Hale-Bopp Research Articles

Cometary deuterium.

Deuterium fractionations in cometary ices provide important clues to the origin and evolution of comets. Mass spectrometers aboard spaceprobe Giotto revealed the first accurate D/H ratios in the water of Comet 1P/Halley. Ground-based observations of HDO in Comets C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp), the detection of DCN in Comet Hale-Bopp, and upper limits for several other D-bearing molecules complement our limited sample of D/H measurements. On the basis of this data set all Oort cloud comets seem to exhibit a similar (D/H)H2O ratio in H2O, enriched by about a factor of two relative to terrestrial water and approximately one order of magnitude relative to the protosolar value. Oort cloud comets, and by inference also classical short-period comets derived from the Kuiper Belt cannot be the only source for the Earth's oceans. The cometary O/C ratio and dynamical reasons make it difficult to defend an early influx of icy planetesimals from the Jupiter zone to the early Earth. D/H measurements of OH groups in phyllosilicate rich meteorites suggest a mixture of cometary water and water adsorbed from the nebula by the rocky grains that formed the bulk of the Earth may be responsible for the terrestrial D/H. The D/H ratio in cometary HCN is 7 times higher than the value in cometary H2O. Species-dependent D-fractionations occur at low temperatures and low gas densities via ion-molecule or grain-surface reactions and cannot be explained by a pure solar nebula chemistry. It is plausible that cometary volatiles preserved the interstellar D fractionation. The observed D abundances set a lower limit to the formation temperature of (30 +/- 10) K. Similar numbers can he derived from the ortho-to-para ratio in cometary water, from the absence of neon in cometary ices and the presence of S2. Noble gases on Earth and Mars, and the relative abundance of cometary hydrocarbons place the comet formation temperature near 50 K. So far all cometary D/H measurements refer to bulk compositions, and it is conceivable that significant departures from the mean value could occur at the grain-size level. Strong isotope effects as a result of coma chemistry can be excluded for molecules H2O and HCN. A comparison of the cometary (D/H)H2O ratio with values found in the atmospheres of the outer planets is consistent with the long-held idea that the gas planets formed around icy cores with a high cometary D/H ratio and subsequently accumulated significant amounts of H2 from the solar nebula with a low protosolar D/H.

Spectral observations of comets with simple instruments

AbstractLow resolution optical spectra of comet Hale-Bopp 1995 O1 taken with a simple equipment are presented. A small fiber feed spectrograph was used linked first to a telescope of 0.1 m aperture and then to another one of 0.25 m aperture. The qualitative survey of the evolution of the Na (λ5893) doublet in comet Hale-Bopp is reported.

Dust shells in the coma of C/1995 O1 (Hale-Bopp)

AbstractThe comet Hale-Bopp was continuously observed from September 1996 to April 1997. We used altogether 316 images of Hale-Bopp to analyse the evolution of dust shells system close to the nucleus. We performed measurements of relative distance of envelopes and velocity of its expansion. Evolution of the spiral arm revealed spin of the nucleus. We estimated the period of rotation to be 11.42 hours. With purpose to confront the real CCD images of dust éjecta and the measured ejection velocities a computer model of formation and expansion of the dust shells has been developed.

Plasma tails: Comets Hale-Bopp and Hyakutake

Comet Hale-Bopp was one of the largest comets ever recorded, and it exhibited both a massive dust tail and a plasma tail, which developed as it approached the Sun over the course of six months in 1996-1997. Because the dust responds to gravity and light pressure, but plasmas also respond to the local solar wind (Coulomb collisions and magnetic fields), there is typically an angular separation between the two tails.

Motion of the near-nucleus shells of comet Hale-Bopp and rotation of its nucleus

Abstract270 images of the near-nucleus region of comet HB were obtained both in integral light (without filters) and using the narrow band interference IHW filters CO+C3C2, RC (red continuum). Prom the measurements of the expanding shells (arcs) the following result was obtained: the period of rotation of the comet nucleus is equal to 11.41±0.05 h; expansion velocities of the shells (arcs)vare in the limits of 0.61-1.99 kms−1, and accelerations of the shellsgin the limits from −18.3 × 10−3ms−2to 4.0 × 10−3ms−2.

The structure of thermal infrared radiation of the inner coma of comet Hale-Bopp

AbstractThe distribution of the thermal infrared radiation from dust in the inner coma of comet Hale-Bopp (1995 O1), based on observations obtained at ESO, LaSilla, Chile, has been mapped in the 10-/μm atmospheric window on July 16, 1997. A preliminary analysis of the infrared data taken with a resolution of 0.498 arcsec per pixel shows a heterogeneous distribution of the infrared radiation in the inner coma. Three asymmetric brightness peaks (in addition to the nucleus) are apparent. Similar behaviour of the distribution of the infrared brightness was found on all observed spectral bands centered around 8.39 μm, 9.78 μm, 11.65 μm, and 12.56 μm. The spectral energy distribution is compared to black-body curves. The observed distribution of the infrared brightness close to the nucleus could be interpreted by multiple sources of ejected dust particles. Simulated aperture photometry shows only minor deviation from a simple spherically symmetric outflow model.

