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1.
Although it is generally accepted that most, if not all, of the molecular hydrogen in interstellar space is formed through recombination reactions on grains, the exact mechanism by which this is accomplished is far from certain. In the past, great emphasis had been placed on the physical adsorption of H atoms on cold dielectric grains and their subsequent recombination and desorption as H2 molecules. However, a careful re-examination of the problem leads us to believe that a rate coefficient ofk10–17 cm3 s–1—the value usually quoted in the literature—is a very strong overestimate. The same thing can be said for the recombination of H atoms on graphite grains. Since two-body gas phase reactions are not sufficient by themselves to account for the observed abundances of H2, an alternate mechanism must exist. It is suggested that the chemisorption of hydrogen on transition metal grains may be just that formation mechanism. After separating the adsorption rate equations from those of desorption and using experimentally determined parameters, it is shown that transition metal grains can successfully catalyze as much H2 as the theoretical maximum predicted for cold ice grains, even though metal grains are probably less than 10% as abundant (by mass) than dielectrics.  相似文献   

2.
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   

3.
We have observed C2H2 and HCN rovibrational transitions near 13µm in absorption against GL2591. We also have observed rotational transitions at 0.6-3 mm of CS, HCN, H2CO, and HCO+. Analysis of the rotational lines, which arise in the extended cloud around the source, shows that no single density model can explain all the data. Models with density and temperature gradients do much better; in particular models withn(r) r –1.5 can reproduce the observed pattern of emission line strengths. The abundances show significant depletion compared to models of gas-phase chemistry. The rovibrational data were analyzed in comparison to the absorption line analysis of CO by Mitchellet al. (1989). Our data are consistent with the C2H2 and HCN absorption arising in the same warm (200 K) and hot (1010 K) components seen in CO, but we see little evidence for the cold (38 K) component seen in CO. The rovibrational lines from higher states (J 21) indicate that the hot HCN deviates from LTE, leading to a density of about 3 × 107 cm–3. Comparison of the two data sets shows that the rovibrational absorption of HCN, rather than arising in the extended envelope, must come from a region with a small angular extent. A model in which early-time gas phase abundances are preserved on grain mantles and released at high temperature can explain the data.  相似文献   

4.
A unified picture of the photodissociation of theC 2 H radical has been developed using the results from the latest experimental and theoretical work. This picture shows that a variety of electronic states ofC 2 are formed during the photodissociation of theC 2 H radical even if photoexcitation accesses only one excited state. This is because the excited states have many avoided corssings and near intersections where two electronic states come very close to one another. At these avoided crossings and near intersections, the excited radical can hop from one electronic state to another and access new final electronic states of theC 2 radical. The complexity of the excited state surfaces also explains the bimodal rotational distributions that are observed in all of the electronic states studied. The excited states that dissociate through a direct path are limited by dynamics to produceC 2 fragments with a modest amount of rotational energy, whereas those that dissociate by a more complex path have a greater chance to access all of phase space and produce fragments with higher rotational excitation. Finally, the theoretical transition moments and potential energy curves have been used to provide a better estimate of the photochemical lifetimes in comets of the different excited states of theC 2 H radical. The photochemically active states are the 22+, 22II, 32II, and 32+, with photodissociation rate constants of 1.0×10–6, 4.0×10–6, 0.7×10–6, and 1.3×10–6s–1, respectively. These rate constants lead to a total photochemical lifetime of 1.4×105 s.  相似文献   

5.
The evolution of the different chemical species are followed in a model of contracting interstellar cloud. The central density increases from n = 10 cm–3 diffuse initial cloud model to a dense cloud with central density number of n >- 105 cm–3 after a time of 1.2 × 107 yr. A network of 622 reactions has been involved. The chemistry of the cloud is integrated simultaneously with the hydrodynamic equations of contraction.The results predict that the different molecular species increase in abundance as the contraction proceeds. The species which enhance significantly are CO, HCO, CS and NO. The fractional abundances of many of the other molecular species increase distinctly with contraction, e.g. CH, C2H, CN, SO2, CO2, H2O, C2, NH3, HCN, SO, OCS and SN. The transformation of the initial diffuse cloud model with small abundances of molecular species to a dense molecular cloud with enhancement of the different molecular species is confirmed. The results predict good agreements of our results with both the observations and other theoretical studies.  相似文献   

