Ion composition and solar wind interaction: Observations of comet C/1995 O1 (Hale-Bopp)

The extraordinary brightness of comet Hale-Bopp (C/1995 O1) offered the rare opportunity to investigate its ion tail by observations over a wide range of wavelengths. For the first time, ion emission lines could be detected in the radio domain, originating from H₃O⁺, HCO⁺ and CO⁺ ions. Observations in the extreme ultraviolet range showed O⁺ emissions. Optical imaging observations show a spectacular detail of structures in the ion tail. The combined measurements of the ion spatial distribution and dynamic provide a wealth of information which needs to be understood in terms of models of the comet-solar wind interaction and ion-neutral molecule reactions in the coma. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comet Hale–Bopp: Velocity Field Of Ions From Fabry-Pérot Imaging

Fabry-Pérot interferograms of comet Hale-Bopp were obtained on several nights in March and April 1997. For this purpose we utilized the 2-channel focal reducer of the Max-Planck-Institute for Aeronomy at the 2-m telescope of the Pik Terskol Observatory. Solid Fabry-Pérot etalons of resolving power 30000 were used in both channels of the focal reducer. The main aim of this study is to measure the velocities and abundances of OH⁺ and H₂O⁺, both ions closely related to the same parent molecule, H₂O. In the blue channel interferograms we identified several individual OH⁺ rotational lines of the A³Π_i - X³Σ⁻ (0-0) transition and measured their Doppler shifts. The target emissions in the red channel were the H₂O⁺ lines of the A²A₁ − X²B₁ (10-0) band. We found that the line of sight velocities, obtained from the Doppler shifted wavelengths of emissions in the comet are higher in sunward direction than in the plasma tail and do not exceed 20 km s^–1. The corresponding values, deprojected in antisolar direction, are consistent with predictions by magnetohydrodynamical models of the solar-wind-comet interaction, when one accounts for the extremely high gas production rate of comet Hale-Bopp. This revised version was published online in July 2006 with corrections to the Cover Date.     

Infrared Observations Of Dust Emission From Comet Hale-Bopp

We present infrared imaging and photometry of the bright, giant comet C/1995 O1 (Hale-Bopp). The comet was observed in an extended infrared and optical observing campaign in 1996–1997. The infrared morphology of the comet was observed to change from the 6 to 8 jet “porcupine” structure in 1996 to the “pinwheel” structure seen in 1997; this has implications for the position of the rotational angular momentum vector. Long term light curves taken at 11.3 μm indicate a dust production rate that varies with heliocentric distance as ∶ r^−1.4. Short term light curves taken at perihelion indicate a rotational periodicity of 11.3 hours and a projected dust outflow speed of ∶ 0.4 km s⁻¹. The spectral energy distribution of the dust on October 31, 1996 is well modeled by a mixture of 70% silicaceous and 30% carbonaceous non-porous grains, with a small particle dominated size distribution like that seen for comet P/Halley (McDonnell et al., 1991), an overall dust production rate of 2 × 10⁵ kg s⁻¹, a dust-to-gas ratio of ∶5, and an albedo of 39%. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hco+ in the Coma of Comet Hale–Bopp

Maps of comet C/1995 O1 (Hale-Bopp) in the millimeter-wave emission of the ion HCO⁺ revealed a local minimum near the nucleus position, with a maximum about 100,000 km in the antisolar direction. These observed features of the HCO⁺ emission require a low abundance of HCO⁺ due to enhanced destruction in the inner coma of the comet, within a region of low electron temperature (T_e). To set constraints on the formation of HCO⁺ in the coma, as well as the location and magnitude of the transition to higher T_e, the data are compared with the results of ion-molecule chemistry models. This revised version was published online in July 2006 with corrections to the Cover Date.     

Observations of the OH Radical in Comet C/1995 O1 (Hale-Bopp) with the Nançay Radio Telescope

We present OH 18-cm observations of comet Hale-Bopp (C/1995 O1) at the Nançay radio telescope. On nucleus and offset position observations allowed us to obtain both OH production rates and quenching radii. The maximum OH production rate was reached around perihelion, at about10³¹ s^-1.     

