共查询到20条相似文献,搜索用时 15 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.
We have constructed a reaction system containing the chemical families of H, C, O, N, S, Si, Cl, metals (Me) and grains. A total of 104 species have been included and a network of 557 reactions has been studied. The chemical kinetic equations were integrated as a function of time by using gear program. The chemical reaction system was followed at low, intermediate and high cloud densities i.e. from 10–107 particles cm-3. The calculated fractional abundances of N2, CN, HCN, and CH which are in good agreement with the results of observations and with those of previous theoretical studies. 相似文献
3.
M. Y. Amin 《Earth, Moon, and Planets》1996,73(2):133-145
A new time-dependent model of chlorine chemistry in cold and warm TMC-1 and Orion clouds was constructed using results of recent laboratory and theoretical studies on the reaction of chlorine molecules. The chemistry of chlorine was found to be fairly simple in dense interstellar clouds. Only the species Cl and HCl have a significant fractional abundance. Our results for the HCl molecule are in agreement with those obtained from the observation and from theoretical values of Orion cloud. 相似文献
4.
In this paper we study the effect of shock waves on the chemical structure of the interstellar clouds. A model of molecular cloud has been assumed. The chemistry is investigated in a time dependent model. Our chemical network contains 56 species in 251 reactions to including molecules of the elements H, O, C, N, S, and Si.The results indicate that the calculated fractional abundance of the molecules NS, H2O, CN, NH, CO, and SO agrees well with the observations. The molecules OH, H2S, CS, H2CS, HS, NO, SiO, CH, CH2, CH3, HCO, C2, and HCN reach high post-shock abundances. 相似文献
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6.
W. W. Duley 《Astrophysics and Space Science》1976,45(2):261-267
Temperature fluctuations induced in very small (r10–3 ) interstellar grains by the absorption of photons from the ultra-violet radiation field or by energy released on molecule formation are shown to lead to significant gas heating due to thermal desorption of condensed atoms or molecules. For clouds with N(H)=1–10×1020 cm–2, heating rates due to this process are comparable to direct heating by cosmic rays or grain photoelectrons. 相似文献
7.
Bell MB Avery LW Matthews HE Feldman PA Watson JK Madden SC Irvine WM 《The Astrophysical journal》1988,326(2):924-930
We have detected the 1(10)-1(01) transition of C3HD at 19.418 GHz at twelve positions in cold, dark clouds and resolved the D hyperfine components in two sources (L1498 and TMC-1C) well enough to derive values for the D quadrupole coupling constants. Simultaneous observations of C3H2 in each source yield relative integrated line intensities in the range 0.10-0.18, from which we derive relative [C3HD]/[C3H2] abundances in the range 0.05-0.15. These are among the highest deuteration ratios yet observed. Within the limits of the observational and modeling uncertainties it is possible to explain the derived [C3HD]/[C3H2] ratios by ion-molecule chemistry if [e-] approximately 3 x 10(-7). 相似文献
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10.
V.I. Shishov 《Astrophysics and Space Science》2001,278(1-2):163-169
Data on interstellar diffraction and refraction scintillation of pulsars are analyzed. Comparison between theory and the observational
data shows that two types of spectra for electron density fluctuations are realized in the interstellar medium: pure power
law and piecewise with a break. The distribution of turbulent plasma in the Galaxy has a three component structure. Component
A is diffuse and it is distributed outside of the spiral arms of the Galaxy. Component BI is cloudy and associated with Galactic
arms. Component BII is extremely nonuniform and associated with HII regions and supernova remnants. The origin of the interstellar
plasma turbulence is considered, and possible sources of turbulent energy are discussed. The contribution of supernova bursts
in the interstellar gas ionization and generation of turbulence are analyzed among other factors.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
11.
The temporal morphology of auroral spectral emission features is investigated. For a given energy distribution of bombarding electrons but a time varying flux magnitude, the emission rates of various auroral radiations exhibit a nonlinear time response due to the variety of reactions that contribute to excitation. Absolute intensities and intensity ratios of various spectroscopic features, therefore, can vary solely with atmospheric interaction effects, indicating the importance of considering the time history of a precipitation event when attempting to infer the characteristics of auroral electrons from optical measurements. 相似文献
12.
Gas phases of the interstellar medium (ISM) coexist locally, penetrate each other and mix by means of dynamical and plasmaphysical
processes. E.g. heat conduction from the hot to the cooler gas leads to energy and mass exchange between the gas phases. Analytical
solutions exist under which evaporation of cloudy material or condensation of hot gas onto the clouds' surface dominate. Since
these results are derived for stationary and static conditions and under ideal assumptions, they do not necessarily hold for
a dynamical ISM. On the other hand, the mass and energy exchange between the gas phases is of great importance for the energy
budget of the ISM and by this influences the evolution of galaxies. This led us to investigate the evolution of interstellar
clouds in a hot gas by means of numerical simulations. At first, we compare static models with the analytical results and
found that interstellar clouds with parameters requiring analytically evaporation are, in contrast, accreting surrounding
material if self-gravitation and cooling are implied. For the more realistic case, where clouds are embedded in a streaming
hot gas, the models show that Kelvin-Helmholtz instability which leads to the disruption of the clouds is suppressed by heat
conduction so that the clouds are stabilized to survive.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
13.
