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1.
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   

2.
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   

3.
The wavelength and Einstein A coefficient are calculated for all rotation–vibration transitions of  4He1H+, 3He1H+, 4He2H+  and  3 He2H+  , giving a complete line list and the partition function for  4HeH+  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   

4.
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   

5.
Recent R -matrix calculations of electron impact excitation rates for transitions among the 2s22p2, 2s2p3 and 2p4 levels of Fe  xxi are used to derive theoretical electron density ( N e) sensitive emission-line ratios involving 2s22p2–2s2p3 transitions in the ∼98–146Å wavelength range. A comparison of these with observations from the PLT tokamak plasma, for which the electron density has been independently determined, reveals generally very good agreement between theory and experiment, and in some instances removes discrepancies found previously. The observed Fe  xxi ratios for a solar flare, obtained with the OSO–5 satellite, imply electron densities which are consistent, with discrepancies that do not exceed 0.2 dex. In addition, the derived values of N e are similar to those estimated for the high-temperature regions of other solar flares. The good agreement between theory and observation, in particular for the tokamak spectra, provides experimental support for the accuracy of the present line-ratio calculations, and hence for the atomic data on which they are based.  相似文献   

6.
We have searched for molecular absorption lines at millimetre wavelengths in 11 gravitational lens systems discovered in the JVAS/CLASS surveys of flat spectrum radio sources. Spectra of only one source 1030+074 were obtained in the 3-, 2- and 1.3-mm bands at the frequencies corresponding to common molecular transitions of CO and HCO+ as continuum emission was not found in any of the other sources. We calculated upper limits to the column density in molecular absorption for 1030+074, using an excitation temperature of 15 K, to be N CO<6.3×1013 cm−2 and N HCO+<1.3×1011 cm−2 , equivalent to hydrogen column density of the order N H<1018 cm−2 , assuming standard molecular abundances. We also present the best upper limits of the continuum at the lower frequency for the other 10 gravitational lenses.  相似文献   

7.
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   

8.
Effective collision strengths for electron-impact excitation of the N-like ion S  x are calculated in the close-coupling approximation using the multichannel R -matrix method. Specific attention is given to the 10 astrophysically important fine-structure forbidden transitions among the 4So, 2Do and 2Po levels in the 2s22p3 ground configuration. The total (e−+ion) wavefunction is expanded in terms of the 11 lowest LS eigenstates of S  x , and each eigenstate is represented by extensive configuration-interaction wavefunctions. The collision strengths obtained are thermally averaged over a Maxwellian distribution of velocities, for all 10 fine-structure transitions, over the range of electron temperatures log  T (K)=4.6–6.7 (the range appropriate for astrophysical applications). The present effective collision strengths are the only results currently available for these fine-structure transition rates.  相似文献   

9.
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   

10.
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   

11.
Effective collision strengths for electron-impact excitation of the N-like ion Ne  iv are calculated in the close-coupling approximation using the multichannel R-matrix method. Specific attention is given to the 10 astrophysically important fine-structure forbidden transitions among the 4So, 2Do and 2Po levels in the 2s22p3 ground-state configuration. The expansion of the total wavefunction incorporates the lowest 11 LS eigenstates of Ne  iv , consisting of eight n  = 2 terms with configurations 2s22p3, 2s2p4 and 2p5, together with three n  = 3 states of configuration 2s22p23s. We present in graphical form the effective collision strengths obtained by thermally averaging the collision strengths over a Maxwellian distribution of velocities, for all 10 fine-structure transitions, over the range of electron temperatures log T (K) = 3.6 to log T (K) = 6.1 (the range appropriate for astrophysical applications). Comparisons are made with the earlier, less sophisticated close-coupling calculation of Giles, and excellent agreement is found in the limited temperature region where a comparison is possible [log T (K) = 3.7 to log T (K) = 4.3]. At higher temperatures the present data are the only reliable results currently available.  相似文献   

12.
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   

13.
It is proposed that some, possibly many, of the unidentified diffuse interstellar absorption bands arise from rovibronic transitions between the ground states of negatively charged molecules and/or small grains, and shallow dipolebound electronic states which lie close to the electron detachment threshold. Under this hypothesis the attributes for the neutral 'molecular' frameworks are electron affinities between 1 and 3 eV and permanent electric dipole moments of 2 debye or greater. Boundbound spectra involving the lowest rotational levels have not been detected in the laboratory, but these proposed carriers appear to be capable of satisfying the main observational astronomical constraints: transitions that lie in the range from the near-ultraviolet to the near-infrared; a wide range of widths; band wavelengths that are invariant; and a large number of related but distinct carriers. The wavelengths of the lowest rotational lines of the band of the transition between the ground and a dipolebound electronic state of the CH2CN molecule appear to be consistent with a diffuse band near 8037 Å.  相似文献   

