共查询到20条相似文献,搜索用时 171 毫秒
1.
Hanner M. S. Gehrz R. D. Harker D. E. Hayward T. L. Lynch D. K. Mason C. C. Russell R. W. Williams D. M. Wooden D. H. Woodward C. E. 《Earth, Moon, and Planets》1997,79(1-3):247-264
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. 相似文献
2.
We present results of polarimetric and photometric observations of bright comet C/1995 O1 (Hale-Bopp) obtained at the 0.7
m telescope of Kharkov University Observatory from June 18, 1996 to April 24, 1997. The IHW and HB comet filters were used.
The C2 and C3 production rates for Hale-Bopp are more than one order of magnitude larger and the dust production rates are more than two
orders of magnitude larger than the Halley ones at comparable distances. Hence, Hale-Bopp was one of the most dusty comets.
The average UC-BC and BC-RC colours of the dust were −0.02 and 0.13 mag, respectively. The polarization of comet Hale-Bopp
at small phase angles of 4.8–13.0° was in good agreement with the date for comet P1/Halley at the same phase angles in spite
of the fact that the heliocentric distances of comments differed nearly twice. However, at intermediate phase angles of 34–49°
the polarization of comet Hale-Bopp was significantly larger than the polarization of the other dusty comets. It is the first
case of such a large difference found in the continuum polarization of comets. The wavelength dependence of polarization for
Hale-Bopp was steeper than for other dusty comets. The observed degree of polarization for the anti-sunward side of the coma
was permanently higher than that for the sunward shell side. The polarization phase dependence of Hale-Bopp is discussed and
compared with the polarization curves for other dusty comets. The peculiar polarimetric properties of comet Hale-Bopp are
most likely caused by an over-abundance of small or/and absorbing dust particles in the coma.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
3.
Pittichovâ J. Sekenina Z. Birkle K. Boehnhardt J. Engels D. Keller P. 《Earth, Moon, and Planets》1997,78(1-3):329-338
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 11h21m, 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 βp ≃ 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. 相似文献
4.
The hypothesis on the genetic connection of near-parabolic comets with Jupiter, Saturn, and the transPlutonian region (5–3000
AU) proposed by E.M. Drobyshevskii is considered. It has been shown that, on average, 5.6 comets per an area of 106 AU2 passed through the transPlutonian region during the whole history of observations. Six-hundred nineteen comets crossed the
ecliptic at heliocentric distances ranging from 0 to 2 AU. As has been shown, from the total number of 945 near-parabolic
comets, eight comets closely approached Jupiter and five closely approached Saturn. The Kreutz comets, 1277 objects, did not
approach Jupiter closer than 3 AU. Their minimal distance to Saturn was 5.5 AU. The minimal distance of the Kreutz comets
from the edge of the transPlutonian region was 28.8 AU. The analysis led to the conclusion that the concept on the origin
of the near-parabolic comets suggested by Drobyshevskii is groundless. 相似文献
5.
Peschke S. B. Grün E. Böhnhardt H. Campins H. Osip D. J. Hanner M. S. Heinrichsen I. Knacke R. F. Leinert Ch. Lemke D. Stickel M. Lisse C. M. Sykes M. Zarnecki J. 《Earth, Moon, and Planets》1997,78(1-3):299-304
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. 相似文献
6.
Lisse C. M. Dennerl K. Englhauser J. Trümper J. Marshall F. E. Petre R. Valinia A. Kellett B. J. Bingham R. 《Earth, Moon, and Planets》1997,77(3):283-291
The discovery of X-ray emission from comets has created a number of questions about the physical mechanism producing the radiation.
There are now a variety of explanations for the emission, from thermal bremsstrahlung of electrons off neutrals or dust, to
charge exchange induced emission from solar wind ions, to scattering of solar X-rays from attogram dust, to reconnection of
solar magnetic field lines. In an effort to understand this new phenomenon, we observed but failed to detect in the X-ray
the very dusty and active comet C/Hale-Bopp 1995 O1 over a two year period, September 1996 to December 1997, using the ROSAT
HRI imaging photometer at 0.1–2.0 keV and the ASCA SIS imaging spectrometer at 0.5–10.0 keV. The results of our Hale-Bopp
non-detections, when combined with spectroscopic imaging 0.08–1.0 keV observations of the comet by EUVE and BeppoSAX, show
that the emission has the same spectral shape and strong variability seen in other comets. Comparison of the ROSAT photometry
of the comet to our ROSAT database of 8 comets strongly suggests that the overall X-ray faintness of the comet was due to
an emission mechanism coupled to gas, and not dust, in the comet’s coma.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
7.
