Hale-Bopp Research Articles

The Infrared Activity of Comet P/Halley 1986 III at Heliocentric Distances from 0.6 to 5.92 AU

We present an analysis of the combined infrared data for comet 1P/Halley 1986 III acquired by Gehrz and Ney, Hanner and coworkers, Tokunaga and coworkers, Green and coworkers, Ryan and Campins, and Bregman and coworkers. This database, the largest single body of infrared photometric data for any comet, spans a wavelength range from 0.7 to 23??m and describes the activity of P/Halley at heliocentric distances from 0.6 to 5.92 AU. Coverage of the thermal infrared spectrum was obtained for heliocentric distances from 0.6 to 2.8 AU. The quantitative corrections and calibration procedures required to intercompare the individual data sets are described. Long-term trends in the heliocentric dependence of P/Halley's grain color temperature, silicate emission optical strength, average grain albedo, apparent luminosity, and infrared monochromatic fluxes are discussed. The normalized thermal infrared flux varied as r-2.2 preperihelion and r-2.1 postperihelion in accord with the slope of the water production rate, indicating a general correlation between dust and gas production. Large-amplitude short-term (hours) fluctuations in all the observable quantities were seen in small apertures; these changes correlated with gas production rates and the presence of jets. Changes in JHK colors and the incomplete correlation between superheat and silicate feature strength imply variability in the grain composition, as well as in the size distribution. Based on a comparison of infrared data, comet Hale-Bopp (C/1995 O1) ejected a much higher relative abundance of submicron grains than Halley, and this pattern persisted to larger heliocentric distances.

The chemistry of C2and C3in the coma of Comet C/1995 O1 (Hale-Bopp) at heliocentric distancesrh≥2.9 AU

The extraordinary activity of comet C/1995 O1 (Hale-Bopp) made it possible to observe the emission bands of the radicals and in the optical wavelengths range at heliocentric distances larger than 3 AU. Based on these observations, we perform an analysis of the formation of and 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 and had to be derived based on the observations. The spatial distributions of and 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 AU. In comet Hale-Bopp, C2H2 and C2H6 were measured directly by infrared observations only at heliocentric distance 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.

Comets, Asteroids and Zodiacal Light as Seen by Iso

ISO performed a large variety of observing programmes on comets, asteroids and zodiacal light — covering about 1% of the archived observations — with a surprisingly rewarding scientific return. Outstanding results were related to the exceptionally bright comet Hale-Bopp and to ISO’s capability to study in detail the water spectrum in a direct way. But many other results were broadly recognised: Discovery of new molecules in comets, the studies of crystalline silicates, the work on asteroid surface mineralogy, results from thermophysical studies of asteroids, a new determination of the asteroid number density in the main-belt and last but not least, the investigations on the spatial and spectral features of the zodiacal light.

IR characterization and radiation chemistry of glycolaldehyde and ethylene glycol ices

Motivated by detections of glycolaldehyde and ethylene glycol in the interstellar medium and, for ethylene glycol, in comet HaleBopp, we report the low-temperature mid-IR spectra of these two molecules, including selected band strengths and positions. Each molecule has also been irradiated with 0.8 MeV protons to simulate cosmic-ray exposure, to measure each molecules radiation lifetime, and to determine radiation chemical products. Evidence is presented that glycolaldehyde is readily formed from irradiated ethylene glycol ices, both in the presence and absence of H2O ice. � 2005 Published by Elsevier Ltd on behalf of COSPAR.

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

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

The Anomalous 14 N/ 15 N Ratio in Comets 122P/1995 S1 (de Vico) and 153P/2002 C1 (Ikeya-Zhang)

High-resolution (R ~ 60,000) spectra of the CN B 2Σ+-X 2Σ+ (0, 0) band (near 3880 A) in the Halley-type comet 122P/1995 S1 de Vico (with a period of 74 yr) and the intermediate-period comet 153P/2002 C1 Ikeya-Zhang (P ~ 370 yr) were obtained with the 2dcoude spectrograph at the 2.7 m Harlan J. Smith telescope of the McDonald Observatory. The comets were within 1 AU from the Sun (0.66 and 0.92 AU, respectively) at the time of the observations. While the measured 12C/13C isotope ratios of both comets (90 ± 10 and 90 ± 25, respectively) are in very good agreement with the solar system value, the 14N/15N ratios (140 ± 20 and 170 ± 50, respectively) are approximately half the value in Earth's atmosphere. The similarity is striking between these ratios and those obtained recently for two other long-period Oort Cloud comets, C/1995 O1 (Hale-Bopp) and C/2000 WM1 (LINEAR). While these optical determinations of 14N/15N are consistent with each other, they disagree with those obtained in comet Hale-Bopp from submillimeter measurements of HCN, generally believed to be the main parent of CN. This puzzling difference points toward the existence of (an)other unknown parent(s) of CN, with an even higher 15N excess. Organic compounds like those found in interplanetary dust particles are good candidates.

