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In July of 2005, the Deep Impact mission collided a 366 kg impactor with the nucleus of Comet 9P/Tempel 1, at a closing speed of 10.2 km s−1. In this work, we develop a first-order, three-dimensional, forward model of the ejecta plume behavior resulting from this cratering event, and then adjust the model parameters to match the flyby-spacecraft observations of the actual ejecta plume, image by image. This modeling exercise indicates Deep Impact to have been a reasonably “well-behaved” oblique impact, in which the impactor-spacecraft apparently struck a small, westward-facing slope of roughly 1/3-1/2 the size of the final crater produced (determined from initial ejecta plume geometry), and possessing an effective strength of not more than . The resulting ejecta plume followed well-established scaling relationships for cratering in a medium-to-high porosity target, consistent with a transient crater of not more than 85-140 m diameter, formed in not more than 250-550 s, for the case of (gravity-dominated cratering); and not less than 22-26 m diameter, formed in not less than 1-3 s, for the case of (strength-dominated cratering). At , an upper limit to the total ejected mass of 1.8×107 kg (1.5-2.2×107 kg) is consistent with measurements made via long-range remote sensing, after taking into account that 90% of this mass would have stayed close to the surface and then landed within 45 min of the impact. However, at , a lower limit to the total ejected mass of 2.3×105 kg (1.5-2.9×105 kg) is also consistent with these measurements. The expansion rate of the ejecta plume imaged during the look-back phase of observations leads to an estimate of the comet's mean surface gravity of (0.17-0.90 mm s−2), which corresponds to a comet mass of mt=4.5×1013 kg (2.3-12.0×1013 kg) and a bulk density of (200-1000 kg m−3), where the large high-end error is due to uncertainties in the magnitude of coma gas pressure effects on the ejecta particles in flight. 相似文献
3.
The findings of Deep Impact on the structure and composition of Tempel-1 are compared with our experimental results on large (20 cm diameter and up to 10 cm high) samples of gas-laden amorphous ice. The mechanical ∼tensile strength inferred for Tempel-1: ∼65 Pa is 30 to 60 times smaller than our experimental findings of 2-4 kPa. This means that Tempel-1 is even fluffier than our very fluffy, talcum like, ice sample. The thermal inertia: is very close to our value of 80. The density of , is close to our value of 250-300 kg m−3, taking into account an ice/silicate ratio of 1 in the comet, while we study pure ice. Surface morphological features, such as non-circular depressions, chaotic terrain and smooth surfaces, were observed in our experiments. The only small increase in the gas/water vapor ratio pre- and post-impact, suggest that in the area excavated by the impactor, the 135 K front did not penetrate deeper than a few meters. Altogether, the agreement between the findings of Deep Impact and our experimental results point to a loose agglomerate of ice grains (with a silicate-organic core), which was formed by a very gentle aggregation of the ice grains, without compaction. 相似文献
4.
The Wide Field Camera (WFC) on the Hubble Space Telescope and the Low Resolution Imaging Spectrograph (LRIS) on the Keck II telescope have been used to image 21 distant dynamically new, long-period (LP) and short-period (SP) Jupiter-family (JF) comet nuclei (near aphelion), as part of a long-term program to search for physical differences between short-period comets and Oort cloud comets. WFC data were obtained on Comets C/1987 H1 (Shoemaker) and C/1984 K1 (Shoemaker) during Cycle 5 (1995 December) and on C/1988 B1 (Shoemaker), C/1987 F1 (Torres), and C/1983 O1 (?ernis) during Cycle 6 (1997 April, May, and June). The HST comets were at heliocentric distances 20.4 < r[AU] < 29.5. Each comet observation was allocated 7 orbits, for ≈3.6 hrs of integration. The most difficult part of the image reduction was the removal of cosmic rays. We present our scheme for cosmic ray removal. None of the HST comet nuclei was detected to the 3-σ level at mR∼27. The inferred upper limits to the nucleus radii are . The SP comets range in radius between , with a median value of RN∼1.61 km. The LP comets ranged in size between <4.0-56 km. Over a range of radii between 1-10 km, the nuclei can be fit with a cumulative distribution N(>RN)∝RN−α with α=1.45±0.05, and for nuclei in the range 2-5 km, α=1.91±0.06. Statistical analysis and modeling shows that the slopes of the observed TNO and JF comet distributions are not compatible, suggesting that the intrinsic distribution of JF comet nuclei is a differential a−3.5 power law truncated at small nucleus radii between 0.3 and 2.0 km. 相似文献
5.
