Isophot Observations Of Comet Hale-Bopp

Comet Hale-Bopp has been observed five times with ISOPHOT, the photometer on board the Infrared Space Observatory (ISO), four times before its perihelion passage at heliocentric distances of 4.92, 4.58, 2.93 and 2.81 AU, and at 3.91 AU postperihelion. Each time, multi-filter photometry covering the range between 3.6–175 μm with eight to ten filters was performed to sample the spectral energy distribution of the comet. These measurements were used to determine dust temperatures for the cometary coma. The evolution of the strength of the silicate feature can be followed in the data as well as the flux deficit at longer wavelengths. This revised version was published online in July 2006 with corrections to the Cover Date.     

CCD Polarimetric Imaging Of Comet Hale-Bopp (C/1995 O1)

Polarimetric observations of the light scattered by dust have been carried out at Pic-du-Midi Observatory with the 2 m telescope in June and September–October 1996, and at Haute-Provence Observatory with the 0.80 m telescope in April 1997. They cover a total number of 11 nights and a large (6.9°–47.7°) phase angle range. The spatial resolution allows to underline structures in the coma, as well in the brightness images as in the polarization maps, with a correlation between the regions of bright structures and the regions of higher polarization. A clear difference appears between the sunward and antisunward side, with higher polarization on the antisunward side. The phase angle coverage allows us to obtain a polarimetric phase curve for the whole coma and to compare it with other cometary phase curves. The degree of polarization is higher for Hale-Bopp than for the comets previously observed. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Thermal Emission From The Dust Coma Of Comet Hale-Bopp And The Composition Of The Silicate Grains

The dust coma of comet Hale-Bopp was observed in the thermal infrared over a wide range in solar heating (R = 4.9–0.9 AU) and over the full wavelength range from 3 μm to 160 μm. Unusual early activity produced an extensive coma containing small warm refractory grains; already at 4.9 AU, the 10 μm silicate emission feature was strong and the color temperature was 30% above the equilibrium blackbody temperature. Near perihelion the high color temperature, strong silicate feature, and high albedo indicated a smaller mean grain size than in other comets. The 8–13 μm spectra revealed a silicate emission feature similar in shape to that seen in P/Halley and several new and long period comets. Detailed spectral structure in the feature was consistent over time and with different instruments; the main peaks occur at 9.3, 10.0 and 11.2 μm. These peaks can be identified with olivine and pyroxene minerals, linking the comet dust to the anhydrous chondritic aggregate interplanetary dust particles. Spectra at 16–40 μm taken with the ISO SWS displayed pronounced emission peaks due to Mg-rich crystalline olivine, consistent with the 11.2 μm peak. This revised version was published online in July 2006 with corrections to the Cover Date.     

Images Of Polarization And Colour In The Inner Coma Of Comet Hale-Bopp

On March 31 and April 1, 1997, simultaneous photometry and polarimetry of comet Hale-Bopp's dust was conducted with the two-channel focal reducer of the Max-Planck-Institute for Aeronomy attached to the 2 m telescope of Pik Terskol Observatory (Northern Caucasus). Interference filters at642 nm and 443 nm selected red and blue narrow-band continuum windows. The observations have been averaged over the one hour of timethe comet could be observed. The polarization maps cover an area of about1 arcmin² around the nucleus. The values of polarization degree measured close to the nucleus agree very well with observations obtained with aperture polarimetry. They are lower than in the surrounding coma by about 1%. In our field of view the polarization increases along the sun-comet line from the solar to the antisolar side by about 3%. The dust shells are visible in the polarization images. The polarization in the shells is higher by 1 to 2%and this increase is higher in the red than in the blue range. Therefore the ratio of red to blue polarization (≈ 1.2) increases in the shells by ≈ 0.03. In principle, the polarization excess in the shells, the ratio of red/blue polarization and the higher integrated polarization as compared to other comets can be explained by an excess of particles of radius of about 0.1 μm. Such particles, however, are subject to strong radiation pressure and will be pushed back into the tail before they reach the observed location of the shells. Real Rayleigh particles cannot explain the observed increase in the ratio of red/blue polarization. One therefore cannot exclude the possibility that the excess polarization in the shells is caused by fluffy aggregates via effects which are presently not well understood. The colour map shows features not well related to intensity and polarization, perhaps another dust shell of a different particle size. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Mid-Infrared Spectro-Imaging Observations Of Comet Hale-Bopp

Comet C/1995 O1 (Hale-Bopp) was observed with camiras, the Saclay mid-infrared camera, mounted on the Nordic Optical Telescope (Roque de los Muchachos, La Palma), from April 14th to April 25th, 1997. This observing run was part of the European campaign devoted to Hale-Bopp observations from the Canary Islands (PI R. West). camiras spectro-imaging capabilities, achieved with a Circular Variable Filter (of spectral resolution R ≈ 50), were used to obtain spectra of the comet in the N atmospheric window (8–13 μm) over a large field of view (52″ × 78″), at a spatial resolution of ∼1 arcsec. Data were also collected at shorter wavelengths (in the L and M bands). The silicate dust feature around 10 μm is clearly apparent in the data. The shape of the feature varies continuously according to the position in the field. A dust model developed at Saclay in the framework of β-Pictoris dust disk studies was used to interpret the observations. The presence of both olivine and pyroxene with a high degree of crystallinity is needed to account for the observations. A change in the size distribution of the grains allows to reproduce the spatial change of the silicate feature according to the position in the field. 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.     

