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1.
Populations of the rotational levels of vibrational and electronic ground state of interstellar HD and CO have been calculated. Ultraviolet pumping and molecular formation have been found to be very important in the case of HD but not so in CO, the population distribution of which is mainly governed either by 3 K or by collisional excitation. The population inversion has been found to occur in HD but not in CO, the excitation temperatures of which pass through maximum for large ultraviolet flux. The population inversion at large density does not occur in HD, probably due to neglect of multipole collision. The observation of HD and CO in the excited states may be possible for suitable values of density and ultraviolet flux.On leave from Tata Institute of Fundamental Research, Bombay, India. 相似文献
2.
Nalin H. SAMARASINHA 《Meteoritics & planetary science》2008,43(6):1063-1073
Abstract— Rotational excitation and damping are discussed in the context of inferring structural properties of asteroids and comets. Opportunities for carrying out deterministic experiments are outlined and basic concepts involving space missions are discussed. Spacecraft carrying an impactor or explosives together with an orbiter are suggested as effective probes of the interiors of asteroid and comets. The feasibility of such missions, especially to near‐Earth objects (NEOs), is highlighted as NEOs provide an appropriate cost‐effective path to explore interiors of asteroids and comets. 相似文献
3.
The cooling of electrons by vibrational and rotational excitation of molecular hydrogen plays an important role in the thermal balance of electrons in atmospheres containing significant amounts of H2. Calculations of vibrational and rotational cooling rates of electrons by H2 are described. Results are presented for a wide range of electron and neutral temperatures. Analytical formulae for some of the cooling rates are also provided. 相似文献
4.
Andrew F. Cheng 《Icarus》2006,184(2):584-588
Three Jupiter family comets have now been observed by spacecraft with the surprising result that these comets lack unambiguous impact craters. Large-scale topography generally appears to be softened on these comets, although sharp topography is preserved at small scales. We find that viscous relaxation of water ice may explain these observations, given reasonable assumptions about ice grain size and temperatures attained in the interiors. We suggest that both the shapes and the cratering records of Jupiter family comets may be substantially modified and no longer reflect cometary formation processes or collisional evolution in the Kuiper Belt. 相似文献
5.
We consider the possibility of the excitation of sodium resonance emission in cometary matter under solar radiation at a heliocentric distance of 5 AU, as was observed when a fragment of Comet Shoemaker-Levy 9 plunged into Jupiter. When the sodium emission is calculated, the multiple scattering in the cometary cloud is taken into account. We use a non-LTE radiative transfer code for a two-level model sodium atom. A comparison of the computed and observed Na I D emission line profiles allows the column density of the sodium atoms for specific times of observations of Comet Shoemaker-Levy 9 to be determined. The observed Na I(D1+D2) line profile was found to agree well with the computed profile for an optically thick sodium cloud. We calculated the column density of the sodium atoms for three comets from the observed intensity of the D2 line emission. We also calculated the D2/D1 intensity ratio for various optical depths of the sodium cloud and various phase angles. 相似文献
6.
《Planetary and Space Science》1999,47(6-7):745-763
An improved magnetohydrodynamic (MHD) model with chemistry is presented. The analysis of the source and sink terms for H2O + shows that for small comets up to 11% of water molecules are finally ionized. For large comets (such as Halley) this fraction decreases to less than 3%. From the MHD scaling laws a similarity law for the individual ion densities is deduced which takes into account that the mother molecules are depleted by dissociation. This is applied to H2O + ions. Radial density profiles from model calculations, observations by Giotto near comet Halley, and ground based observations of three comets confirm this scaling law for H2O + ions. From the similarity law for the density a scaling law for the column density is derived which is more convenient to apply for ground based observations. From these scaling laws methods are derived which allow the determination of the water production rate from the ground based images of the H2O + ions. Finally, the two dimensional images of model column densities are compared with observations. 相似文献
7.
George P. Mantas 《Planetary and Space Science》1981,29(12):1319-1323
The range of applicability of the transport equation for photoelectrons in the ionosphere (Mantas, 1975), is extended to thermal energies. The extension enables one to calculate the photoelectron distribution in the important low energy region (i.e. < 3 eV), where most of the thermal electron gas heating takes place, and thereby to obtain more accurate excitation rates for the low energy: electronic, vibrational and rotational states of the ionospheric constituents, than was formerly possible. 相似文献
8.