Arecibo S-band () radar observations of Comet C/2001 A2 (LINEAR) on 2001 July 7-9 showed a strong echo from large coma grains. This echo was significantly depolarized. This is the first firm detection of depolarization in a grain-coma radar echo and indicates that the largest grains are at least λ/2π or 2 cm in radius. The grains are moving at tens of m s−1 with respect to the nucleus. The nondetection of the nucleus places an upper limit of 3 km on its diameter. The broad, asymmetric echo power spectrum suggests a fan of grains that have a steep (differential number ∼a−4) size distribution at cm-scales, though the observed fragmentation of this comet complicates that picture. 相似文献
6.
We report on observations of the dust trail of Comet 67P/Churyumov-Gerasimenko (CG) in visible light with the Wide Field Imager at the ESO/MPG 2.2 m telescope at 4.7 AU before aphelion, and at with the MIPS instrument on board the Spitzer Space Telescope at 5.7 AU both before and after aphelion. The comet did not appear to be active during our observations. Our images probe large dust grains emitted from the comet that have a radiation pressure parameter β<0.01. We compare our observations with simulated images generated with a dynamical model of the cometary dust environment and constrain the emission speeds, size distribution, production rate and geometric albedo of the dust. We achieve the best fit to our data with a differential size distribution exponent of −4.1, and emission speeds for a β=0.01 particle of 25 m/s at perihelion and 2 m/s at 3 AU. The dust production rate in our model is on the order of 1000 kg/s at perihelion and 1 kg/s at 3 AU, and we require a dust geometric albedo between 0.022 and 0.044. The production rates of large (>) particles required to reproduce the brightness of the trail are sufficient to also account for the coma brightness observed while the comet was inside 3 AU, and we infer that the cross-section in the coma of CG may be dominated by grains of the order of . 相似文献
7.
Masateru Ishiguro 《Icarus》2008,193(1):96-104
A thin, bright dust cloud, which is associated with the Rosetta mission target object (67P/Churyumov-Gerasimenko), was observed after the 2002 perihelion passage. The neckline structure or dust trail nature of this cloud is controversial. In this paper, we definitively identify the dust trail and the neckline structure using a wide-field CCD camera attached to the Kiso 1.05-m Schmidt telescope. The dust trail of 67P/Churyumov-Gerasimenko was evident as scattered sunlight in all images taken between September 9, 2002 and February 1, 2003, whereas the neckline structure became obvious only after late 2002. We compared our images with a semi-analytical dynamic model of dust grains emitted from the nucleus. A fading of the surface brightness of the dust trail near the nucleus enabled us to determine the typical maximum size of the grains. Assuming spherical compact particles with a mass density of 103 kg m−3 and an albedo of 0.04, we deduced that the maximum diameter of the dust particles was approximately 1 cm. We found that the mass-loss rate of the comet at the perihelion was on or before the 1996 apparition, while the mass-loss rate averaged over the orbit reached . The result is consistent with the studies of the dust cloud emitted in the 2002/2003 return. Therefore, we can infer that the activity of 67P/Churyumov-Gerasimenko has showed no major change over the past dozen years or so, and the largest grains are cyclically injected into the dust tube lying along the cometary orbit. 相似文献
8.
