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1.
Anomalous molecular line profile shapes are the strongest indicators of the presence of the infall of gas that is associated with star formation. Such profiles are seen for well-known tracers, such as HCO+, CS and H2CO. In certain cases, optically thick emission lines with appropriate excitation criteria may possess the asymmetric double-peaked profiles that are characteristic of infall. However, recent interpretations of the HCO+ infall profile observed towards the protostellar infall candidate B335 have revealed a significant discrepancy between the inferred overall column density of the molecule and that which is predicted by standard dark cloud chemical modelling.
This paper presents a model for the source of the HCO+ emission excess. Observations have shown that, in low-mass star-forming regions, the collapse process is invariably accompanied by the presence of collimated outflows; we therefore propose the presence of an interface region around the outflow in which the chemistry is enriched by the action of jets. This hypothesis suggests that the line profiles of HCO+, as well as other molecular species, may require a more complex interpretation than can be provided by simple, chemically quiescent, spherically symmetric infall models.
The enhancement of HCO+ depends primarily on the presence of a shock-generated radiation field in the interface. Plausible estimates of the radiation intensity imply molecular abundances that are consistent with those observed. Further, high-resolution observations of an infall-outflow source show HCO+ emission morphology that is consistent with that predicted by this model.  相似文献   

2.
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   

3.
We investigate the possibility of interstellar masers in transitions of the methanol isotopomers CH3OD, 13CH3OH and CH318OH, and of CH3SH. The model used, in which masers are pumped through the first and second torsionally excited states by IR radiation, has accounted successfully for the Class II masers in main species methanol, 12CH316OH. Several potential maser candidates are identified for CH3OD, their detectability depending on the enrichment of this species in star-forming regions. In 13CH3OH and CH318OH the best maser candidates are direct counterparts of the well-known 6.7- and 12.2-GHz methanol masers, but the lower interstellar abundance of these substituted species means that the expected brightness is greatly reduced. The maser candidates in CH3SH are also weak. By comparing these species we find that the large b -component of the dipole moment in methanol plays a significant role in its propensity to form masers, as does the strong torsion–rotation interaction due to the light hydroxyl frame. Thus the exceptional brightness of interstellar methanol masers is due to a favourable combination of molecular properties as well as high interstellar abundance.  相似文献   

4.
An upper limit of the column density of the C5 linear molecule in translucent interstellar clouds is estimated from high-resolution ( R =80 000) and very high signal-to-noise ratio (∼1000) echelle spectra. It is 1012 cm−2 per E ( B − V )=1 (two orders of magnitude lower than that of C2).  相似文献   

5.
The Cepheus A star-forming region has been investigated through a multiline H2S and SO2 survey at millimetre wavelengths. Large-scale maps and high-resolution line profiles reveal the occurrence of several outflows. Cep A East is associated with multiple mass-loss processes: in particular, we detect a 0.6-pc jet-like structure which shows for the first time that the Cep A East young stellar objects are driving a collimated outflow moving towards the south.
The observed outflows show different clumps associated with definitely different H2S/SO2 integrated emission ratios, indicating that the gas chemistry in Cepheus A has been altered by the passage of shocks. H2S appears to be more abundant than SO2 in high-velocity clumps, in agreement with chemical models. However, we also find quite small H2S linewidths, suggestive of regions where the evaporated H2S molecules had enough time to slow down but not to freeze out on to dust grains. Finally, comparison between the line profiles indicates that the excitation conditions increase with the velocity, as expected for a propagation of collimated bow shocks.  相似文献   

