Coma Structure Research Articles

Optical Imaging Polarimetry of Comet 21P/Giacobini–Zinner during Its 2018 Apparition

We conducted polarimetric observations of comet 21P/Giacobini–Zinner in the optical wavelength region using the polarimetric imager PICO equipped with the 50 cm telescope for Public Outreach at the Mitaka Campus of the National Astronomical Observatory of Japan on UT 2018 September 16, at a solar phase angle of 77° for the comet. We used Gunn’s i′-band filter to cover the wavelength region where sunlight reflected by dust grains dominates cometary spectra. The intensity map of comet 21P/Giacobini–Zinner showed an elongated coma structure along the east–west direction. The linear polarization degree map was almost uniform and showed no evidence of jet or arc structures in the inner coma. The linear polarization degree was 22% in the coma, which is consistent with other optical polarization observations of 21P/Giacobini–Zinner for this and previous apparitions reported in the literature. The measured polarization angles (179°) were nearly perpendicular to the position angle of the scattering plane of 271°. There is no evidence of significant changes in the dust properties (composition, porosity, and size distribution) in the comet from its 1985 to 2018 apparitions. The spectral gradient of the linear polarization degree for the comet was nearly flat in the wavelength range of ∼510–830 nm at a solar phase angle of ∼77° during the 2018 apparition.

A SUBLIME 3D Model for Cometary Coma Emission: The Hypervolatile-rich Comet C/2016 R2 (PanSTARRS)

Abstract The coma of comet C/2016 R2 (PanSTARRS) is one of the most chemically peculiar ever observed, in particular due to its extremely high CO/H2O and N 2 + /H2O ratios, and unusual trace volatile abundances. However, the complex shape of its CO emission lines, as well as uncertainties in the coma structure and excitation, has lead to ambiguities in the total CO production rate. We performed high-resolution, spatially, spectrally, and temporally resolved CO observations using the James Clerk Maxwell Telescope and Submillimeter Array to elucidate the outgassing behavior of C/2016 R2. Results are analyzed using a new, time-dependent, three-dimensional radiative transfer code (SUBlimating gases in LIME; SUBLIME, based on the open-source version of the LIne Modeling Engine), incorporating for the first time, accurate state-to-state collisional rate coefficients for the CO–CO system. The total CO production rate was found to be in the range of (3.8 − 7.6) × 1028 s−1 between 2018 January 13 and February 1 (at r H = 2.8–2.9 au), with a mean value of (5.3 ± 0.6) × 1028 s−1. The emission is concentrated in a near-sunward jet, with a half-opening angle of ∼62° and an outflow velocity of 0.51 ± 0.01 km s−1, compared to 0.25 ± 0.01 km s−1 in the ambient (and nightside) coma. Evidence was also found for an extended source of CO emission, possibly due to icy grain sublimation around 1.2 × 105 km from the nucleus. Based on the coma molecular abundances, we propose that the nucleus ices of C/2016 R2 can be divided into a rapidly sublimating apolar phase, rich in CO, CO2, N2, and CH3OH, and a predominantly frozen (or less abundant), polar phase containing more H2O, CH4, H2CO, and HCN.

Diffuse structure of some meteors at the beginning of their trajectories at classic heights

The problem of anomalous meteors with diffuse view and increased size of their comas at the beginning of radiation is considered. Results from processing some meteors detected with high-sensitive observational TV systems of the Super-Isocon type during Leonid storm observations in 2002 are given. As opposed to similar cases described in literature previously, the given meteors had an increased in size diffuse structure of the meteor coma not at extra-high altitudes but below 128 km: 118.06 ± 0.07 km, 123.01 ± 0.02 km, and 124.45 ± 0.10 km. At the trajectory beginning, their absolute stellar magnitudes varied in the range of +6m to +5m, and they reached –0.5m in maximum of brightness. The range of their masses was 0.03–0.06 g. The influence of the TV system’s working mode on the possible appearance of artifacts is considered. While the transparent, diffuse view of a limiting low-light meteor image can be a result of a low signal-to-noise ratio, the increase in spatial size of a meteor coma cannot be explained by technical artifacts. Separation of some extremely low-light meteor images in start frames onto a range of individual point-like objects placed inside a zone of 0.5–1.5 km can serve as an indirect argument for real fragmentation of an initial particle. A conclusion is drawn about the possible fragmentation of some meteoroids from the Leonid stream during a time period of 2–3 weeks before collision with the Earth with velocities of fragment separation on the order of millimeters a second.

