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1.
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   

2.
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   

3.
A theoretical study is made of the visible and UV line radiation of He i atoms and He ii ions from a plane-parallel model flare layer. Solutions are obtained of the statistically steady state equations for a 30 level He i-ii-iii model, with parametric representation of the line and continuum radiation fields. Optical depths and some line intensities are presented for a 1000 km thick layer. Results are given for electron temperatures 104 to 5 × 104 K and electron densities 1010 to 1014 cm–3.Work sponsored by the NASA, Marshall Space Flight Center, Alabama under contract NAS8-27988.  相似文献   

4.
The theory of plasma emission is developed under the assumption that the Langmuir waves are generated by an isotropic distribution of fast electrons. Emission from inverse power-law distributions tend to favor emission at the second harmonic with brightness temperatures up to about 108 K at 100 MHz. The concept of a gap (in velocity space) distribution is developed. Very bright plasma emission can result from a gap distribution. For brightness temperatures between 109 K and 1011 K for the second harmonic the fundamental has a brightness temperature between 108 K and 109 K. For higher brightness temperatures the fundamental is amplified and can be very much brighter than the second harmonic. The maximum brightness temperatures for the fundamental and second harmonic at 100 MHz are about 1016 K and 1013 K respectively. Mechanisms by which a gap distribution might be formed are discussed and two effective mechanisms are identified. The theory is applied to the interpretation of radio bursts of types I, II, stationary IV and V. In each case the suggested mechanism appears to be favorable.  相似文献   

5.
The formation of first molecules, negative Hydrogen ions, and molecular ions in a model of the Universe with cosmological constant and cold dark matter is studied. The cosmological recombination is described in the framework of modified model of the effective 3-level atom, while the kinetics of chemical reactions is described in the framework of the minimal model for Hydrogen, Deuterium, and Helium. It is found that the uncertainties of molecular abundances caused by the inaccuracies of computation of cosmological recombination are approximately 2–3%. The uncertainties of values of cosmological parameters affect the abundances of molecules, negative Hydrogen ions, and molecular ions at the level of up to 2%. In the absence of cosmological reionization at redshift z = 10, the ratios of abundances to the Hydrogen one are 3.08 × 10–13 for H–, 2.37 × 10–6 for H2, 1.26 × 10–13 for H2+, 1.12 × 10–9 for HD, and 8.54 × 10–14 for HeH+.  相似文献   

6.
It is generally assumed that the magnetic fields of millisecond pulsars (MSPs) are ～108 G. We argue that this may not be true and the fields may be appreciably greater. We present six evidences for this: (1) The ～108G field estimate is based on magnetic dipole emission losses which is shown to be questionable; (2) The MSPs in low mass X-ray binaries (LMXBs) are claimed to have <1011 G on the basis of a Rayleygh-Taylor instability accretion argument. We show that the accretion argument is questionable and the upper limit 1011 G may be much higher; (3) Low magnetic field neutron stars have difficulty being produced in LMXBs; (4) MSPs may still be accreting indicating a much higher magnetic field; (5) The data that predict ～108 G for MSPs also predict ages on the order of, and greater than, ten billion years, which is much greater than normal pulsars. If the predicted ages are wrong, most likely the predicted ～108 G fields of MSPs are wrong; (6) When magnetic fields are measured directly with cyclotron lines in X-ray binaries, fields ?108 G are indicated. Other scenarios should be investigated. One such scenario is the following. Over 85% of MSPs are confirmed members of a binary. It is possible that all MSPs are in large separation binaries having magnetic fields >108 G with their magnetic dipole emission being balanced by low level accretion from their companions.  相似文献   

7.
Atmospheric models have been constructed for effective temperatures 4000°, 4500° and 5000° and for hydrogen-to-metal ratios of 1, 102, 103 and 104 times the solar values, and for surface gravities of 2×104 and 2×102. The effect of metal deficiency on the atmospheric structure of these stars are studied.National Academy of Sciences, National Research Council Postdoctoral Research Associate.  相似文献   

