共查询到20条相似文献，搜索用时 31 毫秒
1.
Colom P. Gérard E. Crovisier J. Bockelé-Morvan D. Biver N. Rauer H. 《Earth, Moon, and Planets》1997,78(1-3):37-43
We present OH 18-cm observations of comet Hale-Bopp (C/1995 O1) at the Nançay radio telescope. On nucleus and offset position observations allowed us to obtain both OH production rates and quenching radii. The maximum OH production rate was reached around perihelion, at about1031 s-1. 相似文献
2.
Julio A. Fernández 《Icarus》1985,64(2):308-319
The brightness evolution of short-period comets is discussed in connection with their physical lifetimes. It is shown that changes in the fraction of the free-subliming area of the nuclear surface may be more important than mass decrease in determining brightness variations. The decrease in the activity of short-period comets caused by the buildup of a dust mantle may be interrupted—and partially reversed—by dust blowoffs that leave exposed areas of fresh ices. Short-period comets may thus be subject to random brightness fluctuations that make quite uncertain any derivation of their physical lifetime based on comparisons of their absolute brightness at different apparitions. As an alternate procedure, the numerical integration of the whole sample of short-period comet orbits carried out by A. Carusi, L.Kresák, E. Perozzi and G. B. Valsecchi (1984, Long-Term Evolution of Short-Period Comets. Istituto Astrofisica Spaziale Internal Report 12, Rome) is used to draw conclusions about the transfer rate of their perihelia from Jupiter's region to the region of the terrestrial planets (heliocentric distances<1.5 AU). It is found that about one short-period comet per century reaches the region of the terrestrial planets. From this result and under the assumption of a steady-state comet population, an average lifetime of the order of 6 × 103 years (～103 revolutions) is derived for a typical kilometer-sized short-period comet of perihelion distance q ～ 1 AU. Such a rather long comet lifetime, as compared to some previous derivations, is consistent with the survival of some periodic comets on small-q orbits of long dynamical time scales. 相似文献
3.
Radio emission spectra of the J = 1?0 ground state transition of H12C14N has been detected in comet Kohoutek (1973f) before and after perihelion passage. The HCN gas production rate is about 1028 molecules sec?1 at a heliocentric distance of ～0.4 AU. Multiple Doppler shifts in the observed spectrum suggest jets with velocities ranging up to several km sec?1. 相似文献
4.
Comet 81P/Wild 2 was observed in the thermal infrared over 6 months during its 1997 perihelion passage. The comet was most active in late February, about 3 months preperihelion; dust production declined by a factor of 3 between February and August. For the GIOTTO Halley dust size distribution, maximum dust production rate was ∼2 × 106 g/s. The comet displayed a 10-μm silicate feature about 25% above the continuum, similar to several other Jupiter-family comets, but much lower than that seen in a number of Oort cloud comets.NASA’s STARDUST sample return mission will encounter P/Wild 2 98 days postperihelion in January 2004. Based on our observations at a similar point in the orbit and the Halley size distribution, we predict that the mass fluence on the spacecraft for a 150 km miss distance will be about 8 × 10−6 g/cm2 for particles up to 1 cm in radius. The corresponding areal coverage will be about 10−4. 相似文献
5.
Peter Brown Valerie Marchenko Danielle E. Moser Robert Weryk William Cooke 《Meteoritics & planetary science》2013,48(2):270-288
We propose that the Taurid meteor shower may contain bodies able to survive and be recovered as meteorites. We review the expected properties of meteorite‐producing fireballs, and suggest that end heights below 35 km and terminal speeds below 10 km s?1 are necessary conditions for fireballs expected to produce meteorites. Applying the meteoroid strength index (PE criteria) of Ceplecha and McCrosky (1976) to a suite of 33 photographically recorded Taurid fireballs, we find a large spread in the apparent meteoroid strengths within the stream, including some very strong meteoroids. We also examine in detail the flight behavior of a Taurid fireball (SOMN 101031) and show that it has the potential to be a (small) meteorite‐producing event. Similarly, photographic observations of a bright, potential Taurid fireball recorded in November of 1995 in Spain show that it also had meteorite‐producing characteristics, despite a very high entry velocity (33 km s?1). Finally, we note that the recent Maribo meteorite fall may have had a very high entry velocity (28 km s?1), further suggesting that survival of meteorites at Taurid‐like velocities is possible. Application of a numerical entry model also shows plausible survival of meteorites at Taurid‐like velocities, provided the initial meteoroids are fairly strong and large, both of which are characteristics found in the Taurid stream. 相似文献
6.
