R. J. R. Williams

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.

Highly supersonic motions within the outer features of the η Carinae nebulosity

Spatially resolved HIX and [N II] line profiles have been obtained over striking features in the outer regions of the 11 Carinae nebula. The highly irregular outer shell of low-ionization, [N Il]-bright, knots is shown to exhibit radial velocities between -1200 and + 300 kIn S-1 with respect to the systemic radial velocity, over its perimeter. Furthermore, several knots are found which appear to emit only in the HIX line with radial velocities up to -1450 kIn S-I. However, the most intriguing features are a narrow spike that projects through this outer shell, and a faint arc of emission that extends well beyond it. The spike, which exhibits a change of radial velocity along its length, could be a narrow collimated jet with an outflow velocity of ~ 1000 kIn S-I. In one interpretation the arc is modelled by a conical outflow, and mechanisms for generating [N II] emission from highly supersonic gas are also discussed.

Jet Cocoons and the Formation of Narrow Line Clouds in Seyfert Galaxies

We present non-adiabatic hydrodynamic simulations of a supersonic light jet propagating into a fully ionized medium of uniform density on a scale representative of the narrow-line region (NLR) in Seyfert galaxies with associated radio jets. In this regime the cooling distance of the swept-up gas in the bowshock of the jet is of the same order as the transverse extent of the jet bowshock, as opposed to the more extreme regimes found for more powerful adiabatic large-scale jets or the slow galactic jets which have previously been simulated. We calculate the emissivity for the Hx line and for radio synchrotron emission. We find that the structure of the line-emitting cold envelope of the jet cocoon is strongly dependent on the nonxad stationary dynamics of the jet head as it propagates through the ambient medium. We observe the formation of cloud-like high density regions which we associate with NLR clouds and filaments. We find that some of these clouds might be partially neutral and represent sites of jet-induced star formation. The calculated Hoc flux and the spectral line width are consistent with NLR observations. The simulation of the radio-optical emission with radiative cooling confirms the basic result of the geometric bowshock model developed by Taylor et al., i.e. that the start of noticeable optical line emission can be significantly offset from the hotspot of the radio emission. However, the time-dependent nature of the jet dynamics implies significant differences from Taylor et al.s geometric bowshock model.

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

CLUMPY ULTRACOMPACT HII REGIONS : II. CORES, SPHERES AND SHELLS FROM SUBSONIC FLOWS

We have modelled ultracompact HII regions (UCHIIR) in terms of steady subsonic ionized flows in a clumpy medium. Mass loss from neutral clumps allows the regions to be long-lived. We examine the form of global flows for different dependences of the volume mass injection rate,

BREAKING THE SOUND BARRIER IN RECOMBINATION FRONTS

dot{q}

Wide intermediate scale structures in mass-loaded flows

, on radius and Mach number, and describe the solutions in detail. We find that three observed UCHIIR morphologies are reproduced with these models. Mach number independent flows that include a radial variation can give centre- brightened core--halo morphologies. Mach number dependent flows reproduce naturally the uniform UCHIIR morphology. In a hybrid model, including subsonic and supersonic flows, we allow a supersonic wind to shock in the ionized region. The ionized subsonic gas has a high density and so dominates the emission. The shell produced has a velocity structure very different from that of fully supersonic models. Several morphologies of spherical UCHIIR can be understood in terms of these various models; however, kinematic data are crucial as a discriminant between them.

Line Forming Regions in Active Galaxies and Their Nuclei

We exploit a generic instability in the integration of steady, sonic, near-isothermal flows to find the complete transition diagram for recombination fronts (for a model system of equations). The instability requires the integration of the flow equations for speeds between the isothermal and adiabatic sound speeds to be performed with particular care. As a result of this, the previous work of Newman & Axford on the structure of recombination fronts neglected an important class of solution, that of transonic fronts; our method is readily extensible to a more complete treatment of the ionization structure. Future papers will apply these results in models of the structure of ultracompact HII regions.

Flows and shocks in active galaxies and their nuclei

We showed in earlier work that a clump embedded in tenuous plasma, which is flowing subsonically relative to the clump, will develop a long thin tail whether the gas in the tail behaves adiabatically or isothermally. This paper presents a criterion for determining if viscous dissipation in a tail with result in heating that will cause such a tail to be wide.We argue that the observed properties of the tails in NGC 7293 imply that a very strong effective viscous couple exists across each of them. Assuming a strong couple, we calculate the viscous energy dissipation rate per unit volume and derive an upper bound on the number density in a tail in which viscous heating raises the temperature significantly and thus causes broadening. The densities in observed broad tails in Abell 30 exceed this upper bound, and we suggest that the broadening of those tails is due to a phase transition in the tail gas, caused by photo-absorption heating. We comment on the relationship of intermediate-scale structures in clumpy media to the issue of whether or not a global tenous flow can be considered to be mass-loaded.

On sonic transitions in astrophysical flows

The spectra of active galaxies and their nuclei are rich in emission and absorption line features. A major aim of present research is the development of self-consistent hydrodynamic models for the production of the line-forming regions. We here review such modelling and stress the central role played by shock phenomena induced by winds and explosions on scales ranging from the circumstellar to the intergalactic.