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An Analysis of CO Production in Cometary Comae: Contributions from Gas-phase Phenomena
Abstract
Understanding the sources of CO in cometary comae is important for understanding comet chemistry and the roles comets have played in the development of the solar system. Among comets sampled to date, the CO abundances vary widely and no direct correlation of CO abundance with other known comet properties has been identified. The picture is complicated further by the discovery of CO production in the comae of some comets, most notably comets Halley and Hale-Bopp. In this study, we investigate the conditions under which CO can be produced in the coma via gas-phase phenomena. We include photochemistry of several parent molecules, as well as two-body chemical reactions that involve the parents and their photodissociative daughter and granddaughter products. We also consider the level of excitation of "hot" hydrogen (H*) and O(1D) in the network, because the level of excitation of these reactants strongly influences reaction rates. Our results suggest that the dominant gas-phase contributor to CO formation is the photodissociation of H2CO. Even though typical abundances of H2CO are at ~1% relative to water in the coma, it produces more CO than other processes due to its relatively short photodissociation lifetime. Because other studies have shown H2CO to have a distributed source as well, it suggests that at least some CO formation in the coma is connected to the H2CO distributed source. We take the time to examine the CO2/CO ratio and note that while the CO2/CO ratio in comets Halley, Hale-Bopp, and Hyakutake are noticeably different when only native CO is considered, the CO2/CO ratios show greater similarity when total CO is considered. Although this sample is relatively small, should the relatively similar CO2/COTotal ratio of ~0.25 indeed be constant for comets with distributed CO sources, it suggests that the extended CO source of these comets is tied directly to the overall C, H, O chemistry of comets, as is likely to happen if hydrogenation of CO occurred on icy grains.
... Although the dust and gas counterparts of the comae are both addressed, the main emphasis is on the volatiles (mainly CO), as they are directly involved in generating the distant activity. Studying CO's behavior beyond 4 au is especially important to confirm its natal contributions and study it in isolation (Pierce & A'Hearn 2010). ...
... It would be particularly valuable to compare CO behavior for these two classes of comets when they are beyond the water ice sublimation limit. Also, closer to the Sun, CO may be produced in significant amounts as daughter products from parent species (Pierce & A'Hearn 2010). Such competing sources for CO production near the Sun complicates efforts to account for how much CO is natal and driving the long-term activity. ...
The activity of most comets near the Sun is dominated by the sublimation of frozen water, the most abundant ice in comets. Some comets, however, are active well beyond the water-ice sublimation limit of ~3 au. Three bodies dominate the observational record and modeling efforts for distantly active comets: the long-period comet C/1995 O1 (Hale-Bopp), and the short-period comets (with Centaur orbits) 29P/Schwassmann-Wachmann 1 and 2060 Chiron. We summarize what is known about these three objects with an emphasis on their gaseous comae. We calculate their CN/CO and CO2/CO production rate ratios from the literature and discuss implications, such as HCN and CO2 outgassing are not significant contributors to their comae. Using our own data we derive CO production rates, Q(CO), for all three objects to examine whether there is a correlation between gas production and different orbital histories and/or size. The CO measurements of Hale-Bopp (4–11 AU) and 29P are consistent with a nominal production rate of Q(CO) = 3.5 × 10²⁹ r⁻² superimposed with sporadic outbursts. The similarity of Hale-Bopp CO production rates for pre- and post-perihelion suggests that thermal inertia was not very important and therefore most of the activity is at or near the surface of the comet. We further examine the applicability of existing models in explaining the systematic behavior of our small sample. We find that orbital history does not appear to play a significant role in explaining 29P's CO production rates. 29P outproduces Hale-Bopp at the same heliocentric distance, even though it has been subjected to much more solar heating. Previous modeling work on such objects predicts that 29P should have been devolatilized over a fresher comet like Hale-Bopp. This may point to 29P having a different orbital history than current models predict, with its current orbit acquired more recently. On the other hand, Chiron's CO measurements are consistent with it being significantly depleted over its original state, perhaps due to increased radiogenic heating made possible by its much larger size or its higher processing due to orbital history. Observed spectral line profiles for several volatiles are consistent with the development and sublimation of icy grains in the coma at about 5–6 au for 29P and Hale-Bopp, and this is probably a common feature in distantly active comets, and an important source of volatiles for all comets within 5 au. In contrast, the narrow CO line profiles indicate a nuclear, and not extended, origin for CO beyond ~4 au.
... The likely precursor of CO + , CO, sublimates readily beyond several AU where water sublimation becomes increasingly inefficient (Womack, Sarid & Wierzchos 2017 ), and the neutral CO distribution may be caused by one or more production mechanism(s) (Pierce & A'Hearn 2010 ). Once produced, CO + may serve as a proxy for CO in such distant comets if the emission and production mechanisms can be characterized (Fortenberry, Bodewits & Pierce 2021 ). ...
A new CO+ fluorescence emission model for analyzing cometary spectra is presented herein. Accurate line lists are produced using the PGOPHER software for all transitions between the three electronic states (X 2Σ, A 2Π, B 2Σ) with vibrational states up to vmax = 9, 8, 6, respectively, and maximum rotational states with rotational quantum numbers N ≤ 20. As a result of improved molecular constants and theoretical transition rates, an expansion of the utilized solar spectrum into the infrared, and the substantial expansion of the included rovibronic states, the model provides an update of the fluorescence efficiencies of the CO+ cation. The dependencies on heliocentric velocity and distance are explicitly included. We report, for the first time, quantification of the fluorescence efficiencies for the ground state rovibrational transitions of CO+ and predict the positions and relative intensities of CO+ lines in windows accessible to both ground- and space-based observatories. The computed fluorescence efficiencies show excellent agreement with UV/optical observations of both C/2016 R2 (Pan-STARRS) and 29P/Schwassmann-Wachmann 1. The updated fluorescence efficiencies allow for revised N2/CO abundances for comets 1P/Halley, C/1987 P1 (Bradfield), and C/2016 R2 (Pan-STARRS), which can change by up to 30% when accounting for recent improvements to CO+ and N fluorescence efficiencies. The model code, input files, and fluorescence efficiencies are publicly available and distributed on permanent archives for future uses in cometary analyses.
... Chemical reactions probably would not alter this ratio much, and the ratio is not expected to change with heliocentric distance. However, previous studies have highlighted the presence of a rich CO chemistry in comets (Pierce & A'Hearn 2010;Raghuram & Bhardwaj 2012), suggesting that CO abundances may experience modest changes on the order of 10% due to photochemical processes. Although our conclusions remain unaffected by these potential changes, this is worth consideration. ...
Comets are seen as depleted in nitrogen compared to the protosolar value, but a small number exhibit significantly higher than typical N2/CO ratios: C/1908 R1 (Morehouse), C/1940 R2 (Cunningham), C/1947 S1 (Bester), C/1956 R1 (Arend-Roland), C/1957 P1 (Mrkos), C/1961 R1 (Humason), C/1969 Y1 (Bennett), C/1973 E1 (Kohoutek), C/1975 V1-A (West), C/1986 P1 (Wilson), C/1987 P1 (Bradfield), C/2001 Q4 (NEAT), C/2002 VQ94 (LINEAR), C/2016 R2 (PanSTARRS), and periodic comets 1P/Halley, 29P/Schwassmann-Wachmann 1, and 67P/Churyumov–Gerasimenko. This study examines the composition and dynamical histories of these N2-‘rich’ comets to unearth insights into their formation processes. Using updated N2 fluorescence factors, we re-estimate the N2/CO ratios of this sample and find that they are consistent with the expected values for comets based on estimations of the protosolar nebula. These also often display larger nucleus sizes and show rapid tail morphology variations due to their ionic nature. Numerical simulations reveal no common dynamical history, suggesting that the N2/CO ratio is independent of the number of inner Solar System passages and that N2 is homogeneously distributed within these comets. These volatile-rich comets share an Oort Cloud origin which is consistent with their survival over the past 4.5 Gyr. Our study also suggests that there may be a bias using modern high-resolution spectrometers with narrow slits, which could potentially overlook the ion tail of comets. We advocate for the use of long-slit spectroscopy to potentially detect a wider range of N2-rich comets, thereby enriching our understanding of comet compositions and origins.