The composition of ices in comet C/1995 O1 (Hale-Bopp) from radio spectroscopy

From radio spectroscopic observations of comets, more than 22 molecules, radicals and ions, plus several iso- topologues, were detected, the majority of them being recently revealed in comets C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp). Among them, 6 molecules were detected for the first time (Bockelee-Morvan et al. 2000) in the course of a spectral survey conducted at radio wavelengths in comet Hale-Bopp with the CSO, the IRAM 30-m telescope and Plateau de Bure interferometer. In addition, many species were searched for unsuccessfully, some of them with stringent upper limits. We present here a review of these observations and further analysis of their results. This include: (i) confirmed detection of acetaldehyde (CH3CHO); (ii) limits on small molecules such as ketene (H2CCO) or methanimine (CH2NH); (iii) limits on the abundance ratios in homologous series such as HC5N/HC3N, ethanol/methanol, acetic acid/formic acid; (iv) searches for precursors of key cometary species such as atomic Na and HNC; (v) constraints on more exotic species ranging from water dimer (H2O)2 to glycine; (vi) detection of the H 34 S isotopic species and independent observations of HDO and DCN; (vii) lim- its on several other deuterated species; (viii) limits on several radicals and ions and a tentative detection of the C2H radical; (ix) the presence of unidentified lines. Typical abundance upper limits of 2-5 ×10 −4 relative to water are achieved for many species. Better upper limits are obtained for some linear molecules with high dipole moments. But more complex molecules such as dimethyl ether or glycine are poorly constrained. These results should give important clues to the chemical composition of cometary ices, to the formation mechanisms of cometary material, and to the chemical processes which occur in the inner coma.

Spin Temperatures of Ammonia and Water Molecules in Comets

The nuclear spin temperature, which is derived from the ortho-to-para abundance ratio of molecules measured in cometary comae, is a clue to the formation conditions of cometary materials, especially the physical temperature at which the molecules were formed. In this paper we present new results for the nuclear spin temperatures of ammonia in comets Hale-Bopp (C/1995 O1) and 153P/Ikeya-Zhang based on observations of NH2 at 26 and 32 K, respectively. These results are similar to previous measurements in two other comets, and the nuclear spin temperatures of ammonia in the four comets are concentrated at about 30 K. We emphasize that the nuclear spin temperatures of water measured thus far have also been about 30 K. In particular, the spin temperatures of ammonia and water are equal to each other within ±1 σ error bars in the case of comet Hale-Bopp. These nuclear spin temperatures of ammonia and water were measured under quite different conditions (heliocentric distances and gas production rates). There is no clear trend between the nuclear spin temperatures and the heliocentric distances, the gas production rates, or the orbital periods of the comets. The possibilities of the ortho-to-para conversion in the coma and in the nucleus are discussed. The present data set implies that the ortho-to-para ratios were not altered after the molecules were incorporated into the cometary nuclei. It appears that cometary ammonia and water molecules formed on cold grains at about 30 K.

An Interferometric Study of CO and CS in Comet Hale-Bopp (C/1995 O1)

We used the BIMA Array to observe CO and CS molecules in Comet Hale-Bopp (C/1995 O1) during the 1997 apparition. A spur-like structure around the cometary nucleus is observed. These molecular spurs may be attributed to gas or dust jets. Using Haser model, we find the theoretically predicted CO scale length agrees well with observations. A large fraction of CO is sublimated directly from the nucleus, with the remaining CO coming from icy dust grains in cometary coma. The [CO/H2O] abundance ratio of Hale-Bopp is consistent with that of icy dust grains in star forming regions. We conclude that Hale-Bopp was likely formed at the stage when there were still ample of icy grains in Solar Nebula; therefore, abundances of parent molecules in a comet may reflect the primordial chemical composition of the Nebula.

A survey of 10-μm silicate emission from dust around young sun-like stars

We obtained low resolution ( R=100) mid-infrared (8–13 μm wavelengths) spectra of 8 nearby young main sequence stars with the Keck 1 telescope and Long-Wavelength Spectrometer (LWS) to search for 10 μm silicate (Si–O stretch) emission from circumstellar dust. No stars exhibited readily apparent emission: Spectra were then analyzed by least-squares fitting of a template based on a spectrum of Comet Hale-Bopp. Using this technique, we were able to constrain the level of silicate emission to a threshold 10 times below what was previously possible from space. We found one star, HD 17925, with a spectrum statistically different from its calibrator and consistent with a silicate emission peak of 7% of the photosphere at a wavelength of 10 μm. Excess emission at 60 μm from this star has already been reported.

