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1.
Abstract— A purely physical model is presented describing the depth- and size-dependence of the production of cosmogenic nuclides in meteoroids with radii up to 85 cm and in planetary surfaces by galactic cosmic ray protons. The model is based on Monte Carlo calculations of the intra- and internuclear cascades, by which depth- and size-dependent spectra of primary and secondary protons and of secondary neutrons are derived, and on experimental and theoretical thin-target cross sections of the underlying nuclear reactions. Model calculations are presented for production rates of 53Mn, 26Al, 22Ne, and 21Ne in H- and L-chondrites and of 53Mn and 26Al in lunar surface material and compared with experimental data. From the analysis of 53Mn and 26Al in the Apollo 15 lunar drill core and in the L-chondrite Knyahinya GCR p-spectra and integral particle fluxes at 1 A.U. and in the meteoroid orbits averaged over the last 10 Ma are derived. An analysis of experimental depth profiles in four H- and L-chondrites demonstrates, that the new model is well capable of describing depth- and size-dependences of production rates of cosmogenic nuclides. Moreover, it is possible to determine exposure ages for these meteorites on the basis of the theoretical 21Ne production rates. The model calculations further explain the depth- and size-dependence of 22Ne/21Ne-ratios and the dependences on these ratios of 21Ne, 26Al and 53Mn production rates. The future requirements for model calculations of cosmogenic nuclide production rates in extraterrestrial matter are outlined.  相似文献   

2.
Photochemical calculations for Uranus predict an extensive region of condensation of acetylene, ethane and methane in the vicinity of the temperature inversion layer. This could explain why ethane was not detected on Uranus, unlike Neptune which has a much warmer inversion layer. Subsequent snow-out of the condensibles is expected to result in reduced visibility in the troposphere. Ionospheric calculations for the equatorial region to be probed by Voyager, indicate peak electron concentrations on the order of 5×103 cm?3, if dynamical effects are important. Upper limit to the electron peak is 3×104 cm?3. Exospheric temperatures as high as 200–250K are conceivable.  相似文献   

3.
Evolutionary calculations are presented for spherically symmetric protoplanetary configurations with a homogeneous solar composition and with masses of 10?3, 1.5 × 10?3, 2.85 × 10?4, and 4.2 × 10?4M⊙. Recent improvements in equation-of-state and opacity calculations are incorporated. Sequences start as subcondensations in the solar nebula with densities of ～10?10 to 10?11 g cm?3, evolve through a hydrostatic phase lasting 105 to 107 years, undergo dynamic collapse due to dissociation of molecular hydrogen, and regain hydrostatic equilibrium with densities ～1 g cm?3. The nature of the objects at the onset of the final phase of cooling and contraction is discussed and compared with previous calculations.  相似文献   

4.
Molecular dissociation equilibrium calculations were done for the model atmospheres of DA and non-DA white dwarfs. Our calculations show that He 2 + and HeH+ appear as most abundant molecules in the atmospheres of non-DA white dwarfs while H2 and H 2 + are most abundant molecules in DA white dwarfs. It is suggested that these molecules should be searched for in the atmospheres of white dwarfs.  相似文献   

5.
This paper reports on the microscopic calculation of ground and excited states Gamow-Teller (GT) strength distributions, both in the electron capture and electron decay direction, for 54,55,56Fe. The associated electron and positron capture rates for these isotopes of iron are also calculated in stellar matter. These calculations were recently introduced and this paper is a follow-up which discusses in detail the GT strength distributions and stellar capture rates of key iron isotopes. The calculations are performed within the framework of the proton-neutron quasiparticle random phase approximation (pn-QRPA) theory. The pn-QRPA theory allows a microscopic state-by-state calculation of GT strength functions and stellar capture rates which greatly increases the reliability of the results. For the first time experimental deformation of nuclei are taken into account. In the core of massive stars isotopes of iron, 54,55,56Fe, are considered to be key players in decreasing the electron-to-baryon ratio (Y e ) mainly via electron capture on these nuclide. The structure of the presupernova star is altered both by the changes in Y e and the entropy of the core material. Results are encouraging and are compared against measurements (where possible) and other calculations. The calculated electron capture rates are in overall good agreement with the shell model results. During the presupernova evolution of massive stars, from oxygen shell burning stages till around end of convective core silicon burning, the calculated electron capture rates on 54Fe are around three times bigger than the corresponding shell model rates. The calculated positron capture rates, however, are suppressed by two to five orders of magnitude.  相似文献   

6.
The hydrodynamical problem of nuclear explosion of a dense carbon core of a star with mass 1.40M ⊙ is solved numerically. In calculation the kinetics of carbon burning at the nuclear reaction C12+C12→M24+γ rate is included. Thus the inverse effect of hydrodynamical motion on the process of thermonuclear burning is taken into account, as compared with Bruenn's (1972) calculations. The calculations show that a pulsation regime of burning is realized (actually three pulses were obtained) which evolves to the detonation regime with full combustion and disruption of the star only at the third pulse. The effects of disintegration of iron group nuclei, neutronization of matter and neutrino losses in URCA processes have not yet been considered in calculations. The influence of initial conditions (mainly the temperature distributions) and the above mentioned effects, which have not been included in calculation, on the results of the hydrodynamical problem solution are discussed. The conclusion is made on new possibilities of formation of a gravitationally bound remnant of the explosion and a neutron star.  相似文献   

