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Jet Morphology and Coma Analysis of 103P/Hartley 2
Abstract
We have observed comet 103P/Hartley 2 using the George and Cynthia
Mitchell Spectrograph (formerly VIRUS-P) on the 2.7 m telescope at
McDonald Observatory (Hill et al. 2008). Data for CN, C2,
C3, and NH2 were collected over six nights from
2010 July 15 to November 10. The data were processed to form images of
the coma for each of the observed species. We have performed azimuthal
average division on each of the coma images to examine jet morphology
and have investigated the nature of the production of the radical
species using our modified vectorial model (Ihalawela et al. 2011). This
work enhances the ongoing investigation of the chemistry and outgassing
behavior of Hartley 2 as studied by the EPOXI flyby mission.
... In principle, the OH filter is also effective for such studies, but it suffers from severe atmospheric extinction, and many telescopes have very low throughput at the relevant wavelengths. With recent improvements in integral field unit (IFU) spectroscopy, it is becoming possible to study coma morphology in gas "images" constructed from spectra (Vaughan et al. 2017;Opitom et al. 2019), with the added possibility of studying lines which are too spread out for conventional narrowband filters, like NH 2 . Coma morphology techniques are not limited to near-UV and visible wavelengths. ...
We summarize the collective knowledge of physical and surface properties of comet nuclei, focusing on those that are obtained from remote observations. We now have measurements or constraints on effective radius for over 200 comets, rotation periods for over 60, axial ratios and color indices for over 50, geometric albedos for over 25, and nucleus phase coefficients for over 20. The sample has approximately tripled since the publication of Comets II, with IR surveys using Spitzer and NEOWISE responsible for the bulk of the increase in effective radii measurements. Advances in coma morphology studies and long-term studies of a few prominent comets have resulted in meaningful constraints on rotation period changes in nearly a dozen comets, allowing this to be added to the range of nucleus properties studied. The first delay-Doppler radar and visible light polarimetric measurements of comet nuclei have been made since Comets II and are considered alongside the traditional methods of studying nuclei remotely. We use the results from recent in situ missions, notably Rosetta, to put the collective properties obtained by remote observations into context, emphasizing the insights gained into surface properties and the prevalence of highly elongated and/or bilobate shapes. We also explore how nucleus properties evolve, focusing on fragmentation and the likely related phenomena of outbursts and disintegration. Knowledge of these behaviors has been shaped in recent years by diverse sources: high resolution images of nucleus fragmentation and disruption events, the detection of thousands of small comets near the Sun, regular photometric monitoring of large numbers of comets throughout the solar system, and detailed imaging of the surfaces of mission targets. Finally, we explore what advances in the knowledge of the bulk nucleus properties may be enabled in coming years.
... Given the rotational variability of comets on timescales as brief as a few hours, IFU observations have the advantage of simultaneous measurements of multiple species in contrast to narrow-band observations where different filters need to be cycled to sample each species. The first observations of comets with IFUs at visible wavelengths are relatively recent (Dorman et al. 2013;Vaughan et al. 2017;Opitom et al. 2019b) but they have demonstrated the potential of this type of observation to study the spatial distributions of secondary products and their production mechanisms in the coma. ...
The composition of cometary ices provides key information on the thermal and chemical properties of the outer parts of the protoplanetary disk where they formed 4.6 Gy ago. This chapter reviews our knowledge of composition of cometary comae based on remote spectroscopy and in-situ investigations techniques. Cometary comae can be dominated by water vapour, CO or CO2. The abundances of several dozen of molecules, with a growing number of complex organics, have been measured in comets. Many species that are not directly sublimating from the nucleus ices have also been observed and traced out into the coma in order to determine their production mechanisms. Chemical diversity in the comet population and compositional heterogeneity of the coma are discussed. With the completion of the Rosetta mission, isotopic ratios, which hold additional clues on the origin of cometary material, have been measured in several species. Finally, important pending questions (e.g., the nitrogen deficiency in comets) and the need for further work in certain critical areas are discussed in order to answer questions and resolve discrepancies between techniques.
