Collapse Of Cloud Research Articles

Formation of supermassive stars in the first star clusters

ABSTRACT The formation of supermassive stars is believed to be an essential intermediate step for the formation of the massive black hole seeds that become the supermassive black holes powering the quasars observed in the early Universe. Numerical simulations have shown that supermassive stars can form in atomic-cooling haloes when protostars reach accretion rates higher than ∼10−2 M⊙ yr−1 and fragmentation is suppressed on pc scales. It is, however, still uncertain if a supermassive star still emerges when fragmentation occurs at smaller scales and a cluster of stars is formed instead. In this work, we explore the problem of massive object formation due to the interplay of collisions and accretion in star clusters at low metallicity. We model a small embedded cluster of accreting protostars following subparsec scale fragmentation during the collapse of a primordial gas cloud, and follow its evolution by performing N-body plus hydrodynamical simulations. Our results show that supermassive stars with 103 and 104 M⊙ are always formed due to the interplay of collisions and accretion, and in some cases these objects are part of a binary system. The resulting supermassive star is surrounded by tens of smaller stars with typical masses in the range 1–100 M⊙.

Rotation Periods, Inclinations, and Obliquities of Cool Stars Hosting Directly Imaged Substellar Companions: Spin–Orbit Misalignments Are Common

The orientation between a star’s spin axis and a planet’s orbital plane provides valuable information about the system’s formation and dynamical history. For non-transiting planets at wide separations, true stellar obliquities are challenging to measure, but lower limits on spin–orbit orientations can be determined from the difference between the inclination of the star’s rotational axis and the companion’s orbital plane (Δi). We present results of a uniform analysis of rotation periods, stellar inclinations, and obliquities of cool stars (SpT ≳ F5) hosting directly imaged planets and brown dwarf companions. As part of this effort, we have acquired new values for 22 host stars with the high-resolution Tull spectrograph at the Harlan J. Smith telescope. Altogether our sample contains 62 host stars with rotation periods, most of which are newly measured using light curves from the Transiting Exoplanet Survey Satellite. Among these, 53 stars have inclinations determined from projected rotational and equatorial velocities, and 21 stars predominantly hosting brown dwarfs have constraints on Δi. Eleven of these (52% of the sample) are likely misaligned, while the remaining 10 host stars are consistent with spin–orbit alignment. As an ensemble, the minimum obliquity distribution between 10 and 250 au is more consistent with a mixture of isotropic and aligned systems than either extreme scenario alone—pointing to direct cloud collapse, formation within disks bearing primordial alignments and misalignments, or architectures processed by dynamical evolution. This contrasts with stars hosting directly imaged planets, which show a preference for low obliquities. These results reinforce an emerging distinction between the orbits of long-period brown dwarfs and giant planets in terms of their stellar obliquities and orbital eccentricities.

Enhanced star formation through the high-temperature formation of H2 on carbonaceous dust grains

The microphysics of molecular hydrogen formation has an influence on galactic-scale star-formation rates over cosmic time. H2 is the cooling agent needed to initiate the cloud collapse regulating the star-formation efficiency. H2 formation is inefficient in the gas phase under typical interstellar conditions, requiring dust grain surfaces to act as catalysts. Small carbonaceous grains with sizes from roughly 4 to 100–200 Å, including polycyclic aromatic hydrocarbons (PAHs), have been shown to increase the H2 formation rates due to their large surface-to-volume ratios. H2 formation rates on PAHs were previously thought to reduce above temperatures of 50 K and H atom recombination was believed to be highly efficient only below 20 K. Until now, both laboratory experiments and theoretical modelling have suggested that H2 cannot form on grains with temperatures above 100 K. Here we report evidence, through direct laboratory measurements, of the highly efficient formation of H2 at temperatures up to 250 K on carbonaceous surfaces mimicking interstellar dust. By pushing their formation towards warmer temperatures, the H2 molecules could start contributing substantially to the cooling of warmer gas (temperatures of roughly 50–250 K). This will have a marked impact on our understanding of H2 formation in nearby galaxies and its efficiency in high-redshift galaxies where the Cosmic Microwave Background already pushes dust temperatures to more than 20 K.

