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.     

Observations of High Resolution Spectra of Comet Hale-Bopp (C/1995 O1) at the SAO of the RAS

We present the results of the preliminary study of the comet Hale-Bopp spectrum obtained April 17, 1997 by K. Churyumov and F. Mussayev with the help of the 1-meter Zeiss reflector and the echelle spectrometer (spectral resolutionλ/Δ λ ≈ 50000), CCD and the long slit, oriented along the radius-vector(“Sun-comet direction”). Energy distributions for three selected regions including the C₃, C₂ (0-0) and CN(Δ ν = 0) molecules emissions of the comet Hale-Bopp spectrum were built. The rotational lines of the CN(Δ ν = 0) band were identified. The nature of the high emission peak near λ 4020 Å in the C₃ band is discussed. The presence of the cometary continuum of the nonsolar origin is assumed.     

The Infrared Spectral Energy Distribution And Polarization Of Comet C/1995 O1 (Hale–Bopp) During 1997

Comets, such as C/1995 O1 (Hale-Bopp), are important to studies of the origins of the solar system because they are believed to be frozen reservoirs of the most primitive pre-solar dust grains and ices. Here, we report 1.2–18.5 μm infrared (IR) spectrophotometric and polarimetric observations of comet Hale-Bopp. Our measurements of the spectral energy distribution (SED) and IR polarization near perhelion passage suggest that emission from the coma was dominated by scattering and thermal emission from sub-micron sized dust grains. Hale-Bopp's surprising brightness may have been largely a result of the properties of its coma grains rather than the size of its nucleus. The thermal emission continuum from the grains had a superheat of S = T_color/T_BB ≥ 1.7, the peak of the 10 μm silicate emission feature was 1.7 mags above the carbon grain continuum, and the albedo (reflectivity) of the grains was ≥ 0.4 at a scattering angles, θ ≥ 135° This revised version was published online in July 2006 with corrections to the Cover Date.     

Jets And Arcs In The Coma Of Comet Hale-Bopp From August 1996 To April 1997

An observation program for measuring the activity of Comet Hale-Bopp was conducted at the Observatoire de Haute Provence from August 1996 to April 1997. Narrow band images were obtained from the 120 cm telescope. A set of four filters at 513, 516, 527 and 682 nm was used to measure the C₂ emission in the (1,1) and (0,0) band and the intensity of the dust-scattered continuum. The luminosity of the central part of the coma was measured in the continuum and C₂ emissions, as a function of heliocentric distance before perihelion. The activity of the comet considerably increased between October 1996 and February 1997. The images obtained in August, September and October 1996 show the presence of strong jets. In February, March and April 1997, series of roughly concentric arcs were observed in the sunward hemisphere of the coma. The arcs were also detected in the spatial distribution of the I(682 nm)/I(527 nm) red color ratio and the I(513 nm)/I(516 nm) C₂ emission ratio. Using the I(513 nm)/I(516 nm) ratio as a parameter to measure the degree of excitation of C₂ molecules, we can show that the local production of gas, probably resulting from dust fragmentation, should be taken into account. This revised version was published online in July 2006 with corrections to the Cover Date.     

Narrow-Band Photometry of Comet C/1995 O1 (Hale-Bopp)

Photometric observations of comet C/1995 O1 (Hale-Bopp) carried out at the Stará Lesná Observatory since February to April 1997 are analyzed and discussed. Emission band fluxes and continuum fluxes are presented, from which the total numbers of molecules in the columns of the coma encircled by diaphragms are calculated. The production rates are estimated from the conventional Haser model. We found that the photometric exponent of dust contribution two months prior perihelion was n = 5.2. The photometric exponent n of the cometary magnitude solely to the C₂ emission alone equals 3.3 and that of CN equals 2.5. These values can be explained by a fact that the maximums of production rates of the gases were reached between March 2and 12 and not at the perihelion as it is valid for dust. These results are compared with the values of 1P/Halley (1986 III) under the similar conditions, obtained with the same method and instrument. C/Hale-Bopp exhibited 4.1 times more molecules radiating the CN-emission than 1P/Halley in the same column of the coma. The continuum flux of C/Hale-Bopp was also very strong. The ratios (to 1P/Halley) are 94:1 (Cont. 484.5) and 74:1 (Cont. 365.0). The cometary colour was the same as that of the Sun. This revised version was published online in July 2006 with corrections to the Cover Date.     