... In this study, the comae of four comets were observed with an integral field unit (IFU) spectrograph to learn more about their chemical properties. This spectrograph has been used previously to study the coma of comet 103P/Hartley 2 around the time of the EPOXI flyby encounter (Vaughan et al. 2017). The sample analyzed in this paper includes three Jupiter-family comets (10P/Tempel 2, 168P/Hergenrother, 260P/McNaught) and one long-period comet (C/2009 P1 Garradd). ...
Spectra of the comae of four comets were obtained with an integral field unit spectrograph on the Harlan J. Smith Telescope at McDonald Observatory. The passband of the spectrograph permitted the observation of C 2 , C 3 , CH, CN, and NH 2 transitions for these comets. The classical Haser model was used to derive production rates for each observed species. The production rates obtained for the comets were also used to obtain mixing ratios relative to CN. The relative abundances with respect to CN obtained for these comets vary greatly, but are largely consistent with ranges established from prior comet chemistry surveys. The notable exception is 168P/Hergenrother, which the results suggest is extremely depleted in volatiles, even with respect to many other comets designated as volatile depleted in prior surveys. The results for comet Hergenrother add to a small, but growing, body of data suggesting that there may be a subgroup of carbon-chain-depleted Jupiter-family comets that are also depleted in ammonia.
... As a radical species, it is not inherently present in the nucleus, but is released into the coma via photodissociation of a more complex organic. The leading candidate for this molecule is HCN, though other molecules such as C 2 N 2 and HC 3 N have been suggested, even though currently, there are little observational data constraining the abundances of these species in cometary comae [130]. While HCN can explain the abundance of CN in many comets (e.g., [23]), there are examples of comets where the CN abundance exceeds that of HCN, and therefore HCN cannot account for all the observed CN (e.g., [74,75]). ...
Comets contain primitive material leftover from the formation of the Solar System, making studies of their composition important for understanding the formation of volatile material in the early Solar System. This includes organic molecules, which, for the purpose of this review, we define as compounds with C–H and/or C–C bonds. In this review, we discuss the history and recent breakthroughs of the study of organic matter in comets, from simple organic molecules and photodissociation fragments to large macromolecular structures. We summarize results both from Earth-based studies as well as spacecraft missions to comets, highlighted by the Rosetta mission, which orbited comet 67P/Churyumov–Gerasimenko for two years, providing unprecedented insights into the nature of comets. We conclude with future prospects for the study of organic matter in comets.
We present the first observations of outbursting comet C/2015 ER61 (PANSTARRS) with the Multi-Unit Spectroscopic Explorer (MUSE). The comet was observed on April 9, 2017, only five days after it underwent a significant outburst during which the total brightness increased by a factor of ~7.5. Based on the MUSE observations, we produce simultaneous maps of the CN, C 2 , NH 2 comae, and the dust coma. In turn, we applied image enhancement techniques in order to reveal features in the coma. By comparing the coma morphology for the dust and CN, C 2 , and NH 2 , we investigate the release mechanism of those gas species in the coma of comet ER61. We present evidence that NH 2 could be released by icy or organic-rich dust grains.
We present the concept of BlueMUSE, a blue-optimised, medium spectral resolution, panoramic integral field spectrograph based on the MUSE concept and proposed for the Very Large Telescope. With an optimised transmission down to 350 nm, a larger FoV (1.4 x 1.4 arcmin) and a higher spectral resolution compared to MUSE, BlueMUSE will open up a new range of galactic and extragalactic science cases allowed by its specific capabilities, beyond those possible with MUSE. For example a survey of massive stars in our galaxy and the Local Group will increase the known population of massive stars by a factor 100, to answer key questions about their evolution. Deep field observations with BlueMUSE will also significantly increase samples of Lyman-alpha emitters, spanning the era of Cosmic Noon. This will revolutionise the study of the distant Universe: allowing the intergalactic medium to be detected unambiguously in emission, enabling the study of the exchange of baryons between galaxies and their surroundings. By 2030, at a time when the focus of most of the new large facilities (ELT, JWST) will be on the infra-red, BlueMUSE will be a unique facility, outperforming any ELT instrument in the Blue/UV. It will have a strong synergy with ELT, JWST as well as ALMA, SKA, Euclid and Athena.
