W. J. Henney

Dynamical H II Region Evolution in Turbulent Molecular Clouds

We present numerical radiation-hydrodynamic simulations of the evolution of H II regions formed in an inhomogeneous medium resulting from turbulence simulations. We find that the filamentary structure of the underlying density distribution produces a highly irregul ar shape for the ionized region, in which the ionization front escapes to large distances in some directions within 8 0,000 years. In other directions, on the other hand, neutral gas in the form of dense globules persists within 1 parsec of the central star for the full duration of our simulation (400,000 years). Divergent photoablation flows from these globules maintain a root-mean-squared velocity in the ionized gas that is close to the ionized sound speed. Simulated images in optical emission lines show morphologies that are in strikingly detailed agreement with those observed in real H II regions. Subject headings:H II regions — ISM: clouds — ISM: kinematics and dynamics — stars:formation — turbulence

Radiation‐magnetohydrodynamic simulations of H ii regions and their associated PDRs in turbulent molecular clouds

We present the results of radiation-magnetohydrodynamic simulations of the formation and expansion of H II regions and their surrounding photodissociation regions (PDRs) in turbulent, magnetized, molecular clouds on scales of up to 4 pc. We include the effects of ionizing and non-ionizing ultraviolet radiation and X-rays from population synthesis models of young star clusters. For all our simulations we find that the H II region expansion reduces the disordered component of the magnetic field, imposing a large-scale order on the field around its border, with the field in the neutral gas tending to lie along the ionization front, while the field in the ionized gas tends to be perpendicular to the front. The highest pressure-compressed neutral and molecular gas is driven towards approximate equipartition between thermal, magnetic and turbulent energy densities, whereas lower pressure neutral/molecular gas bifurcates into, on the one hand, quiescent, magnetically dominated regions and, on the other hand, turbulent, demagnetized regions. The ionized gas shows approximate equipartition between thermal and turbulent energy densities, but with magnetic energy densities that are 1-3 orders of magnitude lower. A high velocity dispersion (similar to 8 km s(-1)) is maintained in the ionized gas throughout our simulations, despite the mean expansion velocity being significantly lower. The magnetic field does not significantly brake the large-scale H II region expansion on the length and time-scales accessible to our simulations, but it does tend to suppress the smallest scale fragmentation and radiation-driven implosion of neutral/molecular gas that forms globules and pillars at the edge of the H II region. However, the relative luminosity of ionizing and non-ionizing radiation has a much larger influence than the presence or absence of the magnetic field. When the star cluster radiation field is relatively soft (as in the case of a lower mass cluster, containing an earliest spectral type of B0.5), then fragmentation is less vigorous and a thick, relatively smooth PDR forms.

A Keck High-Resolution Spectroscopic Study of the Orion Nebula Proplyds

We present the results of spectroscopy of four bright proplyds in the Orion Nebula obtained at a velocity resolution of 6 km s-1. After careful isolation of the proplyd spectra from the confusing nebular radiation, the emission-line profiles are compared with those predicted by realistic dynamic/photoionization models of the objects. The spectral line widths show a clear correlation with ionization potential, which is consistent with the free expansion of a transonic, ionization-stratified, photoevaporating flow. Fitting models of such a flow simultaneously to our spectra and HST emission-line imaging provides direct measurements of the proplyd size, ionized density, and outflow velocity. These measurements confirm that the ionization front in the proplyds is approximately D-critical and provide the most accurate and robust estimate to date of the proplyd mass-loss rate. Values of (0.7?1.5) ? 10-6 M? yr-1 are found for our spectroscopic sample, although extrapolating our results to a larger sample of proplyds implies that 0.4 ? 10-6 M? yr-1 is more typical of the proplyds as a whole. In view of the reported limits on the masses of the circumstellar disks within the proplyds, the length of time that they can have been exposed to ionizing radiation should not greatly exceed 104 yr?a factor of 30 less than the mean age of the proplyd stars. We review the various mechanisms that have been proposed to explain this situation, and conclude that none can plausibly work unless the disk masses are revised upward by a substantial amount.

