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.     

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.     

Ir Observations Of An Outburst In Comet Hale-Bopp (C/1995 O1)

The comet Hale-Bopp (C/1995 O1) has been observed in the infrared (1–2.5 μm) with the Nordic Optical Telescope (NOT) equipped with the Arcetri NICMOS3 camera (ARNICA). Two observational campaigns, each one lasting about one week, were made when the comet heliocentric distance was about 3 AU. The first campaign was at the end of August and the second at the end of September 1996. During both runs two major outbursts were observed, the more intense of them started the day before the beginning of the second run. In the images recorded during the first three nights (24.8–26.8 Sept.) of the second run a dust shell expanding in the northern quadrant with a projected velocity of 0.14–0.28 km/s is clearly evident. The dust production rate increased by at least a factor ≈3 at the time of the outburst. Also evident on the first night is a change in the IR color that is well correlated with the dust shell. This is an indication that the material released by the outburst has a different composition and/or size distribution than that in the “quiescent” dust coma. In this paper we present preliminary results about the evolution and the photometric characteristics of the dust shell. This revised version was published online in July 2006 with corrections to the Cover Date.     

Model analysis of the dust tail of comet Hale-Bopp

The paper presents the results of dynamic simulation for the dust tail formation of comet C/1995 O1 (Hale-Bopp). To simulate the dust tail, the trajectories of 2 × 10⁶ dust particles were traced. The sizes, ejection moments, outflow directions and velocities of the dust particles were defined by the Monte Carlo algorithm. The obtained three-dimensional tail was projected on the sky plane to compare it with the observed images. The brightness distribution in the comet tail was fitted to similar model parameters for three different dates. According to our model experiments, the observed tails could be formed by particles with sizes from 0.3 to 8.0 μm, ejection velocities from 0.155 to 0.670 km/s, and power index of the exponential size distribution from −3.6 to −3.7. It is shown that the inclusion of the particles fragmentation processes leads to a noticeable improvement of the simulation results.     

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

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

Spectrocam-10 Thermal Infrared Observations Of The Dust In Comet C/1995 O1 Hale-Bopp

We describe 5 to 18 μm broadband images and R ∼ 100 spectra of comet Hale-Bopp taken with SpectroCam-10 on the 5 m Hale telescope during six runs between 1996 June and 1997 April. Our data show the gradual warming of dust grains as the comet approached perihelion. In June, the 10 μm silicate emission feature was already stronger than observed in any other comet, and it increased to about 3 times the continuum level near perihelion. Spectral structure attributed to a crystalline olivine grain component remained relatively constant, but other features associated with pyroxenes appeared to vary with heliocentric distance. This revised version was published online in July 2006 with corrections to the Cover Date.     

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

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

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.     

Images Of Polarization And Colour In The Inner Coma Of Comet Hale-Bopp

On March 31 and April 1, 1997, simultaneous photometry and polarimetry of comet Hale-Bopp's dust was conducted with the two-channel focal reducer of the Max-Planck-Institute for Aeronomy attached to the 2 m telescope of Pik Terskol Observatory (Northern Caucasus). Interference filters at642 nm and 443 nm selected red and blue narrow-band continuum windows. The observations have been averaged over the one hour of timethe comet could be observed. The polarization maps cover an area of about1 arcmin² around the nucleus. The values of polarization degree measured close to the nucleus agree very well with observations obtained with aperture polarimetry. They are lower than in the surrounding coma by about 1%. In our field of view the polarization increases along the sun-comet line from the solar to the antisolar side by about 3%. The dust shells are visible in the polarization images. The polarization in the shells is higher by 1 to 2%and this increase is higher in the red than in the blue range. Therefore the ratio of red to blue polarization (≈ 1.2) increases in the shells by ≈ 0.03. In principle, the polarization excess in the shells, the ratio of red/blue polarization and the higher integrated polarization as compared to other comets can be explained by an excess of particles of radius of about 0.1 μm. Such particles, however, are subject to strong radiation pressure and will be pushed back into the tail before they reach the observed location of the shells. Real Rayleigh particles cannot explain the observed increase in the ratio of red/blue polarization. One therefore cannot exclude the possibility that the excess polarization in the shells is caused by fluffy aggregates via effects which are presently not well understood. The colour map shows features not well related to intensity and polarization, perhaps another dust shell of a different particle size. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Dust Activity in Comet 67P/Churyumov–Gerasimenko from February 20 to April 20, 2003