... Speculation as to the origins of this material vary since most of the O 2 known to exist in the Earths atmosphere is believed to be of biological provenance, which cannot be the case in the interplanetary medium. Prior to encounter, chemical models (Glinski et al. 2004;Pierce & A'Hearn 2010) predicted that some O 2 could be formed in the comae of comets, but not to the levels detected by Rosetta. The observations, and several suggested mechanisms for O 2 production in comet 67P, have been reviewed recently (Luspay-Kuti et al. 2018;Altwegg et al. 2019). ...
The recent ROSETTA mission to comet 67P/Churyumov–Gerasimenko detected surprisingly high levels of molecular oxygen (O2; hypervolatile species) in the coma. Current models predict that considerable levels of other hypervolatiles (such as molecular nitrogen, N2, methane, CH4, and Argon) should be found at similar levels, whereas they are more depleted. One explanation explored here is that larger (less volatile) parent molecules may have been formed during radiolysis of cometary ices and, upon sublimation, are subsequently broken down within the coma into smaller, more volatile fragments. In support of this hypothesis, this work employs reliable quantum chemical techniques to provide the spectral data necessary for the detection of two candidate precursor "parent" molecules, cyclic carbon trioxide (c-CO3), and cyclic dicarbon trioxide (c-C2O3). Benchmark computations performed for gas-phase CO2 give vibrational frequencies to within 1.5 cm−1 or better for the three fundamentals. Both c-CO3 and c-C2O3 have strong infrared features in the 4.5–5.5 μm (1800–2200 cm−1) range and other notable infrared features closer to 1100 cm−1 (9.10 μm). These molecules are both rotationally active, unlike CO2, and are therefore potentially observable and present new targets for radio telescope observations. Due to the stronger dipole moment, c-CO3 should be more easily detectable than the nearly non-polar c-C2O3. These data may help observations of these molecules and can provide insights as to how radiation-driven derivatization of CO/CO2 precursors could contribute to the generation of higher-mass parent species that subsequently degrade to produce more volatile species, such as O2, observed in cometary comae.
The environment of a comet is a fascinating and unique laboratory to study plasma processes and the formation of structures such as shocks and discontinuities from electron scales to ion scales and above. The European Space Agency’s Rosetta mission collected data for more than two years, from the rendezvous with comet 67P/Churyumov-Gerasimenko in August 2014 until the final touch-down of the spacecraft end of September 2016. This escort phase spanned a large arc of the comet’s orbit around the Sun, including its perihelion and corresponding to heliocentric distances between 3.8 AU and 1.24 AU. The length of the active mission together with this span in heliocentric and cometocentric distances make the Rosetta data set unique and much richer than sets obtained with previous cometary probes. Here, we review the results from the Rosetta mission that pertain to the plasma environment. We detail all known sources and losses of the plasma and typical processes within it. The findings from in-situ plasma measurements are complemented by remote observations of emissions from the plasma. Overviews of the methods and instruments used in the study are given as well as a short review of the Rosetta mission. The long duration of the Rosetta mission provides the opportunity to better understand how the importance of these processes changes depending on parameters like the outgassing rate and the solar wind conditions. We discuss how the shape and existence of large scale structures depend on these parameters and how the plasma within different regions of the plasma environment can be characterised. We end with a non-exhaustive list of still open questions, as well as suggestions on how to answer them in the future.
The Gateway Centaur and Jupiter co-orbital P/2019 LD2 (ATLAS) provides the first opportunity to observe the migration of a solar system small body from a Centaur orbit to a Jupiter Family Comet (JFC) four decades from now. The Gateway transition region is beyond where water ice can power cometary activity, and coma production there is as poorly understood as in all Centaurs. We present contemporaneous multiwavelength observations of LD2 from 2020 July 2–4: Gemini North visible imaging, NASA IRTF near-infrared spectroscopy, and ARO Submillimeter Telescope millimeter-wavelength spectroscopy. Precovery DECam images limit the nucleus’s effective radius to ≤1.2 km and no large outbursts were seen in archival Catalina Sky Survey observations. LD2's coma has g ′ − r ′ = 0.70 ± 0.07 , r ′ − i ′ = 0.26 ± 0.07 , a dust-production rate of ∼10–20 kg s ⁻¹ , and an outflow velocity between v ∼ 0.6–3.3 m s ⁻¹ . We did not detect CO toward LD2 on 2020 July 2–3, with a 3 σ upper limit of Q (CO) < 4.4 × 10 ²⁷ mol s ⁻¹ (⪅ 200 kg s ⁻¹ ). Near-infrared spectra show evidence for water ice at the 1%–10% level depending on grain size. Spatial profiles and archival data are consistent with sustained activity. The evidence supports the hypothesis that LD2 is a typical small Centaur that will become a typical JFC, and thus, it is critical to understanding the transition between these two populations. Finally, we discuss potential strategies for a community-wide, long-baseline monitoring effort.
(60558) 174P/Echeclus is an unusual object that belongs to a class of minor planets called Centaurs, which may be intermediate between Kuiper Belt Objects and Jupiter Family comets. It is sporadically active throughout its orbit at distances too far for water ice to sublimate, the source of activity for most comets. Thus, its coma must be triggered by another mechanism. In 2005, Echeclus had a strong outburst with peculiar behavior that raised questions about the nucleus homogeneity. In order to test nucleus models, we performed the most sensitive search to date for the highly volatile CO molecule via its J=2-1 emission toward Echeclus during 2016 May-June (at 6.1 astronomical units from the Sun) using the Arizona Radio Observatory 10-m Submillimeter Telescope. We obtained a 3.6-sigma detection with a slightly blue-shifted (delta v = -0.55 +- 0.1 km/s) and narrow (FWHM = 0.53 +- 0.23 km/s) line. The data are consistent with emission from a cold gas from the sunward side of the nucleus, as seen in two other comets at 6 AU. We derive a production rate of Q(CO) = (7.7 +- 3.3)x10^26 mol/s, which is capable of driving the estimated dust production rates. Echeclus CO outgassing rate is ~40 times lower than what is typically seen for another Centaur at this distance, 29P/Schwassmann-Wachmann 1. We also used the IRAM 30-m telescope to search for the CO J=2-1 line, and derive an upper limit that is above the SMT detection. Compared to the relatively unprocessed comet C/1995 O1 (Hale-Bopp), Echeclus produces significantly less CO, as do Chiron and four other Centaurs.
The wide range of planetary system architectures and exoplanet properties is certainly linked to a range of properties of their birth-places, the disk-like structures around young stars composed of gas and dust particles. These disks share many of the properties of the solar nebula from which the Sun and our planetary system formed, although their masses, radial dimensions, and internal structures can be very different. Dust spectroscopy has revealed the mineralogical composition of the protoplanetary dust particles that are mostly found in the form of amorphous silicates, crystalline forsterite, water ice, and other molecular ices. There is strong observational evidence that the dust particles in disks can grow in size far beyond the typical sub-micrometer sizes of interstellar dust grains.