Knots in Nearby Planetary Nebulae

HST emission-line images of five of the arguably closest planetary nebulae have shown that there is a progression of characteristics of their knots. This progression begins with dark tangential structures showing no alignment with the central star and location near the main ionization front. At the end of the progression in the largest nebulae, the knots are located throughout much of the ionized zone, where they are photoionized on the side facing the central star and accompanied by long tails well aligned radially. This modification of characteristics is what would be expected if the knots were formed near or outside the main ionization front, obtaining densities high enough to lead to their being only partially ionized as they are fully illuminated by the Lyman continuum (Lyc) radiation field. Their expansion velocities must be lower than that of the main body of the nebular shell. Their forms are altered by exposure to the radiation field from the star, although it is not clear as to the relative role of radiation pressure acting on the dust component vis-a-vis ionization shadowing. The one object that does not fit into this sequence is NGC 2392, which is the most complex nebula in our sample. In this case the inner part of the nebula is composed of a series of loops of material, some being ionization bounded, which cover only a small fraction of the area illuminated by the star. This complex structure may be what gives rise to the large variations in electron temperature inferred from low spatial resolution observations. Cometary-form knots are seen in the outer part of this object, with these objects closely resembling those found in the largest nebula in our sample, NGC 7293.

Enrichment of the interstellar medium by metal-rich droplets and the abundance bias in H ii regions

We critically examine a scenario for the enrichment of the interstellar medium (ISM) in which supernova ejecta follow a long (10 8 yr) journey before falling back onto the galactic disk in the form of metal-rich “droplets”, These droplets do not become fully mixed with the interstellar medium until they become photoionized in H ii regions. We investigate the hypothesis that the photoionization of these highly metallic droplets can explain the observed “abundance discrepancy factors” (ADFs), which are found when comparing abundances derived from recombination lines and from collisionally excited lines, both in Galactic and extragalactic H ii regions. We derive bounds of 10 13 –10 15 cm on the droplet sizes inside H ii regions in order that (1) they should not have already been detected by direct imaging of nearby nebulae, and (2) they should not be too swiftly destroyed by diffusion in the ionized gas. From photoionization modelling we find that, if this inhomogeneous enrichment scenario holds, then the recombination lines strongly overestimate the metallicities of the fully mixed H ii regions. The abundances derived from collisionally excited lines also suffer some bias, although to a much lesser extent. In the absence of any recipe for correcting these biases, we recommend the discarding of all objects showing large ADFs from studies of galactic chemical evolution. These biases must also be kept in mind when comparing the galactic abundance gradients for elements derived from recombination lines with those derived from collisionally excited lines. Finally, we propose a set of observations that could be undertaken to test our scenario and improve our understanding of element mixing in the ISM.

Self-Consistent Dynamic Models of Steady Ionization Fronts. I. Weak-D and Weak-R Fronts

We present a method for including steady state gas flows in the plasma physics code Cloudy, which was previously restricted to modeling static configurations. The numerical algorithms are described in detail, together with an example application to plane-parallel ionization-bounded H II regions. As well as providing the foundation for future applications to more complex flows, we find the following specific results regarding the effect of advection on ionization fronts in H II regions: (1) Significant direct effects of advection on the global emission properties occur only when the ionization parameter is lower than is typical for H II regions. For higher ionization parameters, advective effects are indirect and largely confined to the immediate vicinity of the ionization front. (2) The overheating of partially ionized gas in the front is not large, even for supersonic (R-type) fronts. For subsonic (D-type) fronts we do not find the temperature spike that has been previously claimed. (3) The most significant morphological signature of advective fronts is an electron density spike that occurs at the ionization front whenever the relative velocity between the ionized gas and the front exceeds about one-half the ionized isothermal sound speed. Observational evidence for such a spike is found in [N II] λ6584 images of the Orion bar. (4) Plane-parallel, weak-D fronts are found to show at best a shallow correlation between mean velocity and ionization potential for optical emission lines even when the flow velocity closely approaches the ionized sound speed. Steep gradients in velocity versus ionization, such as those observed in the Orion Nebula, seem to require transonic flows, which are only possible in a diverging geometry when radiation forces are included.