Context. The comet 103P/Hartley 2, target of the EPOXI mission (NASA), was supposed to be observed for 3 days around its perihelion, from October 27 to 29, 2010, but photometric data were obtained only on October 27 and 29, 2010. On both dates, the comet visibility was not optimal due to its proximity to the Moon, as projected on the plane of the sky, whereas on October 28, the comet could not be observed at all. Aims: The goal of the campaign was to give ground support to the EPOXI mission by establishing a baseline of activity at perihelion to be compared with in situ activity observed by the space mission about 7 days later on Nov. 4, 2010. We aimed to assess gas and dust production rates, to study the gas and dust coma morphology, to investigate the behaviour of the refractory component by analysing the dust colour variations with date and with projected cometocentric distance, rho, and to determine the slope of the surface brightness profiles, B, as a function of rho. Methods: Long-slit spectra and optical broad- and narrowband images were acquired with the instrument ACAM mounted on the William Herschel Telescope (WHT) at La Palma Observatory. We investigated the evolution of the dust coma morphology from the images acquired with specific continuum cometary filters (in the blue and red wavelength region) with image-enhancing techniques. We studied (1) the gas and dust production rates; (2) the dust radial brightness profiles; (3) the profiles of the CN, C2, C3 and NH2 column densities, and (4) the CN and C3 coma morphologies. The dust and gas profiles were azimuthally averaged, as well as measured in both the E-W direction (~Sun-antisolar direction) and in a direction defined by the slit orientation at PA 70 to 250 degrees. Results: The morphological analysis of the dust coma reveals only one structure. Aside from the dust tail in the west direction, a bright jet is detected in images acquired on October 27 at 03:00-04:00 UT. This jet turns on and off and it is not clearly detected at any time on the images obtained during October 29. This structure is enhanced by making use of the radial renormalization and the Larson-Sekanina method. It is also confirmed by the distortion of the isophotes at the same position angle (PA). The Afrho parameter, a proxy to the dust production rate, and the gas (CN, C3, C2, and NH2) production rate, Qi, have been measured at perihelion, rh ≈ 1.058 AU. The quotient QC2/QCN ~ 1.3 places 103P/Hartley 2 as a typical comet in terms of long-chain hydrocarbon abundance. The gas-to-dust mass ratio is ~3-6, indicating that 103P/Hartley 2 is a relatively gas-rich comet. At perihelion, Afrho, as measured in a circular aperture of ~4700 km ranges from ~60 cm in the blue to ~110 cm in the red, which indicates an overall change in the optical properties of the dust grains. On the other hand, the Afrho is rather stable in the innermost coma when it is computed from the spectroscopic measurements within several continuum spectral ranges from 482-685 nm. Both 2D dust colour maps and profiles in the directions imposed by the slit indicate that there are variations with rho with a trend towards bluer dust colour with increasing rho. This could indicate sublimation of ices as the cameras on board the EPOXI mission have shown. The average dust reddening is ~24%/100 nm. The azimuthally averaged surface brightness profiles of the continuum from the broad band images can be well fitted with m ~ 1 in the tail direction, whereas in the opposite direction the dust profiles are much flatter at rho
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.
We describe the design, construction, and performance of VIRUS-P (Visible Integral-field Replicable Unit Spectrograph – Prototype), the prototype for 150+ identical fiber-fed integral field spectrographs for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX). VIRUS-P was commissioned in 2007, is in regular service on the McDonald Observatory 2.7 m Smith telescope, and offers the largest field of any integral field spectrograph. The 246-fiber IFU uses a densepak-type fiber bundle with a 1/3 fill factor. It is fed at f/3.65 through a telecentric, two-group dioptric focal reducer. The spectrograph's double-Schmidt optical design uses a volume phase holographic grating at the pupil between the articulating f/3.32 folded collimator and the f/1.33 cryogenic prime focus camera. High on-sky throughput is achieved with this catadioptric system by the use of high reflectivity dielectric coatings, which set the 340-670 nm bandwidth. VIRUS-P is gimbal-mounted on the telescope to allow short fibers for high UV throughput, while maintaining high mechanical stability. The instrument software and the 18 square arcmin field, fixed-offset guider provide rapid acquisition, guiding, and precision dithering to fill in the IFU field. Custom software yields Poisson noise limited, sky subtracted spectra. The design characteristics are described that achieved uniformly high image quality with low scattered light and fiber-to-fiber cross talk. System throughput exceeds requirements and peaks at 40%. The observing procedures are described, and example observations are given.