Jon K. Hillier Simon F. Green Nicolas Altobelli Sascha Kempf Ralf Srama Eberhard Grün 《Icarus》2007,190(2):643-654
During its cruise phase, prior to encountering Jupiter, the Cosmic Dust Analyser (CDA) onboard the Cassini spacecraft returned time of flight mass spectra (TOF MS) of two interplanetary dust particles. Both particles were found to be iron-rich, with possible traces of hydrogen, carbon, nickel, chromium, manganese, titanium, vanadium and minor silicates. Carbon, hydrogen, oxygen and potassium are also present as possible contaminants of the impact target of CDA. Silicates and magnesium do not feature predominantly in the spectra; this is surprising considering the expected dominance of silicate-rich minerals in interplanetary dust particles. The particle masses are and . The corresponding radii ranges for the particles, assuming densities from 7874-2500 kg m−3 are 0.7-4 μm and 2.6-6.8 μm, respectively. With the same density assumptions the β values (ratio of radiation pressure to gravitational force) are estimated as 0.027-0.21 and 0.016-0.06 respectively, allowing possible orbits to be calculated. The resulting orbits are bound and prograde with semi-major axes, eccentricities and inclinations in the region of 0.3-1.26 AU, 0.4-1.0 and 0-60° for the first particle and 0.8-2.5 AU, 0.2-0.9 and 0-30° for the second. The more probable orbits within these ranges indicate that the first particle is in an Aten-like orbit, whilst the second particle is in an Apollo-like orbit, despite both grains having very similar, predominantly metallic compositions. Other possible orbital solutions for both particles encompass orbits which more closely resemble those of Jupiter-family comets. 相似文献
9.
We study the Jupiter family comet (JFC) population assumed to come from the Scattered Disk and transferred to the Jupiter’s zone through gravitational interactions with the Jovian planets. We shall define as JFCs those with orbital periods and Tisserand parameters in the range 2<T?3.1, while those comets coming from the same source, but that do not fulfill the previous criteria (mainly because they have periods ) will be called ‘non-JFCs’. We performed a series of numerical simulations of fictitious comets with a purely dynamical model and also with a more complete dynamical-physical model that includes besides nongravitational forces, sublimation and splitting mechanisms. With the dynamical model, we obtain a poor match between the computed distributions of orbital elements and the observed ones. However with the inclusion of physical effects in the complete model we are able to obtain good fits to observations. The best fits are attained with four splitting models with a relative weak dependence on q, and a mass loss in every splitting event that is less when the frequency is high and vice versa. The mean lifetime of JFCs with radii and is found to be of about 150-200 revolutions (∼. The total population of JFCs with radii within Jupiter’s zone is found to be of 450±50. Yet, the population of non-JFCs with radii in Jupiter-crossing orbits may be ∼4 times greater, thus leading to a whole population of JFCs + non-JFCs of ∼2250±250. Most of these comets have perihelia close to Jupiter’s orbit. On the other hand, very few non-JFCs reach the Earth’s vicinity (perihelion distances ) which gives additional support to the idea that JFCs and Halley-type comets have different dynamical origins. Our model allows us to define the zones of the orbital element space in which we would expect to find a large number of JFCs. This is the first time, to our knowledge, that a physico-dynamical model is presented that includes sublimation and different splitting laws. Our work helps to understand the role played by these erosion effects in the distribution of the orbital elements and lifetimes of JFCs. 相似文献
10.
Brian M. Cudnik 《Planetary and Space Science》2005,53(6):653-658
Hale-Bopp (C/1995 O1) was the most productive recent comet observed in terms of gas and dust output. Since its discovery in 1995 at a distance of 7.14 AU from the Sun, the comet has been well observed, revealing the dynamics of a rare and large comet. Hale-Bopp showed strong emissions of the principle cometary gases CN, C3, and C2, as well as an abundance of dust. The production rates of these gases were found to be 1.45×1028, 1.71×1028, and , respectively, with dust production, in terms of Afρ, , as measured in the green continuum (5260 Å). The observations for this paper are presented in two groups spanning 10 days each, one group centered near 32 days prior to and the other 21 days after perihelion. The averages of dust and gas production rates show a slightly higher value for each prior to perihelion than after perihelion, consistent with a possible peak in production a few weeks prior to perihelion passage. 相似文献
11.
