首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
We have constructed a chemical reaction model in a contracting interstellar cloud including 104 species which are involved in a network of 557 reactions. The chemical kinetic equations were integrated as a function of time by using gear package. The evolution of the system was followed in the density range 10–107 particles cm-3.The calculated fractional abundances of the charged species are in good agreement with those given by other investigators. The charge density has been followed in diffuse, intermediate and dense regions. The most dominant ionic species are metallic ions, HCO+ and H 3 + in the shielded regions and atomic ions H+, C+, Si+, He+, S+ and metal ions in the diffuse and intermediate regions. The abundances of negatively charged ions were found to be negligible. The results of the calculations on the different metallic ions are interpreted.  相似文献   

2.
Abstract. We have constructed a chemical reaction system in a contracting interstellar cloud. In paper (I) we have presented the details of the physical and chemical scheme and the method of solution. The results of our chemical model produce fractional abundances of H2CO, CO, OH, H2O, SO and OCS which are in good agreement with the results of observations. On the other hand, the results of chlorine-bearing species are not in agreement with those of the observations. The calculated abundances of H2CO, CO, OH, H2O, SO, OCS and Cl+ are in agreement with the results of previous theoretical studies.  相似文献   

3.
A chemical evolution of W3 IRS5 have been studied during J-type shock. We modified our code of for one-dimensional J-type shock to satisfy the physical conditions of W3 IRS5. The chemical rate equations and the hydrodynamics of shocks are integrated simultaneously. We find that, a shock of speed V s = 10 km s-1 and magnetic induction B=4.1×10-6 Gauss, propagate into molecular gas density of n=104cm-3, and with initial temperature of 30 K (pre shock) satisfy the physical conditions of W3 IRS5. The results using the extensive chemical network with the most updating reaction rates show that the calculated fractional abundances are in agreement with observation. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

4.
A thermodynamical analysis of the multicomponent system SiTiAlFeMnMgCaNaKPCHO open with respect to CO2, CO, H2O was carried out. Hydration and carbonatization processes are proposed to be geochemical consequences of the hypothesis of quasi-equilibrium conditions between the troposphere and crustal surface rocks. The probable rock-forming hydrated mineral phases are represented by epidote, glaucophane, tremolite, phlogopite, and annite; the carbonatization results in existence of calcite and dolomite as rock-forming minerals of weathered alkaline lavas. The surface rocks are assumed to have high ferric/ferrous iron ratios. The wollastonite equilibrium is rejected as a buffering chemical reaction. Hydrated minerals could be stable at least up to 5-km depths and contribute about 0.1 × 1024 g of H2O whereas about (0.7–0.8) × 1024 g of H2O would be consumed in ferrous iron oxidation with concomitant hydrogen dissipation. The distribution of H2O in the outer planetary shells is possibly a function of their temperatures.  相似文献   

5.
Recently, modelers have expressed a concern that the currently known chemistry of atmospheric NOy is deficient. It is therefore necessary to explore possible sources and sinks of atmospheric NOx that may have been overlooked. In this context, it is noteworthy that the experimentally observed, four-center, Woodward-Hoffman forbidden, reaction 02(B 3Σ) + N2 → NO(X) + NO (X) is atmospherically significant. In the 20 to 30 km region NOx production from this reaction may potentially exceed the production from the “classical” N20 + O(1D) reaction, and may provide a new mechanism to couple the solar UV variability and stratospheric ozone. The avoidance of the non-conservation of the orbital symmetry via the production of one NO in the excited electronic state being endothermic, it appears that the interaction of 02(B 3Σ) with the adjoining 1Λ, 3Λ and 3Σu+ states might be responsible for the reaction. Experimental studies of the reaction as a function of the vibrational levels of the B-state, temperature and pressure are needed for reliable atmospheric applications of this reaction. At altitudes greater than about 50 km the production of NO from 02(B) begins to decrease rapidly. The NO production from 02 (A 3Σ++) + N2 → NO + NO reaction may become important here, if the reaction is confirmed by experiments. These new sources of NOx call for new sinks of this species. In the upper stratosphere and mesosphere the chemical acceleration of NO dissociation via the reactions of electronically and vibrationally excited NO with 02 may help. In the lower atmosphere there is a possibility of the annihilation of NO, N02pair leading to the recreation of a stable NN bond. This might happen if N203 from NO and N02 recombination may photodissociate as N20 + 02. Unfortunately the requirements are stringent, and only experiments can tell whether or not this mechanism operates in the atmosphere.  相似文献   

