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1.
Suresh C. Mehrotra 《Astrophysics and Space Science》1980,71(2):507-514
Using a normalized perturbative, semi-classical approach, collision-induced rotational excitation rates of CO, OCS, SiO, HCN, HC3N due to H2 are computed. The calculated excitation rates for CO–H2 and OCS–H2 systems at 100 K are in good agreement with the results of close coupling approximation at low values ofJ, whereJ is the rotational quantum number. The rates are found to be very sensitive with respect to ortho and para states of H2. 相似文献
2.
We present lightcurves and analysis for four new monolithic fast-rotating asteroids: 2000 AG6, 2000 DO8, 2000 EB14, and 2000 HB24. Their rotation periods of 4.60, 1.30, 107.47, and 13.05 min place them well below the critical threshold for the rotation rate of strengthless prolate ellipsoids, as we demonstrate. These four objects join the five previously identified fast-rotating asteroids. The sharp segregation in spin rates between these nine objects and asteroids with more typical spin rates is somewhat puzzling. No observed objects larger than about 200 m spin with rates faster than the critical rate for strengthless prolate ellipsoids, while no objects smaller than 200 m have shown spin rates slower than this critical limit. We hypothesize that these small, fast-rotating objects are representative of the building blocks of the “rubble pile” asteroids and are in fact derived from impacts into already existing “rubble piles.” 相似文献
3.
Peter Warneck 《Planetary and Space Science》1975,23(11):1507-1518
The evaluation of OH radical reactions as a global sink for many trace gases in the troposphere requires a detailed knowledge of the OH production rate as a function of latitude and altitude. The OH production rate may be expressed as a product of the primary rate P1 from the reaction of O(1D) with water vapor and an amplification factor ? due to a re-cycling mechanism. The primary rate and its diurnal and seasonal averages have been computed as a function of latitude and altitude for the northern hemisphere, using only observational data for the involved parameters. For the southern hemisphere this procedure is not possible at present, because sufficiently detailed ozone measurements are not available. The calculation of the amplification factor requires in addition to the latitudinal distributions of atmospheric mixing ratios of CH4 and CO, those of nitrogen dioxide for which observational data are almost entirely lacking. Accordingly the distribution of NO2 mixing ratios was estimated to obtain values for the amplification factor ?. Effective OH production rates are given assuming that primary OH production rates obtained for the northern hemisphere are applicable also in the southern hemisphere. Due to the many uncertainties entering specifically into the values for the amplification factor the derived OH production rates must be considered a first approximation. 相似文献
4.
P. D. Singh K. Sinha B. M. Tripathi Heloisa M. B. Roberty 《Earth, Moon, and Planets》1989,47(3):231-239
Post-perihelion observed emission fluxes at 388 nm (CN) and 516 nm (C2) of the coma of comets Austin (1982g) and Bradfield (1980t) are analysed in the framework of the Haser model. Ratios of Haser model CN and C2 parent production rates with expansion velocity show that each comet behaves normally. For comet Austin (1982g), the Q
CN/v and Q
c2/v values decrease with increase of heliocentric distance of comet. For an assumed %; activity of the total spherical surface area of the nucleus, the water vaporization theory coupled with derived water production rates from the International Ultraviolet Explorer H and OH flux data yields a nuclear diameter of about 6 km for comet Austin (1982g). For comet Bradfield (1980t), the derived nuclear diameter is expected to be of about 1 km. In each comet, the dust mass production rates as well as ratio of dust-to-gas mass production rates decrease with increase of heliocentric distance of comet. 相似文献
5.
《New Astronomy Reviews》2000,44(4-6):283-285
The statistical investigation of integrated characteristics of the blue compact galaxies from the Second Byurakan survey has been carried out. The tight correlation between the luminosities in the far-infrared range and radio range (radio-continuum at 21 cm) has been revealed. It implies the common origin of this radiation in star forming regions. The current star formation rates have been derived both from the luminosity in the far-infrared range (SFRFIR) and from the luminosity in the radio range (SFR21 cm). The Salpeter shape of the initial mass function (IMF) and low-mass cut-off of 0.1 M⊙ in IMF has been assumed for the SFRFIR calculation. We derive the SFR21 cm assuming that radio free–free emission at λ=21 cm is thermal emission of gas ionised by massive stars. The SFR21 cm is shown to be about 3.5 times as large as SFRFIR for our sample of galaxies. These star formation rates are proposed to be considered as lower and upper limits of true galaxy star formation rates. 相似文献
6.