We consider the connection with Uranus for: (1) 945 near-parabolic comets (the period P > 200 years, the perihelion distance q > 0.1 AU), (2) 1277 Kreutz comets (P > 200 years, q < 0.01 AU), and (3) 414 short-period comets (P < 200 years). It turns out that none of near-parabolic comets passed through Uranus’s activity sphere, none of the Kreutz
comets approach Uranus closer than 11 AU, and only two short-period comets, C/2006 U7 and C/2006 F2, could have a close approach
to Uranus during 5000 years. 相似文献
8.
Wooden Diane H. Harker David E. Woodward Charles E. Koike Chiyoe Butner Harold M 《Earth, Moon, and Planets》1997,78(1-3):285-291
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 rh ≲ 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. 相似文献
9.
The wavelength dependence of the polarization (“polarization spectra”) of cometary dust is discussed. It is shown that, in the case of large phase angles, the wavelength dependence of the polarization is mainly controlled by the complex refractive index of the particle material, whereas the spectral dependence of the intensity is also sensitive to the size of the particles. This suggests that observations of “polarization spectra” may determine the composition of cometary dust. An attempt is made to find the composition of the cometary dust material by comparing the observed polarimetric data with laboratory measurements of complex refractive indices of possible cometary constituents. Silicates, graphite, metals, organics, water ice and their mixtures are considered. It is shown that astronomical silicate must be the most abundant constituent of cometary dust in the range of heliocentric distances from 0.8 to 1.8 AU, whereas the volume fraction of pure graphite or pure metals is less then 1%. A substance similar to that of F-type asteroids may be present in comets. There is evidence for an organic material that is being destroyed between heliocentric distances of 0.8–1.8 AU. 相似文献
10.
We present a light curve of C/1995 O1 (Hale-Bopp) compiledfrom more than 3000 visual observations of the comet made by members
of the The Astronomer Group world-wide. These observations cover the period from discovery through to the end of 1997. The
light curve shows that the rate of brightening of the comet varied widely at different times, with rapid rates of brightening
at high heliocentric distance pre-perhelion and a comparably rapid post-perihelion fade. There is no evidence that the comet
was suffering a large photometric outburst when first discovered, although a small outburst can be identified at perihelion.
At least five difficult brightening regimes can be identified in the light curve between discovery and perihelion. From 2.5
AU to perihelion the rate of brightening with decreasing heliocentric distance was typical for “fairly” new comets(n ∼ 3.5,
where “n” is the power law exponent of the heliocentric distance), although this was preceded by a period of very slow brightening
with n ∼ 1 from r ∼ 4.0 AU to r ∼ 2.8 AU and followed by an initially more rapid brightening which appears to be related to
the on-set of rapid water sublimation activity. We derive the light curve parameters at different stages of the comet's apparition.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
11.
Canaves Marcus V. De Almeida Amaury A. Boice Daniel C. Sanzovo Gilberto C. 《Earth, Moon, and Planets》2002,90(1-4):335-347
The chemistry of both nitrogen and sulfur presents interesting problems in comets.In this paper, we use a model of cometary
comae with gas-phase chemical kineticsand gas dynamics to predict molecular abundances in the inner coma region for twoof
the brightest comets in the past 20 years, Hyakutake (C/1996 B2) and Hale–Bopp(C/1995 O1). In this progress report we concentrate
on the gas-phase chemistry of thenitrogen sulfide (NS) radical at a heliocentric distance of 1 AU to study the abundanceof
NS using a detailed photo and chemical reaction network with over 100 species andabout 1000 reactions. The results are compared
with recent observations of CometHale–Bopp and reveal that conventional gas-phase reactions schemes do not produceNS in sufficient
quantities to explain the observations. We plan to continue therefinement of the model to improve agreement with observational
constraints. 相似文献
12.
G. Tancredi 《Celestial Mechanics and Dynamical Astronomy》1998,70(3):181-200
The dynamics of two families of minor inner solar system bodies that suffer frequent close encounters with the planets is
analyzed. These families are: Jupiter family comets (JF comets) and Near Earth Asteroids (NEAs).
