The lifetime of the OH radical in comets at 1 AU

It has been shown that the photochemical lifetime of OH in comets is a function of the comet's radial velocity. The calculated lifetime at 1 AU can vary between 6.9 × 10⁴ to 2.1 × 10⁵ sec for radial velocities that vary from ?58 to +59 km/sec. A comparison between the scale lengths observed for three comets and those calculated based upon the theoretical lifetime has been made. This comparison shows that in two of the comets the lifetime derived from the scale lengths is a factor of 1.7 larger than the theoretical lifetime. Suggestions are made about the origin of this discrepancy.     

Observation of comets C/2009 R1 (McNaught) and 17P/holmes in the OH line at a wavelength of 18 cm

The observation results for comets C/2009 R1 (McNaught) (June 14–28, 2010) and 17P/Holmes (November 25–December 3, 2007) performed in the line of the OH molecule and at a wavelength of 18 cm are presented. The gas productivity of the OH molecule for comet C/2009 R1 (McNaught) (June 14–28, 2010) as a function of the heliocentric distance (0.42–0.62 au) is determined.     

The organic composition of Comet C/2000 WM1 (LINEAR) revealed through infrared spectroscopy

We investigated the parent volatile composition of the Oort cloud Comet C/2000 WM₁ (LINEAR) on 23-25 November 2001, using the Near Infrared Echelle Spectrograph on the Keck II telescope. Flux-calibrated spectra, absolute production rates, and mixing ratios are presented for H₂O, HCN, CH₄, C₂H₂, C₂H₆, H₂CO, CH₃OH and CO. Compared with “organics-normal” comets, WM₁ is moderately depleted in HCN, CH₄ and CH₃OH, and is even more depleted in C₂H₂ and CO. Its composition is thus intermediate to comets that are severely depleted in their organic volatile composition and those that exhibit “normal” organic volatile abundances. We argue that WM₁ may have formed closer to the young Sun than “organics-normal” comets, but at greater distance than the severely depleted comets, before its ejection to the Oort cloud. The mixing ratios of the above-listed organic volatiles agree day-by-day for 23-25 November 2001. Thus, there is no evidence of macroscopic heterogeneity in chemistry of this comet’s nucleus at the achieved measurement accuracy. As the first comet to show moderate organic depletion in parent volatiles, WM₁ represents an important addition to the emerging taxonomic classification based on chemical composition.     

Observations of the Oh Radio Lines of Comets at Nançay – Recent Developments

The 18 cm lines of OH have been observed in more than 70 comets withthe Nançay radio telescope since 1974. The observations made from1982 to 1999 are now organized in a data base. The Nançay radiotelescope has been recently upgraded, and observations since 2000 aremade with a sensitivity increased by about a factor of two. Twelvecomets were observed from July 2000 to May 2002.     

Chemical Composition Diversity Among 24 Comets Observed At Radio Wavelengths

We present a comparative study on molecular abundances in comets basedon millimetre/submillimetre observations made with the IRAM 30-m,JCMT, CSO and SEST telescopes. This study concerns a sample of 24comets (6 Jupiter-family, 3 Halley-family, 15 long-period) observedfrom 1986 to 2001 and 8 molecular species (HCN, HNC, CH₃CN,CH₃OH, H₂CO, CO, CS, H₂S). HCN was detected in all comets,while at least 2 molecules were detected in 19 comets. From the sub-sample of comets for which contemporary H₂O productionrates are available, we infer that the HCN abundance relative to water variesfrom 0.08% to 0.25%. With respect to other species, HCN is the moleculewhich exhibits the lowest abundance variation from comet to comet. Therefore,production rates relative to that of HCN can be used for a comparative study ofmolecular abundances in the 19 comets. It is found that: CH₃OH/HCN varies from ≤ 9 to 64; CO/HCN varies from ≤ 24 to 180; H₂CO/HCN varies between 1.6 and 10; and H₂S/HCN varies between 1.5 and 7.6. This study does not show any clear correlation between the relative abundancesand the dynamical origins of the comets, or their dust-to-gas ratios.     

