Bright Comets Research Articles

The NH and CH Bands of Comet C/1996 B2 (Hyakutake)

Comet Hyakutake was an ideal target for studying the near-nuclear region of a bright comet. On 1996 March 26.3–26.5 (UT) we observed the comet with an echelle spectrometer at the 4-m telescope atop Kitt Peak. High-resolution spectra of NH and CH are presented and compared with fluorescence models. At a spectral resolution of Δλ ≤ 0.2 Å two multiplets of ND are not blended by the overlying NH line forest. From these lines we derived a 3σ upper limit for the D/H ratio in NH, (D/H)NH≤ 0.006. We reevaluated theg-factors for the NH (0–0) band and presentg-factors for the strongest ND multiplets as a function of heliocentric velocity. Based on our measurements of the OH and NH (0–0) band we inferred a relative production rateQ(NH3)/Q(H2O) = (3.5 ± 1.0) × 10−3. We demonstrate that single-cycle fluorescence models can be used to describe the observed CH bands, if we introduce different rotational temperatures for the two spin states,T(F1) = 100 K andT(F2) = 130 K. These temperatures are believed to be remnants from a previous dissociative excitation. Obviously collisions or radiative processes cannot completely relax CH before the radical decays. The rotational temperature may thus provide clues to the identity of the CH parent(s). A theoretical analysis of CD suggests that several lines of the CDA–Xsystem are not masked by CH lines.

Excitation of X rays in comet Hyakutake (C/1996 B2)

There is great disagreement between current evaluations of the role of bremsstrahlung and line excitation by electron impact and recombination in cometary X rays. To study the problem in detail, we use the exact values of the Roentgen satellite (ROSAT) high‐resolution imager and wide‐field camera instrument response functions and apply them to all nine exposure intervals in the ROSAT observation of comet Hyakutake to determine X ray two‐parameter power law and thermal bremsstrahlung spectra. Thermal bremsstrahlung is a convenient form for two‐parameter spectrum and not a physical reality in comets. Then spectral index is α = 1.97 ± 0.18 ≈ 2, spectral temperature is kT = 360 eV, and the exposure‐mean photon emission is 3 × 1025 ph s−1 and 1.3 × 1016 ergs s−1 in a range of 90–2000 eV and in an aperture radius of 1.2 × 105 km. The exposure‐mean ROSAT spectrum is in excellent agreement with the Extreme Ultarviolet Explorer (EUVE) observation of 7.5 × 1024 ph s−1 in a range of 100–165 eV from comet Hyakutake. Then we follow the same methods as Northrop et al. [1997] to calculate a volume emission rate at the comet brightness maximum and to calculate that from bremsstrahlung with the Vega 2 electron flux. Yet we obtain a much lower emission than Northrop et al., at a level of 0.5% of the observed emission in accordance with results of Krasnopolsky [1996, 1997a] and Bingham et al. [1997]. Electron impacts of molecular, atomic, and ion species in comets produce mostly excitation of the X ray lines C 277 eV and O 525 eV at a level of 2% of the total X ray emission from comet Hyakutake. This value is calculated for the Vega 2 electron flux. The value for the Giotto electron flux is 3 × 10−6 of the measured emission. Processes which are responsible for impurity radiation in fusion plasma either are inefficient or produce ultraviolet photons in cometary plasma, and the method of Bingham et al. is inapplicable to excitation of X ray lines in comets. The calculations show that the expected effects of bremsstrahlung and electron impact are very low in cometary X rays. The obtained spectral index α = 2 differs significantly from α ∼ 5 for scattering of solar X rays by attogram dust. These facts favor charge transfer of heavy solar wind ions as the dominant process of X ray excitation in comet Hyakutake. Charge transfer is not efficient in production of X rays on the Moon and other solid bodies.

Chemistry in cometary comae.

