Outer Coma Research Articles

Coma environment of comet C/2017 K2 around the water ice sublimation boundary observed with VLT/MUSE

We report a new imaging spectroscopic observation of Oort cloud comet C/2017 K2 (hereafter K2) with the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope on its way to perihelion at 2.53 au, around a heliocentric distance where H2O ice begins to play a key role in comet activation. Normalized reflectances over 6500–8500 Å for its inner (cometocentric distance ρ ≈ 103 km) and outer (ρ ≈ 2 × 104 km) comae are 9.7 ± 0.5 and 7.2 ± 0.3 % (103 Å)−1, respectively, the latter being consistent with the slope observed when the comet was beyond the orbit of Saturn. The dust coma of K2 at the time of observation appears to contain three distinct populations: millimeter-sized chunks prevailing at ρ ≲ 103 km; a 105 km steady-state dust envelope; and fresh anti-sunward jet particles. The dust chunks dominate the continuum signal and are distributed over a similar radial distance scale as the coma region with redder dust than nearby. They also appear to be co-spatial with OI1D, suggesting that the chunks may accommodate H2O ice with a fraction (≳1%) of refractory materials. The jet particles do not colocate with any gas species detected. The outer coma spectrum contains three significant emissions from C2(0,0) Swan band, OI1D, and CN(1,0) red band, with an overall deficiency in NH2. Assuming that all OI1D flux results from H2O dissociation, we compute an upper limit on the water production rate QH2O of ~7 × 1028 molec s−1 (with an uncertainty of a factor of two). The production ratio log[QC2/QCN] of K2 suggests that the comet has a typical carbon chain composition, with the value potentially changing with distance from the Sun. Our observations suggest that dust chunks (>0.1 mm) containing water ice and near K2’s nucleus emitted beyond 4 au may be responsible for its very low gas rotational temperature and the discrepancy between its optical and infrared lights reported at similar heliocentric distances.

Solar Wind Protons in the Diamagnetic Cavity at Comet 67P/Churyumov-Gerasimenko.

The plasma environment at a comet can be divided into different regions with distinct plasma characteristics. Two such regions are the solar wind ion cavity, which refers to the part of the outer coma that does not contain any solar wind ions anymore; and the diamagnetic cavity, which is the region of unmagnetized plasma in the innermost coma. From theory and previous observations, it was thought that under usual circumstances no solar wind ion should be observable near or inside of the diamagnetic cavity. For the first time, we report on five observations that show that protons near solar wind energies can also be found inside the diamagnetic cavity. We characterize these proton signatures, where and when they occur, and discuss possible mechanisms that could lead to protons penetrating the inner coma and traversing the diamagnetic cavity boundary. By understanding these observations, we hope to better understand the interaction region of the comet with the solar wind under nonstandard conditions. The protons detected inside the diamagnetic cavity have directions and energies consistent with protons of solar wind origin. The five events occur only at intermediate gas production rates and low cometocentric distances. Charge transfer reactions, high solar wind dynamic pressure and a neutral gas outburst can be ruled out as causes. We suggest that the anomalous appearance of protons in the diamagnetic cavity is due to a specific solar wind configuration where the solar wind velocity is parallel to the interplanetary magnetic field, thus inhibiting mass-loading and deflection.

CO+- and CN-gas Relations in Comet 1P/Halley

This work presents the CO+ and CN relations of comet 1P/Halley, in average column density values, for some diaphragm diameters that correspond to different distances from the nucleus. The results obtained in this paper about the outer coma of the comet (63,000 until 170,000 km) are in a linear concordance with the data of the inner coma (3,900 until 12,000 km).

Measuring molecular abundances in comet C/2014 Q2 (Lovejoy) using the APEX telescope.