6.
Elemental abundances of the VH group of cosmic radiation have been measured in the energy interval 250–550 MeV nucl–1 in a balloon exposure at Sioux Falls (South Dakota) of a plastic detector LeXAN stack. The so obtained abundances have been extrapolated to the sources in the frame of the homogeneous model correcting for energy loss. After taking into account solar modulation, the best fit to model values has led to a escape mean free path e = 5E –0.4 g cm–2, whereE is the energy in GeV nucl–1, forE>1 GeV nucl–1, and a constant e = 5 g cm–2 forE1 GeV nucl–1. When turning to the diffusion model, also including an energy loss term, a diffusion coefficientD=3×1028 cm2 s–1 has been estimated.  相似文献   

7.
The evolution of the charged particles are followed during contraction of a model of an interstellar cloud, with initial density number of n = 10 cm–3. The contraction is followed up to density increase by five orders of magnitude. Special care is given to the details of the negative ions. In addition, we have tested the ambipolar diffusion according to the results of the ion density.The results predict the importance of atomic ions in the diffuse regions. H+ and C+ are distinctly enhanced in the beginning of contraction but decrease as contraction proceeds. Molecular ions enhance as contraction proceeds and becomes important in dense regions. The most enhanced molecular ions are HCO+, O2 +, C2H3 +, H3O+ and SO+, H3 + is less abundant. The atomic ions (except metalic ions) decrease noticeably as density increases. In general the negative ions are of negligible fractional abundances. It has also been found that the time of ambipolar diffusion is shorter than the dynamical time, hence the magnetic field should be weakened in the central core as the central density increases to n = 104 cm–3.  相似文献   

8.
This work is divided into 13 sections and 2 appendices, and aims to elucidate the accretion mechanism, which operates via image-theory forces, whenever two interstellar dust grains come close together. Section 1 is an introduction. Section 2 proposes that the distribution of interstellar grains be taken asn(r) r –4 to avoid distortion of the 3K microwave background by radiation from spinning grains. Section 3 examines each of three types of image force accretion processes, finding them to be dominant compared to radiation or gravitational forces by at least a factor of 1019. Section 4 states that only grains made of conducting material (e.g., graphite, ice, iron) are involved in image theory. Section 5 presents reasons for believing that two grains should coalesce on impact. Section 6 examines the motion of charged interstellar grains in Hi and Hii regions. Section 7 demonstrates, by way of four examples involving dust grains ofr=10–7 cm up tor=10–4 cm, that the image effects on conducting grains are not trivial, and that the dynamics involved is not to be compared at all with elementary Coulomb interaction of two changes. Section 8 concludes that accretion with not take place in Hi clouds if thermal (equipartition) velocities prevail among the dust particles. section 9 examines grain interactions in Hii regions: here, following an argument due to Spitzer, consideration is given to the case of a population of dust grains all streaming in the direction of the local magnetic field B at velocities of order 0.1 km s–1. It is shown that accretion takes place effectively, leading to the formation of interstellar grit, meaning grains of mass 10–8 to 10–7 gm, radius 0.1 mm; and leaving also a population ofr10–6 cm grains, which are observed in polarization and extinction measurements. The existence of the latter is now a deduction and not an ad hoc postulate, as previously, and implies a distribution of the general formn(r) r mean –3 , in approximate agreement with that of Section 2. Section 10 considers the accretion mechanism as a cascade process. Section 11 shows that the existence of grains in space ofr 10–6 cm rules out an origin in supernova or galactic explosions, and supports a passive origin, perhaps in red giants or Mira variables. Section 12 discusses the implications of the results found for polarization observations and cosmogony, the latter being given a new foundation in which planets of different composition form automatically from a solar nebula. Section 13 is a conclusion.  相似文献   

9.
The rate equations of a restricted set of gas-phase chemical reactions occuring in an expanding circumstellar envelope are integrated numerically on the assumption that no chemical evolution has occurred in the stellar atmosphere. Abundances of all species are found to peak at a time on the order ofr 0/u 0, wherer 0 is the initial radius andu 0 the expansion velocity. After this time geometrical dilution dominates. For an initial density of 108 cm–3, on the order of 1% of hydrogen is converted to H2, and CH and CO have comparable densities of 10–6 relative to H, OH and O2 remain very low in abundance. For higher initial densities, H and H2 are more nearly comparable, and nearly all carbon is in CO, CH, OH and O2 remain low in abundance. The relevance of these results to M giants and other objects is discussed.  相似文献   

10.
The existence of condensed carbon in the form of liquid droplets and graphite grains is found in white dwarf atmospheres with parametersg=108 cm s–2, H/He10–3, andT eff6000 K on the basis of model atmospheres techniques. It is shown that the condensation layers are dynamically stable and, consequently, that white dwarfs cannot supply the condensed particles to the interstellar medium. Possible observable effects are considered.  相似文献   