In late 2010 a Jupiter Family comet 103P/Hartley 2 was a subject of an
intensive world-wide investigation. On UT October 20.7 the comet approached the
Earth within only 0.12 AU, and on UT November 4.6 it was visited by NASA's
EPOXI spacecraft. We joined this international effort and organized an
observing campaign. The images of the comet were obtained through narrowband
filters using the 2-m telescope of the Rozhen National Astronomical
Observatory. They were taken during 4 nights around the moment of the EPOXI
encounter. Image processing methods and periodicity analysis techniques were
used to reveal transient coma structures and investigate their repeatability
and kinematics. We observe shells, arc-, jet- and spiral-like patterns, very
similar for the CN and C3 comae. The CN features expanded outwards with the
sky-plane projected velocities between 0.1 to 0.3 km/s. A corkscrew structure,
observed on November 6, evolved with a much higher velocity of 0.66 km/s.
Photometry of the inner coma of CN shows variability with a period of
18.32+/-0.30 h (valid for the middle moment of our run, UT 2010 Nov. 5.0835),
which we attribute to the nucleus rotation. This result is fully consistent
with independent determinations around the same time by other teams. The
pattern of repeatability is, however, not perfect, which is understendable
given the suggested excitation of the rotation state, and the variability
detected in CN correlates well with the cyclic changes in HCN, but only in the
active phases. The revealed coma structures, along with the snapshot of the
nucleus orientation obtained by EPOXI, let us estimate the spin axis
orientation. We obtained RA=122 deg, Dec=+16 deg (epoch J2000.0), neglecting at
this point the rotational excitation.
Global water production rates were determined from the Lyman-{\alpha}
emission of hydrogen around comet 103P/Hartley 2, observed with the SWAN (Solar
Wind ANisotropies) all-sky camera on the SOHO spacecraft from September 14
through December 12, 2010. This time period included the November 4 flyby by
the EPOXI spacecraft. Water production was 3 times lower than during the 1997
apparition also measured by SWAN. In 2010 it increased by a factor of ~2.5
within one day on September 30 with a similar corresponding drop between
November 24 and 30. The total surface area of sublimating water within {\pm}20
days of perihelion was ~0.5 km^2, about half of the mean cross section of the
nucleus. Outside this period it was ~0.2 km^2. The peak value was 90%, implying
a significant water production by released nucleus icy fragments.
We report measurements of eight primary volatiles (H2O, HCN, CH4, C2H6,
CH3OH, C2H2, H2CO, and NH3) and two product species (OH and NH2) in comet
103P/Hartley-2 using high dispersion infrared spectroscopy. We quantified the
long- and short-term behavior of volatile release over a three-month interval
that encompassed the comet's close approach to Earth, its perihelion passage,
and flyby of the comet by the Deep Impact spacecraft during the EPOXI mission.
We present production rates for individual species, their mixing ratios
relative to water, and their spatial distributions in the coma on multiple
dates. The production rates for water, ethane, HCN, and methanol vary in a
manner consistent with independent measures of nucleus rotation, but mixing
ratios for HCN, C2H6, & CH3OH are independent of rotational phase. Our results
demonstrate that the ensemble average composition of gas released from the
nucleus is well defined, and relatively constant over the three-month interval
(September 18 through December 17). If individual vents vary in composition,
enough diverse vents must be active simultaneously to approximate (in sum) the
bulk composition of the nucleus. The released primary volatiles exhibit diverse
spatial properties which favor the presence of separate polar and apolar ice
phases in the nucleus, establish dust and gas release from icy clumps (and
also, directly from the nucleus), and provide insights into the driver for the
cyanogen (CN) polar jet. The spatial distributions of C2H6 & HCN along the
near-polar jet (UT 19.5 October) and nearly orthogonal to it (UT 22.5 October)
are discussed relative to the origin of CN. The ortho-para ratio (OPR) of water
was 2.85 \pm 0.20; the lower bound (2.65) defines Tspin > 32 K. These values
are consistent with results returned from ISO in 1997.
Understanding how comets work—what drives their activity—is crucial to the use of comets in studying the early solar system.
EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation) flew past comet 103P/Hartley 2, one with an unusually
small but very active nucleus, taking both images and spectra. Unlike large, relatively inactive nuclei, this nucleus is outgassing
primarily because of CO2, which drags chunks of ice out of the nucleus. It also shows substantial differences in the relative abundance of volatiles
from various parts of the nucleus.
1. General introduction. 1.1. Historical perspective. 1.2. Discovery.
1.3. Appearance. 1.4. Statistics. 1.5. Importance. 1.6. Brightness. 1.7.
Main characteristics. 1.8. Spacecraft encounters with comets. 1.9. An
overall view -- 2. Dynamics. 2.1. Orbital elements. 2.2. Orbit in space
-- 3. Physical aspects. 3.1. Black body radiation. 3.2. Perfect gas law.
3.3. Dissociative equilibrium. 3.4. Doppler shift. 3.5. Spectroscopy.
3.6. Isotopic effect. 3.7. Franck-Condon factors. 3.8. Intensity of
emitted lines. 3.9. Boltzmann distribution. 3.10. [symbol]-doubling.
3.11. Photochemistry of water. 3.12. Silicate. 3.13. Annealing. 3.14.
Carbon. 3.15. Solar radiation. 3.16. Solar wind -- 4. Spectra. 4.1. Main
characteristics. 4.2. Forbidden transitions. 4.3. Line-to-continuum
ratio -- 5. Spectra of coma. 5.1. Fluorescence process. 5.2. Excitation
temperature. 5.3. Abundances of heavy elements. 5.4. Isotopic
abundances. 5.5. Summary -- 6. Gas production rates in coma. 6.1.
Theoretical models. 6.2. Results. 6.3. Analysis of hydrogen
observations. 6.4. Related studies. 6.5. Parent molecules. 6.6. Chemical
diversity. 6.7. Summary -- 7. Dust tails. 7.1. Dynamics. 7.2. Anti-tail.
7.3. Dust trails. 7.4. Sodium gas tails. 7.5. Dust features -- 8. Light
scattering theory. 8.1. Mie scattering theory. 8.2. Approximate
expressions. 8.3. Computation of cross sections. 8.4. Results. 8.5.
Particles of other types. 8.6. Optical constants -- 9. The nature of
dust particles. 9.1. Visible continuum. 9.2. Polarization. 9.3. Grain
sizes. 9.4. Infrared measurements. 9.5. Spectral feature. 9.6.
Properties derived from direct measurements. 9.7. Radiation pressure
effects. 9.8. Summary -- 10. Ion tails. 10.1. Evidence for the solar
wind. 10.2. Dynamical aberration. 10.3. Theoretical considerations.
10.4. Instabilities and waves. 10.5. Acceleration of cometary ions.
10.6. Large scale structures. 10.7. X-rays. 10.8. Summary -- 11.
Nucleus. 11.1. Morphology. 11.2. Theory of vapourization. 11.3.
Outbursts. 11.4. Albedo and radius. 11.5. Mass, density and surface
gravity. 11.6. Rotation. 11.7. Nucleus composition. 11.8. Mass loss.
11.9. Structure. 11.10. Non-gravitational forces. 11.11. Ortho to para
ratio of molecules. 11.12. Binary systems. 11.13. Summary -- 12. Origin.
12.1. Evidence for the oort cloud. 12.2. Evolution and properties of
oort cloud. 12.3. Origin of the oort cloud. 12.4. Taxonomy. 12.5.
Summary -- 13. Relation to other solar system studies. 13.1. Asteroids.
13.2. Meteorites. 13.3. Meteor streams. 13.4. Particles collected at
high altitudes. 13.5. Primordial material. 13.6. Chemical evolution.
13.7. Terrestrial water. 13.8. Impact of outside bodies. 13.9. Overview
-- 14. Problems and prospects. 14.1. Epilogue. 14.2. Future studies.