Comet composition provides critical information on the chemical and physical processes that took place during the formation of the Solar System. We report here on millimeter spectroscopic observations of the long-period bright comet C/2014 Q2 (Lovejoy) using the Atacama Pathfinder Experiment (APEX) band 1 receiver between UT 16.948 to 18.120 January 2015, when the comet was at heliocentric distance of 1.30 au and geocentric distance of 0.53 au. Bright comets allow for sensitive observations of gaseous volatiles that sublimate in their coma. These observations allowed us to detect HCN, CH3OH (multiple transitions), H2CO and CO, and to measure precise molecular production rates. Additionally, sensitive upper limits were derived on the complex molecules acetaldehyde (CH3CHO) and formamide (NH2CHO) based on the average of the strongest lines in the targeted spectral range to improve the signal-to-noise ratio. Gas production rates are derived using a non-LTE molecular excitation calculation involving collisions with H2O and radiative pumping that becomes important in the outer coma due to solar radiation. We find a depletion of CO in C/2014 Q2 (Lovejoy) with a production rate relative to water of 2.0 %, and relatively low abundances of Q(HCN)/Q (H2O),0.1%, and Q (H2CO)/Q (H2O), 0.2 %. In contrast the CH3OH relative abundance Q (CH3OH)/Q (H2O),2.2 %, is close to the mean value observed in other comets. The measured production rates are consistent with values derived for this object from other facilities at similar wavelengths taking into account the difference in the fields of view. Based on the observed mixing ratios of organic molecules in four bright comets including C/2014 Q2, we find some support for atom addition reactions on cold dust being the origin of some of the molecules.

Optical and Near-infrared Polarimetry of Non-periodic Comet C/2013 US10 (Catalina)

Abstract We present an optical and near-infrared (hereafter NIR) polarimetric study of a comet C/2013 US10 (Catalina) observed on UT 2015 December 17–18 at phase angles of α = 52.°1–53.°1. Additionally, we obtained an optical spectrum and multi-band images to examine the influence of gas emission. We find that the observed optical signals are significantly influenced by gas emission; that is, the gas-to-total intensity ratio varies from 5 to 30% in the R C and 3%–18% in the I C bands, depending on the position in the coma. We derive the “gas-free dust polarization degrees” of 13.8% ± 1.0% in the R C and 12.5% ± 1.1% in the I C bands and a gray polarimetric color, i.e., −8.7% ± 9.9% μm−1 in optical and 1.6% ± 0.9% μm−1 in NIR. The increments of polarization obtained from the gas correction show that the polarimetric properties of the dust in this low-polarization comet are not different from those in high-polarization comets. In this process, the cometocentric distance dependence of polarization has disappeared. We also find that the R C-band polarization degree of the southeast dust tail, which consists of large dust particles (100 μm–1 mm), is similar to that in the outer coma where small and large ones are mixed. Our study confirms that the dichotomy of cometary polarization does not result from the difference of dust properties, but from depolarizing gas contamination. This conclusion can provide a strong support for similarity in origin of comets.

Near-infrared Spectroscopic Observations of Comet C/2013 R1 (Lovejoy) by WINERED: CN Red-system Band Emission

Abstract Although high-resolution spectra of the CN red-system band are considered useful in cometary sciences, e.g., in the study of isotopic ratios of carbon and nitrogen in cometary volatiles, there have been few reports to date due to the lack of high-resolution (R ≡ λ/Δλ > 20,000) spectrographs in the near-infrared region around ∼1 μm. Here, we present the high-resolution emission spectrum of the CN red-system band in comet C/2013 R1 (Lovejoy), acquired by the near-infrared high-resolution spectrograph WINERED mounted on the 1.3 m Araki telescope at the Koyama Astronomical Observatory, Kyoto, Japan. We applied our fluorescence excitation models for CN, based on modern spectroscopic studies, to the observed spectrum of comet C/2013 R1 (Lovejoy) to search for CN isotopologues (13C14N and 12C15N). We used a CN fluorescence excitation model involving both a “pure” fluorescence excitation model for the outer coma and a “fully collisional” fluorescence excitation model for the inner coma region. Our emission model could reproduce the observed 12C14N red-system band of comet C/2013 R1 (Lovejoy). The derived mixing ratio between the two excitation models was 0.94(+0.02/−0.03):0.06(+0.03/−0.02), corresponding to the radius of the collision-dominant region of ∼800–1600 km from the nucleus. No isotopologues were detected. The observed spectrum is consistent, within error, with previous estimates in comets of 12C/13C (∼90) and 14N/15N (∼150).