This is the second paper in a series where we study the influence of
transport processes on the chemical evolution of protoplanetary disks. Our
analysis is based on a flared alpha-model of the DM Tau system, coupled to a
large gas-grain chemical network. To account for production of complex
molecules, the chemical network is supplied with an extended set of surface
reactions and photo-processes in ice mantles. Our disk model covers a wide
range of radii, 10-800 AU (from a Jovian planet-forming zone to the outer disk
edge). Turbulent transport of gases and ices is implicitly modeled in full 2D
along with the time-dependent chemistry. Two regimes are considered, with high
and low efficiency of turbulent mixing. The results of the chemical model with
suppressed turbulent diffusion are close to those from the laminar model, but
not completely. A simple analysis for the laminar chemical model to highlight
potential sensitivity of a molecule to transport processes is performed. It is
shown that the higher the ratio of the characteristic chemical timescale to the
turbulent transport timescale for a given molecule, the higher the probability
that its column density will be affected by diffusion. We find that turbulent
transport enhances abundances and column densities of many gas-phase species
and ices, particularly, complex ones. For such species a chemical steady-state
is not reached due to long timescales associated with evaporation and surface
photoprocessing and recombination. In contrast, simple radicals and molecular
ions, which chemical evolution is fast and proceeds solely in the gas phase,
are not much affected by dynamics. All molecules are divided into three groups
according to the sensitivity of their column densities to the turbulent
diffusion. [Abridged]
We have used the Spitzer Space Telescope Infrared Spectrograph (IRS) to
observe the 5-37 micron thermal emission of comet 73P/Schwassmann-Wachmann 3
(SW3), components B and C. We obtained low spectral resolution (R ~ 100) data
over the entire wavelength interval, along with images at 16 and 22 micron.
These observations provided an unprecedented opportunity to study nearly
pristine material from the surface and what was until recently the interior of
an ecliptic comet - cometary surface having experienced only two prior
perihelion passages, and including material that was totally fresh. The spectra
were modeled using a variety of mineral types including both amorphous and
crystalline components. We find that the degree of silicate crystallinity, ~
35%, is somewhat lower than most other comets with strong emission features,
while its abundance of amorphous carbon is higher. Both suggest that SW3 is
among the most chemically primitive solar system objects yet studied in detail,
and that it formed earlier or farther from the sun than the bulk of the comets
studied so far. The similar dust compositions of the two fragments suggests
that these are not mineralogically heterogeneous, but rather uniform throughout
their volumes. Atomic abundances derived from the spectral models indicates a
depletion of O compared to solar photospheric values, despite the inclusion of
water ice and gas in the models. Atomic C may be solar or slightly sub-solar,
but its abundance is complicated by the potential contribution of spectrally
featureless mineral species to the portion of the spectra most sensitive to the
derication of the C abundance. We find a relatively high bolometric albedo, ~
0.13 for the dust, considering the large amount of dark carbonaceous material,
but consistent with the presence of abundant small particles and strong
emission features.
Absolute rate constants and their temperature dependencies for the deactivation of O(1D) by five important atmospheric gases are reported. O(1D) atoms are produced by photolysis of ozone at 266 nm and the atoms are directly observed in time resolved decay of the O(1D) ↠O(3P) radiation at 630 nm. Gases which only quench O(1D) (O2, N2, and CO2) are observed to have a small negative temperature dependence while O3 and H2O, which also have a reactive channel, display no temperature dependence. Arrhenius expressions for the reactions measured are (A in units of 10−10 cm3/molecule⋅ s, E in cal/mole) O2(T=104–354 K) 0.29 exp(134/RT), N2(T=104–354 K) 0.20 exp(214/RT), CO2(T=139–200 K) 1.2 and (T=200–354 K) 0.68 exp(233/RT). The rate constants for O3 and H2O are 2.4×10−10 and 2.3×10−10 cm3/molecule⋅ s over ranges of 103–393 K and 253–353 K, respectively. The results are compared with other energy dependent measurements and with the theories reported in the literature.
We present quantitative measurements of cometary [C I] 9850 Å emission obtained during observations of comet Hale-Bopp (C/1995 O1) in 1997 March and April. The observations were carried out using a high-resolution (lambda/Deltalambda~40,000) Fabry-Pérot/CCD spectrometer at the McMath-Pierce Solar telescope on Kitt Peak. This forbidden line, the carbon analog of [O I] 6300 Å, is emitted in the radiative decay of C(1D) atoms. In the absence of other sources and sinks, [C I] 9850 Å emission can be used as a direct tracer of CO photodissociation in comets. However, in Hale-Bopp's large, dense coma, other processes, such as collisional excitation of ground-state C(3P), dissociative recombination of CO+, and collisional dissociation of CO and CO2 may produce significant amounts of C(1D). The long C(1D) radiative lifetime (~4000 s) makes collisional deexcitation (quenching) the primary loss mechanism in the inner coma. Thus, a detailed, self-consistent global model of collisional and photochemical interactions is necessary to fully account for [C I] 9850 Å emission in comet Hale-Bopp.
Carbon monoxide was measured in C/1996 B2 (Hyakutake) on four preperihelion dates, UT 1996 March 24.5 and April 10.2–12.2, using CSHELL at the NASA Infrared Telescope Facility on Mauna Kea, Hawaii. These observations provided the first secure ground-based detection of cometary CO at infrared wavelengths. The rotational temperatures retrieved for CO are consistent with a heliocentric dependence Trot = 63 Rh−1.06 K over the range Rh = 1.06–0.64 AU. From long-slit analysis we distinguish native and distributed sources of CO, and we infer the relative abundance of native to total CO to be 0.773 ± 0.054 on April 12.2. If this ratio is constant on UT April 11.2 and 12.2, the mean mixing ratios (relative to water) are 0.149 ± 0.019 for native CO and 0.191 ± 0.022 for the sum of native and extended sources. These mixing ratios are based on direct measurements of H2O made on the same dates with the same instrument and analyzed with the same data processing algorithms. The observed spatial scale for complete development of the extended source on UT 1996 April 12.2 was consistent with that observed for C/1995 O1 (Hale-Bopp) on UT 1997 January 21, after taking into account differences in outflow velocity and heliocentric distance assuming insolation-limited release. This may suggest similar progenitor material in these two comets.
The composition of cometary ices provides key information on the chemical and physical proper- ties of the outer solar nebula where comets formed, 4.6 Gyr ago. This chapter summarizes our current knowledge of the volatile composition of cometary nuclei, based on spectroscopic observations and in situ measurements of parent molecules and noble gases in cometary comae. The processes that govern the excitation and emission of parent molecules in the radio, infrared (IR), and ultraviolet (UV) wavelength regions are reviewed. The techniques used to convert line or band fluxes into molecular production rates are described. More than two dozen parent molecules have been identified, and we describe how each is investigated. The spatial distribution of some of these molecules has been studied by in situ measure- ments, long-slit IR and UV spectroscopy, and millimeter wave mapping, including interferometry. The spatial distributions of CO, H CO, and OCS differ from that expected during direct sublimation from the nucleus, which suggests that these species are produced, at least partly, from extended sources in the coma. Abundance determinations for parent molecules are reviewed, and the evidence for chemical diversity among comets is discussed.
Unattenuated solar photo rate coefficients and excess energies for dissociation, ionization, and dissociative ionization are presented for atomic and molecular species that have been identified or are suspected to exist in the atmospheres of planets, satellites (moons), comets, or as pollutants in the Earth atmosphere. The branching ratios and cross sections with resonances have been tabulated to the greatest detail possible and the rate coefficients and excess energies have been calculated from them on a grid of small wavelength bins for the quiet and the active Sun at 1 AU heliocentric distance.
The v1 and v5 infrared bands of formaldehyde in Halley's comet have been
synthesized, using a line-by-line model for solar infrared fluorescence
from low-temperature H2CO. Fully resolved spectra were convolved to
spectral resolutions comparable with those used on the IKS spectrometer
on the Vega 1 spacecraft. Excellent agreement is obtained between the
observed and modeled spectra, permitting a definite identification of
H2CO in Halley's comet. The retrieved production rate is 4.5 + or - 0.5
x 10 to the 28th molecules/s, and the rotational temperature is probably
less than 150 K, with a best-fit value of about 50 K. Production rates
(or upper limits) are also retrieved from ground-based spectra of Comets
Halley and Wilson (1986), and it is shown that the production rate of
H2CO exhibits significant temporal variability, relative to H2O, in
Comet Halley.
