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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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 model of water and CO activity of Comet C/1995 O1 (Hale-Bopp)

An investigation of the activity of Comet C/1995 O1 (Hale-Bopp) with a thermophysical nucleus model that does not rely on the existence of amorphous ice is presented. Our approach incorporates recent observations allowing to constrain important parameters that control cometary activity. The model accounts for heat conduction, heat advection, gas diffusion, sublimation, and condensation in a porous ice-dust matrix with moving boundaries. Erosion due to surface sublimation of water ice leads to a moving boundary. The movement of the boundary is modeled by applying a temperature remapping technique which allows us to account for the loss in the internal energy of the eroded surface material. These kind of problems are commonly referred to as Stefan problems. The model takes into account the diurnal rotation of the nucleus and seasonal effects due to the strong obliquity of Hale-Bopp as reported by Jorda et al. (Jorda, L., Rembor, K., Lecacheux, J., Colom, P., Colas, F., Frappa, E., Lara, L.M. [1997]. Earth Moon Planets 77, 167-180). Only bulk sublimation of water and CO ice are considered without further assumptions such as amorphous ices with certain amount of occluded CO gas. Confined and localized activity patterns are investigated following the reports of Lederer and Campins (Lederer, S.M., Campins, H. [2002]. Earth Moon Planets 90, 381-389) about the chemical heterogeneity of Hale-Bopp and of Bockelée-Morvan et al. (Bockelée-Morvan, D., Henry, F., Biver, N., Boissier, J., Colom, P., Crovisier, J., Despois, D., Moreno, R., Wink, J. [2009]. Astron. Astrophys. 505, 825-843) about a strong CO source at a latitude of 20°. The best fit to the observations of Biver et al. (Biver, N. et al. [2002]. Earth Moon Planets 90, 5-14) is obtained with a low thermal conductivity of 0.01 W m⁻¹ K⁻¹. This is in agreement with recent results of the Deep Impact mission to 9P/Tempel 1 (Groussin, O., A’Hearn, M.F., Li, J.-Y., Thomas, P.C., Sunshine, J.M., Lisse, C.M., Meech, K.J., Farnham, T.L., Feaga, L.M., Delamere, W.A. [2007]. Icarus 187, 16-25) and with previous thermal simulations (Kührt, E. [1999]. Space Sci. Rev. 90, 75-82). The water production curve matches the production rates well from −4 AU pre-perihelion to the outgoing leg while the model does not reproduce so well the water production beyond 4 AU pre-perihelion. The CO production curve is a good fit to the measurements of Biver et al. (2002) over the whole measured heliocentric range from −7 AU pre- to 15 AU post-perihelion.     

Spectroscopic Monitoring of Comet C/1995 O1 (Hale-Bopp)

Earth, Moon, and Planets -     

Cyanide Chemistry in Comet Hale-Bopp (C/1995 O1)

Observations of comet Hale-Bopp (C/1995 O1) have been carried out near perihelion (1997 March) at millimeter wavelengths using the NRAO 12 m telescope. The J=1-->0, 2-->1, and 3-->2 lines of HCN at 88, 177, and 265 GHz were measured in the comet as well as the J=3-->2 lines of H13CN, HC15N, and HNC. The N=2-->1 transition of CN near 226 GHz was also detected, and an upper limit was obtained for the J=2-->1 line of HCNH+. From the measurements, column densities and production rates have been estimated. A column density ratio of [HCN]/[HNC] = 7+/-1 was observed near perihelion, while it was found that [HCN]/[HCNH+] greater, similar 1. The production rates at perihelion for HCN and CN were estimated to be Q(HCN) approximately 1x1028 s-1 and Q(CN) approximately 2.6x1027 s-1, respectively, resulting in a ratio of [HCN]/[CN] approximately 3. Consequently, HCN is sufficiently abundant to be the parent molecule of CN in Hale-Bopp, and HCNH+ could be a source of HNC. Finally, carbon and nitrogen isotope ratios of 12C/13C = 109+/-22 and 14N/15N = 330+/-98 were obtained from HCN measurements, in agreement with previous values obtained from J=4-->3 data. Such ratios suggest that comet Hale-Bopp formed coevally with the solar system.     

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.     

UV Photopolarimetric Imaging of C/1995 O1 (Hale-Bopp) with the Wide Field Imaging Survey Polarimeter (WISP)

We report on the reduction and analysis of UVpolarimetric images of CI (λ1657 Å) and dust continuum (2696 Å emissions from C/1995 O1 (Hale-Bopp) taken using the Wide Field Imaging Survey Polarimeter (WISP) sounding rocket on 8 April, 1997. These observations represent the first imaging polarimetry of comets in the UV, and were performed in consort with ground based measurements of gas and dust polarization and distribution. The continuum results show 9% polarization across the image field with a polarization phase angle close to the 129° prediction. Comparison with ground based data implies minimal color dependence for Hale-Bopp in either the degree of polarization and in the position angle. The carbon polarimetry implies that most production occurs in the dense inner coma, and that it leaves that area in thermodynamic equilibrium. Its radial profile further constrains the carbon outflow speed to be sufficient to travel ≥5 × 10⁶ km without photoionization.     

Orbit Determination and Evolution of Comet C/1995 O1 (Hale-Bopp)

The situation leading to the determination of the Hale-Bopp orbit is discussed, largely in terms of a procedure that generates two sequences of parabolic orbits. The comet is also considered in relation to the problem of the possibility of impact on the earth. The placement of its orbital nodes near the orbits of the earth and Jupiter is clearly an intriguing feature. The role of the prediscovery observation in 1993 is described, as it appeared both as a boon and a burden. Although evidence has been put forward that the Hale-Bopp nucleus is unusually large, it seems likely that nongravitational forces are noticeably affecting the comet's motion. While discussion of the comet's future long-term motion may be amenable to the usual treatment as a problem of diffusion, it is not entirely improbable that the present situation arose from a recent dramatic approach to Jupiter. It is shown that such a Jupiter encounter in June -2215 is not inconsistent with the non-existence of records at the comet's last perihelion passage, which could then have been the first to occur as close as 0.9 AU to the sun. Of course, the Jupiter encounter might also have given rise to the possible large satellite to Hale-Bopp discussed by Sekanina. This revised version was published online in July 2006 with corrections to the Cover Date.