S. Jane Arthur

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

Hydrodynamics of Cometary Compact H II Regions

We present numerical radiation-hydrodynamic simulations of cometary H II regions for a number of champagne flow and bow shock models. For the champagne flow models we study smooth density distributions with both steep and shallow gradients. We also consider cases in which the ionizing star has a strong stellar wind and cases in which the star additionally has a proper motion within the ambient density gradient. We find that our champagne flow plus stellar wind models have limb-brightened morphologies and kinematics that can see the line-of-sight velocities change sign twice between the head and tail of the cometary H II region, with respect to the rest frame velocity. Our bow shock models show that pressure gradients across and within the shell are very important for the dynamics and that simple analytic models assuming thin shells in ram pressure balance are wholly inadequate for describing the shape and kinematics of these objects at early times in their evolution. The dynamics of the gas behind the shock in the neutral material ahead of the ionization front in both champagne flow and bow shock type cometary H II regions is also discussed. We present simulated emission-measure maps and long-slit spectra of our results. Our numerical models are not tailored to any particular object, but comparison with observations from the literature shows that, in particular, the models combining density gradients and stellar winds are able to account for both the morphology and general radial velocity behavior of several observed cometary H II regions, such as the well-studied object G29.96-0.02.

The Effects of Dust on Compact and Ultracompact H II Regions

We calculate numerical models of dusty H II regions using the Cloudy photoionization code with a grain size distribution. Dust sublimation causes a depletion of grain sizes and types within the ionized region, with large graphite grains being able to exist closer to the star than smaller graphite grains or silicate grains. We investigate the time-dependent hydrodynamic expansion of dusty H II regions and find that the fraction of ionizing photons absorbed by dust decreases with time. Furthermore, dusty H II regions stall earlier and at smaller radii than their dust-free counterparts. Comparison is made between our models and observable parameters, such as the electron density. We find that the electron density in dusty ionized regions estimated from radio continuum observations is likely to be an overestimate, and we quantify the discrepancy. Finally, we calculate the infrared emission from dusty H II regions and their surrounding circumnebular dust shells using the DUSTY code. We find that the far-infrared emission depends strongly on the parameters assumed for the circumnebular dust shell.

From Ultracompact to Extended H II Regions. II. Cloud Gravity and Stellar Motion

The dynamical evolution of HII regions with and without stellar motion in dense, structured molecular clouds is studied. Clouds are modeled in hydrostatic equilibrium, with gaussian central cores and external halos that obey r**-2 and r**-3 density power laws. The cloud gravity is included as a time-independent, external force. Stellar velocities of 0, 2, 8, and 12 km/s are considered. When stellar motion is included, stars move from the central core to the edge of the cloud, producing transitions from ultracompact to extended HII regions as the stars move into lower density regions. The opposite behavior occurs when stars move toward the cloud cores. The main conclusion of our study is that ultracompact HII regions are pressure-confined entities while they remain embedded within dense cores. The confinement comes from ram and/or ambient pressures. The survival of ultracompact regions depends on the position of the star with respect to the core, the stellar life-time, and the core crossing time. Stars with velocities less than the cloud dispersion velocity can produce cometary shapes smaller than 0.1 pc at times of 20,000 yr or more. The sequence Ultracompact to Compact to Extended HII region shows a variety of unpredictable structures due to ionization-shock front instability. Some ultracompact HII regions with a core-halo morphology might be explained by self-blocking effects, when stars overtake and ionize leading, piled-up clumps of neutral gas. We use thermal energy to support the cloud against gravity; the results remain the same if other types of isotropic cloud support are used.

Off-Center H II Regions in Power-Law Density Distributions

The expansion of ionization fronts in uniform and spherically symmetric power-law density distributions is a well-studied topic. However, in many situations, such as a star formed at the edge of a molecular cloud core, an offset power-law density distribution would be more appropriate. In this paper a few of the main issues of the formation and expansion of H II regions in such media are outlined and results are presented for the particular cases where the underlying power laws are r-2 and r-3. A simple criterion is developed for determining whether the initial photoionized region will be unbounded, which depends on the power-law exponent and the ratio of the equivalent Stromgren radius produced by the star in a uniform medium to the stellar offset distance between the star and the center of the density distribution. In the expansion stage, the ionized volumes for the cases studied will eventually become unbounded unless pressure balance with the external medium is reached before the ionization front velocity becomes supersonic with respect to the ionized gas.

The Evolution of the Circumstellar and Interstellar Medium Around Massive Stars

Throughout their lives massive stars modify their environment through their ionizing photons and strong stellar winds. Here, I present coupled radiation-hydrodynamic calculations of the evolution of the bubbles and nebulae surrounding massive stars. The evolution is followed from the main sequence through the Wolf-Rayet stage and shows that structures are formed in the ISM out to some tens of parsecs radius. Closer to the star, instabilities lead to the breakup of swept-up wind shells. The photoevaporated flows from the resulting clumps interact with the stellar wind from the central star, which leads to the production of soft X-rays. I examine the consequences for the different observable structures at all time and size scales and evaluate the impact that the massive star has on its environment.

Photoionized Regions around Supernova Remnants

The radiation produced by the gas cooling behind a fast supernova remnant shock in the interstellar medium is capable of ionizing the undisturbed medium ahead of the shock wave. In this work I investigate the nonequilibrium evolution of these photoionized precursor regions by means of radiation-hydrodynamic simulations of supernova remnant evolution.