8.
Abstract— We report measurements of 26AI, 10Be, 41Ca, and 36Cl in the silicate and metal phases of 11 mesosiderites, including several specimens each of Budulan and Estherville, of the brecciated meteorite Bencubbin, and of the iron meteorite Udei Station. Average production rate ratios (atom/atom) for metal phase samples from Estherville and Budulan are 26Al/10Be = 0.77 ± 0.02; 36Cl/10Be = 5.3 ± 0.2. For a larger set of meteorites that includes iron meteorites and other mesosiderites, we find 26Al/10Be = 0.72 ± 0.01 and 36Cl/10Be = 4.5 ± 0.2. The average 41Ca/36Cl production rate ratio is 1.10 ± 0.04 for metal separates from Estherville and four small iron falls. The 41Ca activities in dpm/(kg Ca) of various silicate separates from Budulan and Estherville span nearly a factor of 4, from <400 to >1600, indicating preatmospheric radii of >30 cm. After allowance for composition, the activities of 26Al and 10Be (dpm/kg silicate) are similar to values measured in most ordinary chondrites and appear to depend only weakly on bulk Fe content. Unless shielding effects are larger than suggested by the 36Cl and 41Ca activities of the metal phases, matrix effects are unimportant for 10Be and minor for 26Al. Noble gas concentrations and isotopic abundances are reported for samples of Barea, Emery, Mincy, Morristown, and Marjalahti. New estimates of 36Cl/36Ar exposure ages for the metal phases agree well with published values. Neon‐21 production rates for mesosiderite silicates calculated from these ages and from measured 21Ne contents are consistently higher than predicted for L chondrites despite the fact that the mesosiderite silicates have lower Mg contents than L chondrites. We suggest that the elevation of the 21Ne production rate in mesosiderite silicates reflects a “matrix effect,” that is, the influence of the higher Fe content of mesosiderites, which acts to enhance the flux of low‐energy secondary particles and hence the 21Ne production from Mg. As 10Be production is relatively insensitive to this matrix effect, 10Be/21Ne ages give erroneously low production rates and high exposure ages. By coincidence, standard 22Ne/21Ne based “shielding” corrections give fairly reliable 21Ne production rates in the mesosiderite silicates.  相似文献   

9.
The methane abundance in the lower Jovian stratosphere is measured using Galilean satellite eclipse light curves. Spectrally selective observations in and between absorption bands are compared. An average mixing ratio at the locations measured is [CH4]/[H2] ～ 1.3 × 10?3, larger than the value 0.9 × 10?3 expected for a solar abundance of carbon. Some zenographic variation of the mixing ratio may occur. Observationally compatible values are 1.3–2.0 × 10?3 in the STZ, 1.3– 2.6 × 10?3 on the GRS/STrZ edge, and 0.7–1.3 × 10?3 in the GRS.  相似文献   

10.
The rapid neutron capture process (r-process) is one of the major nucleosynthesis processes responsible for the synthesis of heavy nuclei beyond iron. Isotopes beyond Fe are most exclusively formed in neutron capture processes and more heavier ones are produced by the r-process. Approximately half of the heavy elements with mass number A > 70 and all of the actinides in the solar system are believed to have been produced in the r-process. We have studied the r-process in supernovae for the production of heavy elements beyond A = 40 with the newest mass values available. The supernova envelopes at a temperature >109 K and neutron density of 1024 cm?3 are considered to be one of the most potential sites for the r-process. The primary goal of the r-process calculations is to fit the global abundance curve for solar system r-process isotopes by varying time dependent parameters such as temperature and neutron density. This method aims at comparing the calculated abundances of the stable isotopes with observation. We have studied the r-process path corresponding to temperatures ranging from 1.0 × 109 K to 3.0 × 109 K and neutron density ranging from 1020 cm?3 to 1030 cm?3. With temperature and density conditions of 3.0 × 109 K and 1020 cm?3 a nucleus of mass 273 was theoretically found corresponding to atomic number 115. The elements obtained along the r-process path are compared with the observed data at all the above temperature and density range.  相似文献   

11.
The URCA neutrino loss rate from a hot stellar environment is investigated. The results indicate that the loss rates for URCA type processes from even mass number nuclei are comparable to the rates from odd mass number nuclei at temperatures above about 109 K. Rates are calculated for some typical odd mass isobar pairs and for the even mass isobar fifty-six for temperatures between 5×108 K to 5×1010 K.Supported in part by National Science Foundation Grant GP13959.  相似文献   

12.
Some weak interaction processes which are important in stars whose central temperatures and densities exceed 109°K and 106 gm/cm3 are discussed. Simple analytic expressions for reaction rates which are convenient for computer studies of the late stages of stellar evolution are given.Of the National Bureau of Standards and University of Colorado.  相似文献   