The study presents an extension of the cometary atmosphere Monte Carlo
particle trajectory model formalism which makes it both physically
correct for heavy species and yet computationally reasonable. The
derivation accounts for the collision path and scattering redirection of
a heavy radical traveling through a fluid coma with a given radial
distribution in outflow speed and temperature. The revised model
verifies that the earlier fast-H atom approximations used in earlier
work are valid, and it is applied to a case where the heavy radical
formalism is necessary: the OH distribution. It is found that a steeper
variation of water production rate with heliocentric distance is
required for a water coma which is consistent with the velocity-resolved
observations of Comet P/Halley.
It has been suggested that the slightly endothermic reaction between H
and DCN to produce HCN and D serves to deplete DCN in hot core regions
of giant molecular clouds. We have calculated the rate of this reaction
for a range of temperatures up to 500 K with a combination of ab initio
quantum chemical and dynamical techniques. We find that the reaction has
two potential barriers lying between the reactants and products. These
barriers permit only a very slow reaction except at high temperatures.
We calculate that the rate coefficient k of the reaction follows the
formula k (cm(3) s(-1) ) = 2 10(-11) Gamma_t exp (-4931/T), where
Gamma_t is a tunneling correction. The estimated rate coefficient at 300
K is 5 10(-18) cm(3) s(-1) ; this rate coefficient is far too small to
be important in the destruction of DCN.
A model of gas-phase chemical kinetics in comet comae has been
developed. Its purpose is to allow characterization of comet nuclei by
predicting abundances of observable coma species. The one-dimensional
calculation follows an isothermal shell of outstreaming molecules,
positive ions, and atoms that starts at the nucleus with a simple set of
mother molecules. Rate equations for 441 photo and chemical reactions
are solved simultaneously at many points in the expansion of the shell.
The calculation includes effects of the ultraviolet opacity of the
mother molecules. Results are presented for four combinations of
hypothetical mother molecules: H2O, CO2, NH3, and CH4. Several species
that are not simple photodestruction products of the mother molecules
reach large abundances. CO(+), C2, C3, HCN, N2(+), and (probably) CN are
underproduced by the model. Additional sources for these species are
discussed.
Among unsolved questions raised by observations of comets is the origin of extended sources, i.e., the distribution of molecules in the coma which cannot be explained by a direct sublimation from the nucleus. Polyoxymethylene [formaldehyde polymer: (-CH2-O-)n, also called POM] is sometimes invoked as a potential parent compound, the degradation of which could produce the required amount of H2CO across the coma, but no quantitative study has ever been undertaken with relevant parameters. From new experimental data, we are now able to consider multiphase chemistry: POM in the solid state on cometary grains slowly degrades by solar photons and heat and produces H2CO in the gaseous phase. This is a new approach to cometary organic chemistry. We show, by considering simple assumptions about the cometary environment, that the hypothesis of POM on grains leads to a very good agreement with Giotto observations if we assume that the cometary grains are ~7% POM by mass at a temperature of 330 K.
Observations of formaldehyde (H2CO) have been conducted toward comets C/1995 O1 (Hale-Bopp), C/2001 Q4 (NEAT), and C/2002 T7 (LINEAR) using the Arizona Radio Observatory (ARO) 12 m telescope at 1.2 and 2 mm. Aperture synthesis maps of H2CO at 3 mm were made using the Berkeley-Illinois-Maryland Association (BIMA) interferometer toward comet Hale-Bopp. These data indicate that the production rate of H2CO is ~3.7 × 1028 s-1 at ~1 AU in comet Hale-Bopp, using a simple Monte Carlo model, if a nuclear origin for the molecule is assumed. However, maps of H2CO in Hale-Bopp, in comparison with CO, show an extended distribution (rs ~ 15,000 km) with small-scale structure oriented roughly along the comet-Sun direction. This result suggests a source of H2CO other than the comet nucleus. The extended source of formaldehyde is probably grains composed of a mixture of silicates and organic material. The production rate for H2CO increases to Q ~ 1.4 × 1029 s-1 assuming such an extended grain source. This value implies a Q/Q(H2O) ~ 1.4%, which is similar to the production rate ratio of Q/Q(H2O) ~ 4% derived from in situ measurements of H2CO in comet Halley. Production rates for H2CO toward comets C/2002 T7 (LINEAR) and C/2001 Q4 (NEAT) are 1.4 × 1027 and 5.6 × 1026 s-1, respectively, modeled using the extended grain source. The spectra of H2CO measured toward comet C/2002 T7 (LINEAR) show evidence for a second velocity component, most likely arising from comet fragmentation.
Unidentified molecular emission bands first observed in optical spectra of the plasma tail of comet Halley (1P/1982 U1) have been discovered in pure ion tail spectra (Δλ~4700-6300 Å, λ/Δλ~1000-4400) of comet Hyakutake (C/1996 B2) and three other recent comets (Nakamura-Nishimura-Machholz P/1994 N1, Austin C/1989 X1, and Brorsen-Metcalf 32P/1989 N1), but not in comet Hale-Bopp (C/1995 O1). The strengths of the bands near 4940, 5310, and 6000 Å are correlated among the comets observed, indicative that the unidentified bands arise from a single molecular species. The bands have an asymmetric spatial distribution that extends in the antisolar direction, resembling molecular ions observed in comet plasma tails. Thus, the source of the unidentified bands is probably a single species of molecular ions. Of known laboratory molecular ion spectra, none can be attributed to the unidentified cometary emission bands. The absence of the unidentified ions in the tail of Hale-Bopp may be explained by a parent species of the unidentified ions that mimics the coma dynamics and photochemistry of H2O more closely than CO. This is inferred from the large tail CO+/H2O+ ratio observed in Hale-Bopp, which arises from the comet's large production rate that extends the ionopause and coma collision zone. Our observations provide evidence that the source region for the ions in plasma tails of comets lies external to the magnetic field-free ionopause.
Formaldehyde and methanol were produced efficiently by the hydrogenation of CO in H2O-CO ice at 10 K in an atomic hydrogen beam experiment. The relative yields to the initial CO were of the order of 10%, 3 orders of magnitude larger than those reported previously. This reveals for the first time experimentally that successive hydrogenation of CO is most likely to produce formaldehyde and methanol on the surface of icy grains in molecular clouds as suggested by theoretical models.
We have constructed a concentric-shell, one-dimensional kinetic model that examines the chemistry of hydrogen and oxygen species in detail. We have studied the effects of the reactions of the reactive OH, O(3 P), and O(1 D) species with themselves and with the abundant stable molecules in the inner coma of moderately and highly active comets. We find that the reactions (1) Oð 1 DÞ þ H 2 O ! 2OH and (2) Oð 3 PÞ þ OH ! O 2 þ H play important roles in the inner comae of active comets. Inclusion of reaction (2) predicts the formation of significant amounts of molecular oxygen. As the densities of O 2 may be as high as 1% those of water in some cases, the possibility of detection exists. We suggest the possibility that the ion O þ 2 may contribute to some previously unassigned features in the optical ion-tail spectra of comets. We also consider the role that reactions of the reactive species might play in the destruction of CO, NH 3 , and organic molecules in the inner coma of the active comet. We find that destruction of formaldehyde, for example, by reaction with OH has a small but essentially negligible effect on the predicted production rate of formaldehyde. Finally, we examine the significance of the reaction of OH with CO in the dense inner coma.