Brian M. Cudnik 《Planetary and Space Science》2005,53(6):653-658
Hale-Bopp (C/1995 O1) was the most productive recent comet observed in terms of gas and dust output. Since its discovery in 1995 at a distance of 7.14 AU from the Sun, the comet has been well observed, revealing the dynamics of a rare and large comet. Hale-Bopp showed strong emissions of the principle cometary gases CN, C3, and C2, as well as an abundance of dust. The production rates of these gases were found to be 1.45×1028, 1.71×1028, and , respectively, with dust production, in terms of Afρ, , as measured in the green continuum (5260 Å). The observations for this paper are presented in two groups spanning 10 days each, one group centered near 32 days prior to and the other 21 days after perihelion. The averages of dust and gas production rates show a slightly higher value for each prior to perihelion than after perihelion, consistent with a possible peak in production a few weeks prior to perihelion passage. 相似文献
7.
The evolution of the perihelion distance distribution in the Oort cloud was studied over the age of the solar system, under the gravitational perturbations of random passing stars, using a statistical approach. These perturbations are accounted for through an empirical relation relating the change in cometary perihelion distance to the closest-approach comet-star distance; this relation is deduced from a previous study [H. Scholl, A. Cazenave, and A. Brahic, Astron. Astrophys.112, 157–166 (1982)]. Two kinds of initial perihelion distances are considered: (a) perihelion distances <2500 AU, associated with an origin of comets as icy planetesimals in the region of the giant planets, and (b) larger perihelion distances (up to 5 × 104 AU), possibly representative of comet formation as satellite fragments in the accretion disk of the primitive solar nebula. Distant star-comet encounters, as well as rare close encounters, are considered. Several quantities are estimated: (i) number of “new” comets entering into the planetary region, (ii) number of comets escaping the Sun sphere of influence or lost by hyperbolic ejection and (iii) percentage of total comet loss over the age of the solar system. From these quantities, the current and original cloud populations are deduced, as well as the corresponding cloud mass, for the two types of formation scenarios. 相似文献
8.
John Byl 《Earth, Moon, and Planets》1983,29(2):121-137
The effect of galactic perturbations on long-period comet orbits is examined via numerical and analytical means. Relations are found between a comet's initial perihelion position and the positions of succeeding perihelia. It was found that the galactic effects were strongest on the comets initially at galactic latitudes close to 40°. In such cases the galactic perturbations caused the orbit to become almost circular before becoming nearly parabolic again. This effect allows comets with semimajor axes of about 25 000 AU to make only a few passages through the inner solar system in a time interval of 109yr. Thus the galactic field is an important factor in the evolution of long-period comet orbits. The observed distribution of perihelia of long-period comets indicates that galactic effects have been active. 相似文献
9.
Narrow band photoelectric measurements of CN(3870), CO+(4250), C2(4700), and C2(5120) were made on twelve nights in December and January of comet Kohoutek. CN and C2 appear to be stronger after perihelion, and CO+ showed a strong post perihelion increase coincident with the first appearance of a strong gas tail and then decreased to a fairly constant level. 相似文献
10.
《Planetary and Space Science》1999,47(6-7):735-744
Understanding the power balance at the surface of the nucleus is essential to study the chemical and physical evolution of a comet. Therefore, we present a detailed energy budget analysis for the surface of a model comet in the orbit of 46P/Wirtanen, target comet of the European space craft mission Rosetta, for a variety of parameters and assumptions. We will show that for a fast spinning Jupiter-family comet such as 46P/Wirtanen with a rotation period of about 6 h, a fast rotator approximation underestimates the effective energy input. This yields lower gas fluxes from the surface. For an 100% active, non-dust covered surface we obtain a water gas flux on the order of about 1.5×1028 molecules s−1 at perihelion, assuming a radius of 600 m. The calculated gas flux of water is within the order of measured values for comet 46P/Wirtanen. But our calculated values are maximum gas fluxes at noon—not averaged over one cometary day or taking the lesser insolation at the polar areas into account. Therefore, we conclude that either the radius of comet 46P/Wirtanen may be much larger than the accepted value of 600 m. A radius in the order of 2 km seems more likely to explain the measurements. Or, an other possibility could be that water-ice particles are blown off from the surface like dust particles. This may also increase the effective surface area of sublimation. 相似文献
11.