11.
Positively charged molecular ions, such as H2O+, which have been observed in cometary. comas, may be efficiently produced by the evaporation of positively charged clathrate grains of radii in the range 10–6–10–5 cm. Such grains may be expelled from nuclei of comets, along with gaseous molecules. Grain charging occurs via interaction with solar ultraviolet photons and/or solar wind protons. Observational data on the total quantities as well as the distributions of H2O and H2O+ in cometary comas are shown to be in accord with detailed model calculations.On leave from: Tata Institute of Fundamental Research, Bombay, India.  相似文献   

12.
Rapid proton capture is supposed to be responsible for the synthesis of a number of proton-rich nuclei. This process of hydrogen burning is considered here for mass elements, the atomic numbers of which range fromZ=10 toZ=20. The possible site for this process is assumed to be the outer envelope of the supernova at a proton number density (n p )ranging fromn p =1022 cm–3 ton p =1028 cm–3 at temperatures in the range ofT=2–3×109 K.The capture path is determined by considering that a dynamical equilibrium between (p, ) and (,p) reactions exists between the reacting nuclei. In this situation, the abundances of elements become proportional to the lifetime of + decaying nuclei at the waiting points.It is suggested that these rapid proton-capture reactions are responsible for the production of a number of nuclei in the rangeA40 during supernova outbursts.  相似文献   

13.
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14.
The magnetic fields observed in the galactic disc are generated by the differential rotation and the helical turbulent motions of interstellar gas. On the scalesl=2k –1 which lie in the intervall 0<l<l e (l 0100 pc is the energy-range scale of the galactic turbulence), the spectral density of the kinetic energy of turbulence (k –5/3) exceeds the magnetic energy spectral density (k –1). The equipartition between magnetic and kinetic energies is reached atl=l e 6 pc in the intercloud medium and is maintained down to the scalel=l d 0.03 pc. In dense interstellar cloudsl e is determined by the individual cloud size andl d 0.1 pc.The internal turbulent velocities in Hi clouds (cloud size is assumed to be 10 pc) lie in the range from 1.8 to 5.6km s–1, fitting well within the observed range of internal rms velocities. Dissipation of the interstellar MHD turbulence leads to creation of temperature fluctuations with amplitudes of 150 K and 65 K in dense clouds and intercloud medium, respectively. The small-scale fluctuations observed in the interstellar medium may arise from such perturbations due to the thermal instability, for instance. Dissipation of the MHD turbulence energy provides nearly half of the energy supply needed to maintain the thermal balance of the interstellar medium.  相似文献   

15.
With the aid of the spectra taken in the years 1959–1968, a physical analysis of the atmosphere of P Cygni has been carried out and the motions of the atmosphere have been studied. The variations of radial velocities, the velocity progressions of Balmer and Hei lines, the high rate of mass loss (2×10–5 M yr–1), the features of the observed line profiles, especially that of H-K lines of Caii andD 1-D 2 lines of Nai confirm the conclusion of Van Blerkom (1978), concerning the assumption of an accelerating atmosphere for P Cygni. The electron density variation with the radius seems to ben e r –5/2, with an average value of 7×1011cm–3 at the lower boundary of the atmosphere.In order to explain the two absorption components of observed lines, an atmospheric model based on the assumption of three envelopes, two of which accelerate gradually with two different velocity laws (up to 11.2r c ), and the third of which accelerates rapidly with a standard velocity law (beyond 11.2r c ) has been developed. From this model and the observed profiles, the geometrical thicknesses of the line-forming regions of H, H, H, and H are derived.The observations were obtained at Haute Provence Observatory (CNRS).  相似文献   

16.
Using a balloon borne double dE/dx x total energy telescope we have determined the isotopic composition of cosmic ray Li, Be and B nuclei in the energy range 100–250 MeV nuc.–1. The measured mass resolution, for these nuclei is 0.3 AMU. The observed isotopic composition is in agreement with that predicted on the basis of interstellar fragmentation with the exception of a deficiency of Be10. If the low abundance of Be10 is attributed to the decay of this radioactive isotope we obtain a mean cosmic ray lifetime of (3.4 –1.3 +3.4 )×106 yr.A recent measurement which we have used in this paper gives this lifetime to be (1.5±0.3)×106 yr (Yiou and Raisbeck, 1972).  相似文献   