Schloerb F. P. Devries C. H. Lovell A. J. Irvine W. M. Senay M. Wootten H. A. 《Earth, Moon, and Planets》1997,78(1-3):45-51
Observations of comets in the 18-cm OH transitions offer a means to probe gas production, kinematics, and OH excitation in
comets. We present initial results of OH observations of comet Hale-Bopp obtained with the NRAO 43 m antenna located in Greenbank,
WV. Maps of the emission provide strong constraints on the amount of quenching of the inversion of the OH ground state Λ-doublet
in the coma. Analysis of the total radio OH flux and maps of its radial brightness distribution indicate a quenched region
on the order of ∼500,000 km during March and April 1997. This large value is generally consistent with previous observations
of radio OH quenching in lower production rate comets when the high production rate of comet Hale-Bopp is considered.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
9.
Eirik Mysen 《Planetary and Space Science》2004,52(10):897-907
A model with subsolar water sublimation on a triaxial, ellipsoidal comet nucleus is presented for the calculation of reactive torques. The resulting differential equations describing the comet's rotation are then Hamiltonian, and gravity-gradients are trivial to include. While effects derived from a weak perturbing function are neither able to change the rotational excitation nor the spin magnitude of the nucleus, it is shown how the spin orientation of comets can change significantly over an orbital run. However, of the four comets studied, 1P, 19P, 46P and the Rosetta target 67P, 19P and 46P were the only objects clearly exhibiting this feature, thereby confirming a technique used to derive the consequences of a more elaborate model of sublimation induced torques. In particular, the rotational parameters of 67P were seen to be very stable, indicating that a highly kinematical model of its rotation for the mapping of the comet's gravitational field during the Rosetta mission can be used. The model's hierarchy with 1P/Halley as the object with highest excitation probability, is consistent with observations. 相似文献
10.
Two processes have been proposed to explain observations of crystalline silicate minerals in comets and in protostellar sources, both of which rely on the thermal annealing of amorphous grains. First, high temperatures generated by nebular shock processes can rapidly produce crystalline magnesium silicate grains and will simultaneously produce a population of crystalline iron silicates whose average grain size is ∼10-15% that of the magnesium silicate minerals. Second, exposure of amorphous silicate grains to hot nebular environments can produce crystalline magnesium silicates that might then be transported outward to regions of comet formation. At the higher temperatures required for annealing amorphous iron silicates to crystallinity the evaporative lifetime of the grains is much shorter than a single orbital period where such temperatures are found in the nebula. Thermal annealing is therefore unable to produce crystalline iron silicate grains for inclusion into comets unless such grains are very quickly transported away from the hot inner nebula. It follows that observation of pure crystalline magnesium silicate minerals in comets or protostars is a direct measure of the importance of simple thermal annealing of grains in the innermost regions of protostellar nebulae followed by dust and gas transport to the outer nebula. The presence of crystalline iron silicates would signal the action of transient processes such as shock heating that can produce crystalline iron, magnesium and mixed iron-magnesium silicate minerals. These different scenarios result in very different predictions for the organic content of protostellar systems. 相似文献
11.
We detected CH4 in eight Oort cloud comets using high-dispersion (λ/Δλ∼2×104) infrared spectra acquired with CSHELL at NASA's IRTF and NIRSPEC at the W.M. Keck Observatory. The observed comets were C/1995 O1 (Hale-Bopp), C/1996 B2 (Hyakutake), C/1999 H1 (Lee), C/1999 T1 (McNaught-Hartley), C/1999 S4 (LINEAR), C/2000 WM1 (LINEAR), C/2001 A2 (LINEAR), and 153/P Ikeya-Zhang (C/2002 C1). We detected the R0 and R1 lines of the ν3 vibrational band of CH4 near 3.3 μm in each comet, with the exception of McNaught-Hartley where only the R0 line was measured. In order to obtain production rates, a fluorescence model has been developed for this band of CH4. We report g-factors for the R0 and R1 transitions at several rotational temperatures typically found in comet comae and relevant to our observations. Using g-factors appropriate to Trot as determined from HCN, CO and/or H2O and C2H6, CH4 production rates and mixing ratios are presented. Abundances of CH4/H2O are compared among our existing sample of comets, in the context of establishing their place of origin. In addition, CH4 is compared to native CO, another hypervolatile species, and no correlation is found among the comets observed. 相似文献
12.