Observations of the OH radical in comets at 18 cm wavelength

We present recent observations of the OH radical at λ 18 cm with the Nançay radio telescope in comets Meier (1978 XXI), Bradfield (1979 X), Meier (1980q), P/Encke (1980), and Bradfield (1980t). The analysis of the OH radio line shape is a powerful tool to study the kinematics of the coma. The expansion velocity of the OH molecules is found to be ≈1.5 km sec^?1 at r_h = 1 AU, and decreases with increasing heliocentric distance. The line profile is generally asymmetric, which demonstrates the Greenstein effect on the fluorescent excitation mechanism and/or anisotropic outgassing of the nucleus. In several cases, especially for comet Meier (1978 XXI), an asymmetry is also found in the east-west brightness distribution of the OH line, showing again the Greenstein effect and/or anisotropic outgassing. An excitation model by uv pumping and fluorescence of the OH radical, which agrees with the observations at least in the first order, and the application of Haser's model lead to the production rate of the parent molecule of OH. There is a close correlation between this gas production rate and the visual brightness of the comet. Our estimates of gas production rates are smaller than or equal to those obtained from uv measurements, but both radio and uv estimates depend heavily on the parameters used in Haser's models.     

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

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

Collisional Quenching of OH Radio Emission from Comet Hale-Bopp

Observations of comets in the 18-cm OH transitions offer a means to probe gas production, kinematics, and OH excitation in comets. We present initial results of OH observations of comet Hale-Bopp obtained with the NRAO 43 m antenna located in Greenbank, WV. Maps of the emission provide strong constraints on the amount of quenching of the inversion of the OH ground state Λ-doublet in the coma. Analysis of the total radio OH flux and maps of its radial brightness distribution indicate a quenched region on the order of ∼500,000 km during March and April 1997. This large value is generally consistent with previous observations of radio OH quenching in lower production rate comets when the high production rate of comet Hale-Bopp is considered. This revised version was published online in July 2006 with corrections to the Cover Date.     

Laboratory simulation of cometary erosion by space plasma

A laboratory experiment has been made where a plasma stream collides with targets made of different materials of cosmic interest. The experiment can be viewed as a process simulation of the solar wind particle interaction with solid surfaces in space — e.g., cometary dust. Special interest is given to sputtering of OH and Na.It is also shown that the erosion of solid particles in interplanetary space at large heliocentric distances is most likely dominated by sputtering and by sublimation near the Sun. The heliocentric distance of the limit between the two regions is determined mainly by the material properties of the eroded surface, e.g., heat of sublimation and sputtering yield, a typical distance being 0.5 AU.It is concluded that the observations of Na in comets at large solar distances, in some cases also near the Sun, is most likely to be explained by solar wind sputtering. OH emission in space could be of importance also from dry, water-free, matter by means of molecule sputtering. The observed OH production rates in comets are however too large to be explained in this way and are certainly the results of sublimation and dissociation of H₂O from an icy nucleus.     

Sulfur Chemistry at Millimeter Wavelengths in C/Hale-Bopp

The recent availability of bright comets has given us an excellent opportunity to study cometary chemistry. Comet Hale-Bopp (1995 O1)gave us the particularly rare opportunity to study a bright and active comet for almost two years. Our program concentrated on millimeter-wave observations of sulfur-bearing molecules in an effort to understand the total sulfur budget of the comet. Using the National Radio Astronomy Observatory 12-m telescope on Kitt Peak we monitored both the long and short-term variations in H₂S, CS, and OCS, as well as observing H₂CS and SO. This was the first observation of H₂CS in any comet (Figure 1). Additionally, we mapped CS with the BIMA interferometer. Variations in the line profiles and changes in line intensity as large as a factor of two were seen in day to day observations of both H₂S and CS. An example for H₂S is shown in Figure 2. This is the first time we can attempt to study the entire group of sulfur-bearing molecules. Models of the sulfur coma have thus far largely been based on observations of the daughter products CS and atomic sulfur made over the last 18 years using the International Ultraviolet Explorer (IUE) satellite, coupled with radio observations of CS and H₂S in several recent comets. Four new sulfur-bearing species have been observed in comets Hale-Bopp and Hyaku take, three of them parent species. The high resolution maps in CS will also allow spatial information to be included in the sulfur model for the first time. C/Hale-Bopp is the first comet in which so many sulfur species have been observed. Analysis of the abundances of these species in comparison to the total atomic sulfur observed should reveal whether or not we can now account for all of the primary sulfur sources in comets. Perhaps the most interesting question that these observations raised was why C/Hale-Bopp appeared to contain so much more SO and SO₂ (as observed by others) than any other comet. This spurred the discovery that the UV fluorescence models of these species were incorrect (S. J. Kim, this issue). Analysis of the data and modeling of the sulfur budget are still underway. This revised version was published online in July 2006 with corrections to the Cover Date.     

P/GE-Wang joins P/Slaughter-Burnham and P/Boethin in the club of comets in 1/1 resonance with jupiter

Three comets are now known to be at or near the 1/1 resonance with Jupiter P/Slaughter-Burnham, P/Boethin and the newly discovered P/Ge-Wang. Their orbital evolutions are compared, using the elliptic three-dimensional restricted three-body model Sun-Jupiter-comet. Although details of the individual orbits differ, the three comets have very similar general dynamical behaviours, and stay during a long time at or near the 1/1 resonance, at least for several thousand years.     