Gaseous Jets in Comet Hale-Bopp (1995 O1)

We report the identification of gas jets in comet Hale-Bopp in OH, NH, CN, C₂ and C₃. This is the first time OH and NH jets without an obvious optical dust jet counterpart have been identified in narrowband comet images. We also confirm the existence of CN jets as reported by Larson et al. (1997) and Mueller et al. (1998). Jet features can be seen in the March and April 1997 datasets, approximately a month before and after perihelion. Our results contribute to the understanding of both the chemical properties of the comet as well as the physical mechanisms necessary to produce these features. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spectrocam-10 Thermal Infrared Observations Of The Dust In Comet C/1995 O1 Hale-Bopp

We describe 5 to 18 μm broadband images and R ∼ 100 spectra of comet Hale-Bopp taken with SpectroCam-10 on the 5 m Hale telescope during six runs between 1996 June and 1997 April. Our data show the gradual warming of dust grains as the comet approached perihelion. In June, the 10 μm silicate emission feature was already stronger than observed in any other comet, and it increased to about 3 times the continuum level near perihelion. Spectral structure attributed to a crystalline olivine grain component remained relatively constant, but other features associated with pyroxenes appeared to vary with heliocentric distance. This revised version was published online in July 2006 with corrections to the Cover Date.     

Observations of the inner coma of C/1995O1 (Hale-Bopp)—gas and dust production

Hale-Bopp (C/1995 O1) was the most productive recent comet observed in terms of gas and dust output. Since its discovery in 1995 at a distance of 7.14 AU from the Sun, the comet has been well observed, revealing the dynamics of a rare and large comet. Hale-Bopp showed strong emissions of the principle cometary gases CN, C₃, and C₂, as well as an abundance of dust. The production rates of these gases were found to be 1.45×10²⁸, 1.71×10²⁸, and , respectively, with dust production, in terms of Afρ, , as measured in the green continuum (5260 Å). The observations for this paper are presented in two groups spanning 10 days each, one group centered near 32 days prior to and the other 21 days after perihelion. The averages of dust and gas production rates show a slightly higher value for each prior to perihelion than after perihelion, consistent with a possible peak in production a few weeks prior to perihelion passage.     

Narrowband Electrophotometry of Comet Hale-Bopp (C/1995 O1) Near Perihelion

We present the results of narrowband photoelectric observations of comet Hale-Bopp near perihelion, which were made with the AZT-14telescope at the station Lisniki of Kyiv University Astronomical Observatory. The standard set of IHW cometary filters was used. The total number of cometary observations was more than 500 during the interval March 13–April 29, 1997. The comet's nuclear gas production rates Q of C₂ and C₃ were calculated using the stellar calibration for these filters. The calculations used the Haser model for a neutral cometary atmosphere. The value Afρ, which is characteristic of the dust production rate, was determined too. Mean values of the production rates near perihelion are log Q(C₂) = 28.4; log Q(C₃) = 27.2; (Afρ)_BC = 6.0. 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.     

Dust Morphology Of The Inner Coma Of C/1995 O1 (Hale-Bopp)

Dust continuum imaging of comet C/1995 O1 (Hale-Bopp) was carried out with the Swedish Vacuum Solar Telescope (SVST)on La Palma in April, 1997. Images were reduced according to standard procedure, aligned, averaged, navigated and enhanced with azimuthal renormalization, rotational derivative, temporal derivative and unsharp masking processing. The rotational period of the nucleus was determined to 11.5 h and the mean projected dust outflow velocity to 0.41 km s⁻¹. Shell envelopes in the sunward side of the coma were separated by a projected distance of ∼15 000–20 000 km and spiralling inwards towards smaller radii in the direction of local evening. Small scale inhomogeneities of size 1 000–2 000 km, interpreted as correlated with variations in dust emission activity, were seen at radii ≤20 000 km. Two overlapping shell systems with a relative lag angle of ∼55° were evident at the time. The north pole of the nucleus was directed towards the Earth. The dust emission pattern is very complex and may be due to several active areas. The shape of the incomplete spiral shell pattern indicates that the angle between the line-of-sight and the rotational axis of the nucleus was not large. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Near-Infrared Imaging Spectrophotometry Of Comet C/1995 O1 (Hale-Bopp) Near Perihelion