The dust activity of comet C/1995 O1 (Hale-Bopp) between 3 AU and 13 AU from the Sun

The active comet C/1995 O1 (Hale-Bopp) was target of an optical long-term monitoring program carried out at the European Southern Observatory (ESO) (Rauer et al. [CITE], [CITE]). Longslit spectra and images were obtained at heliocentric distances from 4.6 AU to 2.9 AU preperihelion and 2.8 AU to 12.8 AU postperihelion. Based on these data, the dust activity of comet Hale-Bopp is analysed. The color of the dust coma and the Afρ parameter are determined. A model for the dust release from the cometary nucleus is presented and used to compute dust production rates. The dust to gas ratio is determined.

Infrared excitation processes of C2H6 in comets

A time-dependent and line-by-line fluorescence model of the v7 band of C2H6 has been constructed. Collisional (neutrals and electrons) and radiative excitation effects have been considered in the calculations of fluorescence efficiency factors (g-factors) of the C2H6 v7 band. Since the lifetime of C2H6 is ∼91,000 seconds at a heliocentric distance of 1 AU, C2H6 molecules far from the nucleus approach fluorescence equilibrium, while molecules within the contact surface should have a much colder rotational distribution due to collisional equilibration with the low temperature gases in that region. We would recommend using “single-cycle” fluorescence models for the analysis of v7 band spectra taken with small apertures centered on the nucleus. We analyzed a v7 band spectrum of comet Hale-Bopp (C/1995 O1) obtained at the IRTF with the CSHELL on 2 March, 1997 (R = 1.1 AU, Δ = 1.5 AU) using a square aperture of 1,000 × 2,000 km, and constructed synthetic spectra to compare with the observation. We analyzed spatial brightness profiles of the R Q0 sub-branch and found that the eastward profile is very well matched by the models, but the observed westward profile is clearly broader than the eastward profile suggesting asymmetric outflow and/or extended sources. We derived a C2H6 production rate of ∼1.7±0.9 × 1028 molec s−1 from the inner coma region of the comet at the time of the observation.

Detection of DCO+in a circumstellar disk

We report the first detection of DCO+ in a circumstellar disk. The DCO+ J=5-4 line at 360.169 GHz is observed with the 15m James Clerk Maxwell Telescope in the disk around the pre-main sequence star TW Hya. Together with measurements of the HCO+ and H13CO+ J=4-3 lines, this allows an accurate determination of the DCO+/HCO+ ratio in this disk. The inferred value of 0.035+-0.015 is close to that found in cold pre-stellar cores and is somewhat higher than that measured in the envelope around the low-mass protostar IRAS 16293 -2422. It is also close to the DCN/HCN ratio obtained for pristine cometary material in the jet of comet Hale-Bopp. The observed DCO+/HCO+ ratio for TW Hya is consistent with theoretical models of disks which consider gas-phase fractionation processes within a realistic 2-D temperature distribution and which include the effects of freeze-out onto grains.

Icy grains as a source of CO in comet 29P/Schwassmann-Wachmann 1

A model of micron-sized icy grains is used to simulate the behaviour of solid material ejected from comet 29P/Schwassmann-Wachmann 1, which resides at ∼6AU from the Sun. It is demonstrated that a population of grains com- posed primarily of water ice can release their ice content on timescales of order 10 6 s. This justifies the idea that icy grains can serve as an extended source of more volatile species, as was assumed when evidence for a coma source of CO was found in radio observations of this comet. The model also predicts short lifetimes for the icy grains inside 3AU from the Sun, removing the extended appearance of the CO production at smaller heliocentric distances, as seen in comet Hale-Bopp. A grain popula- tion sublimating in a time interval similar to that derived for the coma source of CO in comet 29P/Schwassmann-Wachmann 1 must have most of its mass concentrated to small (with radii <5 µm) or porous grains. Those grains must carry a significant ice content, with an refractory to ice ratio of less than ∼30%. By introducing continuous grain fragmentation the extended source activity can be reproduced for a grain population that includes larger particles. This would call for grains to split into smaller pieces on a timescale of ∼5 × 10 5 s.

HCN and HNC in comets C/2000 WM1 (Linear) and C/2002 C1 (Ikeya-Zhang).