The observations of CI 1931 Å emission in the ultraviolet spectra of comets show that a large fraction of carbon atoms in the cometary coma are produced in the metastable 1D excited state. A coupled chemistry-transport model is used to study the chemistry of C(1D) atom and the mechanisms of production of CI 1931 Å in the near nucleus environment. Dissociation of CO by solar UV photon and photoelectron impact are the dominant mechanisms of the C(1D) production in the cometary coma. The CI 1931 Å emission is found to be governed dominantly by the density distribution of CO and C(1D). The model predict height-integrated CI 1931 Å emission intensity of 20 R on comet 46P/Wirtanen, the target of the ROSETTA mission, and intensity averaged over HST FOC field-of-view of 6.7 R. However, the observed intensity could be higher since on comet Halley the model predicted intensity (Bhardwaj, 1999) was about a factor of 5 smaller than that observed by the IUE.
Molecular radio lines were monitored in comet C/1996 B2 (Hyakutake) from 1996 February 10 to June 23, using the James Clerk Maxwell Telescope and the 30 m telescope and the Plateau de Bure interferometer of the Institut de Radioastronomie Millimétrique. We report on observations of HCN, CH_3OH, CO, H_2CO, CS, and H_2S and on the evolution of their production rates with heliocentric distance (r_h), from 1.86 down to 0.24 AU at perihelion. Most production rates increased roughly as r^-2.2_h down to 0.6 AU preperihelion. Closer to the Sun, they stalled before decreasing beyond 0.6 AU postperihelion, when observations resumed. The CS/HCN ratio varied as r^-0.8_h from 1.2 to 0.24 AU. A rapid increase of the mean gas temperature in the coma is measured, and the gas expansion velocity increased from 0.55 to 1.6 km s^-1, as the comet approached the Sun from 1.6 to 0.3 AU. Molecular abundances of the minor species around 1 AU are similar to those observed in other comets, while the CO abundance relative to water is high (~22%). Coarse mapping was used to check the comet's position and to investigate the density distribution of the molecules within the coma. It provides constraints on the size of the extended source of formaldehyde, found to be between 1.2 and 2 times the scale length of H_2CO itself. The density distribution of CS is compatible with its production from the photodissociation of a short-lived molecule such as CS_2. The density distribution observed for CO can be mostly explained by a nuclear source.
The extraordinary activity of comet C/1995 O1 (Hale-Bopp) made it possible to observe the emission bands of the radicals C2 and C3 in the
optical wavelengths range at heliocentric distances larger than 3 AU. Based on these observations, we perform an analysis of the formation of
C2 and C3 in a comet coma at large heliocentric distances.We present the most complete chemical reaction network used until today, computing
the formation of C2 and C3 from C2H2, C2H6, and C3H4 as their parent molecules. The required photodissociation rates of C3H2 and C3 had
to be derived based on the observations. The spatial distributions of C2 and C3 calculated with the chemical model show good agreement with
the observations over the whole range of heliocentric distances covered in this work. Based on the production rates for C2H2, C2H6, and C3H4,
abundance ratios are obtained for heliocentric distances rh ≥ 3 AU. In comet Hale-Bopp, C2H2 and C2H6 were measured directly by infrared
observations only at heliocentric distance rh ≤ 3 AU (Dello Russo et al. 2001). The model presented here greatly extends the heliocentric
distance range over which hydrocarbons can be studied in the coma of comet Hale-Bopp. We discuss possible indications of these abundance
ratios for the formation region of comet Hale-Bopp.
The forbidden red oxygen lines at 6300 and 6364 Å, which results due to 1D→3P transition, provide an important diagnostic tool in the study of comets. These lines cannot be produced by resonance fluorescence excitation of the ground-state oxygen atom, and therefore are mainly produced due to dissociation of H2O and other O-containing species in comets (e.g., OH, CO, CO2, H2CO etc.) by photon and electron impact and in other collision reactions. Since the lifetime of 1D state is quite long (∼110 sec) collisional de-excitation processes are important. We have used a coupled chemistry-transport model in conjunction with an efficient emission production code to study the chemistry of O(1D) atoms and the production of OI 6300 Å emission in comets. The model calculations are made for comet 46P/Wirtanen: the target of the ROSETTA mission. It is found that in the inner coma the density profile of O(1D) is controlled dominantly by the H2O. The model predicts ∼300 R of OI 6300 A brightness on comet Wirtanen.
We show by numerical simulations that the CO distribution in the coma of Comet P/Halley as measured by neutral gas mass spectrometer aboard Giotto spacecraft (P. Eberhardt et al., 1987, Astron. Astrophys. 187, 481-484) can be explained by an extended jet source originating from vent 1 in the rotational model of M. J. S. Belton et al. (1991, Icarus 93, 183-193). This is an alternative interpretation for the origin of the CO distribution where Eberhardt et al. (1987) explained the observations in terms of a spherically symmetric extended source function. We still find nearly 1/3 of CO is due to CO sublimating directly at the nucleus, possibly from the vent itself. Approximately 50% of CO that originates in the extended jet source may be formed via photolysis of H2CO while the rest can be due to CO trapped in grain mantels and other C=O-bearing molecules. The overall production rates of CO in the two models differ by less than 29%. The prediction of the jet source model compares satisfactorily with the distribution of CO observed in the coma by the IUE satellite but unfortunately does not allow one to discriminate between the two models. (C) 1994 Academic Press, Inc.
IAUC 6374 available at Central Bureau for Astronomical Telegrams.
A statistical model of chemical reaction is applied to collisions of O(1D) atoms with the molecules H2, N2, CO, CO2, N2O, O3, and H2O. Rate constants for reaction and deactivation are computed over the temperature range 100−2100°K. Competition among various product channels is investigated. In most cases quantitative agreement with experiment is achieved.
The Neutral Mass Spectrometer on the Giotto spacecraft established that H2O is the dominant species in Comet Halley's volatiles and determined the abundance of more than 10 parent species. The instrument
discovered strong extended H2CO and CO sources in the coma of Comet Halley. Polymerized H2CO associated with the cometary dust and evaporating slowly as the monomer is most likely the extended H2CO source. Photodissociation of the H2CO into CO fully accounts for the extended CO source.
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.
New evaluations of the photodestruction rates for several molecules of cometary interest are presented along with a critical comparison with other estimations from 1976 to 1993, and a summary of the need for future laboratory measurements. Photodestruction rates for a heliocentric distance of 1 AU (assuming the quiet Sun reference spectrum of Huebner and Carpenter) are tabulated for molecules from the water group, hydrocarbons, CO group, CHO species, nitrogen compounds, and sulfur compounds. Inspection of the table shows reasonable agreement between new and previously calculated photodestruction rates. Further work is needed on unstable species, photodissociation channel and quantum yields, temperature effects, kinematics and anistropic ejection of the fragments, and the effects of solar radiation field variations.
Understanding the formation of carbon monoxide in the coma of comets remains one of the most important problems in cometary chemistry. In order to examine the potential impact of photochemistry and chemical reactions on the formation of CO in the coma, we have developed a 1-D gas model to examine the formation of CO in the near-nucleus coma by these processes. In particular, we demonstrate that a chemical reaction between CO2 and hot H dissociated from H2O yields significant amounts of CO. Relative contributions to the abundance of CO from various photochemical processes and chemical reactions will also be discussed.
The electronically excited oxygen atom. O(21D2), has been studied in absorption by time-resolved attenuation of the atomic resonance radiation at λ = 115.2 nm (31D2o←21D2). Absolute quenching coefficients for the removal of O(21D2) by the gases N2, O2, CO, CO2, H2O and O3 are reported. These collisional processes are discussed within the context of the spin and orbital symmetry of reactants and products. A detailed comparison with previous laboratory and atmospheric measurements of these processes is presented.