A NEW 3D MULTI-FLUID MODEL: A STUDY OF KINETIC EFFECTS AND VARIATIONS OF PHYSICAL CONDITIONS IN THE COMETARY COMA

ABSTRACT Physics-based numerical coma models are desirable whether to interpret the spacecraft observations of the inner coma or to compare with the ground-based observations of the outer coma. In this work, we develop a multi-neutral-fluid model based on the BATS-R-US code of the University of Michigan, which is capable of computing both the inner and outer coma and simulating time-variable phenomena. It treats H2O, OH, H2, O, and H as separate fluids and each fluid has its own velocity and temperature, with collisions coupling all fluids together. The self-consistent collisional interactions decrease the velocity differences, re-distribute the excess energy deposited by chemical reactions among all species, and account for the varying heating efficiency under various physical conditions. Recognizing that the fluid approach has limitations in capturing all of the correct physics for certain applications, especially for very low density environment, we applied our multi-fluid coma model to comet 67P/Churyumov–Gerasimenko at various heliocentric distances and demonstrated that it yields comparable results to the Direct Simulation Monte Carlo (DSMC) model, which is based on a kinetic approach that is valid under these conditions. Therefore, our model may be a powerful alternative to the particle-based model, especially for some computationally intensive simulations. In addition, by running the model with several combinations of production rates and heliocentric distances, we characterize the cometary H2O expansion speeds and demonstrate the nonlinear dependencies of production rate and heliocentric distance. Our results are also compared to previous modeling work and remote observations, which serve as further validation of our model.

CHARACTERIZING THE DUST COMA OF COMET C/2012 S1 (ISON) AT 4.15 AU FROM THE SUN

We report results from broadband visible images of comet C/2012 S1 (ISON) obtained with the Hubble Space Telescope Wide Field Camera 3 on 2013 April 10. C/ISON's coma brightness follows a 1/{\rho} (where {\rho} is the projected distance from the nucleus) profile out to 5000 km, consistent with a constant speed dust outflow model. The turnaround distance in the sunward direction suggests that the dust coma is composed of sub-micron-sized particles emitted at speeds of tens of meters s$^{-1}$. A({\theta})f{\rho}, which is commonly used to characterize the dust production rate, was 1340 and 1240 cm in the F606W and F438W filters, respectively, in apertures <1.6" in radius. The dust colors are slightly redder than solar, with a slope of 5.0$\pm$0.2% per 100 nm, increasing to >10% per 100 nm 10,000 km down the tail. The colors are similar to those of comet C/1995 O1 (Hale-Bopp) and other long-period comets, but somewhat bluer than typical values for short-period comets. The spatial color variations are also reminiscent of C/Hale-Bopp. A sunward jet is visible in enhanced images, curving to the north and then tailward in the outer coma. The 1.6"-long jet is centered at a position angle of 291$^\circ$, with an opening angle of about 45$^\circ$. The jet morphology remains unchanged over 19 hours of our observations, suggesting that it is near the rotational pole of the nucleus, and implying that the pole points to within 30 deg of (RA, Dec) = (330$^\circ$, 0$^\circ$). This pole orientation indicates a high obliquity of 50$^\circ$-80$^\circ$.

SuperWASP observations of the 2007 outburst of Comet 17P/Holmes

We present wide-field imaging of the 2007 outburst of Comet 17P/Holmes obtained serendipitously by SuperWASP-North on 17 nights over a 42-night period beginning on the night (2007 October 22-23) immediately prior to the outburst. Photometry of 17P's unresolved coma in SuperWASP data taken on the first night of the outburst is consistent with exponential brightening, suggesting that the rapid increase in the scattering cross-section of the coma could be largely due to the progressive fragmentation of ejected material produced on a very short timescale at the time of the initial outburst, with fragmentation timescales decreasing from t(frag)~2x10^3 s to t(frag)~1x10^3 s over our observing period. Analysis of the expansion of 17P's coma reveals a velocity gradient suggesting that the outer coma was dominated by material ejected in an instantaneous, explosive manner. We find an expansion velocity at the edge of the dust coma of v(exp) = 0.55+/-0.02 km/s and a likely outburst date of t_0=2007 October 23.3+/-0.3, consistent with our finding that the comet remained below SuperWASP's detection limit of m(V)~15 mag until at least 2007 October 23.3. Modelling of 17P's gas coma indicates that its outer edge, which was observed to extend past the outer dust coma, is best explained with a single pulse of gas production, consistent with our conclusions concerning the production of the outer dust coma.

