共查询到20条相似文献，搜索用时 62 毫秒
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Cesare Cecchi-Pestellini Alexander Dalgarno 《Monthly notices of the Royal Astronomical Society》2000,313(1):L6-L8
A model is constructed of the material in front of the star Cygnus OB2 no. 12 in which dense cores are embedded in diffuse clumps of gas. The model reproduces the measured abundances of C2 and CO, and predicts a column density of 91010 cm2 for HCO+ . 相似文献
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E. Herbst R. Terzieva D. Talbi 《Monthly notices of the Royal Astronomical Society》2000,311(4):869-876
The attempt to understand the temperature dependence of the HNC/HCN abundance ratio in interstellar clouds has been long standing and indecisive. In this paper we report quantum chemical and dynamical studies of two neutral–neutral reactions thought to be important in the formation of HNC and HCN, respectively – C+NH2 →HNC+H, and N+CH2 →HCN+H. We find that although these reactions do lead initially to the products suggested by astronomers, there is so much excess energy available in both reactions that the HCN and HNC products are able to undergo efficient isomerization reactions after production. The isomerization leads to near equal production rates of the two isomers, with HNC slightly favoured if there is sufficient rotational excitation. This result has been incorporated into our latest chemical model network of dense interstellar clouds. 相似文献
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Franco Cataldo Giovanni Strazzulla Susana Iglesias-Groth 《Monthly notices of the Royal Astronomical Society》2009,394(2):615-623
The stability of C60 and C70 fullerenes in the interstellar medium deposited on dust surface or embedded in meteorites and comets has been simulated with γ irradiation and with He+ ion bombardment. It is shown by vibrational spectroscopy that a γ radiation dose of 2.6 MGy (1 Gy = 1 joule absorbed energy per kilogram) causes partial oligomerization of both C60 and C70 fullerenes. Oligomers are made by fullerene cages chemically connected each other which can yield back free fullerenes by a thermal treatment. The amount of irreversibly polymerized fullerenes caused by 2.6 MGy as deduced as the toluene insoluble fraction has been determined as 1.7 and 15 per cent by weight, respectively, for C60 and C70 fullerene. The radiation dose generated by radionuclides decay and expected to be delivered to fullerenes buried at a depth of more than 20 m in comets and meteorites is about 3 MGy per 109 yr. Since fullerenes are by far resistant to such radiation dose they can survive for at least some billion years inside comets and meteorites and in fact have been detected inside certain carbonaceous chondrites. On the other hand, the direct exposure of fullerenes to cosmic rays for instance when they are adsorbed or deposited on the surface of carbon dust corresponds to the delivery of a radiation dose comprised between 30 and 65 MGy per 109 yr. Experimental bombardment of both C60 and C70 fullerenes for instance with He+ ions has shown that the complete amorphization occurs at about 250 MGy. Thus in ∼4 Gyr exposure to cosmic rays it is expected a complete amorphization. 相似文献
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The formation of molecular hydrogen (H2 ) in the interstellar medium takes place on the surfaces of dust grains. Hydrogen molecules play a role in gas-phase reactions that produce other molecules, some of which serve as coolants during gravitational collapse and star formation. Thus, the evaluation of the production rate of hydrogen molecules and its dependence on the physical conditions in the cloud are of great importance. Interstellar dust grains exhibit a broad size distribution in which the small grains capture most of the surface area. Recent studies have shown that the production efficiency strongly depends on the grain composition and temperature as well as on its size. In this paper, we present a formula that provides the total production rate of H2 per unit volume in the cloud, taking into account the grain composition and temperature as well as the grain size distribution. The formula agrees very well with the master equation results. It shows that for a physically relevant range of grain temperatures, the production rate of H2 is significantly enhanced due to their broad size distribution. 相似文献
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The distribution of C2 S and NH3 in two Bok globules at different evolutionary stages: CB246 and CB34
Claudio Codella & Flavio Scappini 《Monthly notices of the Royal Astronomical Society》1998,298(4):1092-1096
Two Bok globules, L1253 (CB246) and CB34, have been mapped in the C2 S (21 –10 ) transition and in the NH3 (1, 1) and NH3 (2, 2) inversion transitions, respectively. By comparing the C2 S map of L1253 (CB246) with the NH3 map of the same globule from Lemme et al., a clumped onion structure results as a consequence of the chemical and dynamical evolution of the object. From the derived parameters it appears that both L1253 (CB246) and CB34 are close to virial equilibrium. 相似文献
