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 46 毫秒
1.
The results of recent quantum mechanical calculations of cross-sections for rotational transitions within the vibrational ground state of HD are used to evaluate the rate of radiative energy loss from gas containing HD, in addition to H, He and H2. The cooling function for HD (i.e. the rate of cooling per HD molecule) is evaluated in steady state on a grid of values of the relevant parameters of the gas, namely the gas density and temperature, the atomic to molecular hydrogen abundance ratio and the ortho:para-H2 density ratio. The corresponding cooling function for H2, previously computed by Le Bourlot et al., is slightly revised to take account of transitions induced by collisions with ground-state ortho-H2 ( J =1). The cooling functions and the data required for their calculation are available from http://ccp7.dur.ac.uk/. We then make a study of the rate of cooling of the primordial gas through collisions with H2 and HD molecules. In this case, radiative transitions induced by the cosmic background radiation field and, in the case of H2, collisional transitions induced by H+ ions should additionally be included.  相似文献   

2.
We use a 1-D chemical diffusive model, in conjunction with the measured neutral atmospheric structure, to analyze the Voyager RSS electron density, ne, profiles for the ionospheres of Jupiter and Saturn. As with previous studies we find serious difficulties in explaining the ne measurements. The model calculates ionospheres for both Jupiter and Saturn with ne peaks of 10 times the measured peaks at altitudes which are 900–1000 km lower than the altitude of peaks in the RSS electron densities. Based on our knowledge of neutral atmospheric structure, ionization sources, and known recombination mechanisms it seems that, vibrational excitation of H2 must play some role in the conversion of slowly radiatively recombining H+ ions to the relatively more rapidly recombining H2+ and H3+ ions. In addition, vertical ion flow induced by horizontal neutral winds or electric fields probably also play some role in maintaining the plasma peaks observed both for Jupiter and Saturn to be at high altitudes. For the ionosphere of Saturn, the electron densities are affected by a putative influx of H2O molecules, ΦH2O, from the rings. To reproduce the RSS V2 exit ne results model requires an influx of ΦH2O 2 × 107 molecules cm−2 s−1 without invoking H2f vibrational excitation. To maintain the model ne peak at the measured altitude vertical plasma drift maintained by meridional winds or vertical electric fields is required. The amounts of H2O are consistent with earlier estimates of Connerney and Waite (1984) and do not violate any observational constraints.  相似文献   

3.
The contraction of matter in the primordial medium, to form the first gravitationally bound structures, was mediated by radiative cooling of the gas by H2 and HD. We have computed the initial phases of free-fall collapse, incorporating the results of quantum mechanical calculations of rate coefficients for collisional excitation of H2 and HD by the principal perturbers, H, He, H2 and H+. The structure of shock waves produced when the collapse speed exceeds the local sound speed is determined. In the post-shock gas, radiative cooling by H2 exceeds that by HD, but by a factor of only 4. The intensities of the strongest emission lines of H2– rotational transitions within the vibrational ground state – are calculated. Even with coarse spectral and angular resolution, these transitions might be observable as inhomogeneities in the cosmic background radiation.  相似文献   

4.
The pure rotational spectrum of homonuclear diatomic molecules in the interstellar medium is strongly forbidden, and no such spectrum has been detected. In regions of high excitation, vibrational emission may occur, as is widely detected in the case of H2 in interstellar shocks and photon-dominated regions. However, it is of considerable interest to know the abundance of homonuclear diatomics in quiescent regions. We propose that vibrational emission from homonuclear diatomic molecules in cold clouds may be detectable, where the excitation is mainly through collisions with non-thermal electrons arising from the cosmic-ray ionization of H2. As an example, we estimate the intensity of emission from N2 in cold, dark interstellar clouds. We show that such emission is at the limit of detectability with current technology. Other excitation mechanisms may also contribute and enhance this emission.  相似文献   