A review of radio observations of comets

The current status of cometary radio observations is reviewed. Radio continuum observations made at different wavelengths can be used to model the properties of cometary particles. Continuum observations have been successful for two comets but the interpretation of the data is subject to some disagreement. Radar observations are important for determining the size, angular momentum, direction of motion, and surface properties of the cometary nucleus. One comet, p/Encke, has been successfully observed by radar. The reasons why radio observations can fail are discussed. These include the undue influence of the highly volatile “comet frost” which often coats new comets, small errors in radio ephemerides, the inopportune scheduling of observing periods at less than optimum cometary heliocentric distances and velocities, and poor spectroscopic properties of the molecular transitions chosen for observations. In order to clarify the sometimes confusing observations which have been reported, cometary radio spectroscopy is reviewed in chronological order, comet by comet, starting from the earliest reported searches for polyatomic molecules in the early 1970s through progress in understanding cometary OH and into current searches for glycine, the simplest amino acid. The results of current OH ultraviolet pumping models are briefly discussed and several formalisms for computing molecular production rates arepresented. Radio observational programs which can aid in discriminating between current theories of terrestrial biological evolution are introduced. Both specific and general conclusions are drawn from the available material on cometary radio spectroscopy.     

Stability of C60 and C70 fullerenes toward corpuscular and γ radiation

The stability of C₆₀ and C₇₀ fullerenes in the interstellar medium deposited on dust surface or embedded in meteorites and comets has been simulated with γ irradiation and with He⁺ ion bombardment. It is shown by vibrational spectroscopy that a γ radiation dose of 2.6 MGy (1 Gy = 1 joule absorbed energy per kilogram) causes partial oligomerization of both C₆₀ and C₇₀ fullerenes. Oligomers are made by fullerene cages chemically connected each other which can yield back free fullerenes by a thermal treatment. The amount of irreversibly polymerized fullerenes caused by 2.6 MGy as deduced as the toluene insoluble fraction has been determined as 1.7 and 15 per cent by weight, respectively, for C₆₀ and C₇₀ fullerene. The radiation dose generated by radionuclides decay and expected to be delivered to fullerenes buried at a depth of more than 20 m in comets and meteorites is about 3 MGy per 10⁹ yr. Since fullerenes are by far resistant to such radiation dose they can survive for at least some billion years inside comets and meteorites and in fact have been detected inside certain carbonaceous chondrites. On the other hand, the direct exposure of fullerenes to cosmic rays for instance when they are adsorbed or deposited on the surface of carbon dust corresponds to the delivery of a radiation dose comprised between 30 and 65 MGy per 10⁹ yr. Experimental bombardment of both C₆₀ and C₇₀ fullerenes for instance with He⁺ ions has shown that the complete amorphization occurs at about 250 MGy. Thus in ∼4 Gyr exposure to cosmic rays it is expected a complete amorphization.     

Solar work at Manila observatory

Seventeen comets, having information on sodium D-line emission during their apparition, have been studied. The heliocentric distances corresponding to the sodium emission commencement or termination epoch are found to have a dependence on the phase of the solar cycle. For comets appearing during a solar maximum the sodium emission is detectable out to greater distances than, for the comets appearing during solar minimum. The sodium emission is also found to depend on heliographic latitude of the comet. It is concluded that the spatial properties of the solar wind during a solar maximum and minimum are responsible for the observed dependence.     

Comet classification with new methods for gas and dust spectroscopy

We present the results of a program of comet long-slit spectroscopy with the Kast Dual Spectrograph on the 3-m Shane Telescope at Lick Observatory. A total of 26 comets, from a variety of dynamical families, were observed on 39 different nights from 1996 to 2007. A new statistical method extracted the twilight sky from comet frames, because traditional sky subtraction techniques were inadequate. Because previously published Haser model parent and daughter scale lengths did not fit the data well, unbiased ranges of scale lengths were searched for the best-fitting pairs. Coma gas production rates for OH, CN, C₂, C₃, NH, NH₂, and OH confirmed the widely reported carbon-chain depletion for a sub-class of comets, most notably high-perihelion Jupiter-family comets observed at r_h > 1.5 AU, with different behaviors for C₂ and C₃. Our long-slit spectroscopy data was also adapted for the A(θ)fρ dust production parameter. The assumption that A(θ)fρ is constant throughout the nucleus was not upheld. High dust-to-gas ratios for comets with large perihelia were not a selection effect, and suggest that the dust was released earlier in the formation of the coma than the gas. The dust-to-gas ratio did not exhibit any evolutionary traces between different comet dynamical families. The comet survey illuminates the diversity among comets, including the unusually carbon poor Comet 96P/Machholz.     