The motion of these objects has been considered to be chaotic in a short time scale,and the close encounters are supposed
to be the cause of the fast chaos. For a better understanding of the chaotic behavior we have computed Lyapunov Characteristic
Exponents (LCEs) for all the observed members of both populations. LCEs are a quantitative measure of the exponential divergence
of initially close orbits. We have observed that most members of the two families show a concentration of Lyapunov times (inverse
of LCE) around 50–100yr. The concentration is more pronounced for JF comets than for NEAs, among which a lesser spread is
observed for those that actually cross the Earth's orbit (mean perihelion distance q < 1.05 AU). It is also observed that
a general correspondence exists between Lyapunov times and the time between consecutive encounters.
A simple model is introduced to describe the basic characteristics of the dynamical evolution. This model considers an impulsive
approach, where the particles evolve unperturbedly between encounters and suffer ‘kicks’ in semimajor axis at the encounters.
It also reproduces successfully the short Lyapunov times observed in the numerical integrations and is able to estimate the
dynamical lifetimes of comets during a stay in the Jupiter family in correspondence with previous estimates.
It has been demonstrated with the model that the encounters with the largest effect on the exponential growth of the distance
between initially nearby orbits are neither the infrequent deep encounters, nor the frequent and far ones; instead, the intermediate
approaches have the most relevant contribution to the error growth. Such encounters are at a distance a few times the radius
of the Hill's sphere of the planet (e.g. 3).
An even simpler model allows us to get analytical estimates of the Lyapunov times in good agreement with the values coming
from the model above and the numerical integrations.
The predictability of the medium‐term evolution and the hazard posed to the Earth by those objects are analysed in the Discussion
section.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
13.
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. 相似文献
14.
《Icarus》1987,70(2):269-288
We simulate the Oort comet cloud to study the rate and properties of new comets and the intensity and frequency of comet showers. An ensemble of ∼106 comets is perturbed at random times by a population of main sequence stars and white dwarfs that is described by the Bahcall-Soneira Galaxy model. A cloning procedure allows us to model a large ensemble of comets efficiently, without wasting computer time following a large number of low eccentricity orbits. For comets at semimajor axis a = 20,000 AU, about every 100 myr a star with mass in the range 1M⊙−2M⊙ passes within ∼10,000 AU of the Sun and triggers a shower that enhances the flux of new comets by more than a factor of 10. The time-integrated flux is dominated by the showers for comets with semimajor axes less than ∼30,000 AU. For semimajor axes greater than ∼30,000 AU the comet loss rate is roughly constant and strong showers do not occur. In some of our simulations, comets are also perturbed by the Galactic tidal field. The inclusion of tidal effects increases the loss rate of comets with semimajor axes between 10,000 and 20,000 AU by about a factor of 4. Thus the Galactic tide, rather than individual stellar perturbations, is the dominant mechanism which drives the evolution of the Oort cloud. 相似文献
15.
We present a purely physical model to determine cosmogenic production rates for noble gases and radionuclides in micrometeorites (MMs) and interplanetary dust particles (IDPs) by solar cosmic‐rays (SCR) and galactic cosmic‐rays (GCR) fully considering recoil loss effects. Our model is based on various nuclear model codes to calculate recoil cross sections, recoil ranges, and finally the percentages of the cosmogenic nuclides that are lost as a function of grain size, chemical composition of the grain, and the spectral distribution of the projectiles. The main advantage of our new model compared with earlier approaches is that we consider the entire SCR particle spectrum up to 240 MeV and not only single energy points. Recoil losses for GCR‐produced nuclides are assumed to be equal to recoil losses for SCR‐produced nuclides. Combining the model predictions with Poynting‐Robertson orbital lifetimes, we calculate cosmic‐ray exposure ages for recently studied MMs, cosmic spherules, and IDPs. The ages for MMs and the cosmic‐spherule are in the range <2.2–233 Ma, which corresponds, according to the Poynting‐Robertson drag, to orbital distances in the range 4.0–34 AU. For two IDPs, we determine exposure ages of longer than 900 Ma, which corresponds to orbital distances larger than 150 AU. The orbital distance in the range 4–6 AU for one MM and the cosmic spherule indicate an origin either in the asteroid belt or release from comets coming either from the Kuiper Belt or the Oort Cloud. Three of the studied MMs have orbital distances in the range 23–34 AU, clearly indicating a cometary origin, either from short‐period comets from the Kuiper Belt or from the Oort Cloud. The two IDPs have orbital distances of more than 150 AU, indicating an origin from Oort Cloud comets. 相似文献
16.
The hypothesis suggested by Guliev on transneptunian objects as sources of comets is considered. The motion equations for
137 near-parabolic comets over a time interval of 5000 years were numerically integrated. No interaction between 78 comets
and the transneptunian object 2003 UB 313 has been revealed: the comets were farther than 3 AU from it. It has been shown
that 59 comets—candidates to Pluto’s family-approached Pluto at a distance larger than 0.7 AU. The fulfilled analysis allows
us to conclude that Guliev’s hypothesis on transneptunian objects as sources of comets is groundless. 相似文献
17.