5.
  相似文献   

6.
We report new calculations of the cooling rate of primordial gas by the HD molecule, taking into account its ro-vibrational structure. The HD cooling function is calculated including radiative and collisional transitions for   J ≤ 8  rotational levels, and for the vibrational levels v = 0, 1, 2 and 3. The ro-vibrational level population is calculated from the balance equation assuming steady state. The cooling function is evaluated in the ranges of the kinetic temperatures, T k, from 102 to  2 × 104 K  and the number densities, n H, from 1 to  108 cm−3  . We find that the inclusion of collisional ro-vibrational transitions increases significantly the HD cooling efficiency, in particular for high densities and temperatures. For   n H≳ 105  and   T k∼ 104 K  the cooling function becomes more than an order of magnitude higher than previously reported. We give also the HD cooling rate in the presence of the cosmic microwave radiation field for radiation temperatures of 30, 85 and 276 K (redshifts of 10, 30 and 100). The tabulated cooling functions are available at http://www.cifus.uson.mx/Personal_Pages/anton/DATA/HD_cooling/HD_cool.html . We discuss the relevance to explore the effects of including our results into models and simulations of galaxy formation, especially in the regime when gas cools down from temperatures above ∼3000 K.  相似文献   

7.
The Bethe approximation is used with measured and theoretical values of ionization cross sections and measured values of differential oscillator strengths to derive the initial energy spectrum of auroral secondary electrons. The differential flux of the auroral secondaries is then calculated, using the approximation of continuous energy loss. The calculations are applied to a particular aurora for which rocket data have been published. There is substantial disagreement between theoretical and measured electron spectra. The theoretical spectra show structure at energies less than 20 eV, associated primarily with vibrational and electronic excitation of molecular nitrogen. This structure is largely absent in the measured spectrum. Substantially more high energy electrons were measured than theory predicts. In addition, there are disagreements in the altitude profiles of the total number of non-thermal secondary electrons.

Calculated values of OI green line photon emission rates which result from excitation by secondary electrons and dissociative recombination of O2+ fall short of the measured values. The effect on the excitation rate of varying several parameters is investigated, and it is found that the results are particularly sensitive to competing inelastic processes in N2.  相似文献   


8.
In the mesosphere, water vapor photolysis is the major source of odd hydrogen (H, OH and HO2) under normal conditions. The odd hydrogen produced may then be converted to H2 by the reaction H + HO2→ H2 + O2. This process is responsible for the calculated decrease in the H2O mixing ratio and accompanying increase in the H2 mixing ratio with altitude in the upper mesosphere and lower thermosphere. Charged particle precipitation events are calculated to produce the same effect, particularly in the 70–85 km region, thus temporarily resulting in enhanced conversion of H2O to H2 following such an event. Since odd hydrogen is produced predominantly by water vapor photolysis at these altitudes, decreased odd hydrogen concentrations are also anticipated. Odd hydrogen processes dominate ozone destruction in this region, and so an increase in ozone may occur if odd hydrogen concentrations decrease. We have examined the calculated time behavior of these processes in a numerical model using the August 1972 solar proton event as an example, and we present calculations indicating what might be observed in future events.  相似文献   

9.
We demonstrate that a wide range of molecular hydrogen excitation can be observed in protostellar outflows at wavelengths in excess of 5 μm. Cold H2 in DR 21 is detected through the pure rotational transitions in the ground vibrational level (0–0). Hot H2 is detected in pure rotational transitions within higher vibrational levels (1–1, 1–2, etc.). Although this emission is relatively weak, we have detected two 1–1 lines in the DR 21 outflow with the ISO SWS instrument. We thus investigate molecular excitation over energy levels corresponding to the temperature range 1015–15 722 K, without the uncertainty introduced by differential extinction when employing near-infrared data.
This gas is thermally excited. We uncover a rather low H2 excitation in the DR 21 West Peak. The line emission cannot be produced from single C-shocks or J-shocks; a range of shock strengths is required. This suggests that bow shocks and/or bow-generated supersonic turbulence is responsible. We are able to distinguish this shock-excited gas from the fluoresced gas detected in the K band, providing support for the dual-excitation model of Fernandes, Brand & Burton.  相似文献   