Sungrazing Comets Discovered with the SOHO/LASCO Coronagraphs 1996-1998

The Kreutz sungrazing family of comets is unique because of its small perihelion distance and because of the large number of known members of this family. SOHO/LASCO coronagraph observations beginning in 1996 have revealed an unprecedented number of Kreutz comets. These new coronagraph observations improve upon earlier observations because of a larger field-of-view, increased image cadence, and better photometric measurements. This paper presents the lightcurves of the 141 Kreutz family comets observed from 1996 through 1998. Throughout this period, the number of family members discovered each year is shown to be constant. None of the comets were detected postperihelion. The lightcurves show distinctive characteristics which reveal much about the properties of the nuclei. It is shown that the individual fragments can be related to one of two “standard candles,” which we call Universal Curves. The comets all reach a peak brightness at one of two characteristic distances (both near 12 R_⊙) and that the comets fragment at another characteristic distance (about 7 R_⊙). Also, evidence is seen for line emission, which varies with heliocentric distance.     

New edition of the catalogue of short-period comets

The Institute of Theoretical Astronomy in St. Petersburg and the Astronomical Institute in Bratislava are preparing a new edition of the Catalogue of short-period comets. This edition will be supplemented by short-period comets discovered after the year 1983 and comprises some new features, e. g. the evolution of orbital elements between the years 1750 and 2050, and the perihelion passages of comets within the 1994–2050 years. A new method has been employed for the determination of nongravitational parameters from the osculating elements of a comet based on all its observed returns.The method has been tested on the comets P/Comas Solá and P/Forbes with all returns, except the last one. The results have been compared with the osculating elements of the last return and those used in the old edition of the Catalogue of short-period comets. The new method enables a good prediction of osculating elements for the future, at least for the next return.     

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.     

A survey of debris trails from short-period comets

We observed 34 comets using the 24 μm camera on the Spitzer Space Telescope. Each image contains the nucleus and covers at least ¹⁰⁶ km of each comet's orbit. Debris trails due to mm-sized or larger particles were found along the orbits of 27 comets; 4 comets had small-particle dust tails and a viewing geometry that made debris trails impossible to distinguish; and only 3 had no debris trail despite favorable observing conditions. There are now 30 Jupiter-family comets with known debris trails, of which 22 are reported in this paper for the first time. The detection rate is >80%, indicating that debris trails are a generic feature of short-period comets. By comparison to orbital calculations for particles of a range of sizes ejected over 2 yr prior to observation, we find that particles comprising 4 debris trails are typically mm-sized while the remainder of the debris trails require particles larger than this. The lower-limit masses of the debris trails are typically ¹⁰¹¹ g, and the median mass loss rate is 2 kg/s. The mass-loss rate in trail particles is comparable to that inferred from OH production rates and larger than that inferred from visible-light scattering in comae.     

Radio observations of Jupiter-family comets

Radio observations from decimetric to submillimetric wavelengths are now a basic tool for the investigation of comets. Spectroscopic observations allow us: (i) to monitor the gas production rate of the comets, by directly observing the water molecule, or by observing secondary products (e.g., the OH radical) or minor species (e.g., HCN); (ii) to investigate the chemical composition of comets; (iii) to probe the physical conditions of cometary atmospheres: kinetic temperature and expansion velocity. Continuum observations probe large-size dust particles and (for the largest objects) cometary nuclei.Comets are classified from their orbital characteristics into two separate classes: (i) nearly isotropic, mainly long-period comets and (ii) ecliptic, short-period comets, the so-called Jupiter-family comets (JFCs). These two classes apparently come from two different reservoirs, respectively, the Oort cloud and the trans-Neptunian scattered disc. Due to their different history and—possibly—their different origin, they may have different chemical and physical properties that are worth being investigated.The present article reviews the contribution of radio observations to our knowledge of the JFCs. The difficulty of such a study is the commonly low gas and dust productions of these comets. Long-period, nearly isotropic comets from the Oort cloud are better known from Earth-based observations. On the other hand, JFCs are more easily accessed by space missions. However, unique opportunities to observe JFCs are offered when these objects come by chance close to the Earth (like 73P/Schwassmann-Wachmann 3 in 2006), or when they exhibit unexpected outbursts (as did 17P/Holmes in 2007).About a dozen JFCs were successfully observed by radio techniques up to now. Four to ten molecules were detected in five of them. No obvious evidence for different properties between JFCs and other families of comets is found, as far as radio observations are concerned.