Cometary gas relations 1P/Halley

AbstractPhotographic and photoelectric observations of comet 1P/Halley's ionised gas coma from CO+ at 4,250 Å and neutral gas coma from CN at 3,880 Å were part of the Bochum Halley Monitoring Program, conducted at the European Southern Observatory, La Silla, Chile, from February 17 to April 17, 1986.In this spectral range it is possible to see the continuum formation, motion and expansion of plasma and neutral gas structures. To observe the morphology of these structures, 32 CO+ photos (glass plates) from comet 1P/Halley obtained by means of an interference filter have been analysed. They have a field of view of 28.6 × 28.6 degrees and were obtained from March 29 to April 17, 1986 with exposure times between 20 and 120 minutes.All photos were digitised with a PDS 2020 GM microdensitometer. After digitisation, the data were reduced to relative intensities, and those with proper calibrations were also converted to absolute intensities, expressed in terms of column densities. The CO+ absolute intensity values still contain the continuum intensity. To calculate the CO+ column density it is necessary to subtract this continuum intensity.The relations between CO+ and CN in average column density values (NCO+/NCN) are 11.6 for a circular diaphragm with average diameter (Φ) of 6.1 arcminutes which corresponds to a distance from the nucleus (ρ) equal to 6.3 × 104 km; 20.0 for Φ = 7.1 arcminutes and ρ = 7.3 × 104 km; 8.1 for Φ = 8.5 arcminutes and ρ = 8.7 × 104 km; 35.6 for Φ = 11.9 arcminutes and ρ = 1.2 × 105 km; and 31.3 for Φ = 16.7 arcminutes and ρ = 1.7 × 105 km. These values are in perfect agreement with the data for short distances (ρ from 3.9 × 103 to 1.2 × 104 km) and small slit diameters (Φ from 0.4 to 1.2 arcminutes).With the use of diaphragms with large diameters it is possible to get some information about the outer coma of the comet (in this paper, from 60,000 until 170,000 km away from the nucleus). At these distances, the CO+ column density changes only due to the geometrical dilution, because the CO+ parent molecules are already photoionised or photodissociated.

Electron impact excitation of carbon monoxide in comet Hale‐Bopp

The fourth positive emissions of carbon monoxide in the coma of comet Hale‐Bopp have been assumed to be due mainly to fluorescence induced by sunlight. Based on this assumption they were used to deduce the abundance of carbon monoxide in the comet, giving a value higher than in other comets. Emissions produced by electron impact excitation of CO were not considered. Recent measurements and theoretical calculations of integral cross sections for electron impact excitation of CO allow the contribution of electron impact to be calculated, giving about 40% of the total. This implies that the abundance of CO in the outer coma of comet Hale‐Bopp was only 60% of that previously deduced. However, as the high proportion of CO in comet Hale‐Bopp was also seen in some other measurements, alternative explanations are considered. The method of calculation is tested by successfully predicting the O I emission at 1356 Å, supporting the belief that this line is due to electron impact excitation.

