Non-constant rotation period of Comet C/2001 K5 (LINEAR)

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.     

Radar observations of Comet P/2005 JQ5 (Catalina)

Arecibo radar observations of Comet P/2005 JQ5 (Catalina) have produced the first delay-Doppler images of a comet nucleus and the first radar detection of large-grain ejection from a Jupiter-family comet. The nucleus is small (1.4 km diameter), rough, and rapidly rotating. The large (>cm) grains have low velocities (∼1 m/s) and a low production rate.     

Split Comet C/2001 A2 (LINEAR)

Comet C/2001 A2 experienced several splitting events duringits 2001 perihelion passage. The first break-upevent was observed in March 2001 (IAUC 7616).In this paper we report the first results of ourextensive imaging and spectroscopic monitoring campaignwith ESO telescopes over several weeks before andafter the perihelion passage on May 25 2001.     

A dynamical model with a new inversion technique applied to observations of Comet C/2000 WM1 (LINEAR)

Three continuum images of Comet C/2000 WM₁ (LINEAR) obtained on Nov 10, Nov 19, and Dec 03, 2001, are analyzed with the aid of a dynamical model, i.e. with a model that uses the size-dependent motion of dust grains under solar radiation pressure to determine the dust size distribution and its temporal change. The frames are photometrically calibrated in terms of the albedo filling factor product. On Nov 20.2 the Earth transited the orbital plane of the comet and an anti-tail was recognized in the image of Nov 19. For the determination of the particle fluxes describing the contribution of monodisperse particle shells to the cometary brightness the model uses a new regularization method employing Chebyshev polynomials of selected orders in emission time and particle size. It guarantees positiveness of the particle fluxes and imposes a varying degree of smoothness on their dependence on particle size and emission time. The particle emission velocities are still derived by trial and error. The dynamical model is described in detail. Results are presented for several low orders of the Chebyshev polynomials and are compared in order to understand the limitations imposed by the regularization process. The size distributions derived from the different observations do not always agree. This is particularly true for the earliest and most recent synchrones contributing to an image. In the observations of Nov 10 and Dec 03, i.e. excluding the anti-tail image, the integrated mass loss strongly decreases in the most recent time steps of the model although the comet is still approaching the Sun. This is interpreted as an artifact introduced by the overlap of the shells of large particle size emitted shortly before the observation. The model derives an increasing number of small particles released by the comet in the second half of November. This is at least in part considered as real and attributed to particle fragmentation occurring when the comet was at a heliocentric distance of about 1.4 AU.     

Dust coma of Comet C/1999 S4 (LINEAR): imaging polarimetry during nucleus disruption

Imaging and polarimetry of Comet C/1999 S4 (LINEAR) during its disruption provide information about the physical properties of the scattering dust particles, and some insight into the structure of the nucleus. A significant decrease in the brightness was noticed, together with a drastic change in the shape of the dust coma. The whole-coma polarization increased, which was typical of a comet with a near 27 percent maximum in polarization, the increase being comparable to previous observations for comets suffering a limited fragmentation. An important gradient in the intensity on the solar side corresponds to the ejected material. The degree of polarization in this region is higher than generally observed in jets and it increases with time as the nucleus (or its fragments) breaks up and ejects relatively large and compact particles. In the surrounding coma, these large particles are fragmented on short time-scales, indicated by the decrease of polarization. These results suggest that the fragile nucleus was not, as far as the physical properties of the dust are concerned, differentiated, and that it was possibly built of primordial cometesimals originating from the same formation region.     

Radar observations of Comet 2P/Encke during the 2003 apparition

The nucleus of Comet 2P/Encke was detected with the Arecibo radar during the close approach of November, 2003, making this the first comet to yield radar detections at two different apparitions. Although the measured radar cross section of 1.0 km² was close to that obtained in 1980, the Doppler bandwidth was nearly four times larger. Most of this bandwidth difference can simply be attributed to a different observing aspect relative to the spin axis proposed by Sekanina [1988, Astron. J. 95, 911] and Festou and Barale [2000, Astron. J. 119, 3119]. Comparison of the 2003 Doppler bandwidth with infrared-based size estimates supports an 11-h dominant rotation period and excludes slower 15- and 22-h periods that have also been suggested. If one assumes a short-axis-mode rotation with an 11-h period, then the Doppler bandwidth indicates that the nucleus is an oblong object with a long-axis dimension of 9 km. The estimated radar albedo of 0.05 is similar to that measured for C/IRAS-Araki-Alcock, providing further evidence that comet nuclei have relatively low surface densities of ∼0.5-1.0 g cm⁻³. No broadband echo component was detected from large coma grains despite predictions, based on optical/infrared models, that such a component might be detectable.     