Mosaicked Images and Spectra of [ITAL]J[/ITAL] = 1 → 0 HCN and HCO[TSUP]+[/TSUP] Emission from Comet Hale-Bopp (1995 O1)

We used the BIMA array to map comet Hale-Bopp (1995 O1) in HCN and HCO+ emission at 89 GHz with 10'' angular resolution. The mosaicked images with seven pointing centers were combined with single-dish data to form completely sampled images of a region ~4' in diameter. The HCN image is roughly compatible with a Haser model in which most of the HCN comes from the nucleus. However, clear deviations from circular symmetry are present, and roughly 20% of the HCN does not appear to come from the nuclear source. Frequently, the HCN has an asymmetric, double-peaked profile. The double peak is expected from the Haser model, and the asymmetry observed on April 3 could come from an increased gas production toward the Sun and facing the observer. The abundance of HCN relative to H2O is ~0.3%. The HCO+, in contrast, is smooth, almost completely resolved in the interferometer observations, and offset toward the ion tail by about 1'. This behavior is compatible with expectations from a simple scaling of previous work.

The Preperihelion Dust Environment of C/1995 O1 Hale-Bopp from 13 to 4 AU

Two UK Schmidt plates of comet Hale-Bopp dust tail taken in 1996 May are analyzed by means of the inverse dust tail model. The dust tail fits are the only available tools providing estimates of the ejection velocity, the dust-loss rate, and the size distribution of the dust grains ejected during years preceding the comet discovery. These quantities describe the comet dust environment driven by CO sublimation between 1993 and 1996, when the comet approached the Sun from 13 to 4 AU. The outputs of the model are consistent with the available coma photometry, quantified by the Afρ quantity. The dust mass loss rate increases from 500 to 8000 kg s-1, these values being inversely proportional to the dust albedo, assumed here to be 10%. Therefore, the mass ratio between icy grains and CO results is at least 5. Higher values of the dust-to-gas ratio are probable, because the model infers the dust-loss rate over a limited size range, up to 1 mm sized grains, and because the power-law index of the differential size distribution ranges between -3.5 and -4.0, so that most of the dust mass was ejected in the largest boulders that Hale-Bopp was able to eject. The dust ejection velocity close to the observations, between 7 and 4 AU, was close to 100 m s-1 for grains 10 μm in size, much higher than that predicted by R. F. Probstein's theory, thus confirming previous results of Neck-Line photometry. This result is an indicator of CO superheating with respect to a free sublimating CO ice, in agreement with the high observed CO velocity. The fundamental result of the paper is that such a high dust velocity remained constant between 13 and 4 AU, thus providing a strong constraint to all models of the CO-driven activity of the comet during its approach to the Sun: CO superheating must have been active since 13 AU from the Sun. It might be provided by the abundant dust itself, or by seasonal effects heating the subsurface layers, as was suggested for comet 29P/Schwassmann-Wachmann 1. Another similarity between the two comets is provided by the power-law index of the time-averaged size distributions: -3.6 ± 0.1 for C/1995O1 and -3.3 ± 0.3 for 29P/SW1. However, other characteristics of the dust environments are very different, so that, in general, it is impossible to distinguish a CO-driven comet from a typical water-driven one.

On the sodium tail of comet Hale‐Bopp (C/1995 O1)

Observations of the sodium tail of comet Hale‐Bopp (C/ 1995 O1) were made with the Midcourse Space Experiment (MSX) satellite near perihelion. Observations of the dust continuum, H2O+ emissions, and sodium tail are reported. The measurements show the Na tail to be diffuse and located between the dust and ion tails. The midrange spatial scale of the MSX measurements (104 to 106 km) is complementary to the high spatial resolution measurements of Brown et al. (submitted to Icarus, 1997) and large‐scale measurements of Cremonese et al. [1997], less than 104 km and greater than 3.5×106 km, respectively. The primary Na source appears to be from the inner coma and nucleus region. The observations are fit with a sodium point source model affected only by radiation pressure to obtain excellent agreement.