6.
The first generation of stars was formed from primordial gas. Numerical simulations suggest that the first stars were predominantly very massive, with typical masses M≥100M ⊙. These stars were responsible for the reionization of the universe, the initial enrichment of the intergalactic medium with heavy elements, and other cosmological consequences. In this work, we study the structure of Zero Age Main-Sequence stars for a wide mass and metallicity range and the evolution of 100, 150, 200, 250 and 300M ⊙ galactic and pregalactic Pop III very massive stars without mass loss, with metallicity Z=10−6 and 10−9, respectively. Using a stellar evolution code, a system of 10 equations together with boundary conditions are solved simultaneously. For the change of chemical composition, which determines the evolution of a star, a diffusion treatment for convection and semiconvection is used. A set of 30 nuclear reactions are solved simultaneously with the stellar structure and evolution equations. Several results on the main sequence, and during the hydrogen and helium burning phases, are described. Low-metallicity massive stars are hotter and more compact and luminous than their metal-enriched counterparts. Due to their high temperatures, pregalactic stars activate sooner the triple alpha reaction self-producing their own heavy elements. Both galactic and pregalactic stars are radiation pressure dominated and evolve below the Eddington luminosity limit with short lifetimes. The physical characteristics of the first stars have significant influence in predictions of the ionizing photon yields from the first luminous objects; also they develop large convective cores with important helium core masses which are important for explosion calculations.  相似文献   

7.
Z. Peeters  R.L. Hudson  M.H. Moore 《Icarus》2010,210(1):480-487
The radiation chemistry, thermal stability, and vapor pressure of solid-phase carbonic acid (H2CO3) have been studied with mid-infrared spectroscopy. A new procedure for measuring this molecule’s radiation stability has been used to obtain intrinsic IR band strengths and half-lives for radiolytic destruction. We report, for the first time, measurements of carbonic acid’s vapor pressure (0.290-2.33 × 10−11 bar for 240-255 K) and its enthalpy of sublimation (71 ± 9 kJ mol−1). We also report the first observation of a chemical reaction involving solid-phase carbonic acid. Possible applications of these findings are discussed, with an emphasis on the outer Solar System icy surfaces.  相似文献   

8.
Based on our spectrophotometric observations, we have investigated the envelope of Nova V339 Del in the nebular phase. Our modeling of the Hα line profiles and their comparison with the observed profiles have shown that the Nova envelope consists of circumpolar ejecta and a disk-shaped central component. The inclination of the orbital plane of the binary system, 65°, has been determined in the same way. We have estimated the mean electron density to be ~106 cm-3. Our estimates of the abundances of some chemical elements in the Nova envelope have shown that the concentrations of helium, neon, and iron are nearly solar, while the concentrations of nitrogen and oxygen exceed the solar ones by a factor of 120 ± 60 and 8 ± 1.6, respectively. The mass of the emission envelope in the nebular phase (from 253 to 382 days after the brightness maximum) has been estimated to be ~7 × 10-5 M ⊙.  相似文献   

9.
We have followed the chemistry of W3 IRS4, as one of the most famous regions with a chemistry similar to that found in quiescent molecular clouds. Here the time scale, since star formation, has probably been long enough for the gas-phase ion-molecule chemistry to become dominant again. We have constructed a reaction system containing the chemical families of H, C, O, N, S and metals (Me:Fe, Mg, Na and Si). A total of 282 species have been included and a network of 1270 reactions has been used. The chemical kinetic equations were integrated as a function of pseudo-time-dependent approach. The evolution of the different chemical species are followed in models of density n equal to 106 cm-3 and temperature T equal to 55 K. Different initial abundances have been assumed. Our results are in reasonable agreement with the observation, especially in Model 6. The resulting molecular abundances are compared with those obtained in the intermediate mass star NGC 2264 IRS1, as well as the low mass IRAS 16293-2422. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