We present 1- to 5-μm broadband and CVF images of comet Hale-Bopp taken 1997 February 10.5 UT, 50 days before perihelion. All the images exhibit a nonspherical coma with a bright “ridge” in the direction of the dust tail approximately 10″ from the coma. Synthetic aperture spectrophotometry implies that the optically important grains are of a radius ≤0.4 μm; smallest radius for any comet seen to date. The variation of the integrated surface brightness with radial distance from the coma (ρ) in all the images closely follows the “steady state” ρ⁻¹ model for comet dust ablation (Gehrz and Ney, 1992). The near-infrared colors taken along the dust tail are not constant implying the dust grain properties vary with coma distance. This revised version was published online in July 2006 with corrections to the Cover Date.     

Infrared Spectroscopy of Comet Hale-Bopp

High resolution (λ/δλ ∼ 20,000) spectra of comet C/1995 O1 (Hale-Bopp) in the 2–5 μm region were obtained during UT 2–5 March 1997 using CSHELL at the NASA Infrared Telescope Facility (IRTF) on Mauna Kea. The heliocentric and geocentric distances of the comet were ∼1.1 AU and ∼1.5 AU,respectively. We detected emission lines of the gas-phase molecules H₂O, ₄, C₂H₆, C₂H₂, HCN, and CO and derived absolute production rates and relative abundances for all species. We also used the 2-dimensional nature of the CSHELL data to investigate the spatial distribution of the molecules and find evidence that CO was derived at least partly from an extended source in the coma. This revised version was published online in July 2006 with corrections to the Cover Date.     

Collisional Quenching of OH Radio Emission from Comet Hale-Bopp

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

Photometric studies of comet C/2009 P1 (Garradd) before the perihelion

The results of the photometric observations of comet C/2009 P1 (Garradd) performed at the 60-cm Zeiss-600 telescope of the Terskol observatory have been analyzed. During the observations, the comet was at the heliocentric and geocentric distances of 1.7 and 2.0 AU, respectively. The CCD images of the comet were obtained in the standard narrowband interference filters suggested by the International research program for comet Hale-Bopp and correspondingly designated the “Hale-Bopp (HB) set.” These filters were designed to isolate the BC (λ4450/67 Å), GC (λ5260/56 Å) and RC (λ7128/58 Å) continua and the emission bands of C₂ (λ5141/118 Å), CN (λ3870/62 Å), and C₃ (λ4062/62 Å). From the photometric data, the dust production rate of the comet and its color index and color excess were determined. The concentration of C₂, CN, and C₃ molecules and their production rates along the line of sight were estimated. The obtained results show that the physical parameters of the comet are close to the mean characteristics typical of the dynamically new comets.     

Detection Of Soft X-Ray Emission From Comet C/1995 O1 (Hale–Bopp)

We report the detection of soft X-rays from comet C/1995 O1 (Hale-Bopp) by the Low Energy Concentrator Spectrometer (LECS) on-board the X-ray satellite, BeppoSAX. The observations took place on 1996 September 10–11 approximately 1 day after a large dust outburst (Schulz et al., 1997–1999). After correcting for the comets motion, a 7σ enhancement was found centered (2.1 ± 1.3) x 10⁵ km from the position of the nucleus, in the general solar direction. The total X-ray luminosity in the 0.1–2.0 keV energy band is 5 x 10¹⁶ erg s⁻¹ which is at least a factor of ∼ 3 greater than measured by the Extreme Ultraviolet Explorer (EUVE)4 days later and suggests that the bulk of the emission measured by the LECS is related to the dust outburst. The extracted LECS spectrum is well fit by a thermal bremsstrahlung-like distribution of temperature of 0.29 ± 0.06 keV - consistent with that observed in other comets. We find no evidence for fluorescent carbon or oxygen emission and place 95% confidence limits of 1.0 x 10¹⁵ and 7.8 x 10¹⁵ erg s⁻¹ to narrow line emission at 0.28 and 0.53 keV, respectively. We calculate that if such lines are present, they constitute at most 18% of the 0.1–2.0 keV continuum luminosity. This revised version was published online in July 2006 with corrections to the Cover Date.     