6.
We present ultra-high-resolution (0.35 km s−1 FWHM) observations of the interstellar Ca K line towards seven nearby stars. The spectral resolution was sufficient to resolve the line profiles fully, thereby enabling us to detect hitherto unresolved velocity components, and to obtain accurate measurements of the velocity dispersions ( b values). Absorption components with velocities similar to those expected for the Local Interstellar Cloud (LIC) and the closely associated 'G cloud' were identified towards six of the seven stars. However, in most cases the b values deduced for these components were significantly larger than the b  ≈ 2.2 km s−1 (i.e. T k ≈ 7000 K, v t ≈ 1 km s−1) expected for the LIC, and it is argued that this results from the presence of additional, spectrally unresolved, components having similar velocities and physical conditions. For two stars (δ Vel and α Pav) we detect interstellar components with much smaller b values (1.1 ± 0.3 and 0.8 ± 0.1 km s−1, respectively) than are expected for low-density clouds within the Local Bubble. In the case of the narrow α Pav component, we also find an anomalously large Na  i /Ca  ii column density ratio, which is indicative of a relatively high density. Thus it is possible that, in addition to LIC-type clouds, the local interstellar medium contains a population of previously undetected cooler and denser interstellar clouds.  相似文献   

7.
Two Bok globules, L1253 (CB246) and CB34, have been mapped in the C2S (21–10) transition and in the NH3 (1, 1) and NH3 (2, 2) inversion transitions, respectively. By comparing the C2S 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   

8.
HCO+ has been detected for the first time towards the star Cygnus OB2 No. 12 through emission of the 1–0 rotational transition at 89 GHz. The CO( J =2−1) transition has also been observed. The observations are consistent with a model of dense regions embedded in a low-density clump gas. If actually present, the dense component would have an aggregate size L 1300 au, in agreement with estimates of small-scale density fluctuations observed along diffuse lines of sight.  相似文献   

9.
We present a comprehensive near-infrared study of two molecular bow shocks in two protostellar outflows, HH 99 in R Coronae Australis and VLA 1623A (HH 313) in Rho Ophiuchi. New, high-resolution, narrow-band images reveal the well-defined bow shock morphologies of both sources. These are compared with two-dimensional MHD modelling of molecular bows from which we infer flow inclination angles, shock speeds and the magnetic field in the pre-shock gas in each system. With combined echelle spectroscopy and low-resolution K -band spectra we further examine the kinematics and excitation of each source. Bow shock models are used to interpret excitation (CDR) diagrams and estimate the extinction and, in the case of VLA 1623, the ortho–para ratio associated with the observed H2 population. For the first time, morphology, excitation and kinematics are fitted with a single bow shock model.
Specifically, we find that HH 99 is best fitted by a C-type bow shock model (although a J-type cap is probably responsible for the [Fe  ii ] emission). The bow is flowing away from the observer (at an angle to the line of sight of ∼45°) at a speed of roughly 100 km s−1. VLA 1623A is interpreted in terms of a C-type bow moving towards the observer (at an angle to the line of sight of ∼75°) at a speed of ∼80 km s−1. The magnetic field associated with HH 99 is thought to be orientated parallel to the flow axis; in VLA 1623A the field is probably oblique to the flow axis, since this source is clearly asymmetric in our H2 images.  相似文献   

10.
Long-slit spectra of the molecular outflow Herbig–Haro (HH) 46/47 have been taken in the J and K near-infrared bands. The observed H2 line emission confirms the existence of a bright and extended redshifted counter-jet outflow south-west of HH 46. In contrast with the optical appearance of this object, we show that this outflow seems to be composed of two different emission regions characterized by distinct heliocentric velocities. This implies an acceleration of the counter-jet.
The observed [Fe  ii ] emission suggests an average extinction of 7–9 visual magnitudes for the region associated with the counter-jet.
Through position–velocity diagrams, we show the existence of different morphologies for the H2 and [Fe  ii ] emission regions in the northern part of the HH 46/47 outflow. We have detected for the first time high-velocity (−250 km s−1) [Fe  ii ] emission in the region bridging HH 46 to HH 47A. The two strong peaks detected can be identified with the optical positions B8 and HH 47B.
The H2 excitation diagrams for the counter-jet shock suggest an excitation temperature for the gas of T ex≈2600 K . The lack of emission from the higher energy H2 lines, such as the 4–3 S(3) transition, suggests a thermal excitation scenario for the origin of the observed emission. Comparison of the H2 line ratios with various shock models yielded useful constraints about the geometry and type of these shocks. Planar shocks can be ruled out whereas curved or bow shocks (both J- and C-type) can be parametrized to fit our data.  相似文献   