The EPOXI mission flyby of Comet 103P/Hartley 2 revealed numerous discrete dust jets extending from the nucleus, thereby providing an unprecedented opportunity to visually connect these features to the nuclear surface. The observed distribution of jets yields fresh insight into the conditions under which these cometary features may form. This study examines the geomorphology associated with areas of jet activity and then applies observed topographic correlations in the construction of a 2-D hydrodynamic model of a single dust jet. Visible light images of Hartley 2 show correlations between specific surface structures with both narrow-angle and fan-shaped dust jets; associations include pits, arcuate depressions, scarps, and rimless depressions. Notably, many source regions for jets appear finer than the practical mapping resolution of the imaging instruments (∼12 m). This observation indicates that the processes controlling jet formation operate at significantly finer scales than the resolution of most cometary activity models and motivates a complementary numerical investigation of dust jet formation and evolution. In order to assess controlling variables, our parametric numerical study incorporates different geometries and volatile abundances for the observed source regions. Results indicate that the expression of jet activity not only depends on local topography but also contributes to the evolution and development of surface features. Heterogeneous distributions of volatiles within the nucleus also may contribute to differences in local styles of jet activity.
We conducted high-dispersion spectroscopic observations of Comet 103P/Hartley 2 in the near-infrared wavelength region using the Keck II telescope with NIRSPEC. We obtained observations on four dates, with the last observations performed during the EPOXI closest approach. For this work we focus on the observations performed on UT 2010 October 17 and 21, while observations carried out on UT 2010 September 16 and November 4 have been reported elsewhere. On all dates the spatial distributions in the coma of C2H6, HCN and C2H2 were similar to each other and consistent with the CN-jet morphology observed from optical observations. The spatial distributions in the coma of H2O and CH3OH were also similar to each other, but were generally different from C2H6, HCN and C2H2. There might be two distinct phases of ice in Comet Hartley 2; one is enriched in H2O and CH3OH, and another is enriched in more volatile species (C2H6, C2H2, and HCN). It is possible that highly volatile species like C2H6 were segregated from the H2O matrix when warmer conditions were prevalent. We summarize our spectroscopic observations and report absolute production rates and mixing ratios of parent volatiles. There was no evidence of any significant diversity in the mixing ratios of parent volatiles on different dates. Comet Hartley 2 was normal in CH3OH/H2O, C2H6/H2O, C2H2/H2O, NH3/H2O but depleted in H2CO/H2O and CH4/H2O. OPRs of H2O in Comet Hartley 2 measured in its 2010 apparition were consistent with those observed by the Infrared Satellite Observatory in 1997.
We performed low-dispersion spectroscopic observations of Comet 103P/Hartley 2 in optical wavelengths using the LOSA/F2 mounted on the 1.3 m-Araki telescope at Koyama Astronomical Observatory on UT 2010 November 4 during the close approach of the Deep Impact spacecraft to the nucleus of Comet 103P/Hartley 2 in the EPOXI mission flyby. Our observations have revealed the chemistry of the coma at optical wavelengths; including CN, C3, C2 and NH2 along with H2O from [OI] emission at 6300 Å. Resultant mixing ratios of these radicals put the comet into the normal group in chemical composition. The mixing ratios with respect to H2O obtained in our observations are basically consistent with the previous optical spectro-photometric observations of Comet 103P/Hartley 2 in 1991 by A’Hearn et al. (A’Hearn, M.F., Millis, R.L., Schleicher, D.G., Osip, D.J., Birch, P.V. [1995]. Icarus 118, 223–270), the optical spectroscopic observations in 1998 by Fink (Fink, U. [2009]. Icarus 201, 311–334) and also consistent with the observations on UT 2010 October 27 and 29 by Lara et al. (Lara, L.M., Lin, Z.-Y., Meech, K. [2011]. Astron. Astrophys. 532, A87) (but only for the ratio relative to CN).
We report on the results of a spectroscopic survey of 130 comets that was conducted at McDonald Observatory from 1980 through 2008. Some of the comets were observed on only one night, while others were observed repeatedly. For 20 of these comets, no molecules were detected. For the remaining 110 comets, some emission from CN, OH, NH, C3, C2, CH, and NH2 molecules were observed on at least one occasion. We converted the observed molecular column densities to production rates using a Haser (Haser, L. [1957]. Liege Inst. Astrophysics Reprint No. 394) model. We defined a restricted data set of comets that had at least three nights of observations. The restricted data set consists of 59 comets. We used ratios of production rates to study the trends in the data. We find two classes of comets: typical and carbon-chain depleted comets. Using a very strict definition of depleted comets, requiring C2and C3 to both be depleted, we find 9% of our restricted data set comets to be depleted. Using a more relaxed definition that requires only C2 to be below a threshold (similar to other researchers), we find 25% of the comets are depleted. Two-thirds of the depleted comets are Jupiter Family comets, while one-third are Long Period comets. 37% of the Jupiter Family comets are depleted, while 18.5% of the Long Period comets are depleted. We compare our results with other studies and find good agreement.