The radio recombination line intensities of heavy elements of helium, carbon and oxygen are calculated with accounting for
dielectronic recombination. Dielectronic recombination rates are determined accurate to the second order of a perturbation
theory and the rates are described as function of principal quantum number for helium-like atom or ion. Balance equations
are solved for the departure coefficients from LTE bn. The collision and spontaneous transition rates are accounted for the balance equations, in which non-equilibrium distribution
source is dielectronic recombination. Non-equilibrium amplification coefficients are found as functions of a medium temperature,
density and ion charge z = 1–3 for radio recombination lines.
Optical depths are calculated for the heavy element low-frequency lines with the numbers 300 > n > 1200. For the chosen electronic temperatures and densities Te = 0.8× 104–10× 104 K, Ne = 0.05–0.1 cm−3 the line optical depth is determined by the values τL∼ 0.1× 10−4–100× 10−4. Calculated for free-free transition rates, the optical depth is given by using the value τff∼ 10−2τL. 相似文献
7.
A 1-D collisional Monte Carlo model of Europa's atmosphere is described in which the sublimation and sputtering sources of H2O molecules and their molecular fragments are accounted for as well as the radiolytically produced O2. Dissociation and ionization of H2O and O2 by magnetospheric electron, solar UV-photon and photo-electron impact, and collisional ejection from the atmosphere by the low-energy plasma are taken into account. Reactions with the surface are discussed, but only adsorption and atomic oxygen recombination are included in this model. The size of the surface-bounded oxygen atmosphere of Europa is primarily determined by a balance between atmospheric sources from irradiation of the satellite's icy surface by the high-energy magnetospheric charged particles and atmospheric losses from collisional ejection by the low-energy plasma, photo- and electron-impact dissociation, and ionization and pick-up from the surface-bounded atmosphere. A range of sources rates for O2 to H2O are used with a larger oxygen-to-water ratio than suggested by laboratory measurements in order to account for differences in adsorption onto grains in the regolith. These calculations show that the atmospheric composition is determined by both the water and oxygen photochemistry in the near-surface region, escape of suprathermal oxygen and water into the jovian system, and the exchange of radiolytic water products with the porous regolith. For the electron impact ionization rates used, pick-up ionization is the dominant oxygen loss process, whereas photo-dissociation and atmospheric sputtering are the dominant sources of neutral oxygen for Europa's neutral torus. Including desorption and loss of water enhances the supply of oxygen species to the neutral torus, but hydrogen produced by radiolysis is the dominant source of neutrals for Europa's torus in these models. 相似文献
8.
The electromagnetic and particle cascade resulting from the absorption of galactic cosmic rays in the atmosphere of Titan is shown to be an important mechanism for driving the photochemistry at pressures of 1 to 50 mbar in the atmosphere. In particular, the cosmic ray cascade dissociates N2, a process necessary for the synthesis of nitrogen organics such as HCN. The important interactions of the cosmic ray cascade with the atmosphere are discussed. The N2 excitation and dissociation rates and the ionization rates of the principal atmospheric constituents are computed for a Titan model atmosphere that is consistent with Voyager 1 observations. It is suggested that HCN may be formed efficiently in the lower atmosphere through the photodissociation of methylamine. It is also argued that models of nitrogen and hydrocarbon photochemistry in the lower atmosphere of Titan should include the absorption of galactic cosmic rays as an important energy source. 相似文献
9.
Evaluations are presented of the time-average heating at different latitudes and heights due to energy flux divergence of the equinox diurnal and semidiurnal tides calculated by Forbes (1982a,h)from 0 to 400 km.It is found that diurnal tidal heating maximizes in the region of 80 km and semidiurnal has a sharp maximum at 108 km. Thermospheric diurnal oscillations give rise to a second region of heating that maximizes at 200 km and effectively transports energy from low to high latitudes.Global means are evaluated for the time-averaged vertical energy fluxes and heating rates: below 130 km, the results for the diurnal tide agree with those for the (1,1) mode alone, and for the semidiurnal tide, heating rates below 130 km are the same as those that would he obtained without the thermospheric semidiurnal excitation.Comparisons are made from 90 to 170 km between the combined diurnal and semidiurnal heating rates and previously reported rates due to e.u.v. radiation, Sq currents and gravity waves. 相似文献
10.