10.
We have computed cross-sections and rate coefficients for rovibrational transitions in HD, induced by collisions with atomic and molecular hydrogen. We employed fully quantum-mechanical methods and the potential of Boothroyd et al. for H–HD, and that of Schwenke for H2–HD. The rate coefficients for vibrational relaxation v =1→0 of HD are compared with the corresponding values for H2. The influence of vibrationally excited channels on the rate coefficients for rotational transitions within the v =0 vibrational ground state of HD is shown to be small at T =500 K, where T is the kinetic temperature. The rate coefficients, for 100 T 2000 K, are available from http://ccp7.dur.ac.uk/.  相似文献   

11.
We have taken K -band spectra covering seven cooling flow clusters. The spectra show many of the 1-0S transitions of molecular hydrogen, as well as some of the higher vibrational transitions, and some lines of ionized hydrogen. The line ratios allow us to conclude that the rotational states of the first excited vibrational state are in approximate local thermodynamic equilibrium (LTE), so that densities above 105 cm−3 are likely, but there is evidence that the higher vibrational states are not in LTE. The lack of pressure balance between the molecular gas and the ionized components emphasizes the need for dynamic models of the gas. The ratios of the ionized to molecular lines are relatively constant but lower than from starburst regions, indicating that alternative heating mechanisms are necessary.  相似文献   

12.
The MSX infrared dark cloud G79.2+0.38 has been observed over a 11′×′ region simultaneously in the J=1-0 rotational transition lines of the 12CO and its isotopic molecules 13CO and 18CO. The dense molecular cores defined by the C18O line are found to be associated with the two high-extinction patches shown in the MSX A-band image. The two dense cores have the column density N (H2) (5 – 12) × 1022 cm−2 and the mean number density n (3 ± 1) × 104 cm−3. Their sizes are 1.7 and 1.2 pc in 13CO(1-0) line, 1.2 and 0.6 pc in C18O(1-0) line, respectively. The masses of these cloud cores are estimated to be in the range from 2 × 102 to 2 × 103 M. The profile of radial mean density of the cloud core can be described by the exponential function ¯n(p) p−0.34±0.02. Compared with the cases of typical optical dark clouds, the abundances of the CO isotopic molecules 13CO and C18O in this MSX infrared dark cloud appear to be depleted by a factor of 4–11, but at present there is no evidence for any obvious variation of the relative abundance ratio X13/18 between 13CO and C18O with the column density.  相似文献   

13.
We find a new two-temperature hot branch of equilibrium solutions for stationary accretion discs around black holes. In units of Eddington accretion rate defined as 10 L Edd c 2, the accretion rates to which these solutions correspond are within the range ̇ 1≲ ̇ ≲1, where ̇ 1 is the critical rate of advection-dominated accretion flow (ADAF). In these solutions, the energy loss rate of the ions by Coulomb energy transfer between the ions and electrons is larger than the viscously heating rate and it is the advective heating together with the viscous dissipation that balances the Coulomb cooling of ions. When ̇ 1≲ ̇ ≲ ̇ 2, where ̇ 2∼5 ̇ 1<1, the accretion flow remains hot throughout the disc. When ̇ 2≲ ̇ ≲1, Coulomb interaction will cool the inner region of the disc within a certain radius ( r tr∼several tens of Schwarzschild radii or larger depending on the accretion rate and the outer boundary condition) and the disc will collapse on to the equatorial plane and form an optically thick cold annulus. Compared with ADAF, these hot solutions are much more luminous because of the high accretion rate and efficiency; therefore, we call them luminous hot accretion discs.  相似文献   

14.
We calculate the event rates induced by a 51Cr νe source and by a 90Sr---90Y source in BOREXINO through elastic scattering on electrons, assuming a nonzero neutrino magnetic moment μν. We consider a source activity of about 2 MCi and estimate the solar ν (“source-off”) background for various oscillation scenarios. It is shown that values of μν as low as 0.5 × 10−10μB ( 0.2 × 10−10μB) can be proved with the 51Cr source (90Sr source) in about 100 days of data taking.  相似文献   