14.
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15.
Absorption lines of MgH and CaH N  = 1 − 0 transitions were searched for in foreground molecular clouds towards the continuum sources associated with Sgr B2 (M) and W49A (N). None of these lines was detected with our sensitivity level of ∼20 mK. Millimetric absorption lines of MgO, MgOH, CaO and CaOH were also searched for towards Sgr B2 (M) without success. The fractional abundances relative to molecular hydrogen are ≲ 1.0 × 10−11 for MgH, ≲ 7.9 × 10−13 for MgO, ≲ 1.6 × 10−10 for MgOH, ≲ 1.6 × 10−9 for CaH, ≲ 2.0 × 10−12 for CaO, and ≲ 2.5 × 10−10 for CaOH, respectively. The low abundances measured in absorption indicate that a significant fraction of interstellar magnesium and calcium cannot be tied up in their monohydrides, monoxides and monohydroxides. The low abundance of MgH also implies that grain-surface chemistry involving magnesium is not efficient and that magnesium is depleted on to grains to a factor of ≳ 102.5 in well-shielded molecular clouds.  相似文献   

16.
New R -matrix calculations of electron impact excitation rates for transitions among the 2s22p, 2s2p2 and 2p3 levels of Si  x are presented. These data are subsequently used, in conjunction with recent estimates for proton excitation rates, to derive theoretical electron density sensitive emission-line ratios involving transitions in the ∼253–356 Å wavelength range. A comparision of these with observations of a solar active region and subflare, obtained during the 1989 flight of the Solar EUV Rocket Telescope and Spectrograph ( SERTS ), reveals that the electron densities determined from most of the Si  x line ratios are consistent with one another for both solar features. In addition, the derived densities are also in good agreement with the values of N e estimated from diagnostic lines in other species formed at similar electron temperatures to Si  x , such as Fe  xii and Fe  xiii . These results provide observational support for the general accuracy of the adopted atomic data, and hence line ratio calculations, employed in the present analysis. However, we find that the Si  x 256.32-Å line is blended with the He  ii transition at the same wavelength, while the feature at 292.25 Å is not due to Si  x , but currently remains unidentified. The intensity of the 253.81-Å line in the SERTS active region spectrum is about a factor of 3 larger than expected from theory, but the reason for this is unclear, and requires additional observations to explain the discrepancy.  相似文献   

17.
A multitransition 3-mm molecular line single pointing and mapping survey was carried out towards 29 massive star-forming cores in order to search for the signature of inward motions. Up to seven different transitions, optically thick lines HCO+(1-0), CS(2-1), HNC(1-0), HCN(1-0) and 12CO(1-0), and optically thin lines C18O(1-0) and 13CO(1-0) were observed towards each source. The normalized velocity differences (     ) between the peak velocities of optically thick lines and optically thin line C18O(1-0) for each source were derived. Prominent inward motions are probably present in either HCO+(1-0) or CS(2-1) or HNC(1-0) observations in most sources. Our observations show that there is a significant difference in the incidence of blueshifted asymmetric line profiles between CS(2-1) and HCO+(1-0). The HCO+(1-0) shows the highest occurrence of obvious asymmetric features, perhaps owing to different optical depth between CS(2-1) and HCO+(1-0). HCO+(1-0) appears to be the best inward motion tracer. The mapping observations of multiple line transitions enable us to identify six strong infall candidates: G123.07-6.31, W75(OH), S235N, CEP-A, W3(OH) and NGC 7538. The infall signature is extended up to a linear scale  >0.2 pc  .  相似文献   

18.
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   

19.
In order to interpret H2 quasar absorption-line observations of damped Lyα systems (DLAs) and subDLAs, we model their H2 abundance as a function of dust-to-gas ratio, including H2 self-shielding and dust extinction against dissociating photons. Then, we constrain the physical state of the gas by using H2 data. Using H2 excitation data for DLAs with H2 detections, we derive a gas density  1.5 ≲ log n (cm−3) ≲ 2.5  , temperature  1.5 ≲ log T (K) ≲ 3  , and an internal ultraviolet (UV) radiation field (in units of the Galactic value)  0.5 ≲ log χ≲ 1.5  . We then find that the observed relation between the molecular fraction and the dust-to-gas ratio of the sample is naturally explained by the above conditions. However, it is still possible that H2 deficient DLAs and subDLAs with H2 fractions less than  ∼10−6  are in a more diffuse and warmer state. The efficient photodissociation by the internal UV radiation field explains the extremely small H2 fraction  (≲10−6)  observed for  κ≲ 1/30  (κ is the dust-to-gas ratio in units of the Galactic value); H2 self-shielding causes a rapid increase in, and large variations of, H2 abundance for  κ≳ 1/30  . We finally propose an independent method to estimate the star formation rates of DLAs from H2 abundances; such rates are then critically compared with those derived from other proposed methods. The implications for the contribution of DLAs to the cosmic star formation history are briefly discussed.  相似文献   

20.
The ab initio R -matrix method is used to calculate effective collision strengths for electron-impact excitation of the sulphur-like ion Cl  ii in the close-coupling approximation. All 10 astrophysically important fine-structure forbidden transitions within the 3s23p4     ,     ,     ground configuration levels are considered. The 12 lowest LS target states are included in the calculation. Effective collision strengths are obtained by averaging the electron collision strengths over a Maxwellian distribution of electron velocities. Results are presented for electron temperatures in the range  log  T (K)=3.3  to  log  T (K)=5.5  , appropriate for astrophysical applications. These are the only effective collision strength data that are currently available.  相似文献   

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