The planet Jupiter, like the Earth, possesses a magnetic field, and, therefore, auroral activity is very likely. In this work, the auroral emissions due to electron precipitation are estimated for a model atmosphere with and without helium. The incident primary electrons, which are characterized by representative spectra, are degraded in energy by applying the continuous slow down approximation. All secondaries, tertiaries, and higher generation electrons are assumed to be absorbed locally. A compilation of excitation, dissociation, and ionization cross section data for H, H2, and He are used to model all aspects of the energy deposition process. Volume emission rates are calculated from the total direct excitation rates, and appropriate corrections for cascading are applied. Integrated column intensities of several kiloRayleighs are obtained for the various vibrational levels of the Lyman and Werner bands of H2, as well as the triplet continuum a3Σg+ → b3Σu+. Helium emissions are relatively small because the majority of electrons are absorbed above the region of maximum He concentration. Atomic hydrogen emissions are due mainly to dissociative excitation of molecular hydrogen rather than direct excitation. 相似文献
11.
Abstract— We have measured the δ57Fe of olivines in nine Chainpur chondrules. All are within error of normal (typically 2σ ≤ 1–2%0). Most of the chondules could not have lost more than ～20% of their FeO by Rayleigh evaporation and none can have lost more than ～61%. Yet, the range of Fo contents in these chondrules is Fo78–99.9. The isotopic compositions of the chondrules clearly demonstrate that, for instance, type I chondrules cannot form from type II chondrules by evaporation of FeO under Rayleigh conditions. The isotopic compositions also place constraints on the minimum cooling rates these chondrules could have experienced. These cooling rates must also be equal to or slower than those required to produce the chondrule textures. Assuming flash heating and evaporation rates like those measured in vacuum, the minimum cooling rates necessary to prevent detectable Fe isotopic fractionation via Rayleigh evaporation approach those needed to produce barred and porphyritic textures. The presence of hydrogen in the nebula, non‐linear cooling and other effects will all tend to increase the cooling rates required to prevent δ57Fe > 1–2%0, perhaps by as much as 1–2 orders of magnitude. The two most likely ways that the cooling rates required to prevent δ57Fe >1–2%0 can be kept below those needed to produce barred and porphyritic textures are (1) the pH2 in the nebula was low enough to keep evaporation rates close to those in vacuum, or (2) back reaction of chondrules with Fe in the gas suppressed isotopic fractionation. 相似文献
12.
Emission fluxes of CN, C2 and C3 species observed in the coma of some comets are analysed in the framework of Haser model. CN, C2 and C3 production rates are determined using recently derived fluorescence efficiencies and dependence of CN, C2 and C3 production rates on the heliocentric distance is studied. Evidence for a burst type activity around January 15, 1974 in the post-perihelion period of comet Kohoutek (1973f) is observed. 相似文献
13.
A model of the formation of molecules by surface reactions on interstellar grains is described and assessed numerically. The model predicts that for the molecules—other than H2-likely to be important in the interstellar medium, the formation rates by surface reactions are insensitive to the nature of the surface. The formation rates have magnitudes which are significant when compared with other routes. The model also describes H2 formation in high density clouds and shows it to be parameter dependent. 相似文献
14.
An understanding of the rates of frost grain growth is essential to the goal of relating spectral data on surface mineralogy to the physical history of a planetary surface. Models of grain growth kinetics have been constructed for various frosts based on their individual thermodynamic properties and on the difference in binding energy between molecules on plane vs curved faces. A steady state situation can occur on planetary surfaces in which thermal elimination of small grains competes with their creation, usually by meteorite impact. We utilize predicted grain growth rates to explain telescopic spectral data on condensate surfaces throughout the solar system. On Pluto, predicted CH4 ice grain growth rates are very high despite the low temperature, resulting in a multicentimeter optical path. This explains the strong CH4 absorption band depths, which otherwise would require large amounts of CH4 gas. On the Uranian and Saturnian satellites, extremely slow grain growth rates are predicted because of the low vapor pressure of H2O at the existing average surface temperatures. This may explain evidence for fine grain size and peculiar microstructure. On Io, ordinary thermal exchange is more effective than sputtering in promoting grain growth because of the properties of SO2. Over much of Io's disk, submicron size grains of SO2 could plausibly reconfigure into a surface glaze on a timescale comparable to the resurfacing rate. This may explain the relatively strong SO2 signature in Io's infrared absorption spectrum as opposed to its weaker manifestation in the visible spectrum. In spite of lower sputtering fluxes, sputtering plays a more important role in grain growth for Europa, Ganymede, and Callisto than on Io. This is a result of high rates of thermally activated grain growth and resurfacing on Io. The sequence of H2O-ice absorption band depths (related to the mean grain size) is J2(T) ～ J3(T) > J2(L) > J3(L) ～ J4(T) ～ J4(L), where L = leading and T = trailing. This is to be expected if sputtering were dominant. The calculations show, however, that neither thermalized exchange fluxes nor sputtering exchange fluxes can produce the implied grain growth or the ordering by ice absorption band depths of the six satellite hemispheres. Only sputtering control by simple ejection of H2O from the satellites, as the dominant cause of shorter mean lifetimes for smaller exposed grains, can satisfactorily explain the data. Some observations, which suggest that there are vertical grain size gradients, may result from a steady state balance between intense near surface production of fine frost by comminution, coupled with ongoing ubiquitous grain growth in the vertical column. In certain cases, e.g., Europa and Enceladus, the possibility exists that endogenic activity as well as comminution could affect grain size—at least locally. It is concluded that not only ice identification and mapping, but ice grain size mapping is an important experiment to be conducted on future missions. 相似文献
15.