The role of metastable oxygen O 1D quenching in the production of gaseous compounds in cometary atmospheres is examined, focusing O 1D quenching as a source of OH molecules. The initial conditions of the one-fluid hydrodynamical model of Komitiv and Angelova (1987) are adjusted for Comet Halley parameters obtained by Vega. The radial profiles in the subsolar direction of the H2O, OH, and O 1D densities, and the calculated column densities are illustrated.
The saturated hydrocarbons ethane (C2H6) and methane (CH4) along with carbon monoxide (CO) and water (H2O) were detected in comet C/1996 B2 Hyakutake with the use of high-resolution infrared spectroscopy at the NASA Infrared Telescope
Facility on Mauna Kea, Hawaii. The inferred production rates of molecular gases from the icy, cometary nucleus (in molecules
per second) are 6.4 × 1026 for C2H6, 1.2 × 1027 for CH4, 9.8 × 1027 for CO, and 1.7 × 1029 for H2O. An abundance of C2H6 comparable to that of CH4 implies that ices in C/1996 B2 Hyakutake did not originate in a thermochemically equilibrated region of the solar nebula.
The abundances are consistent with a kinetically controlled production process, but production of C2H6 by gas-phase ion molecule reactions in the natal cloud core is energetically forbidden. The high C2H6/CH4 ratio is consistent with production of C2H6 in icy grain mantles in the natal cloud, either by photolysis of CH4-rich ice or by hydrogen-addition reactions to acetylene condensed from the gas phase.
An analysis of our spectra of 39 comets from 0.55 to 1.0 μm collected
over the last decade is presented. All spectra were obtained with the
154 cm Mount Bigelow telescope of the University of Arizona
Observatories, using our fast f/1.2 spectrograph and an 800 x 800 TI CCD
detector. Roughly 21 of the objects observed displayed emissions while
the other 18 simply yielded a continuum, although three of these gave a
suggestion of possible CN emission. Emission fluxes were measured for
the Δυ = -1 C2 Swan band, the 0,10,0 and 0,8,0
NH2 bands, the O I 1D line at 6300 Å and the
red CN system 2-0 and 1-0 bands. From these fluxes, production rates for
the parents of C2, NH2, O I, and CN were
determined and mixing ratios of the parents of C2,
NH2, and CN with respect to H2O were calculated.
We estimate our standard deviation error in these numbers to be about
10%. We find that about 10% of the comets have deviant composition with
the most common being the P/Giacobini-Zinner class and the most unusual
represented by Yanaka (1988r). The other 90% of our comet sample shows
reasonably uniform mixing ratios with maximum comet-to-comet variations
of a factor of 2-3 and a standard deviation of ˜30%. No distinct
compositional classes could be discerned within that group. We interpret
the observed spread in production rate ratios as chemical composition
variations between individual comets and a measure of the nonuniformity
of the solar nebula during the time of the solar system formation.
Within the frame of the model of agglomerated grains for cometary dust
the possible role of organic polymers as gluing material between the
individual building blocks of submicron size is outlined. In this
context the characteristics of the dust fragmentation, observed in situ
at Comet P/Halley and the importance of proposed dust fragmentation
mechanisms are discussed. By combining the appearance of dust
fragmentation with gas phase observationss in the coma of Halley's comet
(CO release, CN jets, possible in situ identification of organic
material like H2CO and polymerized H2CO) it is concluded that both
phenomena may arise from the sublimation of a gluing material such as
polyoxymethylene, in the grain agglomerates of the dust.
IT has been realized for some time that hydrogen atoms produced by photolysis of the hydrogen halides or of certain other hydrides possess kinetic energy in excess of that corresponding to thermal equilibrium1–5. This arises because the energy of the quantum absorbed is larger than the minimum required to disrupt the molecule into stationary atoms. For example, the dissociation energy of HI is 296 kJ moles−1 (ref. 6) and its first absorption maximum occurs at 220 nm7, corresponding to an energy of 539 kJ moles−1. As a result of conservation of momentum, most of the excess energy appears as translational energy of the hydrogen atom. Such hydrogen atoms are often described as “hot” and their energy is manifested in enhanced reactivity. Their reactions with HI1–3, H2S4, halogens1–4, D2 and several deuterated hydrocarbons (or the corresponding reactions of D or T with H2 or hydrocarbons)8–13 have been demonstrated, and in some cases the variation of reaction probability with wavelength of photolysis has been measured10–12.
The extremely dark nucleus of Comet Halley suggests that its volatile ices contain a few percent of carbonaceous material in the form of graphitic or amorphous carbon. The very high abundance of light elements in the coma dust and the emission feature near 3.4 microns further suggest the presence of a significant organic component, although the identified carbon-containing materials' parent species cannot account for all of such a component. It is presently proposed that an additional contribution from carbon suboxide can account for these observational data, assuming a production rate about 0.03-0.04 times that of water.
Rate constants for the reaction, O+H2O→OH+OH, have been measured by the Flash Photolysis-Shock Tube (FP-ST) technique over the temperature range, 1500–2400 K. This technique combines shock heating with flash photolysis in the reflected shock wave regime, and the transient species, O-atoms in this case, are monitored by atomic resonance absorption spectroscopy (aras). Additional experiments were performed with N2O as a thermal source of O-atoms, and the formation and depletion of [O] were followed by the aras technique. These results require that the decomposition rate behavior of N2O be known. The results obtained by this technique are compared to those obtained by the FP-ST technique and are found to be corroborative. Hence, the combined results are used to describe the rate constants for the title reaction. These results can be represented by the Arrhenius expression: k=(1.12±0.20)×10−10 exp(−9115±304 K/T) cm3 molecule−1 s−1 over the experimental temperature range. The individual data points deviate from this equation by ±17% at the one standard deviation level, and this represents a measure of the precision.The experimental results are compared to earlier work, and rate constants for the title reaction are additionally calculated from published results for the reverse reaction, OH+OH, and the well known equilibrium constant. All results are combined, and the rate behavior for the title reaction is evaluated. The three parameter expression is, k=8.44×10−14 T0.946 exp(−8571 K/T) cm3 molecule−1 s−1 for the temperature range, 250–2400 K. Lastly, the results for both forward and reverse reactions are compared to the theoretical calculations presented recently by Harding and Wagner. It is concluded that theory and experiment are in agreement within experimental error.
We report the results of the low-dispersion spectroscopic observations of comet Ikeya-Zhang (C/2002 C1) performed from 2002 March 10 to 20. The unidentified molecular bands that have been recognized in the plasma tail of several comets are detected in an antisunward coma of the comet Ikeya-Zhang. Our observations show the flux of unidentified bands at 5310 Å is correlated to the flux of H2O+ as reported for three comets by S. Wyckoff et al. The observed column density ratio between H2O+ and CO+, and the flux ratio between the unidentified bands and CO+ varied day by day, by a factor of ≈2 in our observations. However, it appears that the ratios are proportional to each other. We conclude that a parent of unidentified bands is produced from or generates H2O+ directly or indirectly. We propose the hypothesis that H2O+ is the parent of the unidentified bands since similar structures of emission bands are recognized in some laboratory studies on charge transfer collisions between neutral water and Ar+ or N.
157 nm photodissociation of jet-cooled CH3OH and C2H5OH was studied using the high-n Rydberg atom time-of-flight (TOF) technique. TOF spectra of nascent H atom products were measured. Simulation of these spectra reveals three different atomic H loss processes: one from hydroxyl H elimination, one from methyl (ethyl) H elimination, and one from secondary dissociation of the methoxy (ethoxy) radical. The relative branching ratio indicates secondary dissociation of ethoxy is less important than that of methoxy. The average angular anisotropy parameter of methanol is negative (with β≈ −0.3), indicating the transition dipole moment is perpendicular to the C–O–H plane. The slightly more negative β value of ethanol (with β≈ −0.4) implies that ethanol has a longer rotational period. These experimental results indicate that both systems undergo fast internal conversion to the 3s surface after it is excited to the 3px surface, and then dissociate on the 3s surface. The translational energy distribution of the CH3O+H products reveals extensive CH3 rocking or CH3 umbrella excitation in the CH3O radical. However the vibrational structures are not resolved in the C2H5O radical.