10.
From the experimentally measured (p,n) cross-sections for92,94Zr,93Nb,95,96,98Mo,103Rh,107,109Ag,110Pd,115In,117,122Sn nuclides, for proton energies below 7 MeV, thermonuclear reaction rates in the temperature range 1≤T 9≤5(T9=109 K) have been extracted. These reaction rates have been fitted to a three parameter empirical expression proposed by Fowler.  相似文献   

11.
A meteorite which fell at Lahrauli, district Basti, U.P. (India) in 1955 has been examined for mineralogical, chemical and cosmogenic characteristics. It contains pigeonite Wo7.7Fs18 (with Cr2O3 = 1.2% and Al2O3 = 0.4%), olivine Fo79 (with CaO = 0.3% and Cr2O3 = 0.7%) and diamond. The basic similarities of this meteorite to Goalpara, Dyalpur, Havero and Novo Urei indicate that it is a ureilite. Cosmic ray tracks have been measured in the olivine and pyroxene grains. Track density of 2.3 × 106 per cm2 in olivines and VVH/VH ratio of 10?3 is similar to that observed in other meteorites.  相似文献   

12.
Loss processes which remove Si+ ions selectively relative to other meteor-derived atomic ions in the E- and D-regions of the ionosphere have been identified and measured in the laboratory. The major Si+ loss in the E-region is the reaction Si+ + H2O → HSiO+ + H (1) with a rate constant 2.3 ± 0.9 × 10?10 cm3s?1 at 300 K. The corresponding reactions with Fe+, Mg+ and other metallic meteor ions are endothermic. Presumably (1) is followed by a fast dissociative-recombination with electrons to produce neutral SiO or Si. At lower altitudes Si+ ions associate in a three-body reaction with O2 with a much larger rate constant than the corresponding associations of Fe+ and Mg+ with O2.  相似文献   

13.
A new model is presented on how chemically driven cryovolcanism might contribute to episodic outgassing at the icy moon Enceladus and potentially elsewhere including Europa and Kuiper Belt Objects. Exposed water ices can become oxidized from radiolytic chemical alteration of near-surface water ice by space environment irradiation. In contact with primordially abundant reductants such as NH3, CH4, and other hydrocarbons, the product oxidants can react exothermically to produce volatile gases driving cryovolcanism via gas-piston forces on any subsurface liquid reservoirs. Radiolytic oxidants such as H2O2 and O2 can continuously accumulate deep in icy regoliths and be conveyed by rheological flows to subsurface chemical reaction zones over million-year time scales indicated by cratering ages for active regions of Enceladus and Europa. Surface blanketing with cryovolcanic plume ejecta would further accelerate regolith burial of radiolytic oxidants. Episodic heating from transient gravitational tides, radioisotope decay, impacts, or other geologic events might occasionally accelerate chemical reaction rates and ignite the exothermic release of cumulative radiolytic oxidant energy. The time history for the suggested “Old Faithful” model of radiolytic gas-driven cryovolcanism at Enceladus and elsewhere therefore consists of long periods of chemical energy accumulation punctuated by much briefer episodes of cryovolcanic activity. The most probable sequence for detection of activity in the current epoch is a long evolutionary phase of slow but continuous oxidant accumulation over billions of years followed by continuous but variable high activity over the past 107-108 years. Detectable cryovolcanic activity could then later decline due to near-total oxidation of the rheologically accessible ice crust and depletion the accessible reductant abundances, as may have already occurred for Europa in the more intense radiation environment of Jupiter's magnetosphere. Astrobiological potential of Enceladus could correspondingly be higher than at Europa due to a less extreme state of oxidation and greater residual abundance of organics.  相似文献   

14.
Ethane (C2H6), methylacetylene (CH3C2H or C3H4) and diacetylene (C4H2) have been discovered in Spitzer 10-20 μm spectra of Uranus, with 0.1-mbar volume mixing ratios of (1.0±0.1)×10−8, (2.5±0.3)×10−10, and (1.6±0.2)×10−10, respectively. These hydrocarbons complement previously detected methane (CH4) and acetylene (C2H2). Carbon dioxide (CO2) was also detected at the 7-σ level with a 0.1-mbar volume mixing ratio of (4±0.5)×10−11. Although the reactions producing hydrocarbons in the atmospheres of giant planets start from radicals, the methyl radical (CH3) was not found in the spectra, implying much lower abundances than in the atmospheres of Saturn or Neptune where it has been detected. This finding underlines the fact that Uranus' atmosphere occupies a special position among the giant planets, and our results shed light on the chemical reactions happening in the absence of a substantial internal energy source.  相似文献   

