M. P. Redman

HCO+ emission excess in bipolar outflows

A plausible model is proposed for the enhancement of the abundance of molecular species in bipolar outflow sources. In this model, levels of HCO + enhancement are considered based on previous chemical calculations, which are assumed to result from shock-induced desorption and photoprocessing of dust grain ice mantles in the boundary layer between the outflow jet and the surrounding envelope. A radiative transfer simulation that incorporates chemical variations within the flow shows that the proposed abundance enhancements in the boundary layer are capable of reproducing the observed characteristics of the outflow seen in HCO + emission in the star-forming core L1527. The radiative transfer simulation also shows that the emission lines from the enhanced molecular species, which trace the boundary layer of the outflow, exhibit complex line profiles, indicating that detailed spatial maps of the line profiles are essential in any attempt to identify the kinematics of potential infall/outflow sources. This study is one of the first applications of a full three-dimensional radiative transfer code which incorporates chemical variations within the source.

Clumpy ultracompact H II regions – I. Fully supersonic wind-blown models

We propose that a significant fraction of the ultracompact HII regions found in massive star-forming clouds are the result of the interaction of the wind and ionizing radiation from a young massive star with the clumpy molecular cloud gas in its neighbourhood. Distributed mass loading in the flow allows the compact nebulae to be long-lived. In this paper, we discuss a particularly simple case, in which the flow in the HII region is everywhere supersonic. The line profiles predicted for this model are highly characteristic, for the case of uniform mass loading. We discuss briefly other observational diagnostics of these models.

Chemistry and clumpiness in planetary nebulae

We study the chemistry in the slow wind during the transition from asymptotic giant branch (AGB) star to protoplanetary nebula (PPN) and planetary nebula (PN). We show that there is a very rich chemistry of degradation products created by photoprocessing, driven by the gradually hardening radiation field of the central star. Most of these products are, however, photodissociated during the PPN phase if the wind is smooth. By contrast, if the wind is clumpy, possibly because of clumpiness in the AGB atmosphere, then many of these degradation products survive into the PN phase. Thus, chemistry may be used to infer the existence of clumpiness in the AGB phase. We identify potential molecular tracers, and we note that, in the case of clumpiness, large molecules may survive the transport from the stellar atmosphere to the interstellar medium. We compare between our model results with observations of three objects at differing evolutionary stages: CRL618, NGC 7027 and the Helix nebula (NGC 7293).

Molecular gas freeze‐out in the pre‐stellar core L1689B

C 1 7 O J = 2 → 1 observations have been carried out towards the pre-stellar core L1689B. By comparing the relative strengths of the hyperfine components of this line, the emission is shown to be optically thin. This allows accurate CO column densities to be determined and, for reference, this calculation is described in detail. The hydrogen column densities that these measurements imply are substantially smaller than those calculated from SCUBA dust emission data. Furthermore, the C 1 7 O J = 2 → 1 column densities are approximately constant across L1689B, whereas the SCUBA column densities are peaked towards the centre. The most likely explanation is that CO is depleted from the central regions of L1689B. Simple models of pre-stellar cores with an inner depleted region are compared with the results. This enables the magnitude of the CO depletion to be quantified and also allows the spatial extent of the freeze-out to be firmly established. We estimate that within about 5000 au of the centre of L1689B, over 90 per cent of the CO has frozen on to grains. This level of depletion can only be achieved after a duration that is at least comparable to the free-fall time-scale.

Rotation of the pre-stellar core L1689B

The search for the onset of star formation in pre-stellar cores has focused on the identification of an infall signature in the molecular line profiles of tracer species. The classic infall signature is a double-peaked line profile with an asymmetry in the strength of the peaks such that the blue peak is stronger. L1689B is a pre-stellar core and infall candidate, but new James Clerk Maxwell Telescope (JCMT) HCO+ line profile data, presented here, confirm that both blue and red asymmetric line profiles are present in this source. Moreover, a dividing line can be drawn between the locations where each type of profile is found. It is argued that it is unlikely that the line profiles can be interpreted with simple models of infall or outflow, and that rotation of the inner regions is the most likely explanation. A rotational model is developed in detail with a new three-dimensional molecular line transport code, and it is found that the best type of model is one in which the rotational velocity profile is in between solid-body and Keplerian. It is first shown that red and blue asymmetric line profiles can be generated with a rotation model entirely in the absence of any infall motion. The model is then quantitively compared with the JCMT data, and an iteration over a range of parameters is performed to minimize the difference between the data and the model. The results indicate that rotation can dominate the line profile shape even before the onset of infall.

