arXiv subjects

Eric Keto

Publications and source records attributed to Eric Keto.

At least 73 records · Page 4Linked to original sources

arXiv2010

Dynamics of Molecular Clouds

We further develop the model of molecular cloud fragmentation introduced in Field, Blackman and Keto (2007; FBK). We show that external pressure acting on fragments establishes a scale-dependent critical mass. Fragments with masses less than the critical value are confined largely by pressure, while those with masses greater than or equal to the critical value collapse under self gravitation. Both types of fragments are commonly observed. Without specifying the source of the external pressure, and without assuming any other scaling relations, we predict the power - law index in the relation between the rms velocity of supersonic motions and the size of fragments . We then investigate the possibility that the external pressure is due to the kinetic energy of H atoms released by photodissociation of hydrogen molecules in the fragment. This can account approximately for the observed values of external pressure and two additional observations: the value of the scaling coefficient in the power law mentioned above, and the observation of outflowing atomic hydrogen around molecular clouds. A further prediction is HI at fragment edges with column densities of order 1E20 per sq. cm and velocities of a few km/s that should be detectable with high resolution 21 cm observations. Finally, we predict the magnitude of the coefficient of dissipation in the observed supersonic flows.

astro-ph.GA↗

arXiv2010

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. 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. 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. 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.

astro-ph.SR↗

arXiv2009

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 behavior 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. We use our non-LTE radiative transfer code MOLLIE to predict observable CO and N2H+ line spectra, including the non-LTE hyperfine ratios of N2H+, during the contraction. These are compared against observations of the starless core L1544.

astro-ph.GA↗

arXiv2009

Formation of an O-Star Cluster by Hierarchical Accretion in G20.08-0.14 N

Spectral line and continuum observations of the ionized and molecular gas in G20.08-0.14 N explore the dynamics of accretion over a range of spatial scales in this massive star-forming region. Very Large Array observations of NH_3 at 4'' angular resolution show a large-scale (0.5 pc) molecular accretion flow around and into a star cluster with three small, bright HII regions. Higher resolution (0.4'') observations with the Submillimeter Array in hot core molecules (CH_3CN, OCS, and SO_2) and the VLA in NH_3, show that the two brightest and smallest HII regions are themselves surrounded by smaller scale (0.05 pc) accretion flows. The axes of rotation of the large- and small-scale flows are aligned, and the timescale for the contraction of the cloud is short enough, 0.1 Myr, for the large-scale accretion flow to deliver significant mass to the smaller scales within the star formation timescale. The flow structure appears to be continuous and hierarchical from larger to smaller scales. Millimeter radio recombination line (RRL) observations at 0.4" angular resolution indicate rotation and outflow of the ionized gas within the brightest HII region (A). The broad recombination lines and a continuum spectral energy distribution (SED) that rises continuously from cm to mm wavelengths, are both characteristic of the class of HII regions known as "broad recombination line objects". The SED indicates a density gradient inside this HII region, and the RRLs suggest supersonic flows. These observations are consistent with photoevaporation of the inner part of the rotationally flattened molecular accretion flow. We also report the serendipitous detection of a new NH_3 (3,3) maser.

astro-ph.GA↗

arXiv2008

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 the 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 10^5 cm-3. This density is significant as the critical density for gas cooling by gas-dust collisions and also as the critical density for dynamical stability, given the typical properties of the starless cores. The starless cores thus divide into two classes that we refer to as thermally super-critical and thermally sub-critical.This two-class distinction allows an improved interpretation of the different observational data of starless cores within a single model.

astro-ph↗

arXiv2008

Embedded Oscillating Starless Cores

In a previous paper we demonstrated that non-radial hydrodynamic oscillations of a thermally-supported (Bonnor-Ebert) sphere embedded in a low-density, high-temperature medium persist for many periods. The predicted column density variations and molecular spectral line profiles are similar to those observed in the Bok globule B68 suggesting that the motions in some starless cores may be oscillating perturbations on a thermally supported equilibrium structure. Such oscillations can produce molecular line maps which mimic rotation, collapse or expansion, and thus could make determining the dynamical state from such observations alone difficult. However, while B68 is embedded in a very hot, low-density medium, many starless cores are not, having interior/exterior density contrasts closer to unity. In this paper we investigate the oscillation damping rate as a function of the exterior density. For concreteness we use the same interior model employed in Broderick et al. (2007), with varying models for the exterior gas. We also develop a simple analytical formalism, based upon the linear perturbation analysis of the oscillations, which predicts the contribution to the damping rates due to the excitation of sound waves in the external medium. We find that the damping rate of oscillations on globules in dense molecular environments is always many periods, corresponding to hundreds of thousands of years, and persisting over the inferred lifetimes of the globules.