Significant gas-phase chemistry occurs in the comae of bright comets, as is demonstrated here for the case of Comet Hale-Bopp. The abundance ratio of the two isomers, hydrogen cyanide and hydrogen isocyanide, is shown to vary with heliocentric distance in a way that is consistent with production of HNC by ion-molecule chemistry initiated by the photoionization of water. Likewise, the first maps of emission from HCO+ show an abundance and an extended distribution that are consistent with the same chemical model.

Radar detection of the nucleus and coma of comet hyakutake

Radar observations of comet Hyakutake (C/1996 B2) made at the Goldstone Deep Space Communications Complex in California have detected echoes from the nucleus and from large grains in the inner coma. The nucleus of this bright comet was estimated to be only 2 to 3 kilometers in diameter. Models of the coma echo indicate backscatter from porous, centimeter-size grains ejected anisotropically at velocities of tens of meters per second. The radar observations suggest that a comet's activity may be a poor indicator of its size and provide evidence that large grains constitute an important component of the mass loss from a typical active comet.

The Spatial Distribution of Gaseous Atomic Sodium in the Comae of Comets: Evidence for Direct Nucleus and Extended Plasma Sources

Sodium D-line emission (5890 and 5896 Å) has been observed in bright comets at small to moderate heliocentric distances for many years. We present here the first in depth study of a set of spatial profiles of the sodium D-line emission constructed from long-slit spectroscopic observations of Comets Bennett C/1969 Y1, Kohoutek C/1973 D1, and 1P/Halley. Preliminary analysis of these data lead to the suggestion by Combiet al.(1996,A Plasmagenic Source for Gaseous Sodium in Comets.Presented at Asteroids, Comets, Meteors) that a major fraction of the gaseous sodium was produced by an extended source in the tail and that the source was likely to be some charged species. Dissociative recombination of a molecular ion was suggested. The spatial profiles of sodium are not like typical neutral species. The inner region from the nucleus (<2 × 104km) can be explained in terms of a model that accounts for collisional entrainment in the expanding coma and the heliocentric velocity dependent fluorescence rate and radiation pressure acceleration. This source comes either directly from the nucleus or has a very short-lived parent (⪡103s). Away from the nucleus, down the tail and to the sides, the spatial profile slope flattens, indicating a second extended source. The striking similarity of the extended region of sodium spatial profiles with those of ions (H2O+), both along and perpendicular to the tail, is highly suggestive that an ion source is responsible for the production of the extended component of gaseous sodium in the coma. The production rate of the highly variable extended source when present is four to five times that of the direct nucleus source. Observations (Schneideret al., 1991,Science253,1394–1397) and quantitative model analyses (Wilson and Schneider, 1994,Icarus111,31–34) have shown that a dissociative recombination of a sodium bearing molecular ion (NaX+) produces a peculiar component of the neutral sodium near Io. It displays a variable spatial morphology consistent with that of a molecular ion source “picked-up” in the plasma torus corotating with Jupiter's magnetic field. The rapid onset of the appearance of gaseous sodium in bright comets, its spatial distribution in the extended coma and near tail, and recent observations of sodium tails are all consistent with our original suggestion of this plasma source for sodium in comets.

Neutral Sodium from Comet Hale-Bopp: A Third Type of Tail

We report on the discovery and analysis of a striking neutral sodium gas tail associated with comet C/1995 O1 Hale-Bopp. Sodium D line emission has been observed at heliocentric distance r≤1.4 AU in some long-period comets, and the presence of neutral sodium in the tailward direction of a few bright comets has been noted, but the extent, and in particular the source, has never been clear. Here we describe the first observations and analysis of a neutral sodium gas tail in comet Hale-Bopp, which is entirely different from the previously known ion and dust tails. We show that the observed characteristics of this third type of tail are consistent with it being produced by radiation pressure due to resonance fluorescence of sodium atoms and that the lifetime for photoionization is consistent with recent theoretical calculations.