The Hypersonic, Bipolar, Knotty Outflow from the Engraved Hourglass Planetary Nebula MyCn 18

The remarkable velocity structure of the different components of the young planetary nebula MyCn 18 have been revealed by obtaining imagery and spatially resolved spectrometry of the Hα and [N II] λλ6548, 6584 lines with the Manchester echelle spectrometer combined with the 3.9 m Anglo-Australian telescope. The bright, bipolar, nebular core is shown to be composed of two extended hemispherical cavities whose axes are tilted at 52° to the plane of the sky. Ionized flows, at ≤90 km s-1 and parallel to the walls of these cavities, are occurring. The full extent of the elongated bipolar assembly of high-speed knots which apparently lie along the same axis is now revealed in a continuum-subtracted image in the light of the Hα and [N II] λλ6548, 6584 nebular emission lines. Complete spatial coverage of line profiles from these knots is also presented for the first time. In their most likely configuration, these knots are shown to have a range of outflowing speeds of ≤630 km s-1 that are proportional to their distance from the central star. There is some degree of point/velocity symmetry, indicating that some pairs of knots have been ejected in opposing directions at the same speed. Curiously, the line profiles from the knots are very narrow, i.e., from 15 to 30 km s-1. Among several possible explanations of the origin of these hypersonic knots is a recurrent nova-like ejection from a central binary star.

Observations of HCN hyperfine line anomalies towards low- and high-mass star-forming cores

HCN is becoming a popular choice of molecule for studying star formation in both low and high mass regions and for other astrophysical sources from comets to high red shift galaxies. However, a major and often overlooked diculty with HCN is that

Oscillations in the stable starless core Barnard 68

New molecular line observations of the Bok globule Barnard 68 in HCO+ irrefutably confirm the complex pattern of red and blue asymmetric line profiles seen across the face of the cloud in previous observations of CS. The new observations thus strengthen the previous interpretation that Barnard 68 is undergoing peculiar oscillations. Furthermore, the physical chemistry of B68 indicates that the object is much older than the sound crossing time and is therefore long-lived. A model is presented for the globule in which a modest external pressure perturbation is shown to lead to oscillations about a stable equilibrium configuration. Such oscillations may be present in other stable starless cores as manifested by a similar signature of inward and outward motions.

Clumpy ultracompact H II regions - III. Cometary morphologies around stationary stars

ABSTRACT Cometary ultracompact H II regions have been modelled as the interaction of thehypersonicwindfromamovingstarwiththemolecularcloudwhichsurroundsthestar.We here show that a similar morphology can ensue even if the star is stationary withrespect to the cloud material. We assume that the H II region is within a stellar windbubble which is strongly mass loaded: the cometary shape results from a gradient inthe distribution of mass loading sources. This model circumvents problems associatedwith the necessarily high spatial velocities of stars in the moving star models.Key words: hydrodynamics – stars: mass-loss – ISM: structure – H II regions – radiolines: ISM. 1 INTRODUCTIONThe ultracompact H II regions (UCHiiR) found deep withinmolecular clouds provide important information on the earlyphases of the interaction of massive stars with their natalenvironment. The disruption of the cloud material by thehypersonic winds and UV radiation fields of these stars is asevere barrier to an understanding of the process of massivestar formation – only by studying the disruption process willanything be learnt about the innermost regions of the pro-tostellar cloud. The disruption process also involves manyimportant problems of gas dynamics. Considerable theoret-ical effort has gone into modelling the varied morphology ofUCHiiR.Most theoretical attention has so far been given to thecometary regions, which comprise about 20 per cent of ob-served UCHiiR (Churchwell 1990). Perhaps the most de-tailed model, that of Van Buren & Mac Low (1992, andreferences therein), treats them as the steady-state partiallyionized structures behind bow shocks driven by the windsof stars moving through molecular cloud material. Althoughit has been argued that some morphologies which are notapparently cometary (in particular, core–halo) can be ex-plained as cometary structures viewed close to their axes(Mac Low et al. 1991), the shell and multiply peaked mor-phologies cannot, and so other models should also be inves-tigated.There are also a number of unresolved questions withregard to the cometary models. First, the star is assumed tobe moving through relatively homogeneous molecular cloudgas. Yet it is well known that cloud material has a clumpydistribution down to very small scales. Secondly, currentcometary models require rather high stellar velocities, char-acteristically 10–20kms

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, 12 CO J = 2 → 1, 13 CO J = 2 → 1, C 18 O J = 2 → 1 and C 17 O 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 12 CO and 13 CO have a large linewidth due to a high-velocity jet. These profiles replicate the process of more abundant material being susceptible to a jet. C 18 O and C 17 O do not display such a large linewidth as they trace denser quiescent material deep in the cloud.