnaert k parameter is modeled to be 0.680±0.014. The photometric variations on Tempel 1 are relatively small compared to other comets and asteroids, with a ∼20% full width at half maximum of albedo variation histogram, and ∼3% for color. Roughness variations are evident in one small area, with a roughness parameter about twice the average and appearing to correlate with the complex morphological texture seen in high-resolution images. 相似文献
19.
R. Vasundhara 《Icarus》2009,204(1):194-208
The pre-Deep Impact images of Comet Tempel-1 obtained at the Indian Astronomical Observatory are used to investigate the morphology of the dust coma of the comet. We show that the trajectory of a cometary grain under the influence of solar radiation pressure is a reliable diagnostic to estimate its initial velocity. Four main active regions at mean latitudes +45° ± 5°(D), 0° ± 5° (E),−30° ± 5°(A) and−60° ± 5°(F) are found to explain the morphology of the dust coma in the ground-based and published images obtained by the High Resolution Instrument(HRI) cameras aboard the Deep Impact flyby spacecraft. From a χ2 fit of the intensity distribution in the observed and the simulated images, we derive the fraction of the productivity of the active vents to the total dust emission of the comet to be 27%. Of this the southern source alone accounts for 19.8%. The grains are found to be ejected with a velocity distribution with an upper limit of 70 ± 7 m s−1. However, the broad region ‘A’ appears to eject slower grains with an upper limit of 24 ± 2.5 m s−1. This source, that is active throughout the cycle is likely to be driven by CO2 sublimation. We compute the dependence of the percentage contribution of the southern source on the heliocentric distance and show that this ratio varies over the apparition and reaches a maximum at around 260 days before perihelion. The published images of the nucleus of Comet Tempel-1 show significant departure from sphericity. Therefore, the torque exerted by the enhanced activity of the southern region may be significant enough to produce changes in the rotational state of the nucleus before each perihelion passage. 相似文献
20.
We present analyses and results from both narrowband photometry and CCD imaging of Comet 81P/Wild 2 from multiple apparitions, obtained in support of the Stardust mission. These data include photometric measurements from 12 days before the encounter and imaging from 3 days after. Using narrowband photometry from the different apparitions, we analyzed the dust and gas production rates as a function of heliocentric distance, finding a substantial seasonal effect where the production of OH, NH, and dust peaks 11-12 weeks before perihelion. The CN, C2, and C3 production show no such asymmetry, suggesting that there may be heterogeneities among different sources on the nucleus. The water production peaked at a level of approximately in 1997. A comparison of the relative abundances of minor gas species places Wild 2 in the “depleted” category in the A'Hearn et al. (1995, Icarus 118, 223) taxonomic classifications. Continuum measurements at multiple wavelengths indicate that the comet has a low dust-to-gas ratio, with moderately reddened dust. In our images we see a dust tail, an anti-tail and two well-defined jets. The primary jet, which persists for several months and is roughly aligned with the spin axis, has a source latitude >+75°, while the secondary jet is located on the opposite hemisphere between −37° and −62°. We used the apparent position angle of the primary jet to determine the pole orientation, α=281±5°, δ=+13±7°, and surmise that the nucleus is likely in a state of simple rotation. The primary source is continuously illuminated when Wild 2 is inbound and turns away from the Sun at about the time that the comet reaches perihelion, explaining the seasonal effects in the production rates. We measured lightcurves on several observing runs but saw no significant modulation, so no constraints can be set on the rotation rate. Images at different wavelengths show that the jets have the same colors as the dust in other regions in the coma and tail, indicating that the grain properties are similar throughout the coma. Radial profiles of the coma were measured in various directions on a number of different observing runs, and we discuss the findings from these measurements. Finally, we compare our results with other published data and attempt to predict future times at which observations should be obtained to help constrain additional properties. 相似文献