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.     

Semimajor axes of the orbits and ejection velocities of Perseid meteoroids

Photographic orbits of meteors are combined with modeling of the ejection of Perseids during the perihelion passage of comet Swift-Tuttle in 1862 to analyze the most likely ejection velocities of particles from the comet nucleus. Given the scatter of the semimajor axes of observed Perseids with masses greater than 10^–4 g, the most likely interval of ejection velocities spans 0 to 300 m/s for particles ejected in the plane of the comet orbit in the retrograde direction and in the direction of the comets anomalous tail.__________Translated from Astronomicheskii Vestnik, Vol. 39, No. 2, 2005, pp. 184–190.Original Russian Text Copyright © 2005 by Ishmukhametova, Kondrateva.     

Near-Infrared Imaging Spectrophotometry Of Comet C/1995 O1 (Hale-Bopp) Near Perihelion

We present 1- to 5-μm broadband and CVF images of comet Hale-Bopp taken 1997 February 10.5 UT, 50 days before perihelion. All the images exhibit a nonspherical coma with a bright “ridge” in the direction of the dust tail approximately 10″ from the coma. Synthetic aperture spectrophotometry implies that the optically important grains are of a radius ≤0.4 μm; smallest radius for any comet seen to date. The variation of the integrated surface brightness with radial distance from the coma (ρ) in all the images closely follows the “steady state” ρ⁻¹ model for comet dust ablation (Gehrz and Ney, 1992). The near-infrared colors taken along the dust tail are not constant implying the dust grain properties vary with coma distance. This revised version was published online in July 2006 with corrections to the Cover Date.     

Infrared Observations Of Dust Emission From Comet Hale-Bopp

We present infrared imaging and photometry of the bright, giant comet C/1995 O1 (Hale-Bopp). The comet was observed in an extended infrared and optical observing campaign in 1996–1997. The infrared morphology of the comet was observed to change from the 6 to 8 jet “porcupine” structure in 1996 to the “pinwheel” structure seen in 1997; this has implications for the position of the rotational angular momentum vector. Long term light curves taken at 11.3 μm indicate a dust production rate that varies with heliocentric distance as ∶ r^−1.4. Short term light curves taken at perihelion indicate a rotational periodicity of 11.3 hours and a projected dust outflow speed of ∶ 0.4 km s⁻¹. The spectral energy distribution of the dust on October 31, 1996 is well modeled by a mixture of 70% silicaceous and 30% carbonaceous non-porous grains, with a small particle dominated size distribution like that seen for comet P/Halley (McDonnell et al., 1991), an overall dust production rate of 2 × 10⁵ kg s⁻¹, a dust-to-gas ratio of ∶5, and an albedo of 39%. This revised version was published online in July 2006 with corrections to the Cover Date.     

Models for the Origin of the Quadrantids

We present two scenarios for production of the Quadrantid stream based on two different models for its origin: the extinct model in which 2003EH1 was an active comet that released the dust particles during past 5000 years, stopping its activity abruptly in AD 1488; and the split model; in which a catastrophic disruption of an asteroid at AD 1488 released a large number of dust particles in a single event. We calculate the orbital evolution of test particles released in both cases and derive the resulting size distribution of surviving meteoroids in the current Quadrantid stream in the form of s ^−α ds, where s denotes the radius of a meteoroid. We find α = 3.1 in the extinct model and 2.0 in the split model. In addition, the radius of the surviving meteoroids is derived as s >10 μm in the both models. We propose, based on our estimation of the infrared color ratio for the Quadrantid stream derived from both models, that infrared observations of the Quadrantid stream may determine which origin model is more reasonable.     

The Dust in Comet C/1999 S4 (LINEAR) during Its Disintegration: Narrow-Band Images, Color Maps, and Dynamical Models

Comet C/1999 S4 was observed with the 2m-telescopes of the Bulgarian National Observatory and Pik Terskol Observatory, Northern Caucasus, Russia, at the time of its disintegration. Maps of the dust brightness and color were constructed from images obtained in red and blue continuum windows, free from cometary molecular emissions. We analyze the dust environment of Comet C/1999 S4 (LINEAR) taking into account the observed changes apparent in the brightness images and in plots of Afρ profiles as function of the projected distance ρ from the nucleus. We also make use of the syndyne-synchrone formalism and of a Monte Carlo model based on the Finson-Probstein theory of dusty comets. The brightness and color of individual dust particles, which is needed to derive theoretical brightness and color maps of the cometary dust coma from the Monte Carlo model, is determined from calculations of the light scattering properties of randomly oriented oblate spheroids. In general, the dust of Comet C/1999 S4 (LINEAR) is strongly reddened, with reddening values up to 30%/1000 Å in some locations. Often the reddening is higher in envelopes further away from the nucleus. We observed two outbursts of the comet with brightness peaks on July 14 and just before July 24, 2000, when the final disintegration of the comet started. During both outbursts an excess of small particles was released. Shortly after both outbursts the dust coma “turns blue.” After the first outburst, the whole coma was affected; after the second one only a narrow band of reduced color close to the tail axis was formed. This difference is explained by different terminal ejection speeds, which were much lower than normal in case of the second outburst. In particular in the second, final outburst the excess small particles could originate from fragmentation of “fresh” larger particles.     