Epidemiology of acute diabetes complications (coma) according to the Federal Diabetes register of the Russian Federation (2013–2016)

Background: Despite the improvement in the quality of diabetes care in the Russian Federation (RF), coma remain one of the causes of death in patients with diabetes.
 Aim: To assess dynamic of epidemiological characteristic of acute complications in adult patients with T1D and T2D in 201316.
 Materials and methods: The database of the Russian Federal Diabetes register (81 regions). The indicators of coma for 201316 were estimated for 10000 adult patients with diabetes (18 years).
 Results: In 2016, the prevalence of coma in RF was 225.9 with T1D and 11.6/10000 adults with T2D. For the period from 2007 the prevalence of ketoacidotic coma decrease three times in T1D, 4 times for T2D.Totally in 2016, 165 new cases of coma for both types of diabetes were registered, an average of 0.4/10000 adults. Interregional differences in the prevalence of coma were observed 04.2/10000 adults. The frequency of new cases of coma has a tendency to decrease: 0,90,4/10000 adults: T1D 5.73.4, T2D 0.60.2/10000 adults. When evaluating the structure of coma, redistribution is evident in their form. So in 2016 the proportion of hypoglycemic coma increased to 40.7%, and ketoacidotic coma decreased to 56.6% in T1D. With T2D, the difference expressed in a lesser degree. The mean duration of diabetes at the time of coma development increased with T1D from 3.89.1 years, with T2D 3.57.0 years. The maximum frequency of development of coma is recorded with the diabetes duration more than 30 years, regardless of the type. The patients age at the time of coma development in T1D increased to 27.5 years old, and in T2D it was 60.4 years, it didnt change significantly. The assessment of glycemic control showed a significant improvement: a decrease in the proportion of patients with HbA1c 9.0% (23% with T1D, 8.8% with T2D), an increase with HbA1c 7% (32.4% and 51.7%, respectively). The average value of HbA1c in 2016 with T1D 8.21%, with T2D 7.48%.
 Conclusions: It is established that the dynamics of the frequency of development of coma in 201316 in adult patients with diabetes in the RF has a stable tendency to decrease: 1.5 times with T1D and more than 3 times with T2D. It can be assumed that this is due to the improvement in the quality of diabetes care and glycemic control in general, as well as the use of modern medicines. Attention is required to draw to the high frequency of coma in T1D, the development of coma with a longer duration of diabetes, an increase in the proportion of patients with hypoglycemic coma. Significant interregional differences in the frequency of coma registration require additional analysis.

Seed traits favouring dispersal and establishment of six epiphyticTillandsia(Bromeliaceae) species

AbstractThere are a number of studies describing the gross range of morpho-anatomical variability in epiphyticTillandsiaspecies, but the interspecific variation in seed traits remain largely unexplored, although these play an important role in determining dispersal and establishment success. In order to evaluate interspecific variation in seed morphology, anatomy and germination, we sampled sixTillandsiaspecies from the Yucatan peninsula, Mexico, distributed along a precipitation gradient. We studied morpho-anatomical traits (seed length, seed mass, ratio of coma to seed, ratio of embryo to endosperm), seed terminal velocity in still air, and performed histochemical analyses and germination trials under controlled conditions.Tillandsia recurvatadiffers from the other five species in the structure of the plumose coma; it was the only species lacking an endosperm and showed distinct seedling development. Among the species, bigger seeds were related to longer comas, and had higher germinability. Overall, seed terminal velocity was invariably slow, compared with reports of other anemochorous species, suggesting a high dispersal potential. Taxonomical and ecological implications of our results are discussed.