The Distribution, Excitation, and Formation of Cometary Molecules: Methanol, Methyl Cyanide, and Ethylene Glycol

We present an interferometric and single-dish study of small organic species toward Comets C/1995 O1 (Hale-Bopp) and C/2002 T7 (LINEAR) using the BIMA interferometer at 3 mm and the ARO 12 m telescope at 2 mm. For Comet Hale-Bopp, both the single-dish and interferometer observations of CH3OH indicate an excitation temperature of 105 ± 5 K and an average production rate ratio Q(CH3OH)/Q(H2O) ~ 1.3% at ~1 AU. In addition, the aperture synthesis observations of CH3OH suggest a distribution well described by a spherical outflow and no evidence of significant extended emission. Single-dish observations of CH3CN in Comet Hale-Bopp indicate an excitation temperature of 200 ± 10 K and a production rate ratio of Q(CH3CN)/Q(H2O) ~ 0.017% at ~1 AU. The nondetection of a previously claimed transition of cometary (CH2OH)2 toward Comet Hale-Bopp with the 12 m telescope indicates a compact distribution of emission, D < 9'' (<8500 km). For the single-dish observations of Comet T7 LINEAR, we find an excitation temperature of CH3OH of 35 ± 5 K and a CH3OH production rate ratio of Q(CH3OH)/Q(H2O) ~ 1.5% at ~0.3 AU. Our data support current chemical models that CH3OH, CH3CN, and (CH2OH)2 are parent nuclear species distributed into the coma via direct sublimation off cometary ices from the nucleus with no evidence of significant production in the outer coma.

The Grain Impact Analyser and Dust Accumulator (GIADA) Experiment for the Rosetta Mission: Design, Performances and First Results

The Grain Impact Analyser and Dust Accumulator (GIADA) onboard the ROSETTA mission to comet 67P/Churyumov–Gerasimenko is devoted to study the cometary dust environment. Thanks to the rendezvous configuration of the mission, GIADA will be plunged in the dust environment of the coma and will be able to explore dust flux evolution and grain dynamic properties with position and time. This will represent a unique opportunity to perform measurements on key parameters that no ground-based observation or fly-by mission is able to obtain and that no tail or coma model elaborated so far has been able to properly simulate. The coma and nucleus properties shall be, then, clarified with consequent improvement of models describing inner and outer coma evolution, but also of models about nucleus emission during different phases of its evolution. GIADA shall be capable to measure mass/size of single particles larger than about 15 μm together with momentum in the range 6.5 × 10−10 ÷ 4.0 × 10−4 kg m s−1 for velocities up to about 300 m s−1. For micron/submicron particles the cumulative mass shall be detected with sensitivity 10−10 g. These performances are suitable to provide a statistically relevant set of data about dust physical and dynamic properties in the dust environment expected for the target comet 67P/Churyumov–Gerasimenko. Pre-flight measurements and post-launch checkouts demonstrate that GIADA is behaving as expected according to the design specifications.

Deep Impact: Optical spectroscopy and photometry obtained at MIRA

We present spectroscopic and photometric observations, spanning the optical UV to the far red, before, during, and after the NASA Deep Impact event of July 4, 2005. The inner 2000 km of the pre- and post-impact coma was about 0.3 magnitude redder in B–R than in the outer coma. The pre-impact spectrum was a faint reflected solar spectrum dominated by molecular emissions extending > 40 , 000 km from the nucleus. The post-impact light curve in R and I showed a rapid rise consistent with an expanding optically thick cloud during the first 18 min after impact. During the next 8 min the cloud became optically thin. Sixty minutes after impact the impact R-band flux reached a plateau at 7.5 × 10 −15 erg cm −2 s −1 Å −1 , the comet brightening by a factor of ∼4.3 above its pre-impact value observed in a 15″ aperture. The mean expansion velocity of the grains during the first 49 min was 229 ± 49 m s −1 . The spectrum became dominated by scattered sunlight during the first hour after impact. The volume scattering function (VSF) observed 32 min after impact shows strong reddening. At 49 min, however, the VSF shows an additional twofold increase in the blue but only a 20% increase at 5500 Å. Post-impact spectra and R–I photometry showed rapid reddening. The particle size distribution, dominated by 1–2.5 μm particles shortly after impact, changed dramatically during the first hour due to sublimation of water-ice particles of this size. On the night following impact the comet was still substantially brighter than before impact, but R–I had returned to its pre-impact value. B–R remained significantly redder. The ejecta 25 h after impact was fan-shaped subtending ∼180° roughly symmetrical about position angle 225°. The mean expansion velocity 90° from the direction to the Sun was 185 ± 12 m s −1 .