13.
The effect of radiation losses on the dispersion and damping of magnetohydrodynamic waves in the solar corona is studied. The conditions are determined under which radiation losses are most appreciable. A damping of kink modes of coronal loops with plasma temperatures within 106–106.3 K and 106.3–107 K are compared. It is concluded that the radiation damping dominates in the temperature range 106–106.3 K, which can cause the observed fast damping of kink oscillations of coronal loops. Radiation losses should be taken into account in full magnetohydrodynamic equations with radiative transfer.  相似文献   

14.
This paper presents disk models for cataclysmic variables in which convection in the central layers has been included. The calculation of the vertical structure at different points is presented. The models have a central mass of 1M ⊙ and matter fluxes of 10?9, 10?8, and 10?7 M ⊙ yr?1. The corresponding luminosities are 1.86, 1.86×10 and 1.86×102 L ⊙.  相似文献   

15.
E. Grün  H.A. Zook  H. Fechtig  R.H. Giese 《Icarus》1985,62(2):244-272
Taking into account meteoroid measurements by in situ experiments, zodiacal light observations, and oblique angle hypervelocity impact studies, it is found that the observed size distributions of lunar microcraters usually do not represent the interplanetary meteoroid flux for particles with masses ?10?10g. From the steepest observed lunar crater size distribution a “lunar flux” is derived which is up to 2 orders of magnitude higher than the interplanetary flux at the smallest particle masses. New models of the “lunar” and “interplanetary” meteoroid fluxes are presented. The spatial mass density of interplanetary meteoritic material at 1 AU is ～10?16g/m3. A large fraction of this mass is in particles of 10?6 to 10?4 g. A detailed analysis of the effects of mutual collisions (i.e., destruction of meteoroids and production of fragment particles) and of radiation pressure has been performed which yielded a new picture of the balance of the meteoritic complex. It has been found that the collisional lifetime at 1 AU is shortest (～104years) for meteoroids of 10?4 to 1 g mass. For particles with masses m > 10?5g, Poynting-Robertson lifetimes are considerably larger than collisional lifetimes. The collisional destruction rate of meteoroids with masses m ? 10?3g is about 10 times larger than the rate of collisional production of fragment particles in the same mass range. About 9 tons/sec of these “meteor-sized” (m > 10?5g) particles are lost inside 1 AU due to collisions and have to be replenished by other sources, e.g., comets. Under steady-state conditions, most of these large particles are “young”; i.e., they have not been fragmented by collisions and their initial orbits are not altered much by radiation pressure drag. Many more micrometeoroids of masses m ? 10?5g are generated by collisions from more massive particles than are destroyed by collisions. The net collisional production rate of intermediate-sized particles 10?10g ? m ? 10?5g is found to be about 16 times larger at 1 AU than the Poynting-Robertson loss rate. The total Poynting-Robertson loss rate inside 1 AU is only about 0.26 tons/sec. The smallest fragment particles (m ? 10?10g) will be largely injected into hyperbolic trajectories under the influence of radiation pressure (β meteoroids). These particles provide the most effecient loss mechanism from the meteoritic complex. When it is assumed that meteoroids fragment similarly to experimental impact studies with basalt, then it is found that interplanetary meteoroids in the mass range 10?10g ? m ? 10?5g cannot be in temporal balance under collisions and Poynting-Robertson drag but their spatial density is presently increasing with time.  相似文献   

16.
Equations of thermal equilibrium along coronal loops with footpoint temperatures of 2 × 104 K are solved. Three fundamentally different categories of solution are found, namely hot loops with summit temperatures above about 4 × 105 K, cool loops which are cooler than 8 × 104 K along their whole length and hot-cool loops which have summit temperatures around 2 × 104 K but much hotter parts at intermediate points between the summit and the footpoints. Hot loops correspond to the hot corona of the Sun. The cool loops are of relevance for fibrils, for the cool cores observed by Foukal and also for active-region prominences where the magnetic field is directed mainly along the prominence. Quiescent prominences consist of many cool threads inclined to the prominence axis, and each thread may be modelled as a hot-cool loop. In addition, it is possible for warm loops at intermediate summit temperatures (8 × 104K to 4 × 105 K) to exist, but the observed differential emission measure suggests that most of the plasma in the solar atmosphere is in either the hot phase or the cool phase. Thermal catastrophe may occur when the length or pressure of a loop is so small that the hot solution ceases to exist and there are only cool loop solutions. Many loops can be superimposed to form a coronal arcade which contains loops of several different types.  相似文献   