Observations suggest most stars originate in clusters embedded in giant molecular clouds [Lada, C.J., Lada, E.A., 2003. Annu. Rev. Astron. Astrophys. 41, 57-115]. Our Solar System likely spent 1-5 Myrs in such regions just after it formed. Thus the Oort Cloud (OC) possibly retains evidence of the Sun's early dynamical history and of the stellar and tidal influence of the cluster. Indeed, the newly found objects (90377) Sedna and 2000 CR105 may have been put on their present orbits by such processes [Morbidelli, A., Levison, H.F., 2004. Astron. J. 128, 2564-2576]. Results are presented here of numerical simulations of the orbital evolution of comets subject to the influence of the Sun, Jupiter and Saturn (with their current masses on orbits appropriate to the period before the Late Heavy Bombardment (LHB) [Tsiganis, K., Gomes, R., Morbidelli, A., Levison, H.F., 2005. Nature 435, 459-461]), passing stars and tidal force associated with the gas and stars of an embedded star cluster. The cluster was taken to be a Plummer model with 200-400 stars, with a range of initial central densities. The Sun's orbit was integrated in the cluster potential together with Jupiter and Saturn and the test particles. Stellar encounters were incorporated by directly integrating the effects of stars passing within a sphere centred on the Sun of radius equal to the Plummer radius for low-density clusters and half a Plummer radius for high-density clusters. The gravitational influence of the gas was modeled using the tidal force of the cluster potential. For a given solar orbit, the mean density, 〈ρ〉, was computed by orbit-averaging the density of material encountered. This parameter proved to be a good measure for predicting the properties of the OC. On average 2-18% of our initial sample of comets end up in the OC after 1-3 Myr. A comet is defined to be part of the OC if it is bound and has q>35 AU. Our models show that the median distance of an object in the OC scales approximately as 〈ρ〉−1/2 when . Our models easily produce objects on orbits like that of (90377) Sedna [Brown, M.E., Trujillo, C., Rabinowitz, D., 2004. Astrophys. J. 617, 645-649] within ∼1 Myr in cases where the mean density is or higher; one needs mean densities of order to create objects like 2000 CR105 by this mechanism, which are reasonable (see, e.g., Guthermuth, R.A., Megeath, S.T., Pipher, J.L., Williams, J.P., Allen, L.E., Myers, P.C., Raines, S.N., 2005. Astrophys. J. 632, 397-420). Thus the latter object may also be part of the OC. Close stellar passages can stir the primordial Kuiper Belt to sufficiently high eccentricities (e?0.05; Kenyon, S.J., Bromley, B.C., 2002. Astron. J. 123, 1757-1775) that collisions become destructive. From the simulations performed it is determined that there is a 50% or better chance to stir the primordial Kuiper Belt to eccentricities e?0.05 at 50 AU when . The orbit of the new object 2003 UB313 [Brown, M.E., Trujillo, C.A., Rabinowitz, D.L., 2005. Astrophys. J. 635, L97-L100] is only reproduced for mean cluster densities of the order of , but in the simulations it could not come to be on its current orbit by this mechanism without disrupting the formation of bodies in the primordial Kuiper Belt down to 20 AU. It is therefore improbable that the latter object is created by this mechanism. 相似文献
12.