Theodore P. Snow Jr. 《Astrophysics and Space Science》1979,66(2):453-466
Observational results obtained to date on interstellar grains and molecules are briefly reviewed, and several promising areas for further research withSpacelab are suggested. Regarding grains, useful data can be expected on the shape of the ultraviolet extinction curve for new interstellar regions; the nature of UV extinction at short wavelengths, down to the Lyman limit; the presence or absence of structure in the UV extinction curve comparable to the visible-wavelength diffuse bands; the scattering properties of grains in new kinds of clouds and nebulae; and the polarization properties of grains in UV wavelengths. The principal advances which may be expected in observations of molecules will include the ability to probe more heavily-obscured regions, where molecular species are more abundant than in the diffuse clouds observed to date; coverage of wavelength regions (such as 1400–3200) not well-studied with previous instruments such asCopernicus; and the capability of observing in optical absorption species detected in the same line of sight in radio emission, which provides unique information on cloud geometry and physical conditions. 相似文献
14.
I. A. Crawford 《Monthly notices of the Royal Astronomical Society》2000,317(4):996-1004
Intermediate-resolution (60 000 R 120 000) observations of interstellar Na i lines towards 29 stars in the general direction of the Lupus molecular clouds (330°≲ l ≲350°; 0°≲ b ≲25°) are presented. Previously published spectra towards an additional seven stars are also included. Based on the Hipparcos distances to these stars, and the minimum distance at which strong interstellar Na i lines appear in the spectra, I obtain a distance of ~150±10 pc to the Lupus molecular complex. While in agreement with a number of other independent estimates, this result is at odds with the value of 100 pc recently obtained by Knude & Høg from a Hipparcos -based study of interstellar extinction. A possible explanation for this discrepancy is discussed, and it is concluded that the value of 150±10 pc obtained here is to be preferred. In addition, these observations have some other implications for the structure of the interstellar medium in this direction, and these are briefly considered. 相似文献
15.
Using the standard GEANT4 code, we calculated the ionization rate of the Mars atmosphere and the dose absorbed by the planet’s
soil caused by the galactic cosmic rays and the anomalous cosmic ray component in the heliosphere. Cases of the solar system
passing through dense molecular clouds leading to an increase of the energetic particle flux at the orbit of Mars and cases
of thickness variation of the atmosphere itself are considered. 相似文献
16.
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
17.
Evelyne Roueff Guillaume Pineau Des Forêts David R. Flower 《Astrophysics and Space Science》1995,233(1-2):125-137
Shock waves perturb the chemical state of the interstellar gas. We consider the effects of C-shocks on the composition of molecular clouds, for a range of values of the pre-shock gas density and magnetic induction. The time required to re-establish equilibrium in the post-shock gas depends on the initial conditions and can become very large. The significance of the two known chemical phases of dark clouds and of bistability is considered in this context. 相似文献
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Jayadev Rajagopal G. Srinivasan K. S. Dwarakanath 《Journal of Astrophysics and Astronomy》1998,19(3-4):117-131
In the preceding paper (Paper I), we presented HI absorption spectra towards radio sources very close to the lines of sight
towards twenty five bright stars against which optical absorption spectra had been obtained earlier, In this paper we analyse
the results and draw some conclusions.
To summarize briefly, in most cases we found HI absorption at velocities corresponding to the optical absorption features
provided one restricted oneself to velocities ≲10 kms-1. At higher velocities we did not detect any HI absorption down to
an optical depth limit of 0.1 (except in four cases which we attribute to gas in systematic motion rather than clouds in random
motion). After discussing various scenarios, we suggest that this trend should perhaps be understood in terms of the high
velocity interstellar clouds being accelerated, heated and ablated by expanding supernova remnants. 相似文献
20.
Jayadev Rajagopal G. Srinivasan K. S. Dwarakanath 《Journal of Astrophysics and Astronomy》1998,19(3-4):97-116
This investigation is aimed at clarifying the nature of the interstellar gas seen in absorption against bright O and B stars.
Towards this end we have obtained for the first time HI absorption spectra towards radio sources very close to the lines of
sight towards twenty five bright stars previously studied. In this paper we describe the selection criteria, the details regarding
our observations, and finally present the absorption spectra. In the accompanying paper we analyse the results and draw conclusions. 相似文献