《天文和天体物理学研究(英文版)》2020,(8)
Gravitational accretion accumulates the original mass.This process is crucial for us to understand the initial phases of star formation.Using the specific infall profiles in optically thick and thin lines,we searched the clumps with infall motion from the Milky Way Imaging Scroll Painting(MWISP) CO data in previous work.In this study,we selected 133 sources as a sub-sample for further research and identification.The excitation temperatures of these sources are between 7.0 and 38.5 K,while the H_2 column densities are between 10~(21) and 10~(23) cm~(-2).We have observed optically thick lines HCO~+(1-0) and HCN(1-0) using the DLH 13.7-m telescope,and found 56 sources with a blue profile and no red profile in these two lines,which are likely to have infall motions,with a detection rate of 42%.This suggests that using CO data to restrict the sample can effectively improve the infall detection rate.Among these confirmed infall sources are 43 associated with Class O/I young stellar objects(YSOs),and 13 which are not.These 13 sources are probably associated with the sources in the earlier evolutionary stage.In comparison,the confirmed sources that are associated with Class O/I YSOs have higher excitation temperatures and column densities,while the other sources are colder and have lower column densities.Most infall velocities of the sources that we confirmed are between 10~(-1) to 10~0 km s~(-1),which is consistent with previous studies. 相似文献
13.
The relatively low value of Xe/Kr in the atmospheres of Earth and Mars seems to rule out meteorites as the major carriers of noble gases to the inner planets. Laboratory experiments on the trapping of gases in ice forming at low temperatures suggest that comets may be a better choice. It is then possible to develop a model for the origin of inner planet atmospheres based on volatiles delivered by comets added to volatiles originally trapped in planetary rocks. The model will be tested by results from the Galileo Entry Probe. 相似文献
14.
The relatively low value of Xe/Kr in the atmospheres of Earth and Mars seems to rule out meteorites as the major carriers of noble gases to the inner planets. Laboratory experiments on the trapping of gases in ice forming at low temperatures suggest that comets may be a better choice. It is then possible to develop a model for the origin of inner planet atmospheres based on volatiles delivered by comets added to volatiles originally trapped in planetary rocks. The model will be tested by results from the Galileo Entry Probe. 相似文献
15.
Susumu Kato 《Planetary and Space Science》1962,9(12):939-946
Joule heating has been shown to be very effective in increasing electronic temperature in the upper atmosphere. It is found theoretically that the electronic temperature can rise up to several thousands °K soon after certain ionospheric current disturbances occur, while the temperature of neutral particles increases only very slowly. Temperatures in various conditions have been computed and are found to be compatible with observation. It is also possible that the high electronic temperatures may explain the excitation of certain auroral glows. 相似文献
16.
P. Nurmi M.J. Valtonen J.Q. Zheng 《Monthly notices of the Royal Astronomical Society》2001,327(4):1367-1376
Time variation in impact probability is studied by assuming that the periodic flux of the Oort Cloud comets within 15 au arises from the motion of the Sun with respect to the Galactic mid-plane. The periodic flux clearly shows up in the impact rate of the captured Oort Cloud cometary population, with a phase shift caused by the orbital evolution. Depending on the assumed flux of comets and the size distribution of comets, the impact rate of the Oort Cloud comets of 1 km in diameter or greater is from 5 to 700 impacts Myr−1 on the Earth and from 0.5 to 70 impacts per 1000 yr on Jupiter. The relative fractions of impacts are 0.09, 0.11, 0.26 and 0.54 for long-period comets, Halley type comets, Jupiter family comets and near-Earth objects, respectively. For Jupiter, the corresponding fractions in the first three categories are 0.18, 0.31 and 0.51. If we consider physical fading of comet activity that is compatible with the observations, then the impact rates of active comets are two orders of magnitude smaller than the total impact rates by all kinds of comets and cometary asteroids of size 1 km or greater. 相似文献
17.