The dust trail of Comet 67P/Churyumov-Gerasimenko

We report the detection of Comet 67P/Churyumov-Gerasimenko's dust trail and nucleus in 24 μm Spitzer Space Telescope images taken February 2004. The dust trail is not found in optical Palomar images taken June 2003. Both the optical and infrared images show a distinct neck-line tail structure, offset from the projected orbit of the comet. We compare our observations to simulated images using a Monte Carlo approach and a dynamical model for comet dust. We estimate the trail to be at least one orbit old (6.6 years) and consist of particles of size ?100 μm. The neck-line is composed of similar sized particles, but younger in age. Together, our observations and simulations suggest grains 100 μm and larger in size dominate the total mass ejected from the comet. The radiometric effective radius of the nucleus is 1.87±0.08 km, derived from the Spitzer observation. The Rosetta spacecraft is expected to arrive at and orbit this comet in 2014. Assuming the trail is comprised solely of 1 mm radius grains, we compute a low probability (∼¹⁰⁻³) of a trail grain impacting with Rosetta during approach and orbit insertion.     

Observations of Ions in Comets: A Contribution Towards Understanding the Comet-Solar Wind Interaction

Since many years cometary ions have been observed by the authors and their coworkers in order to study the comet-solar wind interaction. Comets with water production rates ranging from 10²⁸(46P/Wirtanen) to 6 10³⁰molecules s⁻¹(C/1995 O1 Hale-Bopp) have been observed. In this paper we briefly introduce the physics of the comet-solar wind interaction. New observations of comet C/1996 Q1 (Tabur) are presented, where for the first time H₂O⁺and CO⁺ions have been recorded exactly simultaneously with a two-channel system. They are compared with previous observations of comets C/1989 X1 (Austin), 46P (Wirtanen) and 109P (Swift-Tuttle). We use a new method of Wegmann et al. (1998), based on the MHD scaling law, to determine the water production of comet Tabur from its H₂O⁺column density map and obtain a value of 3.3 10²⁸water molecules s⁻¹. Nonstationary phenomena like tail rays and so-called tail disconnections are very briefly reviewed. A movie of plasma envelopes observed in the light of OH⁺in comet 1995 O1 (Hale-Bopp) is presented on the attached CD-ROM. This revised version was published online in July 2006 with corrections to the Cover Date.     

Observations of the Dust Cloud in Comet Hale-Bopp on 9–11 May 1997

During our monitoring observations of comet Hale-Bopp, we found several sporadic ejections of dust from the nucleus. The most prominent ejection was observed on May 6–9, 1997, in the post-perihelion phase of the apparition. In this paper, we report preliminary analysis of this event, in which the total mass of the dust cloud is estimated to be 1.6 × 10¹¹ g. This revised version was published online in July 2006 with corrections to the Cover Date.     

Search for New Parent Bodies of Meteoroid Streams Among Comets. I. Showers of Comets 126P/1996 P1 and 161P/2004 V2 with Radiants on Southern Sky

We deal with theoretical meteoroid streams the parent bodies of which are two Halley-type comets in orbits situated at a relatively large distance from the orbit of Earth: 126P/1996 P1 and 161P/2004 V2. For two perihelion passages of each comet in the far past, we model the theoretical stream and follow its dynamical evolution until the present. We predict the characteristics of potential meteor showers according to the dynamical properties of theoretical particles currently approaching the orbit of the Earth. Our dynamical study reveals that the comet 161P/2004 V2 could have an associated Earth-observable meteor shower, although no significant number of theoretical particles are identified with real, photographic, video, or radar meteors. However, the mean radiant of the shower is predicted on the southern sky (its declination is about −23°) where a relatively low number of real meteors has been detected and, therefore, recorded in the databases used. The shower of 161P has a compact radiant area and a relatively large geocentric velocity of ∼53 km s⁻¹. A significant fraction of particles assumed to be released from comet 126P also cross the Earth’s orbit and, eventually, could be observed as meteors. However, their radiant area is largely dispersed (declination of radiants spans from about +60° to the south pole) and, therefore, mixed with the sporadic meteor background. An identification with real meteors is practically impossible.     