15.
Experimental results on the spectral identification of new infrared absorption features and the changes of their absorbances produced through vacuum ultraviolet-extreme ultraviolet (VUV-EUV) photon-induced chemical reactions in the C2H2-H2O mixed ices at 10 K are obtained. To the best of our knowledge, this is the first time that EUV photons have been employed in the study of the photolysis of ice analogues. Two different compositions, i.e., C2H2:H2O=1:4 and 1:1, were investigated in this work. A tunable intense synchrotron radiation light source available at the Synchrotron Radiation Research Center, Hsinchu, Taiwan, was employed to provide the required VUV-EUV photons. In this study, the photon wavelengths selected to irradiate the icy samples corresponded to the prominent solar hydrogen, helium, and helium ion lines at 121.6 nm, 58.4 nm, and 30.4 nm, respectively. The photon dosages used were typically in the range of 1×1015 to 2×1017 photons. Molecular species produced and identified in the ice samples at 10 K resulting from VUV-EUV photon irradiation are mainly CO, CO2, CH4, C2H6, CH3OH, and H2CO. In addition to several unidentified features, we have tentatively assigned several absorption features to HCO, C3H8, and C2H5OH. While new molecular species were formed, the original reactants, i.e., H2O and C2H2, were detectably depleted due to their conversion to other species. The new chemical species produced by irradiation of photons at 30.4 nm and 58.4 nm can be different from those produced by the 121.6-nm photolysis. In general, the product column density of CO reaches saturation at a lower photon dosage than that of CO2. Furthermore, the production yield of CO is higher than that of CO2 in the photon irradiation. In the present study, we also observe that the photon-induced chemical reaction yields are high using photons at 30.4 and 58.4 nm. The results presented in this work are essential to our understanding of chemical synthesis in ice analogues, e.g., the cometary-type ices and icy satellites of planetary systems.  相似文献   

16.
A possible scenario for the evolution of the universe following the big bang at t > 10-5 sec is considered. The necessary conditions that must be present for the formation of stars and stellar systems to be possible are formulated. As a condition for the formation of stars we take kTs≤ GMsmp(3R), and for stellar systems HR ? (GM/R)1/2, where Ts is the temperature of the cosmic plasma, mp is the mass of a proton, Ms is the mass of a star, M is the mass of a stellar cluster, R is the radius of these celestial bodies, and H is the bubble parameter for the corresponding time. In accordance with these criteria, we assume that in the course of cosmological expansion, neutron stars should have been formed first (times 2.10-4 ? t ? 1 sec, densities 0.07 ? ρB? 2.104 g/cm3) and then, in chronological order, appeared white dwarfs (t ≈ 102 sec, ρB ? 5.10-3 g/cm3), ordinary stars (t ≈ 4.106 sec, ?B ≈ 10-11 g/cm3), galactic nuclei (t ≈ 3.1011 sec, ?B ≈ 5.10-19 g/cm3, globular clusters (t ≈ 1013 sec, ?B ≈ 4.10-21 g/cm3), and galaxies (t ≈ 1015 sec, ?B ≈ 10-24 g/cm3), where ?B is the average density of ordinary (baryon) matter in the universe. It is shown that a galactic nucleus is a stellar system in statistical equilibrium and consists mainly of neutron stars and white dwarfs. The formation of some pulsars (neutron stars with angular rotation rates 1 < Ω < 200 sec-1) may occur in a galactic nucleus. Observed pulsars should therefore contain some fraction of neutron stars from the nucleus of the Galaxy that were able to escape it over the relaxation time (the tail of the Maxwell distribution, with star velocities v > v0, where v0 is the velocity corresponding to the work function 2GMMs/R, M being the mass and R the radius of the Galaxy’s nucleus.  相似文献   

17.
Based on our spectrophotometric observations, we have studied the envelope of the HeN Nova Mon 2012. The abundances of some chemical elements in the envelope and its mass have been estimated. Our results show that the helium, nitrogen, oxygen, and neon abundances in the Nova envelope exceed the solar ones by a factor of 1.5, 33, 9, and 95, respectively. The envelope mass has been found to be 2.3 × 10?4 M ⊙.  相似文献   