11.
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   

12.
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   

13.
Solid CO2 is observed to be an abundant interstellar ice component towards both quiescent clouds and active star-forming regions. Our recent models of gas–grain chemistry, appropriate for quiescent regions, severely underproduce solid CO2 at the single assumed gas density and temperature. In this paper, we investigate the sensitivity of our model results to changes in these parameters. In addition, we examine how the nature of the grain surface affects the results and also consider the role of the key surface reaction between O and CO. We conclude that the observed high abundance of solid CO2 can be reproduced at reasonable temperatures and densities by models with diffusive surface chemistry, provided that the diffusion of heavy species such as O occurs efficiently.  相似文献   

14.
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   

15.
We suggest a new way to establish the relation between the electron temperature t 3 within the [O  iii ] zone and the electron temperature t 2 within the [O  ii ] zone in high-metallicity  (12 + log(O/H) > 8.25)  H  ii regions. The   t 2– t 3  diagram is constructed by applying our method to a sample of 372 H  ii regions. We find that the correlation between t 2 and t 3 is tight and can be approximated by a linear expression. The new   t 2– t 3  relation can be used to determine t 2 and accurate abundances in high-metallicity H  ii regions with a measured t 3. It can also be used in conjunction with the ff relation for the determination of t 3 and t 2 and oxygen abundances in high-metallicity H  ii regions, where the [O  iii ]λ4363 auroral line is not detected. The derived   t 2– t 3  relation is independent of photoionization models of H  ii regions.  相似文献   

16.
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   

17.
To better understand the environment surrounding CO emission clumps in the Keyhole Nebula, we have made images of the region in H2 1–0 S(1) (2.122-μm) emission and polycyclic aromatic hydrocarbon (PAH) emission at 3.29 μm. Our results show that the H2 and PAH emission regions are morphologically similar, existing as several clumps, all of which correspond to CO emission clumps and dark optical features. The emission confirms the existence of photodissociation regions (PDRs) on the surface of the clumps. By comparing the velocity range of the CO emission with the optical appearance of the H2 and PAH emission, we present a model of the Keyhole Nebula whereby the most negative velocity clumps are in front of the ionization region, the clumps at intermediate velocities are in it and those which have the least negative velocities are at the far side. It may be that these clumps, which appear to have been swept up from molecular gas by the stellar winds from η  Car, are now being overrun by the ionization region and forming PDRs on their surfaces. These clumps comprise the last remnants of the ambient molecular cloud around η Car.  相似文献   

18.
The formation of CO2 in the gas phase and on a polyaromatic hydrocarbon surface (coronene) via three possible pathways is investigated with density functional theory. Calculations show that the coronene surface catalyses the formation of CO2 on model grain surfaces. The addition of 3O to CO is activated by 2530 K in the gas phase. This barrier is lowered by 253 K for the Eley–Rideal mechanism and 952 K for the hot-atom mechanism on the surface of coronene. Alternative pathways for the formation of CO2 are the addition of 3O to the HCO radical, followed by dissociation of the HCO2 intermediate. The O + HCO addition is barrierless in the gas phase and on the surface and is more than sufficiently exothermic to subsequently cleave the H–C bond. The third mechanism, OH + CO addition followed by H removal from the energized HOCO intermediate, has a gas-phase exit barrier that is 1160 K lower than the entrance barrier. On the coronene surface, however, both barriers are almost equal. Because the HOCO intermediate can also be stabilized by energy dissipation to the surface, it is anticipated that for the surface reaction the adsorbed HOCO could be a long-lived intermediate. In this case, the stabilized HOCO intermediate could react, in a barrierless manner, with a hydrogen atom to form H2+ CO2, HCO2H, or H2O + CO.  相似文献   

19.
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   

20.
We have carried out a systematic search for the molecular ion CO+ in a sample of eight protoplanetary and planetary nebulae in order to determine the origin of the unexpectedly strong HCO+ emission previously detected in these sources. An understanding of the HCO+ chemistry may provide direct clues for the physical and chemical evolution of planetary nebulae. We find that the integrated intensity of the CO+ line may be correlated with that of HCO+, suggesting that the reaction of CO+ with molecular hydrogen may be an important formation route for HCO+ in these planetary nebulae.  相似文献   

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