7.
Observations of light isotopes in cosmic rays provide valuable information on their origin and propagation in the Galaxy. Using the data collected by the AMS-01 experiment in the range ∼0.2–1.5 GeV nucleon−1, we compare the measurements on 1 H, 2 H, 3 He, and 4 He with calculations for interstellar propagation and solar modulation. These data are described well by a diffusive-reacceleration model with parameters that match the B/C ratio data, indicating that He and heavier nuclei such as C–N–O experience similar propagation histories. Close comparisons are made within the astrophysical constraints provided by the B/C ratio data and within the nuclear uncertainties arising from errors in the production cross section data. The astrophysical uncertainties are expected to be dramatically reduced by the data upcoming from AMS-02, so that the nuclear uncertainties will likely represent the most serious limitation on the reliability of the model predictions. On the other hand, we find that secondary-to-secondary ratios such as 2 H/3 He, 6 Li/7 Li or 10 B/11 B are barely sensitive to the key propagation parameters and can represent a useful diagnostic test for the consistency of the calculations.  相似文献   

8.
Laura Schaefer 《Icarus》2004,168(1):215-219
Chemical equilibrium calculations of volatile metal geochemistry on Venus show that high dielectric constant compounds of lead and bismuth such as PbS (galena), Bi2S3 (bismuthite) or Pb-Bi sulfosalts condense in the venusian highlands and may be responsible for the low radar emissivities observed by Magellan and Pioneer Venus. Our calculations also show that elemental tellurium is unstable on Venus' surface and will not condense below 46.6 km. This is over 30 km higher than Maxwell Montes, the highest point on Venus' surface. Elemental analyses of Venus' highlands surface by laser induced breakdown spectroscopy (LIBS) and/or X-ray fluorescence (XRF) can verify the identity of the heavy metal frost on Venus. The Pb-Pb age of Venus could be determined by mass spectrometric measurements of the Pb207/Pb204 and Pb206/Pb204 isotopic ratios in Pb-bearing frosts. All of these measurements are technologically feasible now.  相似文献   

9.
Data on interstellar extinction are interpreted to imply an identification of interstellar grains with naturally freeze-dried bacteria and algae. The total mass of such bacterial and algal cells in the galaxy is enormous, ～ 1040 g. The identification is based on Mie scattering calculations for an experimentally determined size distribution of bacteria. Agreement between our model calculations and astronomical data is remarkably precise over the wavelength intervals 1 μ-1 < λ-1 < 1.94 μ-1and 2.5 μ-1 < λ-1 < 3.0 μ-1. Over the more restricted waveband 4000–5000 Å an excess interstellar absorption is found which is in uncannily close agreement with the absorption properties of phytoplankton pigments. The strongest of the diffuse interstellar bands are provisionally assigned to carotenoid-chlorophyll pigment complexes such as exist in algae and pigmented bacteria. The λ2200 Å interstellar absorption feature could be due to `degraded' cellulose strands which form spherical graphitic particles, but could equally well be due to protein-lipid-nucleic acid complexes in bacteria and viruses. Interstellar extinction at wavelengths λ < 1800 Å could be due to scattering by virus particles.  相似文献   

10.
We have developed a model which aims to reproduce observational data of many kinds related to cosmic-ray (CR) origin and propagation: direct measurements of nuclei, antiprotons, electrons and positrons,γ-rays, and synchrotron radiation. Our main results include evaluation of diffusion/convection and reacceleration models, estimates of the halo size, calculations of the interstellar positron and antiproton spectra, evaluation of alternative hypotheses of nucleon and electron interstellar spectra, and computation of the Galactic diffuseγ-ray emission. Recently our CR propagation code has been generalized to include fragmentation networks of arbitrary complexity. The code can now provide an alternative to leaky-box calculations for full isotopic abundance calculations and has the advantage of including the spatial dimension which is essential for radioactive nuclei. Preliminary predictions for sub-Fe/Fe, 10Be/9Be and 26Al/27Al are presented in anticipation of new experimental isotopic data. We show that combining information from classical CR studies with γ-ray and other data leads to tighter constraints on CR origin and propagation. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

11.
The results of thermal model calculations which assume 26Al as a heat source are presented. The relation between 26Al content, the maximum central temperature obtained, and the time interval after formation until central cooling commences is elucidated. Because of the heating times required, these results constrain maximum permissible planetoid dimensions more severely than do previous calculations which assume a high initial temperature.  相似文献   