Broadband imaging of Comet 67P/Churyumov–Gerasimenko has provided more data on the characterisation of the target of the ESA Rosetta Mission. The comet monitoring between r _h=2.37 and r _h=2.78 AU postperihelion shows a prominent dust coma which extends up to ≈ 25,000 km from the nucleus, and a long dust structure in approximately anti-tail direction, reaching at least 230,000 km, identified as a neck-line structure. The non-isotropic dust emission is detected from the structures in the inner coma, and it is reflected on the slope of linear fits of surface brightness profiles vs. cometocentric projected distance in log–log representation as m ≈ 0.83−0.941. Besides the long dust spike at position angle of 295°, the morphological study of the dust coma confirms the presence of two structures at position angles of 95 and 195° where the overabundance of dust can be as high as 50% at ρ ≤ 30,000 km. The A f ρ parameter derived from our R broadband data is 26.0 and 29.8 cm at r _h=2.37 and 2.48 AU postperihelion. The dust reflectivity S′(λ), a measurement of the dust colour, is 0.061±0.019, a rather neutral colour.     

Nucleus and tail studies of comet P/Swift-tuttle

CCD images of comet P/Swift-Tuttle, obtained in April 1994 with the 2.2m telescope at ESO La Silla/Chile, showed a comaless stellar nucleus. From absolute photometry we estimated the equivalent radius of the cometary nucleus to be about 11 km (assuming an albedo of 0.04 as for P/Halley) for two rotation phase angles which differ by about 75 deg. From that we conclude that the nucleus is either of rather spherical shape or that the viewing geometry was almost pole-on during our observations.An analysis of the plasma tail and inner coma of the comet by means of photographic plates and CCD images through IHW and BVR filters, obtained with the 80cm Schmidt camera and the 1.2m telescope at Calar Alto/Spain in November 1992, revealed several tail rays, head streamers and substructures in brightness excess areas in the coma. While some of the tail rays extended to several million km nuclear distance, most of them can be traced to starting points which lie in a region just 20000–35000 km projected distance tailward from the nucleus.     

Dust Morphology Of Comet Hale-Bopp (C/1995 O1). Ii. Introduction Of A Working Model

A Monte Carlo image simulation code for dust features in comets is applied to comet Hale-Bopp in order to model the object's persistent porcupine-like appearance on high-resolution images taken between May 11 and Nov. 2, 1996. A self-consistent fan model is proposed, with six isolated sources of dust emission assumed at various locations on the surface of the rotating nucleus and with the spin axis undergoing a complex motion in an inertial coordinate system. In the framework of this model, jet pairs represent boundaries of fan-shaped formations described by dust ejected from isolated sources during periods of time when the Sun is above the local horizon. The spin axis is found to have traveled through a field of 10° by 20° during the examined period of nearly six months. Still more successful is a fan model with large diurnal dust-emission fluctuations, which is consistent with an inertially fixed position of the spin axis and requires only three discrete sources. In this scenario, the dust-emission profile is dominated by several brief flare-ups, or “puffs”, in the production of dust from one of the sources. The results are insensitive to the spin rate, but the observed dust coma appearance is more typical of a rapidly rotating comet. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Comparison Between Near-Infrared And Visible Imaging Of The Inner Coma Of Comet Hale-Bopp At Perihelion

We present a comparison between images of comet C/1995 O1 (Hale-Bopp) obtained from 12 March to 9 May 1997, with two telescopes of the Observatorio del Teide (IAC, Tenerife, Spain) in visible (Johnson-V filter) and three near-infrared narrowband filters (Brγ (2.166 μm), CO (2.295 μm), and K_con (2.260 μm)). No significant differences are observed between the images in the three near-infrared bands suggesting that any CO emission is very weak, or produced by an extended flat source. We do not find evidence for the existence of an additional infrared component due to emission by warm dust, or to dust grains of diverse composition and/or size. Visible and infrared images of the same rotational phase look almost identical, supporting the view that the observed jets and shells are mainly dust structures. This revised version was published online in July 2006 with corrections to the Cover Date.     