Jet Morphology and Coma Analysis of Comet 103P/Hartley 2

Abstract Spectral data for the coma of Hartley 2 were acquired across four nights in late 2010 using an integral field spectrometer at McDonald Observatory. For the 30 observations during these four nights, we detected five radical species in the coma: C2, C3, CH, CN, and NH2. Using division by azimuthal mean and division by radial profile, we enhanced 150 images of the coma to reveal subtle coma structure. These images revealed noticeable temporal evolution and spatial variations between species. To quantify the observed variation between species, we partitioned the coma and used analysis of variance (ANOVA) techniques to provide a statistical basis for heterogeneity. Nearly every ANOVA test indicated a spatially diverse distribution in the coma when considering all species collectively. To examine the temporal behavior, we used the works by Belton et al., Thomas et al., and Bruck Syal et al. to predict nucleus orientation and active jet directions at our observation times. Several of these reported jet sites correlated to high radical concentrations, and the sites on the smaller lobe are more closely associated with high radical concentrations. Lastly, we provide constraints for the suspect parent molecules of the detected radicals, and we propose that photolysis reactions occurring at or near extended icy grains are a source for the more enigmatic radicals, such as C3.

Scientific assessment of the quality of OSIRIS images

OSIRIS, the scientific imaging system on board the ESA Rosetta spacecraft, has been imaging the nucleus of comet 67P/Churyumov-Gerasimenko and its dust and gas environment since March 2014. The images serve different scientific goals, from morphology and composition studies of the nucleus surface, to the motion and trajectories of dust grains, the general structure of the dust coma, the morphology and intensity of jets, gas distribution, mass loss, and dust and gas production rates. Aims. We present the calibration of the raw images taken by OSIRIS and address the accuracy that we can expect in our scientific results based on the accuracy of the calibration steps that we have performed. Methods. We describe the pipeline that has been developed to automatically calibrate the OSIRIS images. Through a series of steps, radiometrically calibrated and distortion corrected images are produced and can be used for scientific studies. Calibration campaigns were run on the ground before launch and throughout the years in flight to determine the parameters that are used to calibrate the images and to verify their evolution with time. We describe how these parameters were determined and we address their accuracy. Results. We provide a guideline to the level of trust that can be put into the various studies performed with OSIRIS images, based on the accuracy of the image calibration.

Modeling the heterogeneous ice and gas coma of Comet 103P/Hartley 2

The spectacular images of Comet 103P/Hartley 2 recorded by the Medium Resolution Instrument (MRI) and High Resolution Instrument (HRI) on board of the Extrasolar Planet Observation and Deep Impact Extended Investigation (EPOXI) spacecraft, as the Deep Impact extended mission, revealed that its bi-lobed very active nucleus outgasses volatiles heterogeneously. Indeed, CO2 is the primary driver of activity by dragging out chunks of pure ice out of the nucleus from the sub-solar lobe that appear to be the main source of water in Hartley 2’s coma by sublimating slowly as they go away from the nucleus. However, water vapor is released by direct sublimation of the nucleus at the waist without any significant amount of either CO2 or icy grains. The coma structure for a comet with such areas of diverse chemistry differs from the usual models where gases are produced in a homogeneous way from the surface. We use the fully kinetic Direct Simulation Monte Carlo model of Tenishev et al. (Tenishev, V.M., Combi, M.R., Davidsson, B. [2008]. Astrophys. J. 685, 659−677; Tenishev, V.M., Combi, M.R., Rubin, M. [2011]. Astrophys. J. 732, 104–120) applied to Comet 103P/Hartley 2 including sublimating icy grains to reproduce the observations made by EPOXI and ground-based measurements. A realistic bi-lobed nucleus with a succession of active areas with different chemistry was included in the model enabling us to study in details the coma of Hartley 2. The different gas production rates from each area were found by fitting the spectra computed using a line-by-line non-LTE radiative transfer model to the HRI observations. The presence of icy grains with long lifetimes, which are pushed anti-sunward by radiation pressure, explains the observed OH asymmetry with enhancement on the night side of the coma.

ROTATION OF COMET 103P/HARTLEY 2 FROM STRUCTURES IN THE COMA

The CN coma structure of the EPOXI mission target, comet 103P/Hartley 2, was observed during twenty nights from September to December 2010. These CN images probe the rotational state of the comet's nucleus and provide a ground-based observational context to complement the EPOXI observations. A dynamically excited cometary nucleus with a changing rotational rate is observed, a characteristic not seen in any comet in the past. The lack of rotational damping during the four-month observing interval places constraints on the interior structure of the nucleus.