Deep Impact: Optical spectroscopy and photometry obtained at MIRA

We present spectroscopic and photometric observations, spanning the optical UV to the far red, before, during, and after the NASA Deep Impact event of July 4, 2005. The inner 2000 km of the pre- and post-impact coma was about 0.3 magnitude redder in B–R than in the outer coma. The pre-impact spectrum was a faint reflected solar spectrum dominated by molecular emissions extending > 40 , 000 km from the nucleus. The post-impact light curve in R and I showed a rapid rise consistent with an expanding optically thick cloud during the first 18 min after impact. During the next 8 min the cloud became optically thin. Sixty minutes after impact the impact R-band flux reached a plateau at 7.5 × 10 −15 erg cm −2 s −1 Å −1 , the comet brightening by a factor of ∼4.3 above its pre-impact value observed in a 15″ aperture. The mean expansion velocity of the grains during the first 49 min was 229 ± 49 m s −1 . The spectrum became dominated by scattered sunlight during the first hour after impact. The volume scattering function (VSF) observed 32 min after impact shows strong reddening. At 49 min, however, the VSF shows an additional twofold increase in the blue but only a 20% increase at 5500 Å. Post-impact spectra and R–I photometry showed rapid reddening. The particle size distribution, dominated by 1–2.5 μm particles shortly after impact, changed dramatically during the first hour due to sublimation of water-ice particles of this size. On the night following impact the comet was still substantially brighter than before impact, but R–I had returned to its pre-impact value. B–R remained significantly redder. The ejecta 25 h after impact was fan-shaped subtending ∼180° roughly symmetrical about position angle 225°. The mean expansion velocity 90° from the direction to the Sun was 185 ± 12 m s −1 .

Time-dependent, three-dimensional fluid model of the outer coma, with application to the comet Hale-Bopp gas spirals

We describe here the first tridimensional, time-dependent model of a gas coma, and its first application to observed rotationally induced gas coma structures. The present version of the model uses inviscid flow equations and, because of it, is applicable rigorously only to highly productive comets, such as comet C1995/O1 Hale-Bopp in the inner Solar System. Using parameters suitable for this comet, and an arbitrary aspherical nucleus shape and rotation mode, the model is capable to generate large-scale gas spirals of the kind observed in this comet, thus demonstrating that the existence of such spirals does not require surface heterogeneity (active/inactive areas). This result does not in itself exclude heterogeneity. The question whether, in the absence of direct nucleus observations, any reliable interpretation of such structures is possible is raised.

HCO+Observations toward Comet Hale‐Bopp (C/1995 O1): Ion‐Molecule Chemistry and Evidence for a Volatile Secondary Source

Several millimeter-wave transitions of HCO + have been detected toward comet Hale-Bopp (C/1995 O1) using the Arizona Radio Observatory 12 m telescope. The J ¼ 2 ! 1 transition at 178 GHz was observed toward the comet nucleus near perihelion on 1997 March 10 and 20, as well as the J ¼ 3 ! 2 transition at 268 GHz on 1997 March 9, with angular resolutions of 36 00 and 23 00 , respectively. These data all show a slight velocity shift (� 1.2 km s � 1 ) from the nominal comet velocity, and the J ¼ 3 ! 2 profile is asymmetric with a redshifted wing. These differences likely arise from ion acceleration by the solar wind. A rotational diagram analysis of the data yielded a column density of 1:1;10 12 cm � 2 for HCO + in Hale-Bopp, which corresponds to an average number density of 36 cm � 3 . The data taken on March 9 show a second velocity component redshifted by 7:0 � 0: 6k m s � 1 ,w hich is considerably weaker than the main feature and appears to have a counterpart in the HNC, J ¼ 3 ! 2 data, observed within an hour of the HCO + measurements. The velocity difference between the main and secondary emission lines deprojected onto the extended solar radius vector is � 10 km s � 1 for both HCO + and HNC, and the weak-to-strong line intensity ratios (� 5%) are identical to within observational errors, suggesting a common highvelocity volatile secondary source. A plausible model that may account for the redshifted velocity components is a comoving, localized debris field of submicron refractory grains accelerated by solar radiation pressure located � 10 5 ‐10 6 km from the nucleus. The parent material of the weaker redshifted HNC and HCO + lines may be predominately complex organic polymers. An examination of the production rates for HCO + suggests that the reaction H2 þ CO þ is likely to be an important route to this ion in the outer coma beyond the collisionopause,