18.
New multifrequency spatial and spectral studies of the hot molecular core associated with the ultracompact HII region G34.3+0.15 have demonstrated an extremely rich chemistry in this archetypal hot core and revealed differing spatial structure between certain species which may be a dynamical effect of chemical evolution. The structure of the hot core has been studied with the JCMT in the high excitation J=19-18 and J=18-17 lines of CH3CN and with the Nobeyama Millimetre Array at 4 arc resolution in the J=6-5 transition. Comparison with a VLA NH3(3,3) map shows a displacement between peak emission in the two chemical species which is consistent with chemical processing on a time scale comparable to the dynamical time scale of 105 yrs.A 330-360 GHz spectral survey of the hot core with the JCMT has detected 358 spectral lines from at least 46 distinct chemical species, including many typical of shocked chemistry while other species indicate abundances that reflect the chemistry of a previous cold phase. The first unambiguous detection of ethanol in hot gas has been made. Observations of 14 rotational transitions of this molecule yield a temperature of 125 K and column density 2×1015 cm–2. This large abundance cannot be made by purely gas-phase processes and it is concluded that ethanol must have formed by grain surface chemistry.  相似文献   

19.
In Titan's atmosphere consisting of N2 and CH4, large amounts of atomic hydrogen are produced by photochemical reactions during the formation of complex organics. This atomic hydrogen may undergo heterogeneous reactions with organic aerosol in the stratosphere and mesosphere of Titan. In order to investigate both the mechanisms and kinetics of the heterogeneous reactions, atomic deuterium is irradiated onto Titan tholin formed from N2 and CH4 gas mixtures at various surface-temperatures of the tholin ranging from 160 to 310 K. The combined analyses of the gas species and the exposed tholin indicate that the interaction mechanisms of atomic deuterium with the tholin are composed of three reactions; (a) abstraction of hydrogen from tholin resulting in gaseous HD formation (HD recombination), (b) addition of D atom into tholin (hydrogenation), and (c) removal of carbon and/or nitrogen (chemical erosion). The reaction probabilities of HD recombination and hydrogenation are obtained as ηabst=1.9(±0.6)×10−3×exp(−300/T) and ηhydro=2.08(±0.64)×exp(−1000/T), respectively. The chemical erosion process is very inefficient under the conditions of temperature range of Titan's stratosphere and mesosphere. Under Titan conditions, the rates of hydrogenation > HD recombination ? chemical erosion. Our measured HD recombination rate is about 10 times (with an uncertainty of a factor of 3-5) the prediction of previous theoretical model. These results imply that organic aerosol can remove atomic hydrogen efficiently from Titan's atmosphere through the heterogeneous reactions and that the presence of aerosol may affect the subsequent organic chemistry.  相似文献   

20.
Exploratory models of the collapse of spherical self-gravitating clouds are studied in relation to the problem of the formation of first generation star-systems. The masses which were considered are in the range of 83 to 5.2×1010 M ⊙. For simplicity, the assumed composition includes hydrogen only, which could be in the form of H, H2, H+ or H?. Since the physical conditions that might have prevailed in a primeval nebula are not well known, rather simple initial conditions were chosen: The gas starts from rest and has initially a uniform temperature. We consider the case of rather cool (T 0～100 K) neutral clouds with different initial ionization degrees. Some of the initial density-distributions here considered are uniform while others are decreasing from the center outwards. The assumed initial values for the densities are ～10?24 g cm?3, except for one of the models, for which it is ～10?26 g cm?3. Several atomic processes within the gas, including physical-chemical reactions and the evaluation of radiative emission coefficients are considered. A system of differential equations is set up in order to evaluate the concentrationsn H,n H 2,n H +,n H ? andn e as a function of time. The treatment makes possible the study of the cooling and heating properties of the gas. Furthermore, the dynamical, thermal and chemical evolution of the cloud can be followed during the collapse. The computations apply only to the optically thin stages. The models show the importance of a correct evaluation of the chemical reactions and dissipative mechanisms, which cannot be ignored in a realistic treatment of the collapse of self-gravitating clouds. The influence of the initial conditions on the dynamical and thermal properties during evolution are also analysed.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号