No compelling evidence of distributed production of CO in Comet C/1995 O1 (Hale-Bopp) from millimeter interferometric data and a re-analysis of near-IR lines

Based on long-slit infrared spectroscopic observations, it has been suggested that half of the carbon monoxide present in the atmosphere of Comet C/1995 O1 (Hale-Bopp) close to perihelion was released by a distributed source in the coma, whose nature (dust or gas) remains unidentified. We re-assess the origin of CO in Hale-Bopp’s coma from millimeter interferometric data and a re-analysis of the IR lines. Simultaneous observations of the CO J(1–0) (115 GHz) and J(2–1) (230 GHz) lines were undertaken with the IRAM Plateau de Bure interferometer in single-dish and interferometric modes. The diversity of angular resolutions (from 1700 to 42,000 km diameter at the comet) is suitable to study the radial distribution of CO and detect the extended source observed in the infrared. We used excitation and radiative transfer models to simulate the single-dish and interferometric data. Various CO density distributions were considered, including 3D time-dependent hydrodynamical simulations which reproduce temporal variations caused by the presence of a CO rotating jet. The CO J(1–0) and J(2–1) observations can be consistently explained by a nuclear production of CO. Composite 50:50 nuclear/extended productions with characteristic scale lengths of CO parent L p > 1500 km are rejected. Based on similar radiation transfer calculations, we show that the CO v = 1–0 ro-vibrational lines observed in Comet Hale-Bopp at heliocentric distances less than 1.5 AU are severely optically thick. The broad extent of the CO brightness distribution in the infrared is mainly due to optical depth effects entering in the emitted radiation. Additional factors can be found in the complex structure of the CO coma, and non-ideal slit positioning caused by the anisotropy of dust IR emission. We conclude that both CO millimeter and infrared lines do not provide compelling evidence for a distributed source of CO in Hale-Bopp’s atmosphere.

Shape and obliquity effects on the thermal evolution of the Rosetta target 67P/Churyumov-Gerasimenko cometary nucleus

This work is dedicated to the application to 67P/Churyumov-Gerasimenko of a new quasi-3D approach for non-spherically shaped comet nuclei with the aim to interpret the current activity of the comet in terms of initial characteristics and to predict shape and internal stratification evolution of the nucleus. The model is applied to differently shaped nuclei taking into account the characteristics of Comet 67P/Churyumov-Gerasimenko deduced from observations. We focus our attention on the combined effects that shapes and obliquity have on the comet surface and sub-surface evolution. We discuss the results in terms of activity, local dust mantle formation and disruption, erosion of the surface and internal stratigraphy. The results show that differently shaped nuclei can have different internal structures leading to different activity patterns and behaviors. Our calculations have shown that local variations in the dust and gas fluxes can be induced by the nucleus shape. The distribution of “active” areas on Comet 67P/Churyumov-Gerasimenko is different because of different shapes, reflecting the illumination conditions on the surface. These shapes can influence the structure of the inner coma, but the coma far away from the nucleus is only marginally affected by the nucleus shape. However, different comet behaviors can arise from differently shaped comet nuclei, especially in terms of local activity, surface and sub-surface characteristics and properties. The water flux local distribution is the most influenced by the shape as it is directly linked to the illumination. Irregular shapes have large shadowing effects that can result in activity patterns on the comet surface. The effects of different pole directions are discussed to see the relations with the nucleus activity and internal structure. It is shown that the orientation of the rotation axis plays a strong role on the surface evolution of 67P/Churyumov-Gerasimenko, determining seasonal effects on the fluxes. The activity of the comet changes greatly with the nucleus obliquity leading to pre–post-perihelion differences in the activity and seasonal effects. The effects of the dust deposition and crust formation on the cometary activity have also been simulated and are discussed with respect to 67P/Churyumov-Gerasimenko observations. The dust mantling is also strongly obliquity dependent, with different surface distributions of the dust-covered regions according to the different comet pole orientations. Finally, we show that our model can reproduce the fluxes behavior near perihelion in terms of amplitude and asymmetry, and we estimate 20% of the illuminated surface to be active.