Energy balance and the gas flux from the surface of comet 46P/Wirtanen

Understanding the power balance at the surface of the nucleus is essential to study the chemical and physical evolution of a comet. Therefore, we present a detailed energy budget analysis for the surface of a model comet in the orbit of 46P/Wirtanen, target comet of the European space craft mission Rosetta, for a variety of parameters and assumptions. We will show that for a fast spinning Jupiter-family comet such as 46P/Wirtanen with a rotation period of about 6 h, a fast rotator approximation underestimates the effective energy input. This yields lower gas fluxes from the surface. For an 100% active, non-dust covered surface we obtain a water gas flux on the order of about 1.5×10²⁸ molecules s⁻¹ at perihelion, assuming a radius of 600 m. The calculated gas flux of water is within the order of measured values for comet 46P/Wirtanen. But our calculated values are maximum gas fluxes at noon—not averaged over one cometary day or taking the lesser insolation at the polar areas into account. Therefore, we conclude that either the radius of comet 46P/Wirtanen may be much larger than the accepted value of 600 m. A radius in the order of 2 km seems more likely to explain the measurements. Or, an other possibility could be that water-ice particles are blown off from the surface like dust particles. This may also increase the effective surface area of sublimation.     

Model analysis of the dust tail of comet Hale-Bopp

The paper presents the results of dynamic simulation for the dust tail formation of comet C/1995 O1 (Hale-Bopp). To simulate the dust tail, the trajectories of 2 × 10⁶ dust particles were traced. The sizes, ejection moments, outflow directions and velocities of the dust particles were defined by the Monte Carlo algorithm. The obtained three-dimensional tail was projected on the sky plane to compare it with the observed images. The brightness distribution in the comet tail was fitted to similar model parameters for three different dates. According to our model experiments, the observed tails could be formed by particles with sizes from 0.3 to 8.0 μm, ejection velocities from 0.155 to 0.670 km/s, and power index of the exponential size distribution from −3.6 to −3.7. It is shown that the inclusion of the particles fragmentation processes leads to a noticeable improvement of the simulation results.     

C/1995 O1 (Hale-Bopp): Gas Production Curves And Their Interpretation

The discovery of C/1995 O1 (Hale-Bopp) at 7 AU from the Sun provided the first opportunity to follow the activity of a bright comet over a large range of heliocentric distances r_h. Production rates of a number of parent molecules and daughter species have been monitored both pre- and postperihelion. CO was found to be the major driver of the activity far from the Sun, surpassed by water within 3 AU whose production rate reached 10³¹ s⁻¹ at perihelion. Gas production curves obtained for various species show several behaviours with r_h. Gas production curves contain important information concerning the physical state of cometary ices, the structure of the nucleus and all the processes taking place inside the nucleus leading to outgassing. They are relevant to the study of several other phenomena such as the sublimation from icy grains, dust mantling or seasonal effects. For some species, such as H₂CO or HNC, they permit to constrain their origin in the coma. We discuss models of subsurface gas production in distant comets and predictions of how such a source may vary as the comet moves along its orbit, approaching perihelion and receding again. Features in the observed gas production curves of comet Hale-Bopp are generally interpretable in terms of either subsurface production (typical example: CO at large r_h) or free sublimation (typical example: H₂O). Possible implications for the vertical stratification of the cometary ices are reviewed, and preference is found for a model with crystallization of amorphous ice close to the nuclear surface. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Interaction of C/1995 O1 Hale–Bopp with the Solar Wind as recorded in CoCam Images: A Progress Report