CCD Observations of Comet Hale-Bopp at Shajn Telescope I. Coude spectra

Spectroscopic observations of Comet Hale-Bopp were made at the 2.6 m Shajn Telescope of the Crimean Astrophysical Observatory. Some spectra were obtained with high spectral resolution, FWHM = 0.18–0.4Å, in the coude focus on February 22 and 26, 1997. The observations were made in selected spectral windows (4805–4872 Å,6528–6595 Å, 7186–7253 Å, 8276–8408 Å).The spectrograph slit was centered on the nucleus and had dimensions of 25.2 × 0.6 arcsec² on the plane of the sky. The continuum spatial profiles were extremely asymmetrical toward the Sun. However, the continuum-subtracted spatial profiles of the molecular emissions were symmetrical relative to the nucleus, except for C₂. The shape of the spatial profiles of the C₂ emissions is similar to that one of continuum but is more flattened. So, there are evidences that dust can be an additional source of the C₂ radicals in the cometary coma. The main aim of our research was identification of the cometary emissions. Using recent laboratory spectroscopic data we identified newlines of C₂ associated with the transitions from high rotational levels in the 4805–4872 Å spectral region. We detected cometary H_α emission as well. Emissions of NH₂, H₂O⁺,and C₂ (Phillips system) were found in the red spectral windows. Some emission features are still unassigned.     

The Infrared Spectrum of Comet Hele–Bopp as Seen by the Infrared Space Observatory

Spectra of comet C/1995 O1 (Hale-Bopp) were obtained with the Infrared Space Observatory (ISO) at medium resolution with the grating spectrometer in the photometer (PHT-S) and/or at high resolution with the short wavelength spectrometer (SWS) and long wavelength spectrometer (LWS) in April 1996 (Crovisier et al., 1996), September–October 1996 (Crovisier et al., 1997a, b) and December 1997, at distances from the Sun of 4.6, 2.9 and 3.9 AU, respectively. For the first time, high-resolution spectra of a comet covering the entire 2.4 to 200 μm spectral range were obtained. The vibrational bands of H₂O, CO₂ and CO are detected in emission with PHT-S. Relative production rates of 100:22:70 are derived for H₂O:CO₂:CO at 3 AU pre-perihelion. H₂O is observed at high spectral resolution in the ν₃ group of bands around 2.7 μm and the ν₂ group around 6 μm with SWS, and in several rotational lines in the 100–180 μm region with LWS. The high signal-to-noise ratio of the ν₃ band observed on September–October 1996 allows accurate determinations of the water rotational temperature (28 K) and of its ortho-to-para ratio(2.45 ± 0.10, which significantly differs from the high temperature limit and corresponds to a spin temperature of 25 K). Longward of 6 μm the spectrum is dominated by dust thermal continuum emission, upon which broad emission features are superimposed. The wavelengths of the emission peaks correspond to those of Mg-rich crystalline olivine (forsterite). In the September–October 1996 spectra, emission features at 45 and 65 μm and possible absorption at 2.9–3.2 μm suggest that grains of water ice were present at 3 AU from the Sun. The observations made post-perihelion in late December 1997 led to the detections of H₂O, CO₂ and CO at 3.9 AU from the Sun (Figures 1 and 2). The production rates were ≈3.0 × 10²⁸,3.5 × 10²⁸ and ≈1.5 × 10²⁹ s^-1, respectively. This corresponds to H₂O:CO₂:CO = 100:110:500 and confirms that at such distances from the Sun, cometary activity is dominated by sublimation of CO and CO₂ rather than by H₂O. This revised version was published online in July 2006 with corrections to the Cover Date.     

Observations of C2 Molecules in the Coma of Comets at Mitaka

In order to monitor the various phenomena of comets, we continuously made the near-nucleus imaging observations of comets at National Astronomical Observatory, Mitaka. Here we report on the results of the observations of two spectacular comets. One is the analysis of photometry of C₂ emission images in the coma of comet Hyakutake (C/1996 B2). By comparing the production rates of C₂ radicals with C₂H₂ molecules, we conclude that the most C₂ radicals were originated from C₂H₂, while some of them might be produced from other parents. The second item is the analysis of the morphology of C₂ emission images in the coma of comet Hale-Bopp(C/1995 O1). By applying a ring masking technique, we detected asymmetrical distribution of the C₂ molecules. The results of these two comets might suggest that a portion of C₂ radicals were formed by the disintegration of the small organic dust grains, like the so-called CHON particles. This revised version was published online in July 2006 with corrections to the Cover Date.