The organic composition of Comet C/2001 A2 (LINEAR): I. Evidence for an unusual organic chemistry

We used the NIRSPEC instrument on the Keck-2 telescope atop Mauna Kea, HI to observe Comet C/2001 A2 (LINEAR) in a Target of Opportunity campaign on UT 2001 July 9.5, 10.5 August 4.4, 10.5. We measured seven organic parent volatiles (C₂H₆, C₂H₂, HCN, CH₄, CO, CH₃OH, H₂CO) simultaneously with H₂O. We obtained absolute production rates and relative abundances for parent volatiles, and also measured rotational temperatures for several of these species. The chemical composition of C/2001 A2 differs substantially from any comet we have observed to date. The abundances we measure (relative to H₂O) for C₂H₆, C₂H₂, HCN, and CH₃OH are enriched by a factor of ∼2 to 3 in C/2001 A2 compared with most comets in our database. Other molecular species were detected within the typical range of measured abundances. C/2001 A2 presented a unique opportunity to study the chemistry of a fragmenting comet where pristine areas are exposed to the Sun.     

Dust tail of the active distant Comet C/2003 WT42 (LINEAR) studied with photometric and spectroscopic observations

We present the study of dust environment of dynamically new Comet C/2003 WT42 (LINEAR) based on spectroscopic and photometric observations. The comet was observed before and after the perihelion passage at heliocentric distances from 5.2 to 9.5 AU. Although the comet moved beyond the zone where water ice sublimation could be significant, its bright coma and extended dust tail evidenced the high level of physical activity. Afρ values exceeded 3000 cm likely reaching its maximum before the perihelion passage. At the same time, the spectrum of the comet did not reveal molecular emission features above the reflected continuum. Reddening of the continuum derived from the cometary spectrum is nonlinear along the dispersion with the steeper slop in the blue region. The pair of the blue and red continuum images was analyzed to estimate a color of the comet. The mean normalized reflectivity gradient derived from the innermost part of the cometary coma equals to 8% per 1000 Å that is typical for Oort cloud objects. However, the color map shows that the reddening of the cometary dust varies over the coma increasing to 15% per 1000 Å along the tail axis. The photometric images were fitted with a Monte Carlo model to construct the theoretical brightness distribution of the cometary coma and tail and to investigate the development of the cometary activity along the orbit. As the dust particles of distant comets are expected to be icy, we propose here the model, which describes the tail formation taking into account sublimation of grains along their orbits. The chemical composition and structure of these particles are assumed to correspond with Greenberg’s interstellar dust model of comet dust. All images were fitted with the close values of the model parameters. According to the results of the modeling, the physical activity of the comet is mainly determined by two active areas with outflows into the wide cones. The obliquity of the rotation axis of the nucleus equals to 20° relative to the comet’s orbital plane. The grains occupying the coma and tail are rather large amounting to 1 mm in size, with the exponential size distribution of a^−4.5. The outflow velocities of the dust particles vary from a few centimeters to tens of meters per second depending on their sizes. Our observations and the model findings evidence that the activity of the nucleus decreased sharply to a low-level phase at the end of April–beginning of May 2007. About 190 days later, in the first half of November 2007 the nucleus stopped any activity, however, the remnant tail did not disappear for more than 1.5 years at least.     

Optical polarimetry of Comet NEAT C/2001 Q4

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.     

Circular polarization in comets: Observations of Comet C/1999 S4 (LINEAR) and tentative interpretation

Comet C/1999 S4 (LINEAR) was exceptional in many respects. Its nucleus underwent multiple fragmentations culminating in the complete disruption around July 20, 2000. We present circular polarization measurements along the cuts through the coma and nucleus of the comet during three separate observing runs, in June 28-July 2, July 8-9, and July 21-22, 2000. The circular polarization was detected at a rather high level, up to 0.8%. The left-handed as well as right-handed polarization was observed over the coma with the left circularly polarized light systematically observed in the sunward part of the coma. During our observations the phase angle of the comet varied from 61 up to 122°, which allowed us to reveal variations of circular polarization with the phase angle. Correlation between the degree of circular polarization, visual magnitude, water production rate, and linear polarization of Comet C/1999 S4 (LINEAR) during its final fragmentation in July 2000 was found. The mechanisms that may produce circular polarization in comets and specifically in Comet C/1999 S4 (LINEAR) are discussed and some tentative interpretation is presented.     