17.
As a result of the analysis of the observed interstellar 2800 Mgii absorption line data, an empirical relationship — a positive correlation — between the equivalent widthW(2800) and the effective temperature of the starT was discovered (Figure 1). However, in the case when this doublet is of stellar (photospheric) origin, only a negative correlation betweenW(2800) andT exists. Hence, the existence itself of such a positive correlation betweenW(2800) andT may be viewed as incomprehensible for the present influence of the star on the strength of the absorption line 2800 Mgii of nonstellar origin.On the other hand, we have evidence that the ionizing radiation of hot stars cannot provide for the observed very high degree of ionization of the interstellar magnesium. In particular, the observations give for interstellar magnesium the ratioN +/N 1 1000, while in the case of ionization under the action of stellar radiation only we haveN +/N 1 10.The assumption that circumstellar clouds surround hot stars can naturally explain these and other similar facts. A method for the determination of the general parameters-size, concentration, mass etc. — of the circumstellar clouds is developed. The main results of the application of this method to the relation of more than 20 hot stars are:(1) The circumstellar clouds surround almost (70%) all hot giants and subgiants. In the remaining (30%) cases, the absence of circumstellar envelopes requires additional evidence. (2) The linear sizes of circumstellar clouds vary within wide ranges — from 0.002 pc up to 1 pc. Most frequent are clouds with size of 0.1 pc. (3) The main concentration of hydrogen atoms (electrons) in circumstellar clouds is of the order of 100 cm–3; the minimum value is 20–30 cm–3, the maximum 104 cm–3. In one case (Deneb) the electron concentration rises up to 105 cm–3 for the size of the cloud 0.001 pc=3×1015 cm. (4) Stars of the same spectral and luminosity classes may possess circumstellar clouds characterized by quite different parameters. (5) Hydrogen in circumstellar clouds is completely ionized; for these clouds the optical depth c 1; on the average,T c 0.005. (6) The integrated brightness of circumstellar clouds is substantially fainter (by 8–10m) than that of the central star. This is the reason why these clouds cannot be detected by ground-based observations. (7) The masses of individual circumstellar clouds vary from 1 down to 10–4 . This gives for the mass ejection rate from 10–10 to 10–6 per year in case if these clouds are formed by the braking and accumulation of the ejected mass.The method of 2800 Mgii seems very convenient, fruitful and promising for the detection and study of circumstellar envelopes. Also, this method is very sensitive for a determination of the general parameters of such clouds, and concerns practically all their geometric, physical, kinematic and other properties.  相似文献   

18.
On the basis of Sobolev's method, the population of 30 levels of hydrogen atom is determined allowing for the radiative and collision processes of the heating and ionization of the medium with velocity gradient gradv=10–9–10–11s–1, electron temperatureT e=104 K-2×104 K and electron densityN e=1010 cm–3–1011 cm–3. The central source radiation is characterized by a power spectrum with spectral indices varying from 0 to 2. A region of possible physical conditions is found where the thermal diffuse radiation of the envelope exceeds the emission in the Balmer H line.  相似文献   

19.
Intensities and profiles of the H, H, H, K, and D3 lines are measured in a solar prominence. From the profiles of these lines we estimate T = 6400 K and t = 5.7 km s–1. We construct a simple isothermal model which explains the H intensity and profile for an assumed total particle density n T = 3 × 1011 cm–3, and a filling factor, = 1/6.From this model we find that the source function in the H line is nearly constant through the prominence. We estimate from the model that the radiative energy loss at the center of the prominence is of the order of 107 erg s–1 g–1.  相似文献   

20.
David E. Woon  Jin-Young Park 《Icarus》2009,202(2):642-680
Barrierless reactions between unsaturated hydrocarbons and the ethynyl radical (C2H) can contribute to the growth of organic particulates in the haze-forming regions of Titan's atmosphere as well as in the gas giants and in the interstellar medium. We employed a combination of quantum chemistry and statistical rate theories to characterize reactions between ground state C2H and seven alkenes of the general structure R1R2C′C″R3R4 containing up to six carbons. The alkenes included ethene (C2H4); propene (C3H6); 1-butene, 2-butene, and isobutene (C4H8); trimethylethene (C5H10); and tetramethylethene (C6H12). Density functional theory calculations at the B3LYP/6-31 + G∗∗ level were used to characterize the adducts, isomers, products, and the intervening transition states for the addition-elimination reactions of all seven species. A multiple-well treatment was then employed to determine the outcome distributions for the range of temperatures and pressures relevant to Titan's atmosphere, the interstellar medium, and the outer atmospheres of the gas giants. Finally, trajectory calculations using an ROMP2 potential energy surface were used to calculate kinetic rates for the ethene + C2H reaction, where the agreement between the computed and measured values is very good. At low pressure and temperature, vinyl acetylene is a dominant product of several of the reactions, and all of the reactions yield at least one dominant product with both a double and a triple CC bond.  相似文献   

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