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.
The coma of comet 103P/Hartley 2 has a significant population of large
particles observed as point sources in images taken by the Deep Impact
spacecraft. We measure their spatial and flux distributions, and attempt to
constrain their composition. The flux distribution of these particles implies a
very steep size distribution with power-law slopes ranging from -6.6 to -4.7.
The radii of the particles extend up to 20 cm, and perhaps up to 2 m, but their
exact sizes depend on their unknown light scattering properties. We consider
two cases: bright icy material, and dark dusty material. The icy case better
describes the particles if water sublimation from the particles causes a
significant rocket force, which we propose as the best method to account for
the observed spatial distribution. Solar radiation is a plausible alternative,
but only if the particles are very low density aggregates. If we treat the
particles as mini-nuclei, we estimate they account for <16-80% of the comet's
total water production rate (within 20.6 km). Dark dusty particles, however,
are not favored based on mass arguments. The water production rate from bright
icy particles is constrained with an upper limit of 0.1 to 0.5% of the total
water production rate of the comet. If indeed icy with a high albedo, these
particles do not appear to account for the comet's large water production rate.
The CN coma structure of the NASA EPOXI mission target, comet 103P/Hartley 2, was observed during 20 nights from 2010 September to December. These CN images probe the rotational state of the comet's nucleus and provide a ground-based observational context to complement the EPOXI observations. A dynamically excited cometary nucleus with a changing rotational rate is observed, a characteristic not seen in any comet in the past. The lack of rotational damping during the four-month observing interval places constraints on the interior structure of the nucleus.
We report on photometry and imaging of Comet 103P/Hartley 2 obtained at
Lowell Observatory from 1991 through 2011. We acquired photoelectric photometry
on two nights in 1991, four nights in 1997/98, and 13 nights in 2010/11. We
observed a strong secular decrease in water and all other observed species
production in 2010/11 from the 1991 and 1997/98 levels. We see evidence for a
strong asymmetry with respect to perihelion in the production rates of our
usual bandpasses, with peak production occurring ~10 days post-perihelion and
production rates considerably higher post-perihelion. The composition was
"typical", in agreement with the findings of other investigators. We obtained
imaging on 39 nights from 2010 July until 2011 January. We find that, after
accounting for their varying parentage and lifetimes, the C2 and C3 coma
morphology resemble the CN morphology we reported previously. These species
exhibited an hourglass shape in October and November, and the morphology
changed with rotation and evolved over time. The OH and NH coma morphology
showed hints of an hourglass shape near the nucleus, but was also enhanced in
the anti-sunward hemisphere. This tailward brightness enhancement did not vary
significantly with rotation and evolved with the viewing geometry. We conclude
that all five gas species likely originate from the same source regions on the
nucleus, but that OH and NH were derived from small grains of water and ammonia
ice that survived long enough to be affected by radiation pressure and driven
in the anti-sunward direction. We detected the faint, sunward facing dust jet
reported by other authors, and did not detect a corresponding gas feature. This
jet varied little during a night but exhibited some variations from night to
night, suggesting it is located near the total angular momentum vector.
We present spectral data from 152 observations of 17 comets obtained with an Intensified Dissector Scanner spectrograph at McDonald Observatory. We present the details of the observations and reduction of these data to production rates. These production rates are then analyzed to show that comets seem to be reasonably homogeneous as a group. The ratios of production rates remain constant with activity level and with heliocentric distance, with the exception of Q(NH2)/Q(CN), which shows a heliocentric distance dependence.
A computer code to calculate the time-dependent nonequilibrium chemistry taking place within the coma of a comet has been developed. This code incorporates 1249 chemical reactions involving 128 species. Models were fit to data on Comet P/Stephan-Oterma (A. L. Cochran and E. S. Barker, 1985, Icarus62, 72–81). It was shown that (1) HCN is the parent for CN; (2) C2H2 is a parent for C2; (3) pure gas-phase chemistry with known species cannot adequately reproduce the observed C3 but a single step process can; and (4) at least prior to perihelion, the vaporization rate seems to have been controlled by water vaporization.