Radio observations from decimetric to submillimetric wavelengths are now a basic tool for the investigation of comets. Spectroscopic observations allow us: (i) to monitor the gas production rate of the comets, by directly observing the water molecule, or by observing secondary products (e.g., the OH radical) or minor species (e.g., HCN); (ii) to investigate the chemical composition of comets; (iii) to probe the physical conditions of cometary atmospheres: kinetic temperature and expansion velocity. Continuum observations probe large-size dust particles and (for the largest objects) cometary nuclei.Comets are classified from their orbital characteristics into two separate classes: (i) nearly isotropic, mainly long-period comets and (ii) ecliptic, short-period comets, the so-called Jupiter-family comets (JFCs). These two classes apparently come from two different reservoirs, respectively, the Oort cloud and the trans-Neptunian scattered disc. Due to their different history and—possibly—their different origin, they may have different chemical and physical properties that are worth being investigated.The present article reviews the contribution of radio observations to our knowledge of the JFCs. The difficulty of such a study is the commonly low gas and dust productions of these comets. Long-period, nearly isotropic comets from the Oort cloud are better known from Earth-based observations. On the other hand, JFCs are more easily accessed by space missions. However, unique opportunities to observe JFCs are offered when these objects come by chance close to the Earth (like 73P/Schwassmann-Wachmann 3 in 2006), or when they exhibit unexpected outbursts (as did 17P/Holmes in 2007).About a dozen JFCs were successfully observed by radio techniques up to now. Four to ten molecules were detected in five of them. No obvious evidence for different properties between JFCs and other families of comets is found, as far as radio observations are concerned. 相似文献
18.
H. Rickman G. B. Valsecchi Cl. Froeschlé 《Monthly notices of the Royal Astronomical Society》2001,325(4):1303-1311
We investigate the first stage of the dynamical evolution of Oort cloud comets entering the planetary region for the first time. To this purpose, we integrate numerically the motions of a large number of fictitious comets pertaining to two samples, both with perihelion distances up to 5.7 au and random inclinations; the first sample is composed of comets whose orbits have at least one node close to 5.2 au, while the second is not subject to this constraint. We examine the orbits when the comets come to aphelion after their first perihelion passage within the planetary region, and find that there is a clear statistical dependence of the energy perturbations on the Tisserand parameter. There appear to be two main processes, of comparable importance, governing the shortening of semimajor axes to values of less than 1000 au, i.e. planetary close encounters, especially with Jupiter, and indirect perturbations due to the shifting of the motion from barycentric to heliocentric and back; the former process mostly affects comets crossing the ecliptic at about 5.2 au, or on low-inclination orbits, while the latter mostly affects comets of small perihelion distance. This last result may help to understand the relative paucity of Halley-type comets with perihelion distances larger than about 1.5 au. 相似文献
19.
V. P. Tomanov 《Kinematics and Physics of Celestial Bodies》2007,23(5):196-206
We analyze our earlier data on the numerical integration of the equations of motion for 274 short-period comets (with the period P<200 yr) on a time interval of 6000 yr. As many as 54 comets had no close approaches to planets, 13 comets passed through the Saturnian sphere of action, and one comet passed through the Uranian sphere of action. The orbital elements of these 68 comets changed by no more than ±3 percent in a space of 6000 yr. As many as 206 comets passed close to Jupiter. We confirm Everhart’s conclusion that Jupiter can capture long-period comets with q = 4–6 AU and i < 9° into short-period orbits. We show that nearly parabolic comets cross the solar system mainly in the zone of terrestrial planets. No relationship of nearly parabolic comets and terrestrial planets was found for the epoch of the latest apparition of comets. Guliev’s conjecture about two trans-Plutonian planets is based on the illusory excess of cometary nodes at large heliocentric distances. The existence of cometary nodes at the solar system periphery turns out to be a solely geometrical effect. 相似文献
20.
The orbital stochasticity of comets P/Ciffréo (1985 XVI) and P/Maury (1985 VI), at the present time near the 5/3 and 4/3 resonances with Jupiter, is investigated using Lyapunov Characteristic Indicators. First results indicate a strong stochastic behaviour for the two comets, mainly induced by encounters with Jupiter, which looks roughly like the behaviour of the group of comets in 1/1 resonance with Jupiter. 相似文献