Spin Temperature of Ammonia Determined from NH2 in Comet C/2001 A2 (LINEAR)

The ortho-to-para ratio (OPR) of a cometary molecule is one of primordial character in comets. The OPR which is characterized by a spin temperature, is thought to reflect the formation conditions of the molecule. In this paper we show the high-dispersion spectrum of cometary NH₂ in Comet C/2001 A2 (LINEAR), from which the OPR of NH₂ is determined based on the fluorescence excitation model. Since the NH₂ is a photodissociation product of cometary ammonia, we applied the permutation group theory to the whole reaction system (i.e. the photodissociation reaction of ammonia to NH₂ and H) in order to derive the OPR of ammonia from that of NH₂. The derived OPR of ammonia is 1.12 ± 0.03 in Comet C/2001 A2 (LINEAR). This value corresponds to a spin temperature of 30⁺³ _-2 K. If this reflects the temperature where the comet formed in the protosolar nebula, our result indicates that thiscomet was formed in the region of the giant planets between Jupiter and Neptune.     

Observations of Comet C/2001 A2 (Linear) with the Multipupil Fiber Spectrograph

We present preliminary results of observations of the spectrum of comet C/2001 A2(LINEAR) obtained with the Multipupil Fiber Spectrograph, installed in the PrimeFocus of the 6-m Telescope of the SAO of the RAS (Northern Caucasus, NizhnyArkhyz), on August 11/12, 12/13 and 14/15, 2001. High resolution spectra of thecomet were obtained in the range from 3800–5200 AA. The emission lines in the spectrum of comet C/2001 A2 (LINEAR) on Aug. 12, 2001 were identified. Thefluorescent continuum in the spectrum of comet C/2001 A2 (LINEAR) on Aug. 12,2001 is detected.     

Arecibo radar observations of Phobos and Deimos

In November 2005, we observed the moons of Mars using the Arecibo 2380-MHz (13-cm) radar, obtaining a result for the OC radar albedo of Phobos (0.056±0.014) consistent with its previously reported radar albedo and implying an upper bound on its near-surface bulk density of . We detected Deimos by radar for the first time, finding its OC radar albedo to be 0.021±0.006, implying an upper bound on its near-surface density of , consistent with a high-porosity regolith. We briefly discuss reasons for these low radar albedos, Deimos' being possibly the lowest of any Solar System body yet observed by radar.     

A multi-wavelength study of parent volatile abundances in Comet C/2006 M4 (SWAN)

Volatile organic emissions were detected post-perihelion in the long-period Comet C/2006 M4 (SWAN) in October and November 2006. Our study combines target-of-opportunity infrared observations using the Cryogenic Echelle Spectrometer (CSHELL) at the NASA-IRTF 3-m telescope, and millimeter wavelength observations using the Arizona Radio Observatory (ARO) 12-m telescope. Five parent volatiles were measured with CSHELL (H₂O, CO, CH₃OH, CH₄, and C₂H₆), and two additional species (HCN and CS) were measured with the ARO 12-m. These revealed highly depleted CO and somewhat enriched CH₃OH compared with abundances observed in the dominant group of long-period (Oort cloud) comets in our sample and similar to those observed recently in Comet 8P/Tuttle. This may indicate highly efficient H-atom addition to CO at very low temperature (∼10-20 K) on the surfaces of interstellar (pre-cometary) grains. Comet C/2006 M4 had nearly “normal” C₂H₆ and CH₄, suggesting a processing history similar to that experienced by the dominant group. When compared with estimated water production at the time of the millimeter observations, HCN was slightly depleted compared with the normal abundance in comets based on IR observations but was consistent with the majority of values from the millimeter. The ratio CS/HCN in C/2006 M4 was within the range measured in ten comets at millimeter wavelengths. The higher apparent H-atom conversion efficiency compared with most comets may indicate that the icy grains incorporated into C/2006 M4 were exposed to higher H-atom densities, or alternatively to similar densities but for a longer period of time.     

Infrared observations of Comet 81P/Wild 2 in 1997

Comet 81P/Wild 2 was observed in the thermal infrared over 6 months during its 1997 perihelion passage. The comet was most active in late February, about 3 months preperihelion; dust production declined by a factor of 3 between February and August. For the GIOTTO Halley dust size distribution, maximum dust production rate was ∼2 × 10⁶ g/s. The comet displayed a 10-μm silicate feature about 25% above the continuum, similar to several other Jupiter-family comets, but much lower than that seen in a number of Oort cloud comets.NASA’s STARDUST sample return mission will encounter P/Wild 2 98 days postperihelion in January 2004. Based on our observations at a similar point in the orbit and the Halley size distribution, we predict that the mass fluence on the spacecraft for a 150 km miss distance will be about 8 × 10⁻⁶ g/cm² for particles up to 1 cm in radius. The corresponding areal coverage will be about 10⁻⁴.     