17.
Lidar observations of atmospheric sodium reveal a diurnally steady layer with a maximum concentration near 92 km of 3?5 × 103cm?3. These data imply that both Fe and Mg should have peak concentrations near 105 cm?3 on the basis of their relative abundance to sodium in chondritic meteorites. We review the available metallic ion data below 100 km and find typical maxima of 102 cm?3 for Na and 103 cm?3 for both [Fe+] and [Mg+] for undisturbed daytime conditions. We argue that enhancements of up to 104 cm?3 for Fe+ and Mg+ during aurora support the viewpoint that [Fe] and [Mg] both approach 105 cm?3 near 92 km. We briefly discuss the lack of spectroscopic evidence for such concentrations. In our review of the metallic ion data we find evidence for about a factor of ten diurnal variation in both Fe+ and Mg+ and probably Na+. Minimum concentrations of 102 cm?3 are attained just before dawn for [Fe+] and [Mg+].  相似文献   

18.
The rates of photodissociation of the OH and OD molecules from absorption of solar radiation in he X2Π-A2Σ+ electronic transition are calculated to lie between 3.5 and 6.7 × 10?6 sec?1 for OH for heliocentric velocities between -60 and +60 km sec?1 and to be about 4.7 × 10?7 sec?1 for OD at 1 AU from the Sun. The corresponding lifetimes, which are upper bounds to the actual lifetimes, are generally consistent with the observational cometary data.  相似文献   

19.
Abstract— Isotopic compositions and abundances of boron were measured in sixteen chondrules from seven chondrites by ion microprobe mass spectrometry. The chondrules are of the porphyritic, barred, and radial type and host meteorites include carbonaceous, ordinary, and enstatite chondrites. Boron abundances are generally low with average boron concentrations of between 80 and 500 ppb. These abundances are lower than those of bulk chondrites (0.35 to 1.2 ppm; Zhai et al., 1996), confirming earlier suggestions that boron is mostly contained in the matrix. No significant variation in the 11B/10B ratio is observed among these chondrules, outside our experimental error limits of several permil, and B‐isotopic compositions agree with those reported for bulk chondrites. The lack of a significant isotope fractionation between chondrules and matrix implies that the low boron abundances are not the result of a Rayleigh fractionation during chondrule formation. Isotopic heterogeneities within individual chondrules are constrained to be < ±20%0 at > 95% confidence level at a spatial scale of 20–30 μm, significantly lower than the value of about ±40%0 previously reported for chondrules from carbonaceous and ordinary chondrites (Chaussidon and Robert, 1995, 1998). The observed B‐isotopic homogeneity does not conflict with the presence of decay products from extinct 10Be, with (10Be/9Be)0 ? 10?3, as was inferred for calcium‐aluminum‐rich inclusions. Extinct 10Be in chondrules would shift the abundance ratio 11B/10B at best by several permil because of their commonly observed low Be/B ratios (<2). The results show that potential B‐isotopic heterogeneities in the solar nebula due to the presence of components with different B‐isotopic signatures, such as boron produced by high‐energy galactic cosmic rays (11B/10B ? 2.5), or by the hypothetical low‐energy particle irradiation (11B/10B ? 3.5–11) or boron from type II supernovae (11B/10B >> 1), did not survive the chondrule formation processes to a measurable extent.  相似文献   

20.
By correlating the 1:25,000,000 geologic map of Mars of Scott and Carr (1977) with 4- to 10-km-diameter crater density data from Mariner 9 images, the average crater density for 23 of the equatorial geologic-geomorphic units on Mars was computed. The correlation of these two data sets was accomplished by digitizing both the crater density data and geologic map at the same scale and by comparing them in a computer. This technique assigns the crater density value found in the corresponding location on the geologic data set to a discrete computer file assigned each of the 23 geologic units. By averaging the crater density values accumulated in each file, an “average” crater density for each geologic unit was obtained. Condit believes these average crater density values are accurate indicators of the relative age of the geologic units considered. The statistical validity of these average values is strongest for the geologic units of the largest areal extent. The relative ages as obtained from the average crater density values for the seven largest geologic units, from youngest to oldest, are: Tharsis volcanic material, 21 ± 4 craters/106km2; smooth plains material, 57 ± 14 craters/106km2; rolling plains material, 66 ± 16 craters/106km2; plains materials, 80 ± 17 craters/106km2; ridged plains material, 128 ± 25 craters/106km2; hilly and cratered material, 137 ± 38 craters/106km2; and cratered plateau material, 138 ± 27 craters/106km2.  相似文献   

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