Eric Keto

H II REGIONS: WITNESSES TO MASSIVE STAR FORMATION

We describe the first three-dimensional simulation of the gravitational collapse of a massive, rotating molecular cloud that includes heating by both non-ionizing and ionizing radiation. These models were performed with the FLASH code, incorporating a hybrid, long characteristic, ray-tracing technique. We find that as the first protostars gain sufficient mass to ionize the accretion flow, their H II regions are initially gravitationally trapped, but soon begin to rapidly fluctuate between trapped and extended states, in agreement with observations. Over time, the same ultracompact H II region can expand anisotropically, contract again, and take on any of the observed morphological classes. In their extended phases, expanding H II regions drive bipolar neutral outflows characteristic of high-mass star formation. The total lifetime of H II regions is given by the global accretion timescale, rather than their short internal sound-crossing time. This explains the observed number statistics. The pressure of the hot, ionized gas does not terminate accretion. Instead, the final stellar mass is set by fragmentation-induced starvation. Local gravitational instabilities in the accretion flow lead to the build-up of a small cluster of stars, all with relatively high masses due to heating from accretion radiation. These companions subsequently compete with the initial high-mass star for the same common gas reservoir and limit its mass growth. This is in contrast to the classical competitive accretion model, where the massive stars are never hindered in growth by the low-mass stars in the cluster. Our findings show that the most significant differences between the formation of low-mass and high-mass stars are all explained as the result of rapid accretion within a dense, gravitationally unstable, ionized flow.

The Formation of Massive Stars by Accretion through Trapped Hypercompact H II Regions

The formation of massive stars may take place at relatively low accretion rates over a long period of time if the accretion can continue past the onset of core hydrogen ignition. The accretion may continue despite the formation of an ionized H II region around the star if the H II region is small enough that the gravitational attraction of the star dominates the thermal pressure of the H II region. The accretion may continue despite radiation pressure acting against dust grains in the molecular gas if the momentum of the accretion flow is sufficient to push the dust grains through a narrow zone of high dust opacity at the ionization boundary and into the H II region where the dust is sublimated. This model of massive star formation by continuing accretion predicts a new class of gravitationally trapped, long-lived, hypercompact H II regions. The observational characteristics of the trapped hypercompact H II regions can be predicted for comparison with observations.

THE FORMATION OF MASSIVE STARS: ACCRETION, DISKS, AND THE DEVELOPMENT OF HYPERCOMPACT H ii REGIONS

The hypothesis that massive stars form by accretion can be investigated by simple analytical calculations that describe the effect that the formation of a massive star has on its own accretion flow. Within a simple accretion model that includes angular momentum, that of gas flow on ballistic trajectories around a star, the increasing ionization of a massive star growing by accretion produces a three-stage evolutionary sequence. The ionization first forms a small quasi-spherical H II region gravitationally trapped within the accretion flow. At this stage the flow of ionized gas is entirely inward. As the ionization increases, the H II region transitions to a bipolar morphology in which the inflow is replaced by outflow within a narrow range of angle aligned with the bipolar axis. At higher rates of ionization, the opening angle of the outflow region progressively increases. Eventually, in the third stage, the accretion is confined to a thin region about an equatorial disk. Throughout this early evolution, the H II region is of hypercompact to ultracompact size depending on the mass of the enclosed star or stars. These small H II regions whose dynamics are dominated by stellar gravitation and accretion are different than compact and larger H II regions whose dynamics are dominated by the thermal pressure of the ionized gas.

Observations on the Formation of Massive Stars by Accretion

Observations of the H66� recombination line from the ionized gas in the cluster of newly formed massive stars, G10.6–0.4, show that most of the continuum emission derives from the dense gas in an ionized accretion flow that forms an ionized disk or torus around a group of stars in the center of the cluster. The inward motion observed in the accretion flow suggests that despite the equivalent luminosity and ionizing radiation of several O stars, neither radiation pressure nor thermal pressure has reversed the accretion flow. The observations indicate why the radiation pressure of the stars and the thermal pressure of the H ii region are not effective in reversing the accretion flow. The

On the Evolution of Ultracompact H II Regions

The classic model for the pressure-driven expansion of H II regions is reevaluated to include the gravitational force of the star responsible for the H II region. The model shows that the gravitational attraction of the star maintains a steep density gradient and accretion flow within the ionized gas and prevents the H II region from expanding hydrodynamically unless the radius of ionization equilibrium is beyond the radius where the sound speed of the ionized gas approximates the escape velocity. Once past this critical radius, the H II region will expand rapidly and the accretion flow through the H II region is quickly reduced. However, in contrast to the model without gravity in which the velocity of the ionized gas is everywhere outward, in the model with gravity, the velocity within the H II region is always inward. Newly formed massive stars within dense molecular cores may initially form very small H II regions that at first evolve slowly through an increase in ionizing flux, as would be caused by an increase in the mass or number of stars through continuing accretion through the H II region.