Masateru Ishiguro Yuki Sarugaku Munetaka Ueno Fumihiko Usui Suk Minn Kwon 《Icarus》2007,189(1):169-183
We present observations of the extended dust structures near the orbits of three short-period comets: 2P/Encke, 22P/Kopff, and 65P/Gunn. The dust trails were originally discovered by the Infrared Astronomical Satellite (IRAS). Our observations were made using wide-field optical CCD cameras on the University of Hawaii 2.24-m telescope, the Canada-France-Hawaii 3.6-m telescope, and the Kiso 1.05-m Schmidt telescope. We compared the observed images with models and found that the extended structures seen around 2P/Encke and 22P/Kopff before perihelion passage were most likely “dust trails,” whereas images taken after perihelion passage show a high contamination by recently released particles (i.e., particles in Neck-Line structures are visible). We could not confirm the existence of a dust trail from 65P/Gunn within the field of view of the camera used. The effective sizes of the particles responsible for the scattered light were estimated at 1-100 mm (2P/Encke), 1-10 mm (22P/Kopff), and 100 μm-1 mm (65P/Gunn), respectively, which is consistent with previous studies of dust trails made with infrared space telescopes and optical telescopes. We evaluated the mass loss rates of these comets, averaged over their orbits, as reaching (2P/Encke), (22P/Kopff), and (65P/Gunn). These values are consistent with previous work. Therefore, the total amount of material ejected from these three comets is , which would contribute a considerable fraction of the lost within 1 AU that needs to be replaced if the zodiacal cloud is to be maintained in a steady state. We also found that the particles in the dust structures are significantly redder than the Sun and the zodiacal light, and might be redder than the average short-period comet nuclei. Specifically, the reflectivity gradients of 2P/Encke, 22P/Kopff, and 65P/Gunn are 13±7 (% 103 Å−1), 20±5 (% 103 Å−1), and 15±4 (% 103 Å−1), respectively. We examined the change in color with distance from the nucleus. No clear correlation was detected for 2P/Encke or 22P/Kopff to an accuracy of 3-11%, while the 65P/Gunn tail did show color variation, becoming redder with increasing distance from the nucleus. This dark red material, consisting of particles of sand-cobble size, has marginally escaped from the nuclei and will evolve into finer-grained interplanetary dust particles after subsequent collisions. 相似文献
13.
Compositional and physical results for Rosetta's new target Comet 67P/Churyumov-Gerasimenko from narrowband photometry and imaging 总被引：1，自引：0，他引：1
David G. Schleicher 《Icarus》2006,181(2):442-457
We present compositional and physical results of Comet 67P/Churyumov-Gerasimenko, the new target of ESA's Rosetta mission. A total of 16 nights of narrowband photometry were obtained at Lowell Observatory during the 1982/83 and 1995/96 apparitions, along with one night of imaging near perihelion in 1996. These data encompass an interval of −61 to +118 days from perihelion, corresponding to a range of heliocentric distances before perihelion from 1.48 to 1.34 AU, and an outbound range from 1.30 to 1.86 AU. Production rates were determined for OH, NH, CN, C3, and C2, along with A(θ)fρ, a proxy of the dust production. Water production, based on OH, has a steep () power-law rH-dependence post-perihelion and the minor species are somewhat less steep ( to −4), while the dust is quite shallow (), possibly due to a lingering population of large, slow-moving grains. All species exhibit larger production rates after perihelion, with water having a ∼2×pre/post-perihelion asymmetry, while minor species and dust have larger asymmetries. These asymmetries imply a strong seasonal effect and probable high obliquity of the rotational axis, along with one or more isolated source regions coming into sunlight near perihelion. Peak water production (which occurred about 1 month after perihelion) was and, when combined with a standard water vaporization model, implies an effective active area on the surface of the nucleus of ∼1.5-2.2 km2 or an active fraction of only about 3-4%. Abundances of carbon-chain molecules yield a classification of slightly “depleted” in the A'Hearn et al. [A'Hearn, M.F., Millis, R.L., Schleicher, D.G., Osip, D.J., Birch, P.V., 1995. Icarus 118, 223-270] database. The peak dust production (as measured by A(θ)fρ, and uncorrected for phase angle) was ∼450 cm, while the color of the dust is moderately reddened, and the mean radial profile has a power-law slope of −1.3. Large night-to-night variability is also present, presumably due to the source region(s) rotating in and out of sunlight along with effects due to the use of differently sized apertures. A strong sunward radial feature was detected in images obtained near perihelion, along with a significant asymmetry between the two perpendicular directions from the Sun/tail line. These features may be the result of a mid-latitude source region sweeping out a cone with each rotation, which we are viewing from the side and where the sunward radial feature is one edge of the cone seen in projection. When combined with other constraints on the pole orientation, a possible pole solution is found having an obliquity of about 134° at an RA of about 223° and a Dec of −65°, with a source region located near +50° and in overall agreement with the photometric results. In comparison to the original Rosetta target Comet 46P/Wirtanen, Comet Churyumov-Gerasimenko has essentially the same peak water production but a peak dust production about 3 times greater than does Wirtanen based on A(θ)fρ (i.e., if one assumes that the properties of the dust grains are similar) (cf. Farnham and Schleicher [1998. Astron. Astrophys. 335, L50-L55]). 相似文献
14.