15.
We study the prospects for observing H2 emission during the assembly of primordial molecular cloud kernels. The primordial molecular cloud cores, which resemble those at the present epoch, can emerge around  1+ z ∼20  according to recent numerical simulations. The kernels form inside the cores, and the first stars will appear inside the kernels. A kernel typically contracts to form one of the first generation stars with an accretion rate that is as large as ∼0.01 M⊙ yr−1. This occurs owing to the primordial abundances, which result in a kernel temperature of order 1000 K, and the collapsing kernel emits H2 line radiation at a rate ∼1035 erg s−1. Predominantly   J =5-3   ( v =0)  rotational emission of H2 is expected. At redshift  1+ z ∼20  , the expected flux is ∼0.01 μJy for a single kernel. While an individual object is not observable by any facilities available in the near future, the expected assembly of primordial star clusters on subgalactic scales can result in fluxes at the sub-mJy level. This is marginally observable with ASTRO-F and ALMA. We also examine the rotational   J =2-0   ( v =0)  and vibrational   δv =1  emission lines. The former may possibly be detectable with ALMA.  相似文献   

16.
Molecular R-matrix calculations are performed at a range of energies to give rotational and vibrational excitation and de-excitation cross-sections and, hence, rates for electron collisions with HeH+ up to electron temperatures of 20 000 K. Critical electron densities are also given. The rotational calculations include the Coulomb–Born completion of the cross-sections for high l values. Rates for the transition j  = 0 → 2, which have previously been assumed to be negligible, are found to be up to half those for j  = 0 → 1, raising the prospect of observing the HeH+ j  = 2 → 1 emission line at 74.8 μm.  相似文献   

17.
Recent improvements in rocket-borne mass spectrometer technology have made it possible to measure lower ionospheric ions with greater sensitivity and to extend the measurements to lower heights. The improvements made to the instrument and positive ion results from a flight of this instrument will be reported here. In addition to the previously known ions, such as NO+(H2O)n and H+(H2O)n, new ion species were found. The total fractional count rate of these ions was found to be constant with height indicating an upper altitude source. Possible identifications of these ions are proposed along with possible production mechanisms.  相似文献   

18.
Ammonia inversion lines are often used as probes of the physical conditions in the dense interstellar medium. The excitation temperature between the first two para-metastable (rotational) levels is an excellent probe of the gas kinetic temperature. However, the calibration of this ammonia thermometer depends on the accuracy of the collisional rates with H2. Here, we present new collisional rates for ortho- and para-NH3 colliding with  para-H2( J = 0)  , and investigate the effects of these new rates on the excitation of ammonia. Scattering calculations employ a new, high-accuracy, potential energy surface computed at the coupled-cluster CCSD(T) level with a basis set extrapolation procedure. Rates are obtained for all transitions involving ammonia levels with   J ≤ 3  and for kinetic temperatures in the range 5–100 K. We find that the calibration curve of the ammonia thermometer – which relates the observed excitation temperature between the first two para-metastable levels to the gas kinetic temperature – does not change significantly when these new rates are used. Thus, the calibration of ammonia thermometer appears to be robust. Effects of the new rates on the excitation temperature of inversion and rotation–inversion transitions are also found to be small.  相似文献   

19.
A general theory of rotational discontinuities is developed and the changes in the components of the plasma pressure, p| and p, and in the magnetic induction, B, are found. For a given value of λ=(p| − p) 4πμ/B2 upstream only a limited range of downstream anisotropies are possible. If λ>0.6 upstream then isotropy is not possible downstream. Some special solutions are analysed and the identification of rotational discontinuities is the solar wind is discussed.  相似文献   

20.
The global structure of accretion disks is investigated in a unified scheme. We use a general radiative cooling formula applicable to both optically thick and thin regimes and we include radial advection cooling. Within the -viscosity models, we found distinct families of global solutions. If the accretion rate is low, there are three non-intersecting solutions, corresponding to optically thick and thin, local cooling and optically thin advection cooling. If the accretion rate is high, and the viscosity coefficient is large, the two local cooling solutions coincide at radii R1 and R2 and exist independently below R1 and above R2 while the advection solution is stable at all radii. If the accretion is high and the viscosity is low, the two optically thin solutions will cross each other while the optically thick solution exists at all radii.  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