The Isaac Newton Group’s CoCam instrument obtained wide-field (10°X 20°) CCD images of the H₂O⁺ tail of Hale-Bopp on March 12–15, 19, 20, 25, 28, 29, 31, April1, 7, 9–13, 21, 23–29 and May 4, 1997. A 6185å filter recorded the distribution of H₂O⁺; 6250å-centered continuum images were also obtained. Initial analysis has revealed the CoCam dataset to be an invaluable record of the large-scale structure of the comet's ion tail. Several images show evidence of upstream parabolic envelopes of ions reminiscent of those reported in C/1908 R1 Morehouse (Eddington A. S., Mon. Not. Roy. Astron. Soc. 70, 442–458, 1910). Studies of these features may help towards a better understanding of the tail ray phenomenon. During most of March and early April, the tail had a largely consistent quiescent appearance, with numerous rays exhibited, but no major disturbances; most probably as a result of being in the stream of fast polar solar wind. A significant kink propagated down the tail around April 9/10. Towards the end of April, as the comet entered more variable solar wind at near-equatorial heliographic latitudes, the tail's morphology became significantly more structured. Striking tail disturbances were recorded in late April and early May, when the comet was in the vicinity of the heliospheric current sheet. Analysis of the dataset continues.     

Aperture Polarimetry And Photometry Of Comet Hale-Bopp

We present results of polarimetric and photometric observations of bright comet C/1995 O1 (Hale-Bopp) obtained at the 0.7 m telescope of Kharkov University Observatory from June 18, 1996 to April 24, 1997. The IHW and HB comet filters were used. The C₂ and C₃ production rates for Hale-Bopp are more than one order of magnitude larger and the dust production rates are more than two orders of magnitude larger than the Halley ones at comparable distances. Hence, Hale-Bopp was one of the most dusty comets. The average UC-BC and BC-RC colours of the dust were −0.02 and 0.13 mag, respectively. The polarization of comet Hale-Bopp at small phase angles of 4.8–13.0° was in good agreement with the date for comet P1/Halley at the same phase angles in spite of the fact that the heliocentric distances of comments differed nearly twice. However, at intermediate phase angles of 34–49° the polarization of comet Hale-Bopp was significantly larger than the polarization of the other dusty comets. It is the first case of such a large difference found in the continuum polarization of comets. The wavelength dependence of polarization for Hale-Bopp was steeper than for other dusty comets. The observed degree of polarization for the anti-sunward side of the coma was permanently higher than that for the sunward shell side. The polarization phase dependence of Hale-Bopp is discussed and compared with the polarization curves for other dusty comets. The peculiar polarimetric properties of comet Hale-Bopp are most likely caused by an over-abundance of small or/and absorbing dust particles in the coma. This revised version was published online in July 2006 with corrections to the Cover Date.     

New Molecular Species in Comet C/1995 O1 (Hale-Bopp) Observed with the Caltech Ssubmillimeter Observatory

We present millimeter-wave observations of HNCO, HC₃N, SO, NH₂CHO, H¹³CN, and H₃O⁺ in comet C/1995 O1 (Hale-Bopp)obtained in February–April, 1997 with the Caltech Submillimeter Observatory (CSO). HNCO, first detected at the CSO in comet C/1996B2 (Hyakutake), is securely confirmed in comet Hale-Bopp via observations of three rotational transitions. The derived abundance with respect to H₂O is (4-13) × 10^-4. HC₃N, SO, and NH₂CHO are detected for the first time in a comet. The fractional abundance of HC₃N based on observations of three rotational lines is (1.9 ± 0.2) × 10^-4. Four transitions of SO are detected and the derived fractional abundance, (2-8) ×10^-3, is higher than the upper limits derived from UV observations of previous comets. Observations of NH₂CHO imply a fractional abundance of (1-8) × 10^-4. H₃O is detected for the first time from the ground. The H¹³CN (3-2)transition is also detected and the derived HCN/H¹³CN abundance ratio is 90 ± 15, consistent with the terrestrial¹²C/¹³C ratio. In addition, a number of other molecular species are detected, including HNC, OCS, HCO⁺, CO⁺, and CN(the last two are first detections in a comet at radio wavelengths). This revised version was published online in July 2006 with corrections to the Cover Date.     