Interferometric imaging of carbon monoxide in comet C/1995 O1 (Hale-Bopp): evidence of a strong rotating jet

Observations of the CO J(1-0) 115 GHz and J(2-1) 230 GHz lines in comet C/1995 O1 (Hale-Bopp) were performed with the IRAM Plateau de Bure interferometer on 11 March, 1997. The observations were conducted in both single-dish (ON-OFF) and interferometric modes with 0.13 km s-1 spectral resolution. Images of CO emission with 1.7 to 3" angular resolution were obtained. The ON-OFF and interferometric spectra show a velocity shift with sinusoidal time variations related to the Hale-Bopp nucleus rotation of 11.35 h. The peak position of the CO images moves perpendicularly to the spin axis direction in the plane of the sky. This suggests the presence of a CO jet, which is active night and day at about the same extent, and is spiralling with nucleus rotation. The high quality of the data allows us to constrain the characteristics of this CO jet. We have developed a 3-D model to interpret the temporal evolution of CO spectra and maps. The CO coma is represented as the combination of an isotropic distribution and a spiralling gas jet, both of nucleus origin. Spectra and visibilities (the direct output of interferometric data) analysis shows that the CO jet comprises ~40% the total CO production and is located at a latitude ~20 degrees North on the nucleus surface. Our inability to reproduce all observational characteristics shows that the real structure of the CO coma is more complex than assumed, especially in the first thousand kilometres from the nucleus. The presence of another moving CO structure, faint but compact and possibly created by an outburst, is identified.

Monte-Carlo and multifluid modelling of the circumnuclear dust coma II. Aspherical-homogeneous, and spherical-inhomogeneous nuclei

We use our newly developed Dust Monte-Carlo (DMC) simulation technique [Crifo, J.F., Lukianov, G.A., Rodionov, A.V., Zakharov, V.V., 2005. Icarus 176, 192–219] to study the dynamics of dust grains in the vicinity of some of the benchmark aspherical, homogeneous cometary nuclei and of the benchmark spherical, inhomogeneous nuclei studied by us precedingly. We use the interim unrealistic simplifying assumptions of grain sphericity, negligible nucleus rotation rate, and negligible tidal force, but take accurately into account the nucleus gravitational force, gas coma aerodynamic force, and solar radiation pressure force, and consider the full mass range of ejectable spherical grains. The resulting complicated grain motions are described in detail, as well as the resulting complicated and often counter-intuitive dust coma structure. The results are used to answer several important questions: (1) When computing coma dust distributions, (a) is it acceptable to take into consideration only one or two of the above mentioned forces (as currently done)? (b) to which accuracy must these forces be known, in particular is it acceptable to represent the gravity of an aspherical nucleus by a spherically symmetric gravity? (c) how do the more efficient but less general Dust Multi-Fluid (DMF) computations compare with the DMC results? (2) Are there simple structural relationships between the dust coma of a nucleus at small heliocentric distance r h , and that of the same nucleus at large r h ? (3) Are there similarities between the gas coma structures and the associated dust coma structures? (4) Are there dust coma signatures revealing non-ambiguously a spherical nucleus inhomogeneity or an homogeneous nucleus asphericity? (5) What are the implications of the apparently quite general process of grain fall-backs for the evolution of the nucleus surface, and for the survival of a landed probe?

