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.     

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.     

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.     

Near-Infrared Photometric And Polarimetric Observations Of Comet Hale-Bopp

Near-infrared photometric and polarimetric observations of comet Hale-Bopp (1995 O1) using KONIC (Kiso Observatory Near-Infrared Camera) are reported. Observations were carried out on March 18 UT and April 26 UT 1997, when the heliocentric distances of the comet were 0.94 and 1.02 AU, and the phase angles were 48.5 deg and 32.9 deg, respectively. In the J, H, and K′ bands, we obtained linear polarization of the near-nucleus region of 16.4 ± 1.2, 18.8 ± 1.3, and 15.1 ± 0.9 percent on March 18UT and 7.1 ± 1.1, 8.9 ± 1.0, and 6.9 ± 0.6 percent on April 26, respectively. These values were higher than those observed for 1P/Halley. The maximum polarization was found at H band on both dates. Polarization maps showed higher polarization regions toward the anti-solar direction in the J and H bands. No distinct correlation was found between high polarization regions and bright regions. The projected expansion velocity of the arc structure of the dust jet was 375 ± 35.7 m/s on 17–19 March. The periodicity was found to be 11.1 ± 2.8 hours. 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.     

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.     

Comet Hale-Bopp (1995 O1)

Considerable interest has been raised by the discovery of Comet Hale-Bopp (1995 O1) and the possibility that it might become a very bright object in Spring 1997. The evidence to support either of the conflicting hypothesis (an intrinsically bright comet or a faint comet in a very large outburst) is too limited to reach solid conclusions and may remain so for some months yet. The pre-discovery observations encountered to date provide some limits to photometric models and suggest that the comet may be intrinsically bright, but do not yet permit a firm discrimination, even between extreme scenarios, due to the enormous extrapolation that must be made from the heliocentric distance at discovery, to that of perihelion.     

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.     

A CCD Image Archive Of Comet C/1995 O1 (Hale-Bopp): Dust Expansion Velocities

An optical archive of comet Hale-Bopp was obtained over 28 nights from June 1996 to May 1997 covering a range of heliocentric distances of r = 4.12 − 0.91. The CCD images were obtained with the 0.8-m University of Washington Manastash Ridge Observatory and the 0.25-m Pennsylvania State University at Eriemobile observatory. The reduced images clearly document the evolution of features of the coma, which were dominated by radial jetsfrom r = 4.1 − 2.3 AU, curved fan-shaped jets from r = 1.2 − 1.0 AU, and arcs or shells for 0.9 < r < 1.0 AU. We present the images and derive an average dust expansion velocity of V_dust = 0.67 ± 0.07 km s⁻¹ from measurements of the arc features. The images in thearchive will be made available to other researchers for further study. 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.     

Comet Hale-Bopp (C/1995 O1): UVSTAR-FUV Spectroscopy from the Space Shuttle

We report here on unique post-perihelion (2.3 AU) measurements of Comet Hale-Bopp in the FUV-range (950–1250 Å) by means of the UVSTAR spectrometer from the space shuttle with the main purpose of searching for argon and other FUV emitters. New methods for separating the strong airglow emission at shuttle altitudes are here discussed in detail. Due to our low resolution (15 Å) and S/N ratio the possible rocket-borne detection of argon near perihelion (0.9 AU) could not be confirmed. New species as N₂ are suspected but difficult to separate from the strong airglow emission at shuttle altitudes. From the Lyα brightness (1.30± 0.08 kRy) a water production rate Q = 5.9 ± 0.4 × 10²⁹ molecules s^?1 could be derived and compared with other post-perihelion observations.     

Dust Morphology Of Comet Hale-Bopp (C/1995 O1): I. Pre-Perihelion Coma Structures In

In 1996 comet Hale-Bopp exhibited a porcupine-like coma with straight jets of dust emission from several active regions on the nucleus. The multi-jet coma geometry developed during the first half of 1996. While the jet orientation remained almost constant over months, the relative intensity of the jets changed with time. By using the embedded fan model of Sekanina and Boehnhardt (1997a) the jet pattern of comet Hale-Bopp in 1996 can be interpreted as boundaries of dust emission cones (fans) from four — possibly five — active regions on the nucleus (for a numerical modelling see part II of the paper by Sekanina and Boehnhardt, 1997b). 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.     

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.     

Ir Observations Of An Outburst In Comet Hale-Bopp (C/1995 O1)

The comet Hale-Bopp (C/1995 O1) has been observed in the infrared (1–2.5 μm) with the Nordic Optical Telescope (NOT) equipped with the Arcetri NICMOS3 camera (ARNICA). Two observational campaigns, each one lasting about one week, were made when the comet heliocentric distance was about 3 AU. The first campaign was at the end of August and the second at the end of September 1996. During both runs two major outbursts were observed, the more intense of them started the day before the beginning of the second run. In the images recorded during the first three nights (24.8–26.8 Sept.) of the second run a dust shell expanding in the northern quadrant with a projected velocity of 0.14–0.28 km/s is clearly evident. The dust production rate increased by at least a factor ≈3 at the time of the outburst. Also evident on the first night is a change in the IR color that is well correlated with the dust shell. This is an indication that the material released by the outburst has a different composition and/or size distribution than that in the “quiescent” dust coma. In this paper we present preliminary results about the evolution and the photometric characteristics of the dust shell. 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.     

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.     

Distribution Law For Particle Fragmentation Times In A Theory For Striated Tails Of Dust Comets: Application To Comet Hale-Bopp (C/1995 O1)

Sekanina and Farrell's model for the striated dust tails of comets describes the formation of striae as a two-step fragmentation process that is characterized by an ejection time t_e of parent particles, by their radiation pressure acceleration β_p, and by their fragmentation time t_f. Of these three, t_f is the weakest parameter in that a range oft_f offers a set of nearly equivalent solutions. In this context, we comment on Nishioka et al.'s finite-lifetime model, which is a modification of the fragmentation model. We propose a truncated Gaussian function as a first-approximation distribution law for particle fragmentation times. We apply this generalized model to a stria in comet Hale-Bopp detected on March 5–15, 1997 and analyzed by Pittichová et al. in a recent paper. We find that in order to fit the stria's estimated width of ∼150 000 km, the fragmentation times cannot be distributed over a period of more than approximately 2 to 3 days. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spectra of Comet Hale-Bopp (C/1995 O1) Obtained with 6-M BTA Reflector at Mount Pastukhov

Between July 1996 and April 1997, 92 spectra of comet Hale-Bopp were obtained with the 6-meter BTA telescope of the SAO of the RAS at Mount Pastukhov. The spectra are two-dimensional, which allows one to determine the energy distribution for each emission along the slit and the energy distribution in wavelength for each individual position in the slit. From the 92 two-dimensional spectra covering the inner coma, detailed spectral maps of the total near-nuclear region of the comet are available for July 10, 11 and 12, 1996 and April 15, 1997. We propose an hypothesis about an unknown cometary species near λ = 620 nm in the spectrum obtained July 10, 1996. We also find an effect which may be caused by fluorescence of cometary dust. 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.