We report the detection of 13CH3D in Titan's stratosphere from Cassini/CIRS infrared spectra near 8.7 μm. Fitting simultaneously the ν6 bands of both 13CH3D and 12CH3D and the ν4 band of CH4, we derive a D/H ratio equal to and a 12C/13C ratio in deuterated methane of , consistent with that measured in normal methane. 相似文献
15.
O. Groussin P. Lamy M. Kelley M. A'Hearn J. Licandro S. Lowry C. Snodgrass 《Icarus》2009,199(2):568-1670
We detected the nucleus of Comet 22P/Kopff at 4.87 AU from the Sun with the two IRS peak-up cameras of the Spitzer Space Telescope on April 19, 2007. Using the thermal model of [Groussin, O., and 15 colleagues, 2007. Icarus 187, 16-25], we derive a nucleus size of 1.89±0.16 km, in agreement with [Lamy, P., Toth, I., Jorda, L., Groussin, O., A'Hearn, M.F., Weaver, H.A., 2002. Icarus 156, 442-455], and a thermal inertia . 相似文献
16.
Asteroid 3200 Phaethon resembles a comet in some ways, including a highly-eccentric orbit (e∼0.89) and a strong associated meteor shower (the Geminids). Yet this object has never been observed to exhibit any cometary activity, i.e., gas production. We observed 3200 Phaethon with the Caltech Submillimeter Observatory on two occasions, once while it was near its closest approach to Earth as it neared perihelion, and another while it was further from Earth post-perihelion. Observations of the J=2→1 and J=3→2 rotational transitions of 12CO, typically strong lines in comets and indicative of gas production, yielded no detection. Upper limits on the 12CO production of 1.8×1028 and for Phaethon were determined on these two occasions. 相似文献
17.
The nucleus mass and bulk density of Comet 9P/Tempel 1 have been estimated by utilizing the non-gravitational force modeling technique. Here, the water production rates and non-gravitational perturbations of the orbit are calculated for a large number of model nuclei with different surface ice distribution patterns. By requiring that the empirical water production rate curve is reproduced, a subset of model nuclei are selected, for which masses are calculated by demanding that empirical non-gravitational changes of the orbital period and in the longitude of perihelion (per revolution) are reproduced. We obtain a mass M=5.8(±1.6)×1013 kg, and a bulk density , which compares very well with measurements made by the Deep Impact Science Team. The main goal of the current work is therefore to demonstrate functionality of an indirect method, i.e., mass estimation through non-gravitational force modeling, by comparing such results to ground truth data. Furthermore, the thermal inertia of active areas is estimated as 30-100 MKS, using a comparatively realistic thermophysical model (although a value in the range 100-350 MKS is obtained with a more simple model). An active area fraction of ∼3% is predicted, and these areas are probably confined to the northern hemisphere, being located close to the cometary equator. 相似文献
18.
A comparative study of meteor ablation in the atmospheres of the Earth and Venus is presented. The classical single body meteor ablation model is extended to incorporate a heat penetration depth estimate allowing the simulation of larger meteoroids, than would an isothermal model. The ablation of icy and rocky meteoroids, with densities of 1.0 and 3.4 g cm−3, respectively, and initial radii of up to for rock and for ice (equivalent to an initial mass of in both cases), was simulated in both atmospheres. In general venusian meteors are brighter than terrestrial equivalents. Large, slow, rocky objects may be up to 0.7 mag brighter on Venus, while small, icy particles with entry speeds in the range 30-60 km s−1, are found to be upwards of 2.7 mag brighter than at the Earth. Venusian meteors reach maximum brightness at greater altitudes than would similar particles at the Earth. Rocky meteoroids have their points of maximum brightness some 15-35 km higher up at Venus, between 90 and 120 km, whereas, for icy particles this altitude difference is about 5-25 km higher up than at the Earth, in the range 100-125 km. These findings agree, for the most part, with recent analytical studies. Venusian meteors, which last from 100 ms to , tend to be shorter-lived than terrestrial meteors, with correspondingly shorter visible trails. Large (), slow () icy particles reach a maximum magnitude of ∼−2 at Venus and remain visible for about one second, with a large section of the smaller faster meteoroids simulated here remaining visible for several hundred milliseconds. In light of recent space-based meteor observations at the Earth [Jenniskens, P., Tedesco, E., Muthry, J., Laux, C.O., Price, S., 2002. Meteorit. Planet. Sci. 37, 1071-1078], such brightness, height and duration estimates as suggested in this work, may be used in developing future observational campaigns to be carried out from Venus orbit. 相似文献
19.