MHD Simulation of Comets: The Plasma Environment of Comet Hale-Bopp

MHD simulation results of the interaction of the expanding atmosphere of comet Hale-Bopp with the magnetized solar wind are presented. At the upstream boundary a supersonic and superalfvénic solar wind enters into the simulation box 25 million km upstream of the nucleus. The solar wind is continuously mass loaded with cometary ions originating from the nucleus. The effects of photoionization, recombination and ion-neutral frictional drag are taken into account in the model. The governing equations are solved on an adaptively refined unstructured Cartesian grid using our MUSCL-type upwind numerical technique, MAUS-MHD (Multiscale Adaptive Upwind Scheme for MHD). The combination of the adaptive refinement with the MUSCL-scheme allows the entire cometary atmosphere to be modeled, while still resolving both the shock and the diamagnetic cavity of the comet. Detailed simulation results for the plasma environment of comet Hale-Bopp for slow and fast solar wind conditions are presented. We also calculate synthetic H₂O⁺, CO⁺ and soft x-ray images for observing conditions on April 11, 1997. This revised version was published online in July 2006 with corrections to the Cover Date.     

K-band Radio Observations of Comet Hale-Bopp: Detections of Ammonia and (Possibly) Water

K-band radio observations of comet Hale-Bopp (C/1995 O1) were conducted in March/April 1997 at the 100-m Telescope of the Max-Planck-Institut für Radioastronomie. Emission was firmly detected from the five lowest metastable (J = K)inversion transitions of ammonia. Assuming a thermal distribution for the metastable states of NH₃, we derive a rotational temperature of 104 ± 30 K and an ammonia production rate at perihelion of6.6 ± 1.3 × 10²⁸ s^-1.The updated ammonia-to-water abundance ratio is found to be of the order of 1.0%. We also report a marginal detection of the 6₁₆–5₂₃transition line of water at λ = 1.35 cm. This revised version was published online in July 2006 with corrections to the Cover Date.     

Wide Field Imaging of Ion Tail of Comet C/Hale–Bopp

A sequential imaging observation of the ion tail of Comet C/Hale-Bopp 1995O1 was carried out in February–March 1997 with a wide-field CCD imaging camera using narrow band filters for two ion species; CO⁺ and H₂O⁺ along with those for blue and red continuum. From the surface photometry of the ion tail of two species, we derived a relationship between plasma density and distance from the nucleus. The local velocity of the ion flow as a function of the distance from the nucleus was also estimated on the basis of some assumptions. We report preliminary results of our analysis, and discuss some characteristics of cometary plasma and its interaction with interplanetary magnetic field (IMF). May the source be with you! This revised version was published online in July 2006 with corrections to the Cover Date.     

Measurement of the solar wind velocity with cometary tail rays

Cometary tail rays are traces of the magnetic fields caught in the cometary magnetosphere. Time variations of these rays give us a way to measure the local solar wind velocity at the location of a comet. We introduce a simple method for determining the radial velocity of the solar wind by observing the ray folding motion, and show an example of its application to comet P/Brorsen-Metcalf 1989o, which resulted in 340 ± 35 km s^–1.     