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D. R. Flower & G. Pineau des Forêts 《Monthly notices of the Royal Astronomical Society》1998,297(4):1182-1188
We have computed optical absorption-line profiles of CH+ and CH, as predicted by a model of a C-type shock propagating in a diffuse interstellar cloud. Both these species are produced in the shock wave in the reaction sequence that is initiated by C+ (H2 , H)CH+ . Whilst CH+ flows at the ion speed, CH, which forms in the dissociative recombination reaction CH+ 3 (e− , H2 )CH, flows at a speed which is intermediate between those of the ions and the neutrals. The predicted velocity shift between the CH+ and CH line profiles is found to be no more than approximately 2 km s−1 , which is smaller than has previously been assumed. We also investigate OH and HCO+ , finding that the correlation between their column densities, recently observed in the diffuse interstellar medium, can be reproduced by the model. 相似文献
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We present a set of gas-phase Planck mean and Rosseland mean opacity tables applicable for simulations of star and planet formation, stellar evolution and disc modelling at various metallicities in hydrogen-rich environments. The tables are calculated for gas temperatures between 1000 and 10 000 K and total hydrogen number densities between 102 and 1017 cm−3 . The carbon-to-oxygen ratio is varied from 0.43 to well above 2.0, the nitrogen-to-oxygen ratio between 0.14 and 100.0. The tables are calculated for a range of metallicities down to [M/H]'= log N M / N H =−7.0 . We demonstrate how the mean opacities and the abundances of the opacity species vary with C/O, N/O and [M/H]' . We use the element abundances from Grevesse et al., and we provide additional tables for the oxygen abundance value from Caffau et al. All tables will be available online at http://star-www.st-and.ac.uk/~ch80/datasources.html . 相似文献
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The rates of rotational transitions for HCO+ , the most abundant ion in interstellar space, induced by collision with helium are obtained for temperatures ranging from 10 to 80 K. The calculations are based on a new potential energy surface for the He–HCO+ interaction and on a scattering matrix whose accuracy was checked by pressure broadening and shift measurements. The rates decrease for increasing values of j and Δ j , with a temperature trend depending on the energy involved in the transitions: if it is small, the rates are almost constant, while an increase with T is found for other cases. Comparison with previous and less accurate results shows an agreement within 50 per cent. Comparison between state-to-state and pressure broadening cross-sections allows us to discuss importance and influence of elastic and inelastic collisions. 相似文献
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Elodie A. Engel Natasha Doss Gregory J. Harris Jonathan Tennyson 《Monthly notices of the Royal Astronomical Society》2005,357(2):471-477
The wavelength and Einstein A coefficient are calculated for all rotation–vibration transitions of 4 He1 H+ , 3 He1 H+ , 4 He2 H+ and 3 He2 H+ , giving a complete line list and the partition function for 4 HeH+ and its isotopologues. This opacity is included in the calculation of the total opacity of low-metallicity stars and its effect is analysed for different conditions of temperature, density and hydrogen number fraction. For a low helium number fraction (as in the Sun), it is found that HeH+ has a visible but small effect for very low densities (ρ≤ 10−10 g cm−3 ) , at temperatures around 3500 K. However, for high helium number fraction, the effect of HeH+ becomes important for higher densities (ρ≤ 10−6 g cm−3 ) , its effect being most important for a temperature around 3500 K. Synthetic spectra for a variety of different conditions are presented. 相似文献
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Stefan A. Schulz Joseph E. King Robert J. Glinski 《Monthly notices of the Royal Astronomical Society》2000,312(4):769-773
We have calculated synthetic spectra of perpendicular and parallel rovibronic bands of cumulene carbene molecules of the form C n H2 . The perpendicular bands are consistent with a regularly spaced group of diffuse interstellar bands (DIBs) near 6850 Å. Parallel bands calculated for these molecular structures are consistent with the intrinsic profile of the associated 6614-Å DIB. Both types of bands are expected for an electronic transition that these species should have at those energies. We could not determine if the molecule was charged or if an atom other than carbon terminated the chain-end. Constraints due to molecular geometry and temperature place the chain length at 7–15 carbons to fit the 6850-Å group and 9–13 carbons to fit the 6614-Å DIB. 相似文献