Measurement of Submicron Grains in the Coma of Comet Hale-Bopp C/1995 O1 during 1997 February 15–20 UT

We present 1.20-18.5 μm infrared (IR) spectrophotometric measurements of comet C/1995 O1 (Hale-Bopp) during 1997 February 15 and 20 UT. The spectral energy distribution (SED) was dominated by scattering and thermal emission from submicron sized dust grains that were unusually small. 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=Tcolor/TBB ≈ 1.84, the peak of the 10 μm silicate emission feature was 1.81 mag above the carbon grain continuum, and the albedo (reflectivity) of the grains was ≈ 0.41 at a scattering angle of θ ≈ 144°. These are the highest values for these empirical parameters ever observed in 20 years of optical/IR measurements of bright comets. The observations indicate that the optically important grains dominating the visual scattering and near-IR emission from the coma had an average radius of a≤0.4 μm. The strong silicate feature is produced by grains with a similar size range. These dust radii are comparable to the radii of the grains that condense in the outflows of some novae ("stardust") but still about 10 times larger than the average radius of the grains that produce the general interstellar extinction.

Brightness of long-period comets observed at large heliocentric distances—I. The computer catalogue for 1861–1941

A catalogue of about 3400 magnitude values for 26 long-period comets observed at heliocentric distances greater than 2.5 AU can be obtained from the author. The list contains the dates of observations, the apparent brightness of comets and the types of magnitudes, the methods, the diameters of objectives and the light-gathering powers of the telescopes used, the types of instruments, the names of observers and references to original literature as well as the calculated values of heliocentric and geocentric distances and phase angles for the dates of observations.

Sulfur Chemistry at Millimeter Wavelengths in C/Hale-Bopp

The recent availability of bright comets has given us an excellent opportunity to study cometary chemistry. Comet Hale-Bopp (1995 O1)gave us the particularly rare opportunity to study a bright and active comet for almost two years. Our program concentrated on millimeter-wave observations of sulfur-bearing molecules in an effort to understand the total sulfur budget of the comet. Using the National Radio Astronomy Observatory 12-m telescope on Kitt Peak we monitored both the long and short-term variations in H2S, CS, and OCS, as well as observing H2CS and SO. This was the first observation of H2CS in any comet (Figure 1). Additionally, we mapped CS with the BIMA interferometer. Variations in the line profiles and changes in line intensity as large as a factor of two were seen in day to day observations of both H2S and CS. An example for H2S is shown in Figure 2. This is the first time we can attempt to study the entire group of sulfur-bearing molecules. Models of the sulfur coma have thus far largely been based on observations of the daughter products CS and atomic sulfur made over the last 18 years using the International Ultraviolet Explorer (IUE) satellite, coupled with radio observations of CS and H2S in several recent comets. Four new sulfur-bearing species have been observed in comets Hale-Bopp and Hyaku take, three of them parent species. The high resolution maps in CS will also allow spatial information to be included in the sulfur model for the first time. C/Hale-Bopp is the first comet in which so many sulfur species have been observed. Analysis of the abundances of these species in comparison to the total atomic sulfur observed should reveal whether or not we can now account for all of the primary sulfur sources in comets. Perhaps the most interesting question that these observations raised was why C/Hale-Bopp appeared to contain so much more SO and SO2 (as observed by others) than any other comet. This spurred the discovery that the UV fluorescence models of these species were incorrect (S. J. Kim, this issue). Analysis of the data and modeling of the sulfur budget are still underway.