Thermal Infrared Spectra of Comet Hale-Bopp at Heliocentric Distances of 4 and 2.9 AU

We present 10 and 20 μm spectra of comet Hale-Bopp taken at UKIRT on 1996 June 20 and 1996 September 29.The 10 μm spectra clearly show a strong silicate feature with peaks at 10.0 and 11.2 μm. The 20 μm spectrum on September 29 has strong excess flux relative to a blackbody and a peak near 19μm, in good agreement with the ISO SWS spectra obtained a week later. However, the 20 μm spectrum on June 20 has significantly lower flux than would be expected based on a blackbody extrapolation from the flux at 12.5 μm. This revised version was published online in July 2006 with corrections to the Cover Date.     

Observations of the Dust Cloud in Comet Hale-Bopp on 9–11 May 1997

During our monitoring observations of comet Hale-Bopp, we found several sporadic ejections of dust from the nucleus. The most prominent ejection was observed on May 6–9, 1997, in the post-perihelion phase of the apparition. In this paper, we report preliminary analysis of this event, in which the total mass of the dust cloud is estimated to be 1.6 × 10¹¹ g. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Comparison Between Near-Infrared And Visible Imaging Of The Inner Coma Of Comet Hale-Bopp At Perihelion

We present a comparison between images of comet C/1995 O1 (Hale-Bopp) obtained from 12 March to 9 May 1997, with two telescopes of the Observatorio del Teide (IAC, Tenerife, Spain) in visible (Johnson-V filter) and three near-infrared narrowband filters (Brγ (2.166 μm), CO (2.295 μm), and K_con (2.260 μm)). No significant differences are observed between the images in the three near-infrared bands suggesting that any CO emission is very weak, or produced by an extended flat source. We do not find evidence for the existence of an additional infrared component due to emission by warm dust, or to dust grains of diverse composition and/or size. Visible and infrared images of the same rotational phase look almost identical, supporting the view that the observed jets and shells are mainly dust structures. This revised version was published online in July 2006 with corrections to the Cover Date.     

Infrared Observations Of Volatile Molecules In Comet Hale-Bopp

Infrared observations of comets C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp) benefited from the high spectral resolution and sensitivity of échelle spectrometers now equipping ground-based telescopes and from the availability of the Infrared Space Observatory (ISO). From the ground, several hydrocarbons were unambiguously detected for the first time: CH₄, C₂H₂, C₂H₆. Water was observed through several of its hot vibrational bands, escaping telluric absorption. CO, HCN, NH₃ and OCS were also observed, as well as several radicals. This permitted the evaluation of molecular production rates, of rotational temperature, and — taking advantage of the 1-D imaging of long-slit spectroscopy — of the space distribution of these species. With ISO, carbon dioxide was directly observed for the second time in a comet (after its detection from the Vega probes in P/Halley). The spectrum of water was investigated in detail (several bands of vibration and far-infrared rotational lines), permitting the evaluation of the rotational temperature of water, and of it spin temperature from the ortho-to-para ratio. Water ice was identified in the grains of Hale-Bopp as far as 7 AU from the ground and possibly at 3 AU with ISO. The composition of cometary volatiles appears to be strikingly similar to that of interstellar ices. This revised version was published online in July 2006 with corrections to the Cover Date.     

Discovery Of Mg-Rich Pyroxenes In Comet C/1995 O1 (Hale-Bopp): Pristine Grains Revealed At Perihelion

The NASA Ames HIFOGS spectrometer observed comet C/1995 O1 (Hale-Bopp) at epochs including 96 Oct 7–14 UT (2.8 AU), 97 Feb 14–15 UT (1.2 AU), 97 Apr 11 UT (0.93 AU), and 97 Jun 22, 25 UT (1.7 AU). The HIFOGS 7.5–13.5 μm spectrophotometry (R = 360 - 180) of the silicate feature at 2.8 AU is identical in shape to the ISO SWS spectra of comet Hale-Bopp (Crovisier et al., 1997); the strong 11.2 μm peak in the structured silicate feature is identified as olivine. Upon close passage to the sun, the HIFOGS spectra at 1.2 AU and 0.93 AU reveals strong peaks at 9.3 μm and 10.0 μm. The post-perihelion 10 μm silicate feature at 1.7 AU is weaker but has nearly the same shape as the pre-perihelion spectra at 1.2 AU, reverting to its pre-perihelion shape: there is no change in the dust chemistry by close passage to the sun. The appearance of the strong peaks at 9.3 μm and 10.0 μm at r_h ≲ 1.7 AU is attributed to the rise in the contribution of pryoxenes (clino-pyroxene and orthopyroxene crystals) to the shape of the feature, and leads to the hypothesis that the pyroxenes are significantly cooler than the olivines. The pyroxenes are radiating on the Wien side of the blackbody at 2.8 AU and transition to the Rayleigh-Jeans tail of the blackbody upon closer approach to the Sun. Composite fits to the observed 10 μm silicate features using IDPs and laboratory minerals shows that a good empirical fit to the spectra is obtained when the pryoxenes are about 150 K cooler than the olivines. The pyroxenes, because they are cooler and contribute signficantly at perihelion, are more abundant than the olivines. The perihelion temperature of the pyroxenes implies that the pyroxenes are more Mg-rich than the other minerals including the olivines, amorphous olivines, and amorphous pyroxenes. The PUMA-1 flyby measurements of comet P/Halley also indicated an overabundance of Mg-rich pryoxenes compared to olivines. Comet Hale-Bopp's pyroxenes are similar to pyroxere IDPs from the ’Spray‘ class, known for their D-richness and their unaltered morphologies: Hale-Bopp's Mg-rich pyroxenes may be pristine relic ISM grains. This revised version was published online in July 2006 with corrections to the Cover Date.     