Zdenek Sekanina 《Icarus》1976,27(1):123-133
A theory of the probability of encounter of the Sun with an interstellar comet at a distance comparable to the Earth-Sun distance is formulated, and a general expression is derived establishing the relationship among the influx rate of interstellar comets, the perihelion distance, the space density of the comets, the Maxwellian distribution of comet velocities in the interstellar cloud, and the cloud's systematic velocity relative to the Sun. The fact that no comet with a strongly hyperbolic orbit has so far been observed is used to determine an upper limit of 6 × 10?4 solar masses per cubic parsec (4 × 10?26 gcm?3) for the space density of interstellar comets. The theoretical distribution of semimajor axes of interstellar comets is derived to show that a strong hyperbolic excess must be present in the orbits of a majority of interstellar comets regardless of the dynamical characteristics of the comet cloud, except when the cloud is moving along with the Sun and the distribution of individual velocities has a very low dispersion. This case, however, implies a possibility of capture by the Sun and thus becomes a problem of an Oort-type cloud. 相似文献
12.
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.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
13.
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. 相似文献
14.
Spectropolarimetric observations from 5000 to 8000 Å have been obtained for comets P/Austin (1982g) and P/Churyumov-Gerasimenko (1982f). The observations were spaced over phase angles of 50–125° for comet Austin and 10–40° for comet Churyumov-Gerasimenko. The use of spectropolarimetry allowed an evaluation of continuum polarization without molecular line contamination. Especially for comet Churyumov-Gerasimenko, the curve of polarization versus phase angle resembles curves for asteroids, where the polarization is negative (electric vector maximum parallel to the scattering plane) for phase angles less than 20° and the most negative polarization is from ?1 to ?2%. The negative polarization at backscattering angles may be due to multiple scattering in agglomerated grains, as assumed for asteroids, or to Mie scattering by small dielectric particles. If multiple scattering is important in comet dust, polarization measurements may imply a low albedo, less than 0.08. The polarization of comet Austin remained steady during a large change in the dust production rate. Both comets increased continuum flux by a factor of 2 near perihelion. The continuum of comet Churyumov-Gerasimenko had the shape of the solar spectrum with derivations less than 5%. The equivalent width of spectral features of C2, NH2, and O varied as r?2. 相似文献
15.
Lisse C. M. Fernández Y. R. A'hearn M. F. Kostiuk T. Livengood T. A. Käufl H. U. Hoffmann W. F. Dayal A. Ressler M. E. Hanner M. S. Fazio G. G. Hora J. L. Peschke S. B. Grün E. Deutsch L. K. 《Earth, Moon, and Planets》1997,78(1-3):251-257
We present infrared imaging and photometry of the bright, giant comet C/1995 O1 (Hale-Bopp). The comet was observed in an
extended infrared and optical observing campaign in 1996–1997. The infrared morphology of the comet was observed to change
from the 6 to 8 jet “porcupine” structure in 1996 to the “pinwheel” structure seen in 1997; this has implications for the
position of the rotational angular momentum vector. Long term light curves taken at 11.3 μm indicate a dust production rate
that varies with heliocentric distance as ∶ r−1.4. Short term light curves taken at perihelion indicate a rotational periodicity of 11.3 hours and a projected dust outflow
speed of ∶ 0.4 km s−1. The spectral energy distribution of the dust on October 31, 1996 is well modeled by a mixture of 70% silicaceous and 30%
carbonaceous non-porous grains, with a small particle dominated size distribution like that seen for comet P/Halley (McDonnell
et al., 1991), an overall dust production rate of 2 × 105 kg s−1, a dust-to-gas ratio of ∶5, and an albedo of 39%.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
16.
We present here a review of the radio observations of the remarkable comet Hale-Bopp C/1995 O1 in which most major radio astronomical
facilities have been involved. These observations started in August 1995, soon after the discovery of the comet (it was then
at ∼7 AU from the sun), and well before its perihelion on April 1st, 1997; they are still going on, hopefully up to end of
1998. Extended cartographies have been obtained using multibeam receivers and on-the-fly techniques. High spatial resolution
(a few ″) has been achieved with interferometers. Submillimetric observations are playing an increasing role, and high resolution
(R ∼ 106−107) spectroscopy of cometary lines is now performed from decimetric to submillimetric wavelengths.