The Morphological Change of the Sodium Tail in Comet Hale-Bopp (C/1995 O1)

The narrow sodium tail was observed in comet Hale-Bopp in the middle of 1997 April and the beginning of 1997 May. However, it was not observed in the middle of 1997 March. Wilson and coworkers recently proposed the hypothesis that the nucleus source of the sodium atoms, which produced the sodium narrow tail, would have commenced (or increased) between 1997 March 17 and April 16. We reproduce the narrow sodium tail on March 17 and on April 19 using a Monte Carlo simulation based on our near-nucleus spectroscopic observations. Our result shows that the narrow sodium tail (which originated from the near-nucleus source) was weak and short on March 17, while it was strong and long on April 19. The sodium production rate increased by a factor of 2 between 1997 March 17 and April 19 according to our observations. However, the g-factor of sodium atoms in the sodium tail on April 19 is 1 order of magnitude larger than that on March 17. The morphological change of the narrow sodium tail was mainly caused by the dependence of the radiation pressure and g-factor of the sodium atoms on the heliocentric radial velocity of the atoms rather than by the change of the nucleus source activity.

BIMA and VLA Observations of Comet Hale-Bopp at 22–115 GHz

We observed comet Hale-Bopp with the Berkeley-Illinois-Maryland Association (BIMA) array at frequencies between 85 and 115 GHz and with the VLA at 43 and 22.5 GHz during 1997 March and April. The continuum radiation of the comet was easily detected in the 3 mm wavelength band, where a peak flux density of 12.8 ± 0.65 mJy beam-1 was obtained for all the data averaged together (scaled to a geocentric distance of 1.322 AU). The beam size for this observation was 119 × 88. The total flux density was 47 ± 4 mJy. We believe that only about 2 mJy can be attributed to thermal emission from the nucleus itself; the remaining emission is thermal emission from large (roughly millimeter-sized or greater ) grains in the comet's coma. Roughly 25% of the emission comes from an unresolved source at the center (i.e., an area with a radius of 4500 km), which is surrounded by a much larger area of low-level emission, visible in the BIMA images up to distances of at least ~25,000 km. Most of this emission can be reconciled with thermal emission from dust flowing away from the nucleus, although there is also clear evidence of distinct emission features ~25,000 km to the southeast and southwest of the nucleus, suggesting former outbursts and/or dust jets. The spectral index of the millimeter emission is consistent with a thermal blackbody spectrum, while a comparison with data at submillimeter wavelengths suggests a steepening in spectral index at shorter wavelengths. The total mass of the dust in the coma is at least a few times 1011 kg, and the dust production rate is of the order ~106 kg s-1.

Influence of Solar Photogravitation on the Motion of Comet Hale-Bopp

On the basis of photogravitational theory, the influence of solar photogravitation on the motion of comet Hale-Bopp is discussed. It is shown that for plasma tail of comet Hale-Bopp there is only one collinear point L3 and for a few of dust tail there can be two coplanar points L6, L7 and three collinear points L21, L22, L3. The regions in which the coplanar points exist and the dust grain of comet Hale-Bopp can move are also obtained.

Chemical processing in the coma as the source of cometary HNC.

The discovery of hydrogen isocyanide (HNC) in comet Hyakutake with an abundance (relative to hydrogen cyanide, HCN) similar to that seen in dense interstellar clouds raised the possibility that these molecules might be surviving interstellar material. The preservation of material from the Sun's parent molecular cloud would provide important constraints on the processes that took place in the protostellar nebula. But another possibility is that HNC is produced by photochemical processes in the coma, which means that its abundance could not be used as a direct constraint on conditions in the early Solar System. Here we show that the HNC/HCN ratio determined for comet Hale-Bopp varied with heliocentric distance in a way that matches the predictions of models of gas-phase chemical production of HNC in the coma, but cannot be explained if the HNC molecules were coming from the comet's nucleus. We conclude that HNC forms mainly by chemical reactions in the coma, and that such reactions need to be considered when attempting to deduce the composition of the nucleus from observations of the coma.

First images of a comet with adaptive optics

Using the ADONIS adaptive optics system at ESO (La Silla, Chile), on 8 March 1996, we made the very first observation of a comet with adaptive optics. Comet C/1996 B2 (Hyakutake) has been imaged in the near-infrared (J, H, K bands at 1.2, 1.6 & 2.2 μm respectively). The achieved spatial resolution of ≈ 0.2″, has allowed us to isolate two distinct dust-colour regions in the immediate neighbourhood of the cometary nucleus. On a [J-H] colour index map, a bluer region has been detected in the sunward-facing hemisphere, while a redder one is located symmetrically in the anti-solar direction. The major dust component in the coma remains silicates. These new observations should help to constrain the current models for dust components in comets. They clearly demonstrate the feasibility of observing comets with adaptive optics systems. Future observations of comet Hale-Bopp should greatly benefit from this new technique.

