Molecular line profiles as diagnostics of protostellar collapse: modelling the 'blue asymmetry' in inside-out infall

The evolution of star-forming core analogues undergoing inside-out collapse is studied with a multipoint chemodynamical model which self-consistently computes the abundance distribution of chemical species in the core. For several collapse periods the output chemistry of infalling tracer species such as HCO⁺, CS and N₂H⁺ is then coupled to an accelerated Λ-iteration radiative transfer code, which predicts the emerging molecular line profiles using two different input gas/dust temperature distributions. We investigate the sensitivity of the predicted spectral line profiles and line asymmetry ratios to the core temperature distribution, the time-dependent model chemistry, as well as to ad hoc abundance distributions. The line asymmetry is found to be strongly dependent on the adopted chemical abundance distribution. In general, models with a warm central region show higher values of blue asymmetry in optically thick HCO⁺ and CS lines than models with a starless core temperature profile. We find that in the formal context of Shu-type inside-out infall, and in the absence of rotation or outflows, the relative blue asymmetry of certain HCO⁺ and CS transitions is a function of time and, subject to the foregoing caveats, can act as a collapse chronometer. The sensitivity of simulated HCO⁺ line profiles to linear radial variations, subsonic or supersonic, of the internal turbulence field is investigated in the separate case of static cores.     

A multitransition molecular line study of inward motions towards massive star-forming cores

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  .     

Discovery of a dense bipolar outflow from a new class 0 protostar in NGC 2068/LBS 17

We report the discovery of high-velocity dense gas from a bipolar outflow source near NGC 2068 in the L1630 giant molecular cloud. CO and HCO⁺ J =3→2 line wings have a bipolar distribution in the vicinity of LBS 17-H with the flow orientated roughly east–west and perpendicular to the elongation of the submillimetre dust continuum emission. The flow is compact (total extent ∼0.2 pc) and contains of the order of 0.1 M_⊙ of swept-up gas. The high-velocity HCO⁺ emission is distributed over a somewhat smaller area <0.1 pc in extent.
A map of C¹⁸O J =2→1 emission traces the LBS 17 core and follows the ambient HCO⁺ emission reasonably well, with the exception of the direction towards LBS 17-H where there is a significant anticorrelation between the C¹⁸O and HCO⁺. A comparison of beam-matched C¹⁸O and dust-derived H₂ column densities suggests that CO is depleted by up to a factor of ∼50 at this position if the temperature is as low as 9 K, although the difference is substantially reduced if the temperature is as high as 20 K. Chemical models of collapsing clouds can account for this discrepancy in terms of different rates of depletion on to dust grains for CO and HCO⁺.
LBS 17-H has a previously known water maser coincident with it but there are no known near-infrared, IRAS or radio continuum sources associated with this object, leading to the conclusion that it is probably very young. A greybody fit to the continuum data gives a luminosity of only 1.7 L_⊙ and a submillimetre-to-bolometric luminosity ratio of 0.1, comfortably satisfying the criteria for classification as a class 0 protostar candidate.     

C-type shocks in the interstellar medium: profiles of CH+ and CH absorption lines

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.     

A search for CO+ in planetary nebulae

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.     

Molecular line mapping of the giant molecular cloud associated with RCW 106 – III. Multimolecular line mapping

We present multimolecular line maps obtained with the Mopra telescope towards the southern giant molecular cloud (GMC) complex G333, associated with the H  ii region RCW 106. We have characterized the GMC by decomposing the 3D data cubes with gaussclumps , and investigated spatial correlations among different molecules with principal component analysis (PCA). We find no correlation between clump size and linewidth, but a strong correlation between emission luminosity and linewidth. PCA classifies molecules into high- and low-density tracers, and reveals that HCO⁺ and N₂H⁺ are anticorrelated.     

The CO Cameron band emission in the Red Rectangle

The origin of the strong CO Cameron band emission in the Red Rectangle is investigated. From a comparison of laboratory spectroscopic data and astronomical observations in combination with chemical modelling, it is concluded that the Cameron band emission of CO originates from the dissociative recombination of HCO⁺, and its intensity provides a measure of the product of the density of HCO⁺ ions and the electron density, integrated along the line of sight.     

Shocked molecular gas towards the supernova remnant G359.1–0.5 and the Snake

We have found a bar of shocked molecular hydrogen (H₂) towards the OH(1720 MHz) maser located at the projected intersection of supernova remnant (SNR)  G359.1–0.5  and the non-thermal radio filament known as the Snake. The H₂ bar is well aligned with the SNR shell and almost perpendicular to the Snake. The OH(1720 MHz) maser is located inside the sharp western edge of the H₂ emission, which is consistent with the scenario in which the SNR drives a shock into a molecular cloud at that location. The spectral line profiles of ¹²CO, HCO⁺ and CS towards the maser show broad-line absorption, which is absent in the ¹³CO spectra and most probably originates from the pre-shock gas. A density gradient is present across the region and is consistent with the passage of the SNR shock, while the H₂ filament is located at the boundary between the pre-shock and post-shock regions.     

Chemical evolution ahead of Herbig–Haro objects

Compact regions of enhanced HCO⁺ and NH₃ emission have been detected close to a number of Herbig–Haro objects. An interpretation of these detections is the following: a transient clump within the molecular cloud has been irradiated by the shock that generates the Herbig–Haro object. The irradiation releases icy mantles from the grains within the transient clump and initiates a photochemistry. On the basis of this picture, we have developed an extensive chemical model which predicts that a wide range of species, other than NH₃ and HCO⁺, should also be detectable. These include CH₃OH, H₂S, C₃H₄, H₂CO, SO, SO₂, H₂CS and NS. The chemical effects should last ∼  10⁴ yr  .     