Modelling Gas And Dust Release From Comet Hale–Bopp

Numerical simulations of the evolving activity of comet Hale-Bopp are presented, assuming a porous, spherical nucleus, 20 km in radius, made of dust and gas-laden amorphous ice. The main effects included are: crystallization of amorphous ice and release of occluded gas, condensation, sublimation and flow of gases through the pores, changing pore sizes, and flow of dust grains. The model parameters, such as initial pore size and porosity, emissivity, dust grain size, are varied in order to match the observed activity. In all cases, a sharp rise in the activity of the nucleus occurs at a large heliocentric distance pre-perihelion, marked by a few orders of magnitude increase in the CO and the CO2 fluxes and in the rate of dust emission. This is due to the onset of crystallization, advancing down to a few meters below the surface, accompanied by release of the trapped gases. A period of sustained, but variable, activity ensues. The emission of water molecules is found to surpass that of CO at a heliocentric distance of 3 AU. Thereafter the activity is largely determined by the behaviour of the dust. If a dust mantle is allowed to build up, the water production rate does not increase dramatically towards perihelion; if most of the dust is ejected, the surface activity increases rapidly, producing a very bright comet.

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.

Cometary volatiles

Comets are believed to be the most pristine material surviving from the origin of the Solar System. The nature of their volatiles should in principle provide chemical and physical clues to their formation environment in the solar nebula. Another important aspect of the study of comets is the extent to which they may incorporate relatively unprocessed interstellar material.Our knowledge of the composition of cometary ices has been greatly improved in this last decade. The years 1995-1996 were rich in new results, with the close approach to Earth of comet C/1996 B2 Hyakutake and the discovery of the bright comet C/1995 O1 Hale-Bopp at large heliocentric distance. We will present the current knowledge of the nature and abundances of cometary volatiles and mention some of the outstanding issues related to the origin of comets and their links to interstellar material.

C/1995 O1 (Hale-Bopp): Gas Production Curves And Their Interpretation

The discovery of C/1995 O1 (Hale-Bopp) at 7 AU from the Sun provided the first opportunity to follow the activity of a bright comet over a large range of heliocentric distances rh. Production rates of a number of parent molecules and daughter species have been monitored both pre- and postperihelion. CO was found to be the major driver of the activity far from the Sun, surpassed by water within 3 AU whose production rate reached 1031 s−1 at perihelion. Gas production curves obtained for various species show several behaviours with rh. Gas production curves contain important information concerning the physical state of cometary ices, the structure of the nucleus and all the processes taking place inside the nucleus leading to outgassing. They are relevant to the study of several other phenomena such as the sublimation from icy grains, dust mantling or seasonal effects. For some species, such as H2CO or HNC, they permit to constrain their origin in the coma. We discuss models of subsurface gas production in distant comets and predictions of how such a source may vary as the comet moves along its orbit, approaching perihelion and receding again. Features in the observed gas production curves of comet Hale-Bopp are generally interpretable in terms of either subsurface production (typical example: CO at large rh) or free sublimation (typical example: H2O). Possible implications for the vertical stratification of the cometary ices are reviewed, and preference is found for a model with crystallization of amorphous ice close to the nuclear surface.

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.

Substantial outgassing of CO from comet Hale-Bopp at large heliocentric distance.

When comet C/1995 O1 (Hale-Boop) was discovered, at a distance of seven astronomical units from the sun, it was more than one hundred times brighter than comet Halley at the same distance. A comet's brightness is derived from the reflection of sunlight from dust grains driven away from the nucleus by the sublimation of volatile ices. Near the sun, sublimation of water ice (a main constituent of comet nuclei) is the source of cometary activity; but at its current heliocentric distance, Hale-Boop is too cold for this process to operate. Other comets have shown activity at large distances, and in the case of comet Schwassmann-Wachmann 1, carbon monoxide has been detected in quantities sufficient to generate its observed coma. Here we report the detection of CO emission from Hale-Boop, at levels indicating a very large rate of outgassing. Several other volatile species were searched for, but not detected. Sublimation of CO therefore appears to be responsible for the present activity of this comet, and we anticipate that future observations will reveal the onset of sublimation of other volatile species as the comet continues its present journey towards the sun.