The conversion of formaldehyde (H2CO) to methanol (CH3OH) by successive hydrogenation on H2O ice was measured at 10, 15, and 20 K using atomic hydrogen beams of 30 and 300 K. The conversion rates and CH3OH yields under the 30 K beam are very similar to those under the 300 K beam at all ice temperatures, demonstrating that the reaction is independent of beam temperature. The dependence of the conversion rates on ice temperature is consistent with that for previous experiments on CO hydrogenation. The conversion rate for H2CO → CH3OH at 15 K was found to be about half that for CO → H2CO. The dependence of the reactions on the initial thickness of H2CO was also measured. More than 80% of H2CO was converted to CH3OH for H2CO layers of less than 1 monolayer in average thickness. Irradiation of CH3OH with H atoms did not produce H2CO, demonstrating that the reverse process, CH3OH → H2CO (H abstraction), is minor compared to the forward process.
We report the detection of CO Cameron band emission in the ultraviolet spectra of four moderately active comets (including 1P/Halley) obtained by the International Ultraviolet Explorer (IUE) satellite. The motivation was the observation of Cameron band emission, a tracer of the CO2 production rate, in the spectra of two comets observed by the Hubble Space Telescope (HST). We examined IUE spectra of those comets for which CO Fourth Positive emission was detected so that the CO production rate could be derived simultaneously. Also, we used the (1, 0) band at 1993 Å, at the extreme end of the spectral range of the short-wavelength prime (SWP) spectrograph rather than the (0, 0) or (0, 1) bands at 2063 and 2155 Å, respectively, which fall at the low-sensitivity end of the long-wavelength camera. For 1P/Halley, the CO2 production rate that we derive, based on a (1, 0) Cameron band emission observation made by IUE at the time of the Giotto encounter, is in good agreement with the in situ result. With the exception of comet Levy, the production rate of CO2 relative to water in all of these comets is found to have a value close to that found for 1P/Halley. However, the two comets observed by HST appear to be deficient in CO (but not in CO2) relative to those observed by IUE. Since these two comets had lower water production rates than any of those observed by IUE, this suggests, albeit from a very small sample, that the overall level of activity of a comet may be related to its relative CO abundance.
We have investigated the role that energetic hydrogen atoms, produced in cometary comae by the photodissociation of water molecules, could have in driving chemical reactions that are endothermic, or possess activation energy barriers. We have developed a model of the density and energy spectrum of these atoms in the coma and have incorporated a number of reactions driven by fast H atoms into our existing coma model. We find that, in high-activity comets close to the Sun, such reactions are competitive with direct photodissociation as the principal destruction mechanism for molecules with long lifetimes in the solar radiation field. We show that measurements of the CH2OH : CH3O ratio may be used to assess the importance of suprathermal reactions in the coma. We also confirm that these reactions are probably unable to account for the observed HNC : HCN ratios.
We have modelled the chemistry occurring in the coma of Comet Lee and have critically evaluated the possible routes leading
to HNC. We show that the observed levels of HNC cannot be produced by ion—molecule chemistry, or by reactions of energetic
H atoms with HCN. Rather, it appears that HNC is injected into the coma following the photodestruction of an unknown precursor.
We discuss the possible nature of the parent of HNC and conclude that photofragmentation of large HCN polymers, such as polyaminocyanomethylene
(PACM), is responsible. The degradation of hydrogen cyanide polymers may constitute a common source of HNC in comets, accounting
for HNC/HCN ratios in the range measured in Lee and Hyakutake (≈0.06–0.12). The high HNC/HCN ratio measured in Hale—Bopp (≈0.2) and its heliocentric variation may, however, require an additional source.
CO was observed on March 11, 1997 in comet Hale–Bopp with theIRAM Plateau de Bure interferometer. The maps show evidence for
asymmetrical patterns, due to the Existence of CO jets. Analysis of the spectra and their velocity shifts shows that there
is a spiral CO jet rotating in a plane almost perpendicular to the sky plane.This is the first time that rotating jets are
observed for parent molecules.We have developed a 3-D model simulating rotating spiral jets of CO gas.We present here the
comparison between the observations and our model.
The infrared instrument IKS flown on board the VEGA space probes was designed for the detection of emission bands of parent molecules, and for a measurement of the size and temperature of the thermal emitting nuclear region. The instrument had three channels with cooled detectors: an “imaging channel” designed to modulate the signal of the nucleus and two spectroscopic channels operating at 2.5–5 and 6–12 μm, respectively, equipped with circular variable filters of resolving power ∼50. This paper presents and discusses the results from the spectral channels. On VEGA 1, usable spectra were obtained at distances D from the comet nucleus ranging from 250,000 to 40,000 km corresponding to fields of view 4000 and 700 km in diameter, respectively. The important internal background signal caused by the instrument itself, which could not be cooled, had to be eliminated. Since no sky chopping was performed, we obtain difference spectra between the current spectrum and a reference spectrum with little or no cometary signal taken at the beginning of the observing sequence (D ∼ 200,000 km). Final discrimination between cometary signal and instrumental background is achieved using their different time evolution, since the instrumental background is proportional to the slow temperature drift of the instrument, and the cometary signal due to parent molecules or dust grains is expected to vary in first order as D−1.The 2.5–5 μm IKS spectra definitely show strong narrow signals at 2.7 and 4.25 μm, attributed to the ν3 vibrational bands of H2O and CO2, respectively, and a broader signal in the region 3.2–3.5 μm, which may be attributed to CH-bearing molecules. All these signals present the expected D−1 intensity variation. Weaker emission features at 3.6 and 4.7 μm could correspond to the ν1 and ν5 bands of H2CO and the (1 - 0) band of CO, respectively. Molecular production rates are derived from the observed emissions, assuming that they are due to resonance fluorescence excited by the Sun's infrared radiation. For the strong bands of H2O and CO2, the rovibrational lines are optically thick, and radiative transfer is taken into account. We derive production rates, at the moment of the VEGA 1 flyby, of ∼1030 sec−1 for H2O, ∼2.7 × 1028 sec−1 for CO2, ∼5 × 1028 sec−1 for CO, and 4 × 1028 sec−1 for H2CO, if attributions to CO and H2CO are correct. The production rate of carbon atoms in CH-bearing molecules is ∼9 × 1029 sec−1 assuming fluorescence of molecules in the gas phase, but could be much less if the 3.2–3.5 μm emission is attributed to CH stretch in polycyclic aromatic hydrocarbons or small organic grains. In addition, marginal features are present at 4.85 and 4.45 μm, tentatively attributed to OCS and molecules with the CN group, respectively. Broad absorption at 2.8–3.0 μm, as well as a narrow emission at 3.15 μm, which follow well the D−1 intensity variation, might be due to water ice. Emission at 2.8 μm is also possibly present, and might be due to OH created in vibrationally excited states after water photodissociation. The 6–12 μm spectrum does not show any molecular emission, nor emission in the 7.5-μm region. The spectrum is dominated by silicate emission showing a double structure with maxima at 9.0 and 11.2 μm, which suggests the presence of olivine.