Comets have been suggested as a possibly significant source of organic molecules to the early Earth. Hydrogen cyanide (HCN) is important in models of prebiotic chemistry, but may be difficult to form in the early terrestrial environment, while hydrogen isocyanide (HNC) is a 'classical' tracer of interstellar ion-molecule chemistry. We have observed both HCN and HNC in 2 recent comets, bringing the number of comets with published measurements of the HNC/HCN abundance ratio to 6. The HNC/HCN ratio in comet Ikeya-Zhang appears to increase with decreasing heliocentric distance, as was previously observed for comet Hale-Bopp, indicating that the HNC is produced at least in part by processes in the cometary coma (atmosphere) and is not simply a constituent of the nuclear ices. Both comets C/2000 WMI (Linear) and C/2002 C1 (Ikeya-Zhang) exhibit values of the HNC/HCN ratio that appear to be too large (approximately 0.09-0.19) to be matched by current models of coma chemistry. Cometary HNC may be a photodissociation product of organic grains or large organic polymers stored in the nucleus. We have also set a limit on the emission from the NO radical in comet WM1.

Measurements of [Ci] Emission from Comet Hale‐Bopp

We present quantitative measurements of cometary [C I] 9850 A 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 (λ/Δλ ≈ 40,000) Fabry-Perot/CCD spectrometer at the McMath-Pierce Solar telescope on Kitt Peak. This forbidden line, the carbon analog of [O I] 6300 A, is emitted in the radiative decay of C(1D) atoms. In the absence of other sources and sinks, [C I] 9850 A 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 A emission in comet Hale-Bopp.

Imaging polarimetry of cometary dust: different comets and phase angles

Polarimetric observations of the light scattered by dust in cometary comae have been carried out at Pic-du-Midi Observatory and at Haute-Provence Observatory (France) between 1990 and 2001. The whole coma polarization can easily be inferred from integration of the fluxes on the aperture. Maps are obtained for seven comets by CCD imaging polarimetry, which allow to observe three main regions in a cometary coma: the background coma, a circumnuclear halo and bright structures (jets or arcs) which correspond to different values of the polarization. The polarization maps of short period comets are compared to the maps obtained for comet Hale-Bopp, for phase angles smaller than 50° ( α smaller than 15° with negative polarization, α near the inversion angle of approximately 20°, α between 30° and 50° on the linear part of the phase curve). At α≅60° the polarization map of comet C/2000 WM1 is presented. Between 109° and 121° the evolution of brightness images and polarization maps of comet C/1999 S4 is followed during the complete disruption of its nucleus. The polarimetric phase curve for the circumnuclear halo presents highly negative values for α<20°. On the opposite for the same phase angles range, the polarization in jets is positive for the four studied comets. The differences between the regions are discussed in terms of physical properties of the particles (size distribution, packing density, mixtures of materials) by comparison to other observational techniques and laboratory measurements.

Multi-transitional observations of methanol in Comet Hale-Bopp (1995 O1)

We have observed 9 rotational lines of methanol in comet Hale-Bopp (1995 O1) using the 45-m radio telescope at Nobeyama Radio Observatory in the frequency range 36–104 GHz. The observed transitions have upper state rotational energy levels of Eu = 7–159 K. Assuming the Haser model for density distribution, we estimated the excitation temperature and the production rate to be K and Q(CH3OH) = molecule s-1, respectively. We also observed the HCN line at 88 GHz to derive the HCN production rate. We compared the production rate of CH3OH with that of HCN and that of other molecules whose production rates have been determined previously. The abundance ratio Q(CH3OH)/Q(X) is estimated to be in the range 3–121, where X denotes HCN, HNC, H2CO, CS and CH3CN. When we compare these abundance ratios with those in interstellar clouds: the bipolar flow L1157 B1/B2, the star-forming region SgrB2(M) and the dark cloud TMC-1, they are close to the values in L1157 B1/B2 or in SgrB2(M). We found that the abundance ratio Q(CH3OH)/Q(H2O) was similar to solid methanol ratio relative to water ice in interstellar medium. These results support the hypothesis that cometary nuclei are formed from dust grains outside of the primordial solar nebula so that the grain composition of the nebula remains in cometary nuclei.

Short-term nongravitational effect due to activity in comet Hale-Bopp's nucleus

Can activity of comet's nucleus, such as jets, cause observable short-term nongravitational effect? We have analysed the data on several nuclear events of Comet Hale-Bopp during late February-early April, 1997 and found the observed position deviated markedly from the position calculated according to the ephemeris after the observing error and the offset of the nucleus mass-center from the comet's light-center have been allowed for. Thus, nuclear activity can indeed cause observable short-term nongravitational effect. Possible short-term variation of sun-comet distance due to the effect is estimated and the relation between the effect and the relevant activity is discussed.

The shell of Comet Hale-Bopp

A film showing the shells in the coma of Comet Hale-Bopp, taken on March 4, 1997, is digitized and measured by means of a PDS. The method of fitting leads to the following results: (1)The first shell (innermost one), derived from a jet formation, is seen as an Archimedean spiral and the velocity of the ejecta from the cometary nucleus is 1.6 km/sec; (2)All of the 2nd, 3rd and 4th shells are parabolas or catenaries.