New experimental results on the degradation of polyoxymethylene: Application to the origin of the formaldehyde extended source in comets

The formaldehyde (H2CO) observed in cometary atmospheres presents a so‐called “extended source,” meaning that its distribution in the coma cannot be explained by sublimation from the nucleus alone; a production inside the coma has to be included. Polyoxymethylene (formaldehyde polymers: (‐CH2‐O‐)n, also called POM) is sometimes evoked as a parent molecule for this extended source. This solid polymer on cometary grains could release gaseous formaldehyde through thermal and photolytic degradation. We have developed an experimental program in order to study the chemical reactions of degradation of POM by UV photolysis and heating. It provides identification of the degradation products and the determination of the photo degradation quantum yields or the thermal degradation kinetics. In this paper we present the improvements of our experimental setup and new measurements on the kinetics of gaseous formaldehyde production by thermal degradation of two types of commercial POM on a greater temperature range than the previous ones. These new data are then included in a model of the outer coma in order to interpret observations. This model takes into account the production of gaseous molecules from solid material present on grains and leads to a very good agreement with Giotto observations of comet 1P/Halley, if we assume that the cometary grains contain 1–16% of POM by mass. Thus, without being final evidence of this polymer presence on comets, we conclude that the degradation of solid POM‐like polymers on cometary grains is to date the best explanation of the H2CO extended source.

An Optical Survey of the Active Centaur C/NEAT (2001 T4)

ABSTRACTWe present the results of optical observations of C/NEAT (2001 T4) taken on three dates between 2001 October and 2002 September at the University of Hawaii 2.2 m telescope. Coma was present for each observation, but the activity level was variable. We present surface brightness profiles, dust production rates, and evidence for structure in the coma. There is a radial gradient in coma color, with the outer coma exhibiting the least red colors. The object’s V−R and R−I colors indicate that this is one of the reddest objects with a Centaur‐type orbit.

Production, Outflow Velocity, and Radial Distribution of H2O and OH in the Coma of Comet C/1995 O1 (Hale‐Bopp) from Wide‐Field Imaging of OH

Observations of OH are a useful proxy of the water production rate (Q(sub H2O)) and outflow velocity (V(sub out)) in comets. We use wide field images taken on 03/28/1997 and 04/08/1997 that capture the entire scale length of the OH coma of comet C/1995O1 (Hale-Bopp) to obtain Q(sub H2O) from the model-independent method of aperture summation. We also extract the radial brightness profile of OH 3080 angstroms out to cometocentric distances of up to 10(exp 6) km using an adaptive ring summation algorithm. Radial profiles are obtained as azimuthal averages and in quadrants covering different position angles relative to the comet-Sun line. These profiles are fit using both fixed and variable velocity two-component spherical expansion models to determine VOH with increasing distance from the nucleus. The OH coma of Hale-Bopp was more spatially extended than in previous comets, and this extension is best matched by a variable acceleration of H2O and OH that acted across the entire coma, but was strongest within 1-2 x 10(exp 4) km from the nucleus. This acceleration led to VOH at 10(exp 6) km that was 2-3 times greater than that obtained from a 1P/Halleytype comet at 1 AU, a result that is consistent with gas-kinetic models, extrapolation from previous observations of OH in comets with Q(sub H2O) > 10(exp 29)/s, and radio measurements of the outer coma Hale-Bopp OH velocity profile. When the coma is broken down by quadrant, we find an azimuthal asymmetry in the radial distribution that is characterized by an increase in the spatial extent of OH in the region between the orbit-trailing and anti-sunward directions. Model fits to this area and comparison with radio OH measurements suggest greater acceleration in this region, with VOH UP to 1.5 times greater at 10(exp 6) km radial distance than elsewhere in the coma.