Behavior of Comet 9P/Tempel 1 around the Deep Impact event

Context. Comet 9P/Tempel 1, the target of the Deep Impact (DI) Mission, has been monitored from June 18 until July 12, 2005, as a follow up to the long-term campaign started in Jan. 2005. Aims. The goal of this campaign is to characterize the comet before the DI event and to detect changes in activity and coma morphology produced by the DI experiment on July 04.226 UT 2005. Methods. Optical broadband images and long-slit spectroscopic measurements have been acquired with the instrument CAFOS mounted at the 2.2 m telescope at Calar Alto Observatory (CSIC-MPG) from July 1 to 11, whereas only imaging has been performed with Versarray at the 1.5 m telescope of the Sierra Nevada Observatory (IAA-CSIC) on June 26 and 29, 2005. Results. Fifteen hours after the impact, the ejecta cloud extends over ~240° in position angle (PA) with a symmetry axis at PA ~ 235°. The effect of the solar radiation pressure is already visible as a slight deviation from a fully symmetric plume and the ejecta dust is already feeding the tail. The exhaustive analysis of the broadband images has revealed that no new long-lasting coma structure is produced by the impact. The structures existing in the coma before the event are recovered after the ejecta plume has moved out. The maximum projected expansion velocity of the ejecta dust results in ~230 and ~150 m/s 15 and 40 h after impact, respectively. Surface brightness profiles of the continuum, either azimuthally averaged profiles from the broadband images or in the north-south direction from the long-slit spectra can generally be well fit with a slope m of -0.94 $\le m \le$ -1.49 in $\log B - \log \rho$ representation. A few exceptions occur on July 2 and 8-10 when much flatter continuum profiles are detected that are possibly related to fragmentation processes and to the reported outbursts occurring around those dates. Normalized color $S^\prime$ of the dust inside the coma does not show spatial variations excluding July 04.875 UT, our first observation after the impact. At that time, the dust inside the ejecta plume is undoubtedly bluer than the surrounding coma (8.2$ \pm $0.4 % /100 nm versus 14.5 ± 0.8% /100 nm). The dust color averaged at $\rho \le$ 10 000 km returns to a ~12% /100 nm on July 07.875 UT, the same value measured a few days before the projectile impact. A lower limit to the mass in the ejecta can be given from our optical observations resulting in 1.2$\times$10 6 kg, which represents about 14 h of quiet (i.e., steady state) pre-impact activity. The value of $A f \rho$ is remarkably variable during the 18 days monitoring as several outbursts took place, beside the one induced by the DI experiment. Apart from outburst periods, $A f \rho \sim$110-12 cm. The gas activity represented by the CN, C 2 , and C 3 production rates ( Q ), are relatively constant from July 1 to 6 excluding the immediate post-impact period on July 4. The number of molecules of CN, C 2 , and C 3 produced by the DI were equal to 2.13$\times$10 29 , 2.07$\times$10 29 , and 1.49$\times$10 28 as measured 15 hrs after impact. The amount of their potential parent species detected at other wavelengths seems to indicate that a large fraction of the daughter species measured 15 h after the impact might originate from the ejected dust grains. Conclusions. General conclusions include (i) new post-impact refractory material different from that seen pre-impact was ejected during the DI experiment, i.e., these dust grains had either different optical properties or a size distribution peaking at smaller sizes, (ii) the DI event did not initiate a long-lasting period of sustained cometary activity, and (iii) in many ways the artificial impact looked very much like a natural outburst of the comet, also (iv) some fraction of gaseous daughter species within the coma after the DI experiment might have originated in the ejected dust grains.

Remote sensing of a comet at millimeter and submillimeter wavelengths from an orbiting spacecraft

The ESA Rosetta Spacecraft, launched on March 2, 2004 with the ultimate destination being Comet 67P/Churyumov–Gerasimenko, carries a relatively small and lightweight millimeter–submillimeter spectrometer instrument, the first of its kind launched into deep space. The instrument, named Microwave Instrument for the Rosetta Orbiter (MIRO), consists of a 30-cm diameter, offset parabolic reflector telescope, which couples energy in the millimeter and submillimeter bands to two heterodyne receivers. Center-band operating frequencies are near 190 GHz (1.6 mm) and 562 GHz (0.5 mm). Broadband, total power continuum measurements can be made in both bands. A 4096-channel spectrometer with 44 kHz resolution is connected to the submillimeter receiver. The spectral resolution is sufficient to observe individual, thermally broadened spectral lines ( T⩾10 K). The submillimeter radiometer/spectrometer is fixed tuned to measure four volatile species—CO, CH 3OH, NH 3 and three isotopes of water, H 2 16O, H 2 17O and H 2 18O. The MIRO experiment will use these species as probes of the physical conditions within the nucleus and coma. The basic quantities measured by MIRO are surface temperature, gas production rates and relative abundances, and velocity and excitation temperature of each species, along with their spatial and temporal variability. This information will be used to infer coma structure and outgassing processes, including the nature of the nucleus/coma interface.