Comet Hale–Bopp: Velocity Field Of Ions From Fabry-Pérot Imaging

Fabry-Pérot interferograms of comet Hale-Bopp were obtained on several nights in March and April 1997. For this purpose we utilized the 2-channel focal reducer of the Max-Planck-Institute for Aeronomy at the 2-m telescope of the Pik Terskol Observatory. Solid Fabry-Pérot etalons of resolving power 30000 were used in both channels of the focal reducer. The main aim of this study is to measure the velocities and abundances of OH⁺ and H₂O⁺, both ions closely related to the same parent molecule, H₂O. In the blue channel interferograms we identified several individual OH⁺ rotational lines of the A³Π_i - X³Σ⁻ (0-0) transition and measured their Doppler shifts. The target emissions in the red channel were the H₂O⁺ lines of the A²A₁ − X²B₁ (10-0) band. We found that the line of sight velocities, obtained from the Doppler shifted wavelengths of emissions in the comet are higher in sunward direction than in the plasma tail and do not exceed 20 km s^–1. The corresponding values, deprojected in antisolar direction, are consistent with predictions by magnetohydrodynamical models of the solar-wind-comet interaction, when one accounts for the extremely high gas production rate of comet Hale-Bopp. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dust Shells Around Carbon Mira Variables

The spectral energy distributions and mid-infrared spectra of 44 carbon Mira variables are fitted using a dust radiative transfer model. The periods cover the entire range observed for carbon Miras. The luminosities are calculated from a period-luminosity relation. Parameters derived are the distance, the dust mass loss rate and the ratio of silicon carbide to amorphous carbon dust. The total mass loss rate is derived from a modified relation between the photon momentum (L/c) and the momentum in the wind (M υ_∞). Mass loss rates between 1 × 10⁻⁸ and 4 × 10⁻⁵ M_⊙ yr⁻¹ are found. We find good correlations between mass loss rate and pulsation period, and mass loss rate and luminosity. The dust-to-gas ratio appears to be almost constant up to periods of about 500 days, corresponding to about 7900 L_⊙, and then to increase by a factor of 5 towards longer periods and higher luminosities. A comparison is made with radiation-hydrodynamical calculations including dust formation. The mass loss rates predicted by these models are consistent with those derived in this paper. The main discrepancy is in the predicted expansion velocities for models with luminosities below ∼5000 L_⊙. The radiation-hydrodynamical calculations predict expansion velocities which are significantly too large. This is related to the fact that these models need to be calculated with a large C/O ratio to get an outflow in the first place. This is contrary to observations. It indicates that a principle physical ingredient in these radiation-hydrodynamical calculations is still missing. Possibly the winds are ‘clumpy’ which may lead to dust formation on a local scale, or there is an additional outwards directed force, possibly radiation pressure on molecules. This revised version was published online in September 2006 with corrections to the Cover Date.     

Comet Tempel-Tuttle and the Leonid meteors

The distribution of dust surrounding periodic comet Tempel-Tuttle has been mapped by analyzing the associated Leonid meteor shower data over the 902–1969 interval. The majority of dust ejected from the parent comet evolves to a position lagging the comet and outside the comet's orbit. The outgassing and dust ejection required to explain the parent comet's deviation from pure gravitational motion would preferentially place dust in a position leading the comet and inside the comet's orbit. Hence it appears that radiation pressure and planetary perturbations, rather than ejection processes, control the dynamic evolution of the Leonid particles. Significant Leonid meteor showers are possible roughly 2500 days before or after the parent comet reaches perihelion but only if the comet passes closer than 0.025 AU inside or 0.010 AU outside the Earth's orbit. Although the conditions in 1998–1999 are optimum for a significant Leonid meteor shower, the event is not certain because the dust particle distribution near the comet is far from uniform. As a by-product of this study, the orbit of comet Tempel-Tuttle has been redetermined for the 1366–1966 observed interval.     

Photographic observations of comet Hale-Bopp at the Pulkovo Observatory: The detection of dust envelopes

The photographic observations of comet Hale-Bopp with the 26-inch Pulkovo Observatory refractor in March–April, 1998, revealed three hemispherical gas-dust envelopes and one spiral jet in the comet head. We determined the angular distances of these envelopes from the comet nucleus and estimated their velocities. The masses and sizes of dust grains were estimated. We conclude that submicron-sized grains dominate in the envelopes. We also estimated the time scale of the comet nuclear activity, which manifests itself in dust ejection, the initial velocity of the ejected dust grains, and the ratio of the radiation-pressure force exerted on dust grains to the force of their gravitational attraction to the Sun. Our observations yielded an estimate for the radius of the comet nucleus, ～30 km.     

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.     