From Interstellar Dust to Comets: Infrared Emission from Comet Hale-Bopp (C/1995 O1)

The 3-20 μm IR thermal emission spectrum of comet Hale-Bopp (C/1995 O1) at a heliocentric distance of rh=1.15 AU has been calculated using fluffy aggregates of silicate core-organic refractory mantle interstellar dust as the comet dust model. The principal dust properties, similar to those obtained for other comets, provide good fits to the observation. Our results are significantly different from the general opinion that Hale-Bopp is rich in submicron grains. The mean grain masses derived here are 2-3 orders of magnitude higher than those estimated from the empirical superheat method or from the IR emission modeling, both of which use compact silicate/carbon particles. It has been shown that the IR emission alone cannot give a reliable dust production rate. We have also modeled the crystalline silicate emission spectrum.

HCO+ imaging of comet Hale-Bopp (C/1995 O1).

The HCO+ J = 1-0 rotational transition at 89.189 GHz has been mapped in comet Hale-Bopp (C/1995 O1) over a total of 38 individual days spanning the period 1997 March 10-June 20 with the Five College Radio Astronomy Observatory 14 m antenna. HCO+ is detectable over an extended region of the comet, with the peak emission commonly located 50,000-100,000 km in the antisolar direction. Maps made throughout the apparition show significant variability in the structure of the HCO+ coma, sometimes on timescales of several hours. The HCO+ brightness is usually depressed at the nucleus position, and on some occasions, the emission is spread into a ring around the position of the nucleus. Individual spectra within the maps display broad (approximately 4 km s-1) lines redshifted by 1-2 km s-1 or more from the nominal velocity of the nucleus, with the redshift typically increasing in the antisolar direction. The spectra and maps may be generally explained by models in which the ions are accelerated tailward at a rate on the order of 10 cm s-2, provided that HCO+ is destroyed within 50,000-100,000 km of the nucleus.

Observation of an explosion in Comet Hale-Bopp

Using the Shanghai Observatory 1.56-m telescope and a Series 200 CCD camera we discovered an explosion on Comet Hale-Bopp between 1996-04-21 and 1996-04-23. On 22, the magnitude was 0.2 mag brighter than on 21, and it was more or less back to the previous brightness on the next day. The half-intensity diameter of the head showed corresponding variations.

Can the Sodium Tail of Comet Hale-Bopp Have a Dust-Impact Origin?

Consideration is given to the possible mechanisms responsible for the production of the atomic sodium tail of comet Hale-Bopp. It is shown that both photosputtering and ion sputtering of nonvolatile dust grains might not be the main driving force. Instead, the generation of impact vapor by collisional interaction between the cometary dust coma of micron-sized particles and the very small grains (VSGs) of 10-100 Å size could possibly account for the observations if a large amount of VSGs existed in the coma region.

OH Observations of Comet Hale-Bopp at 1.667 GH[CLC]z[/CLC] and Maser Amplification of a Background Source by the Comet

OH spectra of comet Hale-Bopp were obtained at 1.667 GHz between 1997 February 19 and June 1 using the 26 m parabolic antenna of the Dominion Radio Astrophysical Observatory. Spectra showed strong (2 Jy) emission in mid-February. The comet's flux density then decreased, until it was barely detectable by March 11. An absorption line dominated the spectrum for several days in March. Flux density remained low for most of the subsequent observations. At the end of April a strong emission line appeared close to the expected cometary velocity. It is argued that this line did not arise in the comet itself, but came from a background source, probably IRAS 04361+2547, whose radiation had undergone maser amplification in the coma of the comet.

On the Outgassing Profile of Comet Hale‐Bopp

When Comet Hale-Bopp was first discovered at a heliocentric distance of ~7.2 AU, its activity appeared to be controlled by the outgassing of the highly volatile CO molecule. Its gas production remained CO-driven until about 4 AU when the outgassing became controlled by the less volatile H2O molecule. While the outgassing around this distance is consistent with the sublimation from a large dirty snowball composed dominantly of H2O, the subsequent outgassing at smaller heliocentric distance up to perihelion (d ≈ 0.914 AU) falls consistently below the predictions of such a model. In this paper, we use the earliest chemical differentiation model of the cometary nucleus developed by Houpis et al., with a few modifications, to show that the continuous chemical differentiation of a deepening layer of the cometary nucleus which has made multiple passages into the inner solar system can explain comet Hale-Bopp's observed pre-perihelion production rate curve. It also predicts a hysteresis with the gas production rate profiles falling systematically lower post-perihelion. The model suggests that comet Hale-Bopp is a complex combination of dust, clathrate, and volatile (mainly CO), the gas production of which changes from being CO-driven to H2O driven around 4 AU. Furthermore, the thermal insulation provided by a growing dust mantle inside ~3.5 AU causes the H2O production rate curve to flatten-out.