astro-ph↗

arXiv2008

The Ionization of Accretion Flows in High Mass Star Formation: W51e2

Previous observations show that the hypercompact HII region W51e2 is surrounded by a massive molecular accretion flow centered on the HII region. New observations of the H53alpha radio recombination line made with the VLA at 0.45 arc second angular resolution show a velocity gradient in the ionized gas within the HII region of > 500 kms-1 pc-1 comparable to the velocity gradient seen in the molecular accretion flow. New CO line observations made with the SMA at arc second angular resolution detect a molecular bipolar outflow immediately around the W51e2 HII region and extending along the axis of rotation of the molecular flow. These observations are consistent with an evolutionary phase for high mass star formation in which a newly formed massive star first begins to ionize its surroundings including its own accretion flow.

astro-ph↗

arXiv2008

Time Variation in G24.78+0.08 A1: Evidence for an Accreting Hypercompact H II Region?

Over a timescale of a few years, an observed change in the optically thick radio continuum flux can indicate whether an unresolved H II region around a newly formed massive star is changing in size. In this Letter we report on a study of archival VLA observations of the hypercompact H II region G24.78+0.08 A1 that shows a decrease of ~ 45 % in the 6-cm flux over a five year period. Such a decrease indicates a contraction of ~ 25 % in the ionized radius and could be caused by an increase in the ionized gas density if the size of the H II region is determined by a balance between photoionization and recombination. This finding is not compatible with continuous expansion of the H II region after the end of accretion onto the ionizing star, but is consistent with the hypothesis of gravitational trapping and ionized accretion flows if the mass-accretion rate is not steady.

astro-ph↗

arXiv2007

The Early Evolution of Massive Stars: Radio Recombination Line Spectra

Velocity shifts and differential broadening of radio recombination lines are used to estimate the densities and velocities of the ionized gas in several hypercompact and ultracompact HII regions. These small HII regions are thought to be at their earliest evolutionary phase and associated with the youngest massive stars. The observations suggest that these HII regions are characterized by high densities, supersonic flows and steep density gradients, consistent with accretion and outflows that would be associated with the formation of massive stars.

astro-ph↗

arXiv2007

A Model of Cloud Fragmentation

We present a model in which the supersonic motions observed in molecular clouds are driven by gravitational energy released as large structures fragment into smaller ones. The fragmentation process begins in large molecular clouds, and continues down to fragments of a critical mass, at which gravitational confinement may be replaced by pressure confinement. The power laws that describe the scaling of density and mass, and number spectra and mass spectra of the fragments are given in terms of that of the observed velocity dispersion of the fragments. The results agree reasonably well with observations over the range from several to about a third of a million solar masses.

astro-ph↗

arXiv2007

Oscillating Starless Cores: The Nonlinear Regime

In a previous paper, we modeled the oscillations of a thermally-supported (Bonnor-Ebert) sphere as non-radial, linear perturbations following a standard analysis developed for stellar pulsations. The predicted column density variations and molecular spectral line profiles are similar to those observed in the Bok globule B68 suggesting that the motions in some starless cores may be oscillating perturbations on a thermally supported equilibrium structure. However, the linear analysis is unable to address several questions, among them the stability, and lifetime of the perturbations. In this paper we simulate the oscillations using a three-dimensional numerical hydrodynamic code. We find that the oscillations are damped predominantly by non-linear mode-coupling, and the damping time scale is typically many oscillation periods, corresponding to a few million years, and persisting over the inferred lifetime of gobules.

astro-ph↗

arXiv2007

The formation of massive stars: accretion, disks and the development of hypercompact HII 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 HII region gravitationally trapped within the accretion flow. At this stage the flow of ionized gas is entirely inward. As the ionization increases, the HII region transitions to a bipolar morphology in which the inflow is replaced by outflow within a narrow range of angle with about 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 HII region is of hypercompact to ultracompact size depending on the mass of the enclosed star or stars. These small HII regions whose dynamics are dominated by stellar gravitation and accretion are different than compact and larger HII regions whose dynamics are dominated by the thermal pressure of the ionized gas.

astro-ph↗

arXiv2006

The Mid-infrared Fine-structure Lines of Neon as an Indicator of Star For mation Rate in Galaxies

The fine-structure lines of singly ([Ne II] 12.8 micron) and doubly ([Ne III] 15.6 micron) 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.