The apparition of Comet C/1996 B2 (Hyakutake) offered an unexpected and rare opportunity to probe the inner atmosphere of a comet with high spatial resolution and to investigate with unprecedented sensitivity its chemical composition. We present observations of over 30 submillimeter transitions of HCN, H13CN, HNC, HNCO, CO, CH3OH, and H2CO in Comet Hyakutake carried out between 1996 March 18 and April 9 at the Caltech Submillimeter Observatory. Detections of the H13CN (4–3) and HNCO (160,16–150,15) transitions represent the first observations of these species in a comet. In addition, several other transitions, including HCN (8–7), CO (4–3), and CO (6–5) are detected for the first time in a comet as is the hyperfine structure of the HCN (4–3) line. The observed intensities of the HCN (4–3) hyperfine components indicate a line center optical depth of 0.9 ± 0.2 on March 22.5 UT. The HCN/HNC abundance ratio in Comet Hyakutake at a heliocentric distance of 1 AU is similar to that measured in the Orion extended ridge— a warm, quiescent molecular cloud. The HCN/H13CN abundance ratio implied by our observations is 34 ± 12, similar to that measured in giant molecular clouds in the galactic disk but significantly lower than the Solar System12C/13C ratio. The low HCN/H13CN abundance ratio may be in part due to contamination by an SO2line blended with the H13CN (4–3) line. In addition, chemical models suggest that the HCN/H13CN ratio can be affected by fractionation during the collapse phase of the protosolar nebula; hence a low HCN/H13CN ratio observed in a comet is not inconsistent with the solar system12C/13C isotopic ratio. The abundance of HNCO relative to water derived from our observations is (7 ± 3) × 10−4. The HCN/HNCO abundance ratio is similar to that measured in the core of Sagittarius B2 molecular cloud. Although a photo-dissociative channel of HNCO leads to CO, the CO produced by HNCO is a negligible component of cometary atmospheres. Production rates of HCN, CO, H2CO, and CH3OH are presented. Inferred molecular abundances relative to water are typical of those measured in comets at 1 AU from the Sun. The exception is CO, for which we derive a large relative abundance of 30%. The evolution of the HCN production rate between March 20 and March 30 suggests that the increased activity of the comet was the cause of the fragmentation of the nucleus. The time evolution of the H2CO emission suggests production of this species from dust grains.
We present the results of narrowband photometry of 85 comets observed over a period of 17 years. The data have been reduced homogeneously to molecular production rates and a proxy for the dust production rate. We confirm previous investigations, both our own and those of others, showing that there is no differentiation with depth in the cometary nucleus, that most comets are very similar to each other in chemical composition, and that the dust-to-gas ratio does not vary with the dynamical age of the comet. There is little variation of relative abundances with heliocentric distance, implying that for the species we observe the role of density-dependent processes in the coma is small. There is also little variation from one apparition to the next for most short-period comets.
The release of carbon monoxide from Comet C/1995 O1 Hale-Bopp was studied between June 1996 and September 1997 using high resolution infrared spectroscopy near 4.7 μm. The excitation of CO molecules in the coma was assessed through measurement of the rotational temperature on several dates at an angular resolution of ∼1 arcsecond. An increase in Trot with distance from the nucleus was revealed, most likely because of photolytic heating by fast H-atoms. Observed temperature profiles varied from date to date, but overall the degree of heating was most pronounced near perihelion. The similar rotational temperatures observed for CO and HCN may indicate control of rotational populations by collisions with electrons.The spatial distribution of CO molecules in the coma revealed two distinct sources for CO, one being CO ice native to the nucleus, and another being CO released from a progenitor distributed in the coma. Only the native source was seen when the comet was beyond 2 AU from the Sun. Based on pre- and post-perihelion observations on five dates with heliocentric distance Rh between 4.10 and 2.02 AU, a heliocentric dependence QCO,native=(1.06±0.44)×1030 Rh−1.76±0.26 molecules s−1 was obtained. Within Rh∼1.5 AU, however, both native and distributed sources were consistently present on all dates of observation. The total CO produced was the sum of the two sources and, based on seven dates, obeyed QCO,total=(2.07±0.20)×1030 Rh−1.66±0.22 molecules s−1. This heliocentric dependence was consistent with that found for water (QH2OαRh−1.88±0.18 between 0.93 and 1.49 AU) and for mm-sized dust (Rh−1.7±0.2 between 0.9 and 2.5 AU). Our derived total mixing ratio for CO was QCO,total/QH2O=0.241±0.009, with native and distributed sources each contributing an abundance of approximately 12 percent that of water. This was the case even after correcting measured CO and H2O column densities, and hence production rates, for opacity in the solar pump. The distributed source exhibited behavior consistent with thermal destruction of a precursor material. The observed variations in its production rate and spatial distribution along the slit suggested contributions from both a diffuse source in the coma and possibly from one or more jets enriched in CO or CO-containing material, such as CHON grains.
We report on high spectral resolution observations of Comet C/1999 S4 (LINEAR) obtained at McDonald Observatory in June and July 2000. We report unequivocal detections of the O (1S) and O (1D) metastable lines in emission in the cometary spectrum. These lines are well separated from any telluric or cometary emission features. We have derived the ratio of the two red doublet lines and show that they are consistent with the predictions of the branching ratio. We also derived a ratio of 0.06±0.01 for the green line flux to the sum of the red line fluxes. This ratio is consistent with H2O as the dominant parent for atomic oxygen. We have measured the widths of the lines and show that the widths imply that there must be some parent of atomic oxygen in addition to the H2O.
High resolution infrared spectra of Comet C/1995 O1 (Hale–Bopp) were obtained during 2–5 March 1997 UT from the NASA Infrared Telescope Facility on Mauna Kea, Hawaii, when the comet was at r≈1.0 AU from the Sun pre-perihelion. Emission lines of CH4, C2H6, HCN, C2H2, CH3OH, H2O, CO, and OH were detected. The rotational temperature of CH4 in the inner coma was Trot=110±20 K. Spatial profiles of CH4, C2H6, and H2O were consistent with release solely from the nucleus. The centroid of the CO emission was offset from that of the dust continuum and H2O. Spatial profiles of the CO lines were much broader than those of the other molecules and asymmetric. We estimate the CO production rate using a simplified outflow model: constant, symmetric outflow from the peak position. A model of the excitation of CO that includes optical depth effects using an escape probability method is presented. Optical depth effects are not sufficient to explain the broad spatial extent. Using a parent+extended-source model, the broad extent of the CO lines can be explained by CO being produced mostly (∼90% on 5 March) from an extended source in the coma. The CO rotational temperature was near 100 K. Abundances relative to H2O (in percent) were 1.1±0.3 (CH4), 0.39±0.10 (C2H6), 0.18±0.04 (HCN), 0.17±0.04 (C2H2), 1.7±0.5 (CH3OH), and 37–41 (CO, parent+extended source). These are roughly comparable to those obtained for other long-period comets also observed in the infrared, though CO appears to vary.
The H2CO production rates measured in Comet C/1995 O1 (Hale–Bopp) from radio wavelength observations [Biver, N., and 22 colleagues, 2002a. Earth Moon Planets 90, 5–14] showed a steep increase with decreasing heliocentric distance. We studied the heliocentric evolution of the degradation of polyoxymethylene (formaldehyde polymers: (CH2O)n, also called POM) into gaseous H2CO. POM decomposition can indeed explain the H2CO density profile measured in situ by Giotto spacecraft in the coma of Comet 1P/Halley, which is not compatible with direct release from the nucleus [Cottin, H., Bénilan, Y., Gazeau, M.-C., Raulin, F., 2004. Icarus 167, 397–416]. We show that the H2CO production curve measured in Comet C/1995 O1 (Hale–Bopp) can be accurately reproduced by this mechanism with a few percents by mass of solid POM in grains. The steep heliocentric evolution is explained by the thermal degradation of POM at distances less than 3.5 AU. This study demonstrates that refractory organics present in cometary dust can significantly contribute to the composition of the gaseous coma. POM, or POM-like polymers, might be present in cometary grains. Other molecules, like CO and HNC, might also be produced by a similar process.