Trapping of N2, CO and Ar in amorphous ice—Application to comets

Recent attempts using high resolution spectra to detect N⁺₂ in several comets were unsuccessful [Cochran, A.L., Cochran, W.D., Baker, E.S., 2000. Icarus 146, 583-593; Cochran, A.L., 2002. Astrophys. J. 576, L165-L168]. The upper limits on N⁺₂ in comparison with the positively detected CO⁺ for Comets C/1995 O1 Hale-Bopp, 122P/1995 S1 de Vico and 153P/2002 C1 Ikeya-Zhang range between . Ar was not detected in three recent comets [Weaver, H.A., Feldman, P.D., Combi, M.R., Krasnopolsky, V., Lisse, C.M., Shemansky, D.E., 2002. Astrophys. J. 576, L95-L98], with upper limits of Ar/CO<(3.4-7.8)×¹⁰⁻² for Comets C/1999 T1 McNaught-Hartley, C/2001 A2 LINEAR and C/2000 WM1 LINEAR. The Ar detected by Stern et al. [Stern, S.A., Slater, D.C., Festou, M.C., Parker, J.Wm., Gladstone, G.R., A'Hearn, M.F., Wilkinson, E., 2000. Astrophys. J. 544, L169-L172] for Comet C/1995 O1 Hale-Bopp, gives a ratio Ar/CO=7.25×¹⁰⁻², which was not confirmed by Cosmovici et al. [Cosmovici, C.B., Bratina, V., Schwarz, G., Tozzi, G., Mumma, M.J., Stalio, R., 2006. Astrophys. Space Sci. 301, 135-143]. Trying to solve the two problems, we studied experimentally the trapping of N₂+CO+Ar in amorphous water ice, at 24-30 K. CO was found to be trapped in the ice 20-70 times more efficiently than N₂ and with the same efficiency as Ar. The resulting Ar/CO ratio of 1.2×¹⁰⁻² is consistent with Weaver et al.'s [Weaver, H.A., Feldman, P.D., Combi, M.R., Krasnopolsky, V., Lisse, C.M., Shemansky, D.E., 2002. Astrophys. J. 576, L95-L98] non-detection of Ar. However, with an extreme starting value for N₂/CO = 0.22 in the region where the ice grains which agglomerated to produce comet nuclei were formed, the expected N₂/CO ratio in the cometary ice should be 6.6×¹⁰⁻³, much higher than its non-detection limit.     

The HNC/HCN Ratio in Comets

The abundance ratio of the isomers HCN and HNC has been investigated in comet Hale-Bopp (C/1995 O1) through observations of the J = 4−3 rotational transitions of both species for heliocentric distances 0.93 < r < 3 AU, both pre- and post-perihelion. After correcting for the optical depth of the stronger HCN line, we find that the column density ratio of HNC/HCN in our telescope beam increases significantly as the comet approaches the Sun. We compare this behavior to that predicted from an ion-molecule chemical model and conclude that the HNC is produced insignificant measure by chemical processes in the coma; i.e., for comet Hale-Bopp, HNC is not a parent molecule sublimating from the nucleus. This revised version was published online in July 2006 with corrections to the Cover Date.     

What is the Parent of CH in the Coma of Comet C/1995 O1 (Hale-Bopp)?

The ratio of CH₄/CO in comets is an important indicator of the region of their formation. However, it is difficult to measure the quantity of CH⁺. The Giotto Ion Mass Spectrometer experiment observed a quantity of CH⁺ which seemed inconsistent with the amount of CH⁺. Thus, it was proposed that a source of the CH⁺ was a distributed source in the dust. We tested this hypothesis by observing the CH emission strength in comet Hale-Bopp as a function of dustiness. We see no strong correlation between the dust and the gas. Thus, for Hale-Bopp,dust is unlikely to be a dominant source of CH. This revised version was published online in July 2006 with corrections to the Cover Date.     

Lessons of Comet Hale-Bopp for Coma Chemistry: Observations and Theory

Many new cometary molecules — both parents and daughters — were detected in the exceptionally productive comet C/1995 O1 (Hale-Bopp).The space distribution of several of these species could be investigated from radio interferometry or from long-slit spectroscopy in the infrared. The distinction between parent species — directly sublimated from nucleus ices — and secondary species — resulting from chemical processing in the coma or produced by a secondary source — is not always clear. It is important to assess whether or not observed minor species (HCOOCH₃, HCOOH...) could be synthesized by chemical reactions favoured by the high density of the coma of comet Hale-Bopp. Chemical modelling by Rodgers and Charnley suggests that this is notthe case. CO and H₂CO are abundant cometary species which partly come from distributed sources. The nature of these sources is still a mystery. A special case, now well documented, is that of HNC, for which the abundance evolution with heliocentric distance could be observed in comet Hale-Bopp and which was observed in several much less productive comets.