John J. Matese Patrick G. Whitman Daniel P. Whitmire 《Celestial Mechanics and Dynamical Astronomy》1997,69(1-2):77-87
We investigate the distribution of Oort cloud comet perihelia. The data considered includes comets having orbital elements
of the two highest quality classes with original energies designated as new or young. Perihelion directions are determined
in galactic, ecliptic and geocentric equatorial coordinates. Asymmetries are detected in the scatter and are studied statistically
for evidence of adiabatic galactic tidal dynamics, an impulse-induced shower and observational bias. The only bias detected
is the well-known deficiency of observations with perihelion distances q > 2.5 AU. There is no significant evidence of a seasonal
dependence. Nor is there a substantive hemispherical bias in either ecliptic or equatorial coordinates. There is evidence
for a weak stellar shower previously detected by Biermann which accounts for ≈ 10% of the total observations. Both the q bias
and the Biermann star track serve to weaken the evidence for a galactic tidal imprint. Nevertheless, statistically significant
asymmetries in galactic latitude and longitude of perihelia remain. A latitude asymmetry is produced by a dominant tidal component
perpendicular to the galactic disk. The longitude signal implies that ≈ 20% of new comets need an additional dynamical mechanism.
Known disk non-uniformities and an hypothetical bound perturber are discussed as potential explanations. We conclude that
the detected dynamical signature of the galactic tide is real and is not an artifact of observational bias, impulsive showers
or poor data.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
18.
We interpret the historical activity of comet 55P/Tempel–Tuttle in terms of the observed characteristics of present-day short
period comets. In this respect, it is now realized that such comets are liable to undergo significant outburst and mantle
loss events at intervals separated by of order a few hundred years. On this basis one might well expect comet 55P/Tempel–Tuttle
to have undergone several outbursts since its earliest sighing in 1366. The limited absolute magnitude data available for
55P/Tempel–Tuttle is not inconsistent with the suggestion that the comet underwent outbursts during its 1699 and 1865 perihelion
returns. If the outbursts of comet 55P/Tempel–Tuttle are interpreted in terms of mantle loss events then the bright, electrophonic
sound producing fireballs reported during the great Leonid meteor storm of 1833 may have been due to the Earth sampling mantle
material ejected during the outburst of 1699.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
19.
Weaver H. A. Brooke T. Y. Chin G. Kim S. J. Bockelée-Morvan D. Davies J. K. 《Earth, Moon, and Planets》1997,78(1-3):71-80
High resolution (λ/δλ ∼ 20,000) spectra of comet C/1995 O1 (Hale-Bopp) in the 2–5 μm region were obtained during UT 2–5 March
1997 using CSHELL at the NASA Infrared Telescope Facility (IRTF) on Mauna Kea. The heliocentric and geocentric distances of
the comet were ∼1.1 AU and ∼1.5 AU,respectively. We detected emission lines of the gas-phase molecules H2O, 4, C2H6, C2H2, HCN, and CO and derived absolute production rates and relative abundances for all species. We also used the 2-dimensional
nature of the CSHELL data to investigate the spatial distribution of the molecules and find evidence that CO was derived at
least partly from an extended source in the coma.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
20.
H. McNamara J. Jones B. Kauffman R. Suggs W. Cooke S. Smith 《Earth, Moon, and Planets》2004,95(1-4):123-139
In an attempt to overcome some of the deficiencies of existing meteoroid models, NASA’s Space Environments and Effects (SEE)
Program sponsored a 3 year research effort at the University of Western Ontario. The resulting understanding of the sporadic
meteoroid environment – particularly the nature and distribution of the sporadic sources – were then incorporated into a new
Meteoroid Engineering Model (MEM) by members of the Space Environments Team at NASA’s Marshall Space Flight Center. This paper
discusses some of the revolutionary aspects of MEM which include (a) identification of the sporadic radiants with real sources
of meteoroids, such as comets, (b) a physics-based approach which yields accurate fluxes and directionality for interplanetary
spacecraft anywhere from 0.2 to 2.0 astronomical units (AU), and (c) velocity distributions obtained from theory and validated
against observation. Use of the model, which gives penetrating fluxes and average impact speeds on the surfaces of a cube-like
structure, is also described along with its current limitations and plans for future improvements. 相似文献