The Mid-Infrared Fine-Structure Lines of Neon as an Indicator of Star Formation Rate in Galaxies

The fine-structure lines of singly ([Ne II] 12.8 μm) and doubly ([Ne III] 15.6 μm) ionized neon are among the most prominent features in the mid-infrared spectra of star-forming regions and have the potential to be a powerful new indicator of the star formation rate in galaxies. Using a sample of star-forming galaxies with measurements of the fine-structure lines available from the literature, we show that the sum of the [Ne II] and [Ne III] luminosities obeys a tight, linear correlation with the total infrared luminosity over 5 orders of magnitude in luminosity. We discuss the formation of the lines and their relation with the Lyman continuum luminosity. A simple calibration between star formation rate and the [Ne II]+[Ne III] luminosity is presented.

Dynamics and depletion in thermally supercritical starless cores

In previous studies, we identified two classes of starless cores, thermally subcritical and supercritical, distinguished by different dynamical behaviour and internal structure. Here, we study the evolution of the dynamically unstable, thermally supercritical cores by means of a numerical hydrodynamic simulation that includes radiative equilibrium and simple molecular chemistry. From an initial state as an unstable Bonnor-Ebert (BE) sphere, a contracting core evolves towards the configuration of a singular isothermal sphere by inside-out collapse. We follow the gas temperature and abundance of CO during the contraction. The temperature is predominantly determined by radiative equilibrium, but in the rapidly contracting centre of the core compressive heating raises the gas temperature by a few degrees over its value in static equilibrium. The time-scale for the equilibration of CO depends on the gas density and is everywhere shorter than the dynamical time-scale. The result is that the dynamics do not much affect the abundance of CO which is always close to that of a static sphere of the same density profile, and CO cannot be used as a chemical clock in starless cores. We use our non-local thermodynamic equilibrium (non-LTE) radiative transfer code MOLLIE to predict observable CO and N 2 H + line spectra, including the non-LTE hyperfine ratios of N 2 H + , during the contraction. These are compared against observations of the starless core L1544. The comparison indicates that the dust in L1544 has an opacity consistent with ice-covered rather than bare grains, the cosmic ray ionization rate is about 1 × 10 -17 s -1 and the density structure of L1544 is approximately that of a BE sphere with a maximum central density of 2 x 10 7 cm -3 , equivalent to an average density of 3 x 10 6 cm -3 within a radius of 500 au. The observed CO linewidths and intensities are reproduced if the CO desorption rate is about 30 times higher than the rate expected from cosmic ray strikes alone, indicating that other desorption processes are also active.

The Different Structures of the Two Classes of Starless Cores

We describe a model for the thermal and dynamical equilibrium of starless cores that includes the radiative transfer of gas and dust and simple CO chemistry. The model shows that the structure and behavior of the cores is significantly different depending on whether the central density is either above or below about 105 cm−3. This density is significant as the critical density for gas cooling by gas-dust collisions and as the critical density for dynamical stability, given the typical properties of the starless cores. Starless cores thus divide into two classes that we refer to as thermally supercritical and thermally subcritical. This two-class distinction allows an improved interpretation of the different observational data of starless cores within a single model.

An Ionized Accretion Flow in the Ultracompact H II Region G10.6–0.4

Analysis of the H66α radio recombination line velocities in the ultracompact H II region G10.6-0.4 indicates inward motion of the ionized gas within the H II region. Comparison with the velocities of molecular lines from the surrounding neutral gas suggests that a molecular accretion flow passes through the ionization boundary of the H II region and continues inward as an ionized accretion flow. Interpreted in the context of a model for star formation in hot molecular cores, these observations indicate that G10.6-0.4 is at a stage of development just after the massive star or stars at the center of the accretion flow have first formed an ionized H II region within the hot molecular core.

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.