H3+ in diffuse interstellar gas

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⁺.     

Detection of HCO+ towards Cygnus OB2 No. 12

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.     

Turbulence in Class 0 and I protostellar envelopes

We estimate the levels of turbulence in the envelopes of class 0 and I protostars using a model based on measurements of the peak separation of double-peaked asymmetric line profiles. We use observations of 20 protostars of both class 0 and I taken in the  HCO⁺(J = 3 → 2)  line that show the classic double-peaked profile. We find that some class 0 sources show high levels of turbulence, whilst others demonstrate much lower levels. In class I protostars, we find predominantly low levels of turbulence. The observations are consistent with a scenario in which class 0 protostars form in a variety of environments and subsequently evolve into class I protostars. The data do not appear to be consistent with a recently proposed scenario in which class 0 protostars can only form in extreme environments.     

HE 1127–1304: the largest radio quasar

By considering the propagation of low-amplitude magnetohydrodynamic waves in partially ionized plasmas, it is shown that the ion-neutral drift (ambipolar diffusion) induced by the waves can have specific effects on the molecular chemistry of cold material. The chemistry occurring in gas swept by Alfvén waves is described and it is shown that this leads to spatial variations in the deuterium fractionation ratios of, for example, HCO⁺ and N₂H⁺, on spatial scales of a few hundredths of a parsec, depending upon the fractional ionization of the ambient medium. The possibility of detecting interstellar Alfvén waves by molecular spectroscopy and their effect of producing small-scale chemical abundance gradients in molecular clouds are briefly discussed.     

CO abundances in a protostellar cloud: freeze-out and desorption in the envelope and outflow of L483

CO isotopes are able to probe the different components in protostellar clouds. These components, core, envelope and outflow have distinct physical conditions, and sometimes more than one component contributes to the observed line profile. In this study, we determine how CO isotope abundances are altered by the physical conditions in the different components. We use a 3D molecular line transport code to simulate the emission of four CO isotopomers, ¹²CO   J = 2 → 1, ¹³CO J = 2 → 1  , C¹⁸O   J = 2 → 1  and C¹⁷O   J = 2 → 1  from the Class 0/1 object L483, which contains a cold quiescent core, an infalling envelope and a clear outflow. Our models replicate James Clerk Maxwell Telescope (JCMT) line observations with the inclusion of freeze-out, a density profile and infall. Our model profiles of ¹²CO and ¹³CO have a large linewidth due to a high-velocity jet. These profiles replicate the process of more abundant material being susceptible to a jet. C¹⁸O and C¹⁷O do not display such a large linewidth as they trace denser quiescent material deep in the cloud.     

Electron-impact rotational excitation of linear molecular ions

Molecular R -matrix calculations are performed to give rotational excitation rates for electron collisions with linear molecular ions. Results are presented for CO⁺, HCO⁺, NO⁺ and H₂⁺ up to electron temperatures of 10 000 K. De-excitation rates and critical electron densities are also given. It is shown that the widely used Coulomb–Born approximation is valid for Δ j =1 transitions when the molecular ion has a dipole greater than about 2D, but otherwise is not reliable for studying electron-impact rotational excitation. In particular, transitions with Δ j >1 are found to have appreciable rates and are found to be entirely dominated by short-range effects.     

The neutral carbon distribution in a bipolar outflow/molecular disc source configuration: G35.2−0.74N

Maps are presented of ³ P ₁→³ P ₀[C  i ] and J =2→1 C¹⁸O line emission from the interstellar molecular cloud G35.2−0.74N. The maps are interpreted with reference to a previous model for the structure of the cloud in which opposing jets from a central object, embedded in a rotating interstellar disc, precess and drive a bipolar molecular outflow. The C¹⁸O emission traces the rotating interstellar disc, but the [C  i ] emission shows several features. An unresolved component is observed which probably results from dissociation of CO in the centre of the disc by UV radiation from the central source. Background [C  i ] emission is also observed which shares the rotation of the disc on larger scales. The C  i /CO ratio in these components is typically a few per cent. High-velocity [C  i ] emission, where C  i /CO is high (>0.1–0.4), is observed between the CO molecular outflow and the cavity exacavated by the jet. This material has probably been accelerated by the jet but dissociated by far-UV radiation propagating through the cavity. The C  i /CO ratio falls as the shocked outflow later sweeps up CO.     

State-to-state rotational transition rates of the HCO+ ion by collisions with helium

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.     

Gas-phase models for the evolved planetary nebulae NGC 6781, M4-9 and NGC 7293

We have studied the chemistry of the molecular gas in evolved planetary nebulae. Three pseudo-time-dependent gas-phase models have been constructed for dense (10⁴–10⁵ cm⁻³) and cool ( T ∼15 K) clumpy envelopes of the evolved nebulae NGC 6781, M4-9 and NGC 7293. The three nebulae are modelled as carbon-rich stars evolved from the asymptotic giant branch to the late planetary nebula phase. The clumpy neutral envelopes are subjected to ultraviolet radiation from the central star and X-rays that enhance the rate of ionization in the clumps. With the ionization rate enhanced by four orders of magnitude over that of the ISM, we find that resultant abundances of the species HCN, HNC, HC₃N and SiC₂ are in good agreement with observations, while those of CN, HCO⁺, CS and SiO are in rough agreement. The results indicate that molecular species such as CH, CH₂, CH₂⁺ , HCl, OH and H₂O are anticipated to be highly abundant in these objects.     

IRAM observations of JVAS/CLASS gravitational lenses

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.     

On the possibility of observing H2 emission from primordial molecular cloud kernels

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.