polarimetric and photometric observations of split comet C/2001 A2 (LINEAR)

We present the results of polarimetric and photometricobservations of split Comet C/2001 A2 (LINEAR), which wereobtained at the 70-cm telescope of the Astronomical Observatoryof Kharkiv National University between 30 June and 31 July 2001.The brightest fragment of the comet, nucleus B, was observed.Eight narrowband cometary filters in the continuum and in emissionbands and a wideband red filter have been used. The comet wasobserved through apertures of 88, 33, and 19 arcsec. Polarizationphase curves were obtained for the continuum and for the firsttime, for NH₂(0, 7, 0) emission. The degree of polarization ofthe light scattered by the dust decreases with the increase ofaperture size. An important temporal variation of the polarizationwith a rotation of the polarization plane was observed at twophase angles (26.5° and 36.2°). Molecular column densities and production rates of CN, C₂,C₃, and NH₂ species are calculated in the framework of theHaser model. A comparative analysis of the temporal variations ofthe visual magnitudes, gas and dust production rates, dust colorand polarization are presented.     

Radar observations of 8P/Tuttle: A contact-binary comet

Arecibo radar imagery of Comet 8P/Tuttle reveals a 10-km-long nucleus with a highly bifurcated shape consistent with a contact binary. A separate echo component was also detected from large (>cm-size), slow-moving grains of the type expected to contribute to the Ursid meteor stream.     

Observations of the long-lasting activity of the distant Comets 29P Schwassmann-Wachmann 1, C/2003 WT42 (LINEAR) and C/2002 VQ94 (LINEAR)

We investigated three comets, which are active at large heliocentric distances, using observations obtained at the 6-m BTA telescope (SAO RAS, Russia) in the photometric mode of the focal reducer SCORPIO. The three comets, 29P/Schwassmann-Wachmann 1, C/2003 WT42 (LINEAR), and C/2002 VQ94 (LINEAR), were observed after their perihelion passages at heliocentric distances between 5.5 and 7.08 AU. The dust production rates in terms of Afρ was measured for these comets. Using the retrieved values, an average dust production rate was derived under different model assumptions. A tentative calculation of the total mass loss of the comet nucleus within a certain observation period was executed. We calculated the corresponding thickness of the depleted uppermost layer where high-volatile ices completely sublimated. The results obtained in our study strongly support the idea that the observed activity of Comet SW1 requires a permanent demolition of the upper surface layers.     

Comparison of X-rays from Comets LINEAR (C/1999 S4) and McNaught-Hartley (C/1999 T1)

The Chandra X-ray Observatory (CXO) observations of Comets McNaught-Hartley (MH) and LINEAR S4 (S4) have been processed in the same way to compare X-rays from those comets. The X-ray isophotes are crescent-like in S4 and more circular in MH because of the different phase angles (98° and 44°, respectively). The peak X-ray brightness is greater in S4 than that in MH by a factor of 1.5 and smaller by a factor of 1.7 after the correction for heliocentric distance. The X-ray luminosities of MH and S4 are equal to 8.6 and 1.4×10¹⁵ erg s⁻¹ inside the apertures of ρ=1.5 and 0.5×10⁴ km, respectively. (Brightness is 20% of the peak value at these ρ.) Efficiencies of X-ray excitation corrected to the solar wind flow are similar and equal to 4.3×10⁻¹⁴ erg AU^3/2 in both comets. This confirms the solar wind excitation of X-rays in comets. Spectra of the comets were extracted with a special care of the background correction and using an energy-dependent spectral resolution code. The MH spectrum consists of ten emissions instead of nine emissions in the previously published spectrum. The new emission at 307 eV fills in a strong minimum in the previous spectrum and removes the major difference between that spectrum and the synthetic spectrum. This emission is assigned to the C⁺⁴ and Mg⁺⁹ lines. The positions of the other emissions and their identification are similar to those in the previous spectrum. The S4 spectrum consists of eight emissions, and four emissions are the same as in MH. The line identification is given. Ion ratios in the solar wind have been extracted from the spectra. O⁺⁸/O⁺⁷ is equal to 0.29±0.04 and 0.14±0.02 in MH and S4, and this difference correlates with the higher solar wind speed in S4. Ne⁺⁹/O⁺⁷ is (15±6)×10⁻³ and (19±7)×10⁻³, and these are the first data on Ne⁺⁹ in the solar wind. C⁺⁶/O⁺⁷ is 0.7±0.2 in both MH and S4. X-ray spectroscopy of comets may be used as a diagnostic tool to study the solar wind composition.