12.
Empirical formulae are presented for calculating the displaced mass, depth, diameter, and effects of oblique trajectories for impact craters formed in dense crystalline rocks. The formulae are applicable to craters with diameters from approximately 10?3–103 cm that require, respectively, impact kinetic energies of approximately 10 to 1016 ergs for their formation. The experimental results are in poor agreement with Öpik’s theoretical calculations and raise questions on the validity of his theoretical model.  相似文献   

13.
Abstract— The cosmogenic radionuclide 60Fe (T1/2 = 1.49 Ma) was measured in two iron meteorites as well as in the magnetic fractions of a stony-iron and a stony meteorite by means of accelerator mass spectrometry (AMS). The measured 60Fe/Fe ratios range from 1.5 × 10-14 to 6.3 × 10-14 and show a significant correlation to the respective concentrations of Ni, which is the main target element. The resulting 60Fe specific activities correspond to production rates in meteorites in the order of 1 dpm/kg Ni. In addition, model calculations of depth- and size-dependent 60Fe production rates were performed. Although there are no experimental data for the production cross sections, our measurements are in surprisingly good agreement with the theoretical predictions using production cross sections that were determined by computer model calculations.  相似文献   

14.
Measurements from the 1225 to 1340 Å region by the ultraviolet detectors on Mars-3 are presented. Model calculations of the intensity of the OI triplet lines at 1304 Å are compared with the measurements made on December 27, 1971, and February 17, 1972. Agreement is found between experimental data and a model in which the neutral oxygen density at 100 km is 2–8 × 109 cm?3.  相似文献   

15.
The effects of the mutual gravitational attraction between asteroids were analyzed by two N-body calculations, in which N=4,516 (the Sun, the nine planets, and 4,506 asteroids). In one calculation the gravity of the asteroids was taken into account, and in the other it was ignored. These calculations were carried out for a time period of about 100 years. The largest difference in the positions of the asteroids between these two calculations is about 10–3 AU. For the orbital elements of the semimajor axis, the eccentricity, and the inclination, the largest differences were 9 × 10–6 AU, 4 × 10–6, and 5 × 10–4 degrees, respectively. It was found that the distribution of the differences of the semimajor axis between the two calculations is quite similar to the Cauchy distribution.  相似文献   

16.
The effects of the mutual gravitational attraction between asteroids were analyzed by two N-body calculations, in which N=4,516 (the Sun, the nine planets, and 4,506 asteroids). In one calculation the gravity of the asteroids was taken into account, and in the other it was ignored. These calculations were carried out for a time period of about 100 years. The largest difference in the positions of the asteroids between these two calculations is about 10?3 AU. For the orbital elements of the semimajor axis, the eccentricity, and the inclination, the largest differences were 9 × 10?6 AU, 4 × 10?6, and 5 × 10?4 degrees, respectively. It was found that the distribution of the differences of the semimajor axis between the two calculations is quite similar to the Cauchy distribution.  相似文献   

17.
G.W. Wetherill 《Icarus》1974,21(1):94-95
Taff (1973) has concluded that asteroidal collisions rates are much lower than those found by previous authors. His calculations are found to be in error as a consequence of inclusion of an extraneous and incorrect factor of ～10?5. The assumption of molecular chaos in the asteroid belt, while not strictly correct, is not an important source of error in calculations of asteroidal collision rates.  相似文献   

18.
Lunar surface temperatures from apollo 12   总被引：4，自引：0，他引：4  
The diurnal variation of temperatures in the lunar surface layer is calculated using the measured properties of the Apollo 12 samples. The results are compared with similar calculations made using data from the Apollo 11 samples and with previous infrared temperature measurements. Comparisons are also made with prior calculations which used assumed properties. These are based on an effective value of the thermal parameter [ = (kqc)–1/2] of 1034 which is obtained from integrated average values of the specific heat and thermal conductivity of the Apollo 12 fines.  相似文献   

19.
Abstract We report on a series of measurements of 14C in samples of the L-chondrite Knyahinya, as a function of depth. The results show 14C concentrations ranging from 37 to 58 dpm/kg. These measurements are compared to the levels of 21Ne, 10Be and noble-gas ratios in this meteorite reported by Graf et al (1990). We also compare the 14C data to the expected profile based on two sets of model calculations. The behavior of this radioisotope with depth is similar to that expected from the models, and good agreement with the profiles of other nuclides is also observed.  相似文献   

20.
Following our recently published measurements of the rate coefficients for mutual neutralization, α, of the ionospherically important reactions NO+ + NO2?(α1) and NO+ + NO3?(α2) carried out in ion-ion flowing afterglow plasmas at 300 K, we have determined the mutual neutralization rates for the water cluster ion H3O+ · (H2O)3 with a mixture of several negative ions which are known to exist in the D region. The α coefficients for these cluster ion reactions do not differ significantly from alpha;1 and α2, all of these reactions having α ?6 × 10?8 cm3/sec which is significantly smaller than values usually adopted in ionospheric calculations. Current information on the ionic composition of the D region and the implications of the present results to de-ionization rate calculations are discussed.  相似文献   

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