The results of the multiaperture photometry of Comet Shoemaker-Levy 1991 T2 in the pre-perihelion and P/deVico in the post-perihelion period with the narrowband CN, C2 and Blue Continuum (BC) IHW filters are presented. A Haser model of the molecular coma was used for the determination of the parent and daughter scale-lengths and production rates of the radicals. The comets showed some substantial differences between their parent scale-lengths. The CN parent scale-length (at 1.0 AU) was 16×103 km for Comet Shoemaker-Levy and 39×103 for P/deVico, the C2 parent scale-lengths were respectively 29×103 and 54×103 km. Such divergences could be interpreted in the frame of different scenarios of emission of cometary parents, either from a nucleus or from a volume source. The daughter scale-lengths for these comets were quite similar, namely: 306×103 and 318×103 km for CN and 69×103 and 66×103 km for C2. We determined the Afρ parameter for apertures of different radii. A Monte Carlo model of the dust coma was used to obtain the dust ejection velocity. It was of the order of 0.1 km s−1 for both comets. The power index of the distribution of the β-parameter of dust particles (ratio of light pressure to the solar gravitation) was of the order of 3 for C/Shoemaker-Levy and close to 2 for P/deVico. The dependence on heliocentric distance (rh) of the radical and dust production rates for P/deVico in the range of 0.7-1.0 AU was described by the power law function with a power index equal to: 5.55±0.14 for CN, 5.70±0.24 for C2 and 5.22±0.19 for dust. Relative abundances of the dynamically new Comet Shoemaker-Levy and short-period P/deVico were quite similar with an enhancement of C2 comparing with standard values taken from A'Hearn et al. (1995). 相似文献
20.
The article presents results of CCD photometry in R-band of a dynamically new Comet C/2001 K5 (LINEAR), obtained at a heliocentric distance of about 5.6 AU, after the perihelion passage. Being so distant from the Sun, this comet was extremely active (Afρ close to 2000 cm), exhibiting quite well developed dust coma and tail. During the observations, general photometric behavior of the comet with heliocentric distance r was well described by the 2.5nlog(r) function with coefficient n=5. The radial profiles of the coma were found to be undulated, with mean slope of the dependence between cometary magnitude and 2.5log of aperture radius (at comet distance) equal to . The light curve of Comet LINEAR exhibited short-term variability which we attributed to cyclic changes of dust emission, induced by nucleus rotation. Model computations by some authors have revealed that active comets can change their spin status quite substantially even during a single orbital revolution. Thus, attempting to search for a rotation frequency, we have modified the classical PDM approach by including the spin acceleration term. Such DynamicalPDM (DPDM) method revealed the most reliable solution for the frequency f0=0.019048±0.000013 h−1 and its first time-derivative (index “zero” denotes reference to the mid time of the whole observing run), indicating a rapid spin-down of the nucleus. These parameters are equivalent to the rotation period of 52.499±0.036 h and its relative increment of 0.02729±0.00013. We present the most probable evolution of the rotation frequency of Comet LINEAR, based on the results of periodicity analysis and a simple, almost parameter independent, dynamical model of nucleus rotation. It is also shown that the DPDM may be an effective tool for determination of a nucleus radius, which provided us with the value of 1.53±0.25 km for Comet LINEAR. 相似文献