Surface Photometry of Comet Hale-Bopp Carbon Images

A very long series of photographic observations of the comet Hale-Bopp has been made during January–April 1997 at the double astrograph (400/2000) of the Main Astronomical Observatory (Kyiv, Ukraine). Some of the cometary photos were obtained with two wide-band filter combinations. One of these combinations isolates C₂ emission, another — the nearby dust continuum. The images were digitized by means of AMDPH-XY machine and then calibrated following the standard procedure. After subtraction of the dust continuum the distribution of surface brightness in the C₂ emission coma of comet Hale-Bopp was studied. We found an asymmetric brightness distribution both pre- and post-perihelion. On 21.77 April 1997 a secondary brightness peak is found at the distance of 1.03 × 10⁵ km from the nucleus. It is possible that this peak is related to the extended source of the C₂ molecules. This revised version was published online in July 2006 with corrections to the Cover Date.     

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

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

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

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

Atmospheric Losses Under Dust Bombardment in the Ancient Atmospheres

We have considered the new process of atmospheric losses - “sputtering” under bombardment by interplanetary dust. It is demonstrated that “sputtering” due to collisions with the interplanetary dust is an effective way of atmospheric gas loss (10^–4–10^–3 of the dust particles' accreting mass) and that it changes the composition of the atmospheric gases. In calculations we have taken that the dust particles collide elastically with the atoms and molecules of the atmosphere. Estimation of the effects of inelastic collisions was also considered. As a result of these collisions a part of the atmospheric atoms and molecules will have “upward” velocity and enough energy to escape. It was considered that escaping atoms can collide with the atoms of the “main” gas of the upper atmosphere. The atmospheric gas composition is assumed to be just as in the modern Martian atmosphere - the “main” gases in the upper atmosphere were taken to be O and CO₂. In our computations we pay particular attention to the abundance of noble gases in planetary atmospheres since these gases are very important for theories of atmospheric origin. We computed that under “sputtering” by the interplanetary dust, atmospheres were enriched by the “heavy” elements and isotopes in the wide range of the upper atmospheric parameters O/CO₂, T/g (O/CO₂– on the level of homosphere;T is temperature of the exosphere,g is gravitational acceleration). However the loss efficiency for “heavy” gases is relatively high compared to other known gas loss processes. In the case of noble gases for the specific parameters of the upper atmosphere (small T/g ratio; high O/CO₂ on the level of homosphere) we have got the unique result: despite the diffusion separation in the upper atmosphere the loss efficiency of Xe > Kr > Ar. The effect of “sputtering” of the planetary atmospheres was strongest during the early stages of the planetary evolution - when the rate of the dust accretion was intrinsically higher than now because of collisions of planetesimals. In light of the new escape process, the main peculiarities of the noble gases abundance in the planetary atmospheres could be explained. This revised version was published online in July 2006 with corrections to the Cover Date.     

Detection of a Satellite Orbiting The Nucleus of Comet Hale–Bopp (C/1995 O1)

This paper reports on the detection of a satellite around the principal nucleus of comet Hale-Bopp. As shown elsewhere, a successful morphological model for the comet's dust coma necessitates the postulation of overlapping jet activity from a comet pair. The satellite has been detected digitally on images taken with the Hubble Space Telescope's Wide Field Planetary Camera 2 in the planetary mode on five days in May–October 1996. An average satellite-to-primary signal ratio is 0.21 ± 0.03, which implies that the satellite is ∼30 km in diameter, assuming the main nucleus is ∼70 km across. To avoid collision, the separation distance must exceed 50–60 km at all times. The satellite's projected distances on the images vary from 160 to 210 km, or 0.06 to 0.10 arcsec. The satellite was not detected in October 1995, presumably because of its subpixel separation from the primary. The radius of the gravitational sphere of action of the principal nucleus 70 km in diameter is 370–540 km at perihelion, increasing linearly with the Sun's distance: the satellite appears to be in a fairly stable orbit. Its orbital period at ∼180 km is expected to be ∼2–3 days, much shorter than the intervals between the HST observations. If the main nucleus should be no more than 42 km across, Weaver et al.'s upper limit, the satellite's orbit could become unstable, with the object drifting away from the main nucleus after perihelion. Potentially relevant ground-based detections of close companions are reported. Efforts to determine the satellite's orbit and the total mass of the system will get under way in the near future. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.