16.
Photoelectric spectrophotometric scans of Comet 1973 XII Kohoutek were made on November 25.85 and 28.85 UT, 1973. The vibrational temperature of C2, the total numbers of CN and of C2, and the production rates for CN and C2 are derived from the observed fluxes. The temperature of C2 was 4900 K on 25.85 November and 4300 K on 28.85 November. Mean abundance ratio of C2 to CN was about 2.7. From the variation of production rates with heliocentric distance, it is shown that there was an outburst on 25.85 November in both CN and C2 productions. For C2, the production rates are derived by using two coma models, i.e. the Haser's parent-daughter model and the model relevant to a proposition of Yamamoto (1981b) that C2 is formed via two-step photodissociation of its parent molecules. By comparing the production rate derived from the two models, it can be supported that C2 is formed via two-step photodissociation rather than one-step photodissociation. In consequence, it is shown that the variation of the production rate with heliocentric distance is largely modified compared with that derived from the Haser's model. 相似文献
17.
The variations in production rates for Comet 46P/Wirtanen for the species H2O and the parents of C2 and CN are examined from the point of view of a variety of commonly used scale lengths. The calculations are carried out as a function of heliocentric distance. It is shown that, by using a common set of scale lengths, the results of various investigators can be brought into acceptable accord. The resulting production rates of H2O and the parents of C2 and CN versus heliocentric distance are recalculated and plotted versus the heliocentric distance rH. The curves show reasonable agreement with a slope of ∼rH−4. The water production rate near perihelion of 46P/Wirtanen is close to . 相似文献
18.
A simplified D-region model consisting of O2+, NO+ and their respective cluster ions grouped as Zo2+ and ZNO+ is used to reproduce the available rocket data on positive ion relative composition and effective clustering rates for the height range 70–90 km. The results of this analysis for a winter anomalous day (Sardinia, 40°N) are in good agreement with the presently known ideas on NO densities, O2+ production rates, mesospheric temperature, negative ion to electron density ratio and effective loss coefficient for electrons. Mesospheric nitric oxide density and temperature profiles from this study are in excellent agreement with the findings of Zbinden et al. (1975) and Hidalgo (1977) for the anomalous day at Sardinia. 相似文献
19.
Models are developed for the photochemistry of a CO2H2ON2 atmosphere on Mars and estimates are given for the concentrations of N, NO, NO2, NO3, N2O5, HNO2, HNO3, and N2O as a function of altitude. Nitric oxide is the most abundant form of odd nitrogen, present with a mixing ratio relative to CO2 of order 10?8. Deposition rates for nitrite and nitrate minerals could be as large as 3× 105 N equivalent atoms cm?2 sec?1 under present conditions and may have been higher in the past. 相似文献
20.
Translationally hot O(3P) atoms are produced in the atmosphere of Earth by photolysis of O2 and O3 and quenching of O(1D). A rigorous kinetic theory analysis of this problem is developed and compared with the approach previously employed by Logan and McElroy [Planet. Space Sci.25, 117 (1977)]. It is shown that the kinetic theory employed by the previous workers is somewhat deficient. With the line-of-centers cross-section, the rates of reactions of the translationally hot O(3P) atoms with other atmospheric gases are calculated and found to be in some instances many orders of magnitude larger than the equilibrium rates. Though the non-equilibrium reaction rates with O(3P) are substantially increased, they are still not competitive with the corresponding reaction rates with O(1D). 相似文献