Direct statistical simulation of the near-surface layers of the cometary atmosphere. I. A spherical nucleus

The key to the explanation of the variety of processes associated with the activity of comets lies in a more profound understanding of the events that occur in the near-surface layer of the cometary nucleus and in the inner part of the cometary atmosphere, which is formed under the action of solar radiation. A complete description of the mass- and energy-transfer processes inside and outside the cometary nucleus, which can be presented as a body with a complex morphology and high porosity and which is composed of mineral, organic, and volatile components, is an extremely complicated problem of space physics. The nucleus and the inner part of the coma are closely related to each other as physical subsystems, and physical processes in these regions are in close symbiosis because of the permanent exchange of both energy and mass. The thermophysical model of the circumnuclear coma cannot be confined only to the consideration of the processes in the nucleus—the entire system should be simulated as a whole. A consistent model of the inner part of a cometary atmosphere of this kind has been developed in this paper within the framework of the gas-kinetic approach. Based on direct statistical simulation with weights, we simulated the two-dimensional gas flow from the cometary nucleus. The nucleus was assumed to be spherical. We considered different models of the nucleus morphology that determined the effective gas production: the homogeneous nucleus, the “spotty” nucleus with its active area in the vicinity of the subsolar point, and the “spotty” nucleus with its active area in the form of a spherical segment. For the first time, the boundary conditions at the inner boundary of the simulation domain required for the kinetic modeling of the inner coma were found from a self-consistent model of the heat and mass transfer in a porous cometary nucleus. The model was earlier developed by the authors. The basic new features of the model included taking into account the volumetric character of the radiation absorption in a porous medium, the kinetic simulation of the transport of sublimation products in the pores, and the consideration of the backward fluxes from the coma due to the intermolecular collisions. The kinetic simulation was performed using the SMILE program package. The parallel computer implementation of the model made it possible to calculate the spatial fields of the basic macrocharacteristics of the gas flow. It was demonstrated that the structure of the inner coma is essentially dependent on variations in the effective gas production. In all the cases considered, the main part of the coma is far from thermodynamic equilibrium. This means that the entire structure of the inner cometary atmosphere can be accurately studied only within the framework of the kinetic approach. In a general case, it can result from the interaction of gas flows generated by different active subregions on the nucleus surface. Thus, fine structures are formed and local efficient gas recondensation from the coma occurs in the model for some regimes with “spotty” activity (where considerable variations in gas production take place on small spatial scales).

Estimating the nucleus density of Comet 19P/Borrelly

The nucleus bulk density of Comet 19P/Borrelly has been estimated by modeling the sublimation-induced non-gravitational force acting upon the orbital motion, thereby reproducing the empirical perihelion advance (i.e., the shortening of the orbital period). The nucleus has been modeled as a prolate ellipsoid, covered by various surface activity maps which reproduce the observed water production rate. The theoretical water production rate of active areas has been obtained by applying a sophisticated thermophysical model. This model takes into account net sublimation of ice and thermal reradiation from the surface, solid state conductivity, sub-surface sublimation and recondensation, mass and heat transport by diffusing gas, layer absorption of solar energy, a full treatment of local time-dependent illumination conditions, and a detailed consideration of nucleus/coma interaction mechanisms. The outgassing properties of the modeled nucleus are physically consistent with the gas kinetic structure of the innermost coma since the molecular backflux and surface gas density required in the thermophysical model (as functions of the nucleus surface temperature and the sub-surface temperature profile) have been obtained from Direct Simulation Monte Carlo modeling of inelastic intermolecular collisions in the cometary Knudsen layer. The calculation of local normal forces acting on the nucleus due to outgassing has been made within the same framework—recoil and/or impact momentum transfer to the nucleus caused by sublimating molecules and by recondensing and/or scattered coma molecules is therefore evaluated in accordance with local nucleus/coma conditions. According to this model, the density is found to be 100–300 kg m −3 , depending on the applied spin axis orientation and surface activity map. This range can be narrowed down to 180–300 kg m −3 by also requiring that the empirical changes (per orbital revolution) of the argument of perihelion and the longitude of the ascending node are reproduced.