Polarimetry Of Comet Hale-Bopp: Linear And Circular Polarization, Stellar Occultation

Comet Hale-Bopp was observed with the 2.6-m and 1.25-m telescopes of the Crimean Astrophysical Observatory on March 9 and 11, 1997. We determined the linear and circular polarization in the coma. For the dust coma, the linear polarization varied from 7.8% to 12.4%. The degree of circular polarization was always negative and did not exceed 0.3% with an accuracy of ±0.04% on average. The passage of a bright star through the cometary coma was monitored with polarimetry and photometry. The wavelength dependence and spatial variations of optical thickness of dust are obtained from the stellar occultation. This revised version was published online in July 2006 with corrections to the Cover Date.     

An Early Investigation Of The Striated Tail Of Comet Hale-Bopp (C/1995 O1)

The Sekanina-Farrell particle fragmentation model for the striated tails of dust comets is successfully applied to two images of comet Hale-Bopp to study the motions of 12 striae in a time span of March 12–15, 1997. There is evidence for recurring outbursts with a periodicity of 11^h21^m, consistent with results based on analysis of dust jets. The ejecta in all the striae appear to have been released from one source on the nucleus between the end of January and the second half of February 1997, some 60 to 40 days before perihelion. The parent particles were subjected to a radiation pressure acceleration of β_p ≃ 0.55 and their fragmentation lifetimes in 11 of the 12 striae were practically constant and equal to 13–15 days, when normalized to 1 AU from the Sun. Brief analysis of Watanabe et al.'s measurements of striae on their images from March 5–9, 1997 shows even shorter fragmentation lifetimes for the parent particles, mostly about 7–11 days at1 AU. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dust Morphology Of Comet Hale-Bopp (C/1995 O1). Ii. Introduction Of A Working Model

A Monte Carlo image simulation code for dust features in comets is applied to comet Hale-Bopp in order to model the object's persistent porcupine-like appearance on high-resolution images taken between May 11 and Nov. 2, 1996. A self-consistent fan model is proposed, with six isolated sources of dust emission assumed at various locations on the surface of the rotating nucleus and with the spin axis undergoing a complex motion in an inertial coordinate system. In the framework of this model, jet pairs represent boundaries of fan-shaped formations described by dust ejected from isolated sources during periods of time when the Sun is above the local horizon. The spin axis is found to have traveled through a field of 10° by 20° during the examined period of nearly six months. Still more successful is a fan model with large diurnal dust-emission fluctuations, which is consistent with an inertially fixed position of the spin axis and requires only three discrete sources. In this scenario, the dust-emission profile is dominated by several brief flare-ups, or “puffs”, in the production of dust from one of the sources. The results are insensitive to the spin rate, but the observed dust coma appearance is more typical of a rapidly rotating comet. 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.     

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.     

Imaging polarimetry of comet Hale-Bopp (C/1995 O1) around perihelion

The polarization distribution of the inner coma of comet Hale-Bopp was measured by CCD imaging around perihelion. The dust shell positions correlate well with relative maxima of polarization. The images taken in the I-band identify different shell systems, each showing individual polarization properties. This could be related to differences in the dust properties of the associated jets. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparative Study Of The Dust Polarimetric Properties In Split And Normal Comets

Our polarimetric database contains six comets, C/1975 V1 (West), 16P/Brooks 2,C/1988 A1 (Liller), D/1996 Q1 (Tabur), C/1999 S4 (LINEAR), and C/2001 A2(LINEAR), which can be related to the group of split comets. Comets West, S4(LINEAR) and A2 (LINEAR) were observed during splitting. We compare thepolarimetric measurements of the dust particles in these comets, sometimes togetherwith available photometric and colorimetric data, with those in normal comets. Weconclude that there is no significant evidence for differences of polarization betweentidally split comets (e.g., Brooks 2), dissipating comets (e.g., Tabur), non-tidally splitcomets (e.g., West) and normal comets. The total disintegration of Comet S4 (LINEAR), however, did produce significant changes in the observed properties of dust.