The number of species observed at radio wavelengths now reaches ∼28,when it was ∼14 for comet C/1996 B2 Hyakutake. Most of
these species are parent molecules. However, ions have been observed for the first time at radio wavelengths, and their velocities
measured. Several isotopic species (involving D,13C,34S,15N) have been sought, allowing isotopic enrichment determinations.
The abundances of cometary molecules present many similarities and some differences with the abundances of interstellar molecules
in regions where grain mantles are believed to be evaporated to the gas phase (hot cores, bipolar flows). They will be discussed
for their implications on the origin of cometary ices and of comets themselves.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
17.
The meteoroid streams associated to short-period comets 9P/Tempel 1 (the target of the Deep Impact mission). and 67P/Churyumov-Gerasimenko (the target of the Rosetta mission) are studied. Their structure is overwhelmingly under the
control of Jupiter and repeated relatively close encounters cause a reversal of the direction of the spatial distribution
of the stream relative to the comet* an initial stream trailing the comet as usually seen eventually collapses, becomes a
new stream leading the comet and even splits into several components. Although these two comets do not produce meteor showers
on Earth, this above feature shows that meteor storms can occur several years before the perihelion passage of a parent body. 相似文献
18.
We have examined the effects of vaporization from the nucleus of a comet and show that a latitude dependence of vaporization can, in some cases, explain asymmetries in cometary light curves. We also find that a non-uniform distribution of solar radiation over a comet can considerably shorten the vaporization lifetime compared to the results normally obtained by assuming that the nuclear surface is isothermal.Independent of any latitude effects, comets with CO2-dominated nuclei and with perihelion distances less than 0.5 AU have vaporization lifetimes less than or comparable to their dynamical ejection times. This may explain the observed deficit of comets with small perihelion distances. Similarly comets with CO2-dominated nuclei and perihelia near Jupiter's orbit have vaporization lifetimes that are shorter than the time for capture into short-period orbits. We suggest, therefore, that at least some new comets are composed in large part of CO2, while only H2O-dominated comets, with lower vaporization rates, can survive to be captured into short-period orbits. 相似文献
19.
Results of Ondřejov radar observation of Taurid complex meteor showers, i.e. ζ Perseids, β Taurids, S and N Taurids, performed
in 2003, are presented. We have found some mass segregation within ζ Perseid, β Taurid and S Taurid showers. We have also
established conspicuous lack of long duration echoes (with T ⩾ 3 s resp. T ⩾ 5 s) in S and N Taurid showers. The lack within remaining showers is not so pronounced but still persists. 相似文献
20.
M. R. Voelzke 《Meteoritics & planetary science》2019,54(4):938-945
This work is based on a systematic analysis of images of comet 1P/Halley collected during its penultimate and ultimate approaches, i.e., in 1910 and 1986. This research has identified, characterized, classified, and compared tail structures of comet 1P/Halley, namely disconnection events (DEs), wavy structures, and solitons. The images of the comet during its 1910 passage, as illustrated in the Atlas of Comet Halley 1910 II (Donn et al. 1986), were compared with those of its approach in 1986 as illustrated in The International Halley Watch Atlas of Large‐Scale Phenomena (Brandt et al. 1992). Two onsets of DEs were discovered after the perihelion passage in 1910 with an average value of the corrected cometocentric velocity (Vc) of 57 ± 15 km s?1. Ten onsets of DEs were discovered after the perihelion passage in 1986 with an average Vc equal to 130 ± 37 km s?1. The mean value of the corrected wavelength λc of wavy structures in 1910 is equal to 1.7 ± 0.1 × 106 km, as compared to 2.2 ± 0.2 × 106 km in 1986. The mean value of the amplitude A of the wave in 1910 is equal to 1.4 ± 0.1 × 105 km and 2.8 ± 0.5 × 105 km in 1986. The goals of this research were to report the results obtained from the analysis of the P/Halley's images from 1910 and 1986, to provide empirical data for comparison, and to form the input for future physical/theoretical work. 相似文献