astro-ph↗

arXiv2006

Oscillations of starless cores

If the split, asymmetric molecular spectral line profiles that are seen in many starless cores are interpreted as indicative of global collapse or expansion of the core then one possible implication is that most starless cores have short lifetimes on the order of the collapse or sound crossing time scale. An alternative interpretation of the line profiles as indicative of perturbations on an underlying equilibrium structure leads to the opposite implication, that many cores have long lifetimes. While evidence suggests that some cores are collapsing on a free-fall time scale, we show that observations of some other starless cores can be reproduced by a model of non-radial oscillations about the equilibrium configuration of a pressure-bounded, thermally-supported sphere (Bonnor-Ebert sphere). We model the oscillations as linear perturbations following a standard analysis developed for stellar pulsations and compare the column densities and molecular spectral line profiles predicted from a particular model to observations of the Bok globule B68.

astro-ph↗

arXiv2005

Observations on the Formation of Massive Stars by Accretion

Observations of the H66a 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 HII region are not effective in reversing the accretion flow. The observed rate of the accretion flow, 0.001 solar masses/yr, is sufficient to form massive stars within the time scale imposed by their short main sequence lifetimes. A simple model of disk accretion relates quenched HII regions, trapped hypercompact HII regions, and photo-evaporating disks in an evolutionary sequence.

astro-ph↗

arXiv2005

M82, Starbursts, Star Clusters, and the Formation of Globular Clusters

We observed the nearby starburst galaxy M82 in CO in the higher frequency (2--1) transition to achieve an angular resolution below 1 arc second or 17 pc at the target. We resolved the molecular gas into a large number of compact clouds, with masses ranging from about 2x10^3 to 2x10^6 solar masses. The mass spectrum scales as N(M) ~ M^-1.5, similar to the mass spectra of young massive star clusters suggesting that individual molecular clouds are transformed in the starburst into individual star clusters. The larger clouds are surrounded by supernovae and HII regions suggesting that star formation proceeds from the outside of the clouds and progresses inward consistent with triggering by a sudden increase in external pressure. The clouds with internal star formation have velocity gradients and inverse P-Cygni spectral line profiles indicating inward motions of 35 kms consistent with shock driven compression. Diffuse free-free radio emission and X-ray emission around the clouds provides evidence for superheated ionized gas sufficient to drive the compression. Clouds with spectral lines indicating expansion show little internal star formation suggesting that the dynamics precedes and is responsible for the star formation rather than the inverse. M82 is known to be in interaction with neighboring M81. The overall picture is consistent with the formation of massive star clusters from individual giant molecular clouds crushed by a sudden galactic scale increase in external pressure generated by the changing dynamics that result from a near-collision with a neighboring galaxy. Present day globular clusters may have formed in a similar fashion in primordial galaxies.

astro-ph↗

arXiv2005

Dark cloud cores and gravitational decoupling from turbulent flows

We test the hypothesis that the starless cores may be gravitationally bound clouds supported largely by thermal pressure by comparing observed molecular line spectra to theoretical spectra produced by a simulation that includes hydrodynamics, radiative cooling, variable molecular abundance, and radiative transfer in a simple one-dimensional model. The results suggest that the starless cores can be divided into two categories: stable starless cores that are in approximate equilibrium and will not evolve to form protostars, and unstable pre-stellar cores that are proceeding toward gravitational collapse and the formation of protostars. The starless cores might be formed from the interstellar medium as objects at the lower end of the inertial cascade of interstellar turbulence. Additionally, we identify a thermal instability in the starless cores. Under par ticular conditions of density and mass, a core may be unstable to expansion if the density is just above the critical density for the collisional coupling of the gas and dust so that as the core expands the gas-dust coupling that cools the gas is reduced and the gas warms, further driving the expansion.

astro-ph↗

arXiv2005

An Infalling Torus of Molecular Gas Around the Ultra-Compact HII Region G28.20-0.05

We present new observations of the ultra-compact HII region G28.20-0.05 in 23 GHz continuum, and the NH3(1,1), NH3(2.2), and NH3(3,3) lines. To explain the complicated kinematics of the molecular gas, we propose a model consisting of two components. One component is an infalling, equatorial torus of molecular gas, whose dense central region has been ionized to form the ultra-compact HII region. The second component is a larger expanding molecular shell driven by some type of wide-angle outflow or wind. We estimate that the infall component includes more than 18 Msun of molecular gas. We calculate the central mass to be 79 Msun, probably comprising more than one star. The arrangement of the molecular material suggests a connection to the other disk-like structures seen around massive young stars. The central star in this case is more massive, and the whole region may be more evolved than other similar objects such as IRAS 20126+4104, IRAS 18089-1732, G192.16-3.84, and AFGL5142.

astro-ph↗