The 18-cm radio lines of the OH radical were observed in comet 103P/Hartley 2
with the Nan\c{c}ay radio telescope in support to its flyby by the EPOXI
mission and to observations with the Herschel Space Observatory. The OH lines
were detected from 24 September to 15 December 2010. These observations are
used to estimate the gas expansion velocity within the coma to 0.83 \pm 0.08
km/s in October 2010. The water production increased steeply but progressively
before perihelion, and reached 1.9 \pm 0.3 X 10E28 s-1 just before the EPOXI
flyby.
The nuclei of active comets emit molecules anisotropically from discrete vents. As the nucleus rotates, we expect
to observe periodic variability in the molecular emission line profiles, which can be studied through millimeter/
submillimeter spectroscopy. Using this technique we investigated the HCN atmosphere of comet 103P/Hartley 2,
the target of NASA’s EPOXI mission, which had an exceptionally favorable apparition in late 2010. We detected
short-term evolution of the spectral line profile, which was stimulated by the nucleus rotation, and which provides
evidence for rapid deceleration and excitation of the rotation state. The measured rate of change in the rotation
period is +1.00 ± 0.15 minutes day−1 and the period itself is 18.32 ± 0.03 hr, both applicable at the epoch of the
EPOXI encounter. Surprisingly, the spin-down efficiency is lower by two orders of magnitude than the measurement
in comet 9P/Tempel 1 and the best theoretical prediction. This secures rotational stability of the comet’s nucleus
during the next few returns, although we anticipate a catastrophic disruption from spin-up as its ultimate fate.
We report on narrowband CN imaging of Comet 103P/Hartley 2 obtained at Lowell
Observatory on 39 nights from 2010 July until 2011 January. We observed two
features, one generally to the north and the other generally to the south. The
CN morphology varied during the apparition: no morphology was seen in July; in
August and September the northern feature dominated and appeared as a mostly
face-on spiral; in October, November, and December the northern and southern
features were roughly equal in brightness and looked like more side-on
corkscrews; in January the southern feature was dominant but the morphology was
indistinct due to very low signal. The morphology changed smoothly during each
night and similar morphology was seen from night to night. However, the
morphology did not exactly repeat each rotation cycle, suggesting that there is
a small non-principal axis rotation. Based on the repetition of the morphology,
we find evidence that the fundamental rotation period was increasing: 16.7 hr
from August 13-17, 17.2 hr from September 10-13, 18.2 hr from October 12-19,
and 18.7 hr from October 31-November 7. We conducted Monte Carlo jet modeling
to constrain the pole orientation and locations of the active regions based on
the observed morphology. Our preliminary, self-consistent pole solution has an
obliquity of 10 deg relative to the comet's orbital plane (i.e., it is centered
near RA = 257 deg and Dec=+67 deg with an uncertainty around this position of
about 15 deg) and has two mid-latitude sources, one in each hemisphere.
The scale lengths are examined for the Haser (1957) model using new, photoelectric, high signal-to-noise ratio data on a variety of comets. The comets have helicocentric distances from 0.75 to 1.81 AU. It is shown that the scale lengths for C2 and C3 compiled by A'Hearn and Newburn and Spinrad (1984) do not adequately represent the data for most of the comets considered. New scale lengths are derived for these two molecules. CN scale lengths are discussed. It is shown that an R-sub-h squared variation for the scale lengths works for most molecules but that the C2 parent scale length must vary by R-sub-h to the 2.5 power.
A method for constructing physically realistic photochemical models, taking into account the isotropic ejection of dissociated molecular fragments as well as radiation pressure acceleration, has been developed using Monte Carlo techniques. The effect of the isotropic ejection, as opposed to the radial motion arbitrarily assumed in Haser's model, is adequately described by a simple average random walk model. It is shown that measured radial (Haser) scale lengths are in fact only lower limits to a range of possible true scale lengths for a given brightness profile, which explains the current discrepancies between observed scale lengths and those predicted by photochemistry.