The Structural Determination of Phosphosulfolactate Synthase from Methanococcus jannaschii at 1.7-Å Resolution: AN ENOLASE THAT IS NOT AN ENOLASE

Members of the enolase mechanistically diverse superfamily catalyze a wide variety of chemical reactions that are related by a common mechanistic feature, the abstraction of a proton adjacent to a carboxylate group. Recent investigations into the function and mechanism of the phosphosulfolactate synthase encoded by the ComA gene in Methanococcus jannaschii have suggested that ComA, which catalyzes the stereospecific Michael addition of sulfite to phosphoenolpyruvate to form phosphosulfolactate, may be a member of the enolase superfamily. The ComA-catalyzed reaction, the first step in the coenzyme M biosynthetic pathway, likely proceeds via a Mg2+ ion-stabilized enolate intermediate in a manner similar to that observed for members of the enolase superfamily. ComA, however, has no significant sequence similarity to any known enolase. Here we report the x-ray crystal structure of ComA to 1.7-A resolution. The overall fold for ComA is an (alpha/beta)8 barrel that assembles with two other ComA molecules to form a trimer in which three active sites are created at the subunit interfaces. From the positions of two ordered sulfate ions in the active site, a model for the binding of phosphoenolpyruvate and sulfite is proposed. Despite its mechanistic similarity to the enolase superfamily, the overall structure and active site architecture of ComA are unlike any member of the enolase superfamily, which suggests that ComA is not a member of the enolase superfamily but instead acquired an enolase-type mechanism through convergent evolution.

The Dependence of the Circumnuclear Coma Structure on the Properties of the Nucleus: IV. Structure of the Night-Side Gas Coma of a Strongly Sublimating Nucleus

The structure of the nightside coma in the vicinity of a strongly active comet nucleus of pure ice is investigated by solving gasdynamic equations for the flow of water vapour sublimated from—or condensed onto—the nucleus surface. To guarantee the physical validity of the solution, both Euler and Navier–Stokes Equations are solved, and the solutions are compared. A spherical nucleus is considered first and then a triaxial ellipsoidal nucleus. The results show that (1) a fluid coma of significant extent and very complicated physical structure is formed; (2) for low heat conduction transfer across the nucleus from the dayside to the nightside surface, a narrow conical weak shock appears near to the antisolar axis; the whole nightside surface acts as a cold trap for the vapor, part of which recondenses onto it; (3) for intermediate heat conduction, part of the nightside surface becomes weakly sublimating, and a different weak shock pattern is formed; and (4) at high heat conduction, the whole nightside surface is weakly sublimating, and the resulting flow pattern becomes similar to that existing in a coma formed by diffusion from the nucleus interior (see Crifo, Rodionov and Bockelée-Morvan, 1999, Icarus 138, 83–106). The results are compared to related model results by other authors, and a discussion is made of their relevance to the 1996 observation of the near-nucleus nightside coma of Comet C/1996 B2 Hyakutake.

The Inner Coma and Nucleus of Comet Hale–Bopp: Results from a Stellar Occultation

We discuss the properties of the nucleus and inner coma of Comet Hale–Bopp (C/1995 O1) as derived from observations of its occultation of Star PPM 200723 on 5 October 1996, while the comet was 2.83 AU from the Sun. Compared to previous occultations by active comets, this is possibly the closest to the nucleus one has ever observed. Three chords (lightcurves) through the comet's inner coma were measured, though only one chord has a strong indication of measuring the occultation, and that was through thin cirrus. We have constrained the radius of the nucleus and properties of the coma using a simple model; there is a large valid section of parameter space. Our data show the optical depth of the coma was ≥1 within 20 to 70 km of the center of the (assumed spherical) nucleus, depending on the coma's structure and the nucleus' size. The dependence of the dust coma's opacity on cometocentric distance, ρ, was steeper than expected for force-free, radial flow, being probably as steep as or steeper than 1/ρ1.4 within 100 km of the nucleus (though it is marginally possible to fit one coma hemisphere with a 1/ρ law). Assuming the dust coma flowed radially from a spot at the center of the nucleus and that the coma's profile was not any steeper than ρ−2, the upper limit to the radius of the nucleus is about 30 km, though relaxing these assumptions limits the radius to 48 km. The chord through the coma does not show the same coma structure within 100 km of the nucleus as that which is apparent in larger-scale (∼700 km/pixel) imaging taken just before the event, suggesting that (a) the star's path sampled the acceleration region of the dust, and/or (b) azimuthal variation in the inner coma is different than that seen in the outer coma.