Mass Loss and Jet Outflow in the Orion Nebula Proplyd LV 2

We have obtained Hubble Space Telescope (HST) Space Telescope Imaging Spectrometer high-resolution spectra of the Orion proplyd LV 2 in the C III doublet at 1906.68 and 1908.73 ?. New images at the 6 cm wavelength with MERLIN complement earlier HST images at a similar spatial resolution. This object is one of the closest proplyds to ?1 Ori C, the source of the photoionizing and photoevaporating radiation. Combining the spectra with the HST images and detailed theoretical models has allowed a determination of the mass-loss rate as 8.2 ? 10-7 M? yr-1 ?10%. This rate of mass loss is used to address the conundrum of the continued existence of proplyds. Even though they should be photoevaporated in only about 105 yr, there is no evidence for their destruction. It is concluded that the only explanation is that the age of ?1 Ori C is less than 105 yr. These spectra and previously unpublished ground-based spectra in [O III] also show the presence of a monopolar microjet, redshifted by about 100 km s-1 with respect to the systemic velocity. This jet is more visible in the 6 cm MERLIN images than in HST images, and this image together with the spectra are used to determine the flow parameters for the jet. Our spectra also include the stand-off shock that lies between LV 2 and ?1 Ori C. This is the result of the high-velocity wind coming from the hot star ?1 Ori C with the low-velocity wind coming from the proplyd. As expected, this shock is at rest with respect to the two objects.

The Three-Dimensional Dynamic Structure of the Inner Orion Nebula

The three-dimensional structure of the brightest part of the Orion Nebula is assessed in the light of published and newly established data. We find that the widely accepted model of a concave blister of ionized material needs to be altered in the southwest direction from the Trapezium, where we find that the Orion-S feature is a separate cloud of very optically thick molecules within the body of ionized gas, which is probably the location of the multiple embedded sources that produce the optical and molecular outflows that define the Orion-S star formation region. Evidence for this cloud comes from the presence of H2CO lines in absorption in the radio continuum and discrepancies in the extinction derived from radio-optical and optical-only emission. We present an equilibrium Cloudy model of the Orion-S Cloud, which successfully reproduces many observed properties of this feature, including the presence of gas-phase H2CO in absorption. We also report the discovery of an open-sided shell of [O III] surrounding the Trapezium stars, revealed through emission-line ratio images and the onset of radiation shadows beyond some proplyds. We show that the observed properties of the shell are consistent with it being a stationary structure, produced by shock interactions between the ambient nebular gas and the high-velocity wind from θ1 Ori C. We examine the implications of the recently published evidence for a large blueshifted velocity of θ1 Ori C with respect to the Orion molecular cloud, which could mean that this star has only recently begun to photoionize the Orion Nebula. We show that current observations of the nebula do not rule out such a possibility, so long as the ionization front has propagated into a pre-existing low-density region. In addition, a young age for the nebula would help explain the presence of nearby proplyds with a short mass-loss timescale to photoablation.

Photoevaporating Flows from the Cometary Knots in the Helix Nebula (NGC 7293)

We explain the Hα emission of the cometary knots in the Helix Nebula (NGC 7293) with an analytical model that describes the emission of the head of the globules as a photoevaporated flow produced by the incident ionizing radiation of the central star. We compare these models with the Hα emission obtained from the Hubble Space Telescope (HST) archival images of the Helix Nebula. From a comparison of the Hα emission with the predictions of the analytical model we obtain a rate of ionizing photons from the central star of about 5 × 1045 s-1, which is consistent with estimates based on the total Hβ flux of the nebula. We also model the tails of the cometary knots as a photoevaporated wind from a neutral shadow region produced by the diffuse ionizing photon field of the nebula. A comparison with the HST images allows us to obtain a direct determination of the value of the diffuse ionizing flux. We compare the ratio of diffuse to direct stellar flux as a function of radius inside an H II region with those obtained from the observational data through the analytical tail and head wind model. The agreement of this model with the values determined from the observations of the knots is excellent.

High Spatial Velocity Features in the Orion Nebula

We have used widely spaced in time Hubble Space Telescope images to determine tangential velocities of features associated with outflows from young stars. These observations were supplemented by ground-based telescope spectroscopy, and from the resultant radial velocities, space velocities were determined for many outflows. Numerous new moving features were found and grouped into known and newly assigned Herbig-Haro objects. It was found that stellar outflow is highly discontinuous, as frequently is the case, with long-term gaps of a few hundred years, and that these outflow periods are marked by staccato bursts over periods of about ten years. Although this has been observed in other regions, the Orion Nebula Cluster presents the richest display of this property. Most of the large-scale Herbig-Haro objects in the brightest part of the Orion Nebula appear to originate from a small region northeast of the strong Orion-S radio and infrared sources. With the possible exception of HH 203, we are not able to identify specific stellar sources, but do identify candidate sources for several other bright Herbig-Haro objects. We find that there are optical features in the BN-KL region that can be related to the known large-scale outflow that originates there. We find additional evidence for this outflow originating 500-1000 years ago.