Simulating H2O Maser Emission in the Mass Outflows from Evolved Stars

The combination of a time-dependent spherically symmetric hydrodynamic model of stellar atmosphere pulsation and a radiation transport code, which incorporates maser saturation theory, enabled us to synthesise maps and spectra of H₂O maser emission from the circumstellar envelopes of long period variable stars. The synthetic maps and spectra compare favourably with observed 22, 321 and 325 GHz H₂O maser emission. As is observed in H₂O maser regions the peak emission occurs between 3–8 stellar radii from the star. The calculated H₂O maser regions are in conditions of n_H2 = 10⁶ − 10⁸ cm⁻³, assuming a fractional abundance of 10⁻⁴; kinetic temperatures of 550–3000 K; dust ensemble temperatures of 500–1200 K and an accelerating velocity field. The IR radiation field is explicitly included in the radiation transport model, incorporating the latest absorption efficiency data for silicates from Draine. We reproduce the features seen in high angular resolution MERLIN spectral line datacubes. This shows that a mass outflow model which extends the photosphere using pulsations and incorporates radiation pressure on silicate based dust particles can produce the observed data on small (10-mas) angular scales. This revised version was published online in September 2006 with corrections to the Cover Date.     

Aperture Polarimetry And Photometry Of Comet Hale-Bopp

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.     

Sodium In Comets

A great deal of attention has been given to the production and spatial distribution of sodium in comets after the discovery of the sodium tail, by Cremonese et al. (1997a), on Hale-Bopp. The sodium has been observed in several comets in the past, but the Hale-Bopp represent the first time where it will be deeply analyzed considering the several data and scientists working on that. The sodium tail stimulated different studies trying to explain the mechanism source and provided the new lifetime for photoionization of the neutral sodium atom. We took into account other sodium observations performed in this century and we focalized our attention to comet Hale-Bopp to understand the main sources responsible for the sodium features observed. We analyzed the sodium tail observations performed by Cremonese et al. (1997b) and Wilson et al. (1998) finding that the Hale-Bopp had four different tails. The wide field images and the high resolution spectroscopy performed along the sodium tail provided very important clues to distinguish the two sodium tails observed and their possible sources. Considering most of the data reported in several papers has been possible to draw a real sketch on what has occurred to the comet during March and April 1997. We are going to demonstrate that the sodium tail observed by Wilson et al. (1998) was not the same reportedby Cremonese et al. (1997a) and in the images taken by the European Hale-Bopp Team there were two distinct sodium tails. The observations allowed us to define “narrow sodium tail” the tail reported by Cremonese et al. (1997a), and “diffuse sodium tail” the tail overimposed to the dust tail. We suggest that the narrow sodium tail was due to a molecular process instead of the diffuse one due to the release of sodium atoms by the dust particles. Such a conclusion is supported by the spatial distribution of sodium on the nucleus and in the coma as reported from other authors. This revised version was published online in July 2006 with corrections to the Cover Date.     

Detection Of Soft X-Ray Emission From Comet C/1995 O1 (Hale–Bopp)

We report the detection of soft X-rays from comet C/1995 O1 (Hale-Bopp) by the Low Energy Concentrator Spectrometer (LECS) on-board the X-ray satellite, BeppoSAX. The observations took place on 1996 September 10–11 approximately 1 day after a large dust outburst (Schulz et al., 1997–1999). After correcting for the comets motion, a 7σ enhancement was found centered (2.1 ± 1.3) x 10⁵ km from the position of the nucleus, in the general solar direction. The total X-ray luminosity in the 0.1–2.0 keV energy band is 5 x 10¹⁶ erg s⁻¹ which is at least a factor of ∼ 3 greater than measured by the Extreme Ultraviolet Explorer (EUVE)4 days later and suggests that the bulk of the emission measured by the LECS is related to the dust outburst. The extracted LECS spectrum is well fit by a thermal bremsstrahlung-like distribution of temperature of 0.29 ± 0.06 keV - consistent with that observed in other comets. We find no evidence for fluorescent carbon or oxygen emission and place 95% confidence limits of 1.0 x 10¹⁵ and 7.8 x 10¹⁵ erg s⁻¹ to narrow line emission at 0.28 and 0.53 keV, respectively. We calculate that if such lines are present, they constitute at most 18% of the 0.1–2.0 keV continuum luminosity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evolution of C/2006 M4 (SWAN) comet tails

We studied variations in the structure of plasma and dust tails of the C/2006 M4 (SWAN) comet during a long observation period (September–December 2006). We found sizes of grains ejected by the comet from the synchronic-syndynamic analysis of comet images. We calculated solar wind speed for high heliographic latitudes from calculations of the aberration angle of the comet plasma tail. Rapid changes in the calculated values of the solar wind speed are caused by its variable transversal component.