Model of the comet outbursts based on a fragmentation of ice grains in its atmosphere

Outbursts of the comet's brightness play a significant role in the study of the physical nature and evolution of comets. The causes of cometary outbursts are still not properly understood. It is known, however, that there is correlation between the outbursts and the high-velocity flux of solar wind. The evidences of such causal relationship are the following: the existence of the correlation between the geomagnetic perturbations and the outburst activity of the comets [1]; two-peaks distribution of the comet's outbursts, which depends on the 11-year solar cycle phase with maxima at phases 0.2–0.3 and 0.7–0.8, in coincidence with the distribution of physical characteristics of high-velocity fluxes [2]; the repeated character of the outbursts with main intervals of 7–8, 14–15, 22–24 and 30 days, this corresponds to the four-sector structure of the interplanetary magnetic field [3].

On the brightness of comet P/Halley in the past

This paper deals with the brightness changes of come P/Halley in the past. On the basis of the analysis of Sun-Earth-comet configuration at the time of perihelion passage within 30 apparitions of the comet since 240 BC, several groups of apparitions of similar observing geometry were specified. These groups were analysed with emphasis on brightness variations of the comet on different, long time scales. Absolute magnitudes and duration of naked-eye visibility both in pre-perihelion and post-perihelion periods of the comet are the main criteria for the aim of this study. Some brightness increase of the comet in 912–1222 is indicated.

Statistical trends in cometary magnitude data

The brightness distributions of long-period comets with P > 200 years, and those short-period comets with P < 15 years (Jupiter family of comets) were modelled by Pareto power-law distributions. The models were fitted using maximum likelihood methods and analysed by a new trend analysis approach. The long-period comets were divided into a number of physical parameter ranges, notably by perihelion, inverse semi-major axis and discovery date, and examined for appropriate physical trends. Similarly, the short-period comets were examined for trends and compared with the long-period comets. Overwhelming evidence was found that the brightness indices increase with discovery date for the long-period comets. There was no statistical evidence that the brightness distribution of the long-period comets significantly altered as the 1/ a values decreased. No trends were apparent in the brightness of long-period comets in terms of their perihelion distance for perihelia less than 2.5 a.u.

The frequency of appearance of bright naked eye comets since 1750

It is a typical lament of the astronomers of today that there has been a dearth of bright comets in the 20th century and, particularly in its latter half. Many go on to point out the large number of very bright comets that were observed in the 18th and 19th century to reinforce the unfavourable comparison with modern times. Analysis of bright naked eye comet apparitions since 1750 demonstrates that there was a superavit of very bright comets in the second half of the last century. The second half of the twentieth century has shown exactly the average of one bright naked eye comet per 2.5 years that has been seen over two and a half centuries. The probable cause of theapparent lack of bright comets is the great increase in light pollution and the shift of population to the big cities. The observed statistics for the appearance of bright comets show no obvious evidence of perturbations of the Kuiper Belt by Planet X.

The Velocity Distribution of Cometary Hydrogen: Evidence for High Velocities?

We present high-velocity and spatial resolution two-dimensional spectra of Hα 6563 Å emission in comets Austin (1989c 1) and Levy (1990c) obtained using the Hamilton Echelle spectrograph on the 3-m Shane telescope at Lick Observatory. Because of the high signal-to-noise of these spectra, achieved through long integrations with long-slit spectroscopy on these relatively bright comets, we can directly observe the hydrogen velocity distribution. The hydrogen velocity distributions in both comets confirm the presence of the components expected from water dissociation and collisional thermalization in the inner coma. In addition, the velocity distribution in comet Austin apparently has a spatially symmetric high-velocity component not predicted by current theory, although the ubiquitous emissions from a unidentified cometary molecule could coincidentally mimic the signature of this fast hydrogen. Comet Levy also shows high-velocity hydrogen, but, unlike the spatially symmetric emission in Austin, the fast hydrogen in Levy is located exclusively on the sunward side of the nucleus. The cause of the Levy fast hydrogen is unknown, but the source could be charge exchange with the solar wind related to the variability and sunward dust ejection observed in that comet by IUE and HST.