Spherical symmetric gravitational collapse of a dust cloud: Polymerized dynamics in reduced phase space

Based on the effective dynamics in the $\bar \mu$ scheme of the spherical symmetry reduced model in the reduced phase space formulation of loop quantum gravity (LQG), we investigate the gravitational collapse of a homogeneous dust cloud, with Gaussian dust serving as both the reference field and the source of the gravitational collapse. The effective dynamics from the considered model for a homogeneous dust cloud reduces precisely to the effective dynamics of loop quantum cosmology (LQC) with extrinsic curvature based K-quantization, indicating that the LQC effective dynamics lives as a subsector of the model presented here. In both the marginally bound and the bound cases of the collapse in effective dynamics, the singularity is resolved and replaced by a bounce. Though quantum geometric modification from spatial curvature is not directly included in the K-quantization it does affect the qualitative dynamics of the collapsing dust cloud in the sense that on the one hand for the marginally bound case, the dust cloud bounces once at fixed maximum energy density and on the other hand for the bound case, the dust cloud undergoes infinite cycles of contraction and expansion at energy densities dependent on the dust mass. Finally, the mass threshold for the formation of a trapped surface in each case is found and the matching conditions between the interior collapsing spacetime and an effective exterior static solution are discussed.

Simulating the Collapse of Rotating Primordial Gas Clouds to Study the Possibility of the Survival of Population III Protostars

It has been argued that the low-mass primordial stars (m Pop III ≤ 0.8 M ⊙) are likely to enter the main sequence and hence may possibly be found in present-day galaxies. However, due to limitations in existing numerical capabilities, current three-dimensional (3D) simulations of disk fragmentation are only capable of following a few thousand years of evolution after the formation of the first protostar. In this work, we use a modified version of the Gadget-2 smoothed particle hydrodynamics code to present the results of the nonlinear collapse of the gas clouds associated with various degrees of initial solid body rotation (parameterized by β) using a piecewise polytropic equation of state. The 3D simulations are followed until the epoch that occurs when 50M ⊙ of mass has been accreted in protostellar objects, which is adequate enough to investigate the dynamics of the protostars with the surrounding gaseous medium and to determine the mass function, accretion rate, and possibility of the survival of these protostellar objects to the present epoch. We found that evolving protostars that stay within slow-rotating parent clouds can become massive enough to survive, due to accretion in the absence of radiative feedback, whereas 10%–12% of those formed within fast-rotating clouds (β ≥ 0.1) could possibly be ejected from the gravitational bound cluster as low-mass stars.

Molecular Gas Properties in Young Stellar Clusters with a Suppressed Star Cluster Wind

In compact and dense star-forming clouds a global star cluster wind could be suppressed. In this case stellar feedback is unable to expel the leftover gas from the cluster. Young massive stars remain embedded in a dense residual gas and stir it by moving in the gravitational well of the system. Here we present a self-consistent model for the molecular gas distribution in such young, enshrouded stellar clusters. It is assumed that the cloud collapse terminates and the star formation ceases when a balance between the turbulent pressure and gravity and between the turbulent energy dissipation and regeneration rates is established. These conditions result in an equation that determines the residual gas density distribution that, in turn, allows one to determine the other characteristics of the leftover gas and the star formation efficiency. It is shown that our model predictions are in good agreement with several observationally determined properties of cloud D1 in the nearby dwarf spheroidal galaxy NGC 5253 and its embedded cluster.

High-resolution Study of Planetesimal Formation by Gravitational Collapse of Pebble Clouds

Planetary embryos are built through the collisional growth of 10–100 km-sized objects called planetesimals, a formerly large population of objects, of which asteroids, comets, and Kuiper Belt objects represent the leftovers from planet formation in our solar system. Here, we follow the paradigm that turbulence created overdense pebble clouds, which then collapse under their own self-gravity. We use the multiphysics code GIZMO to model the pebble cloud density as a continuum, with a polytropic equation of state to account for collisional interactions and capturing the phase transition to a quasi-incompressible “solid” object, i.e., a planetesimal in hydrostatic equilibrium. Thus, we study cloud collapse effectively at the resolution of the forming planetesimals, allowing us to derive an initial mass function for planetesimals in relation to the total pebble mass of the collapsing cloud. The redistribution of angular momentum in the collapsing pebble cloud is the main mechanism leading to multiple fragmentation. The angular momentum of the pebble cloud and thus the centrifugal radius increases with distance to the Sun, but the solid size of the forming planetesimals is constant. Therefore we find that with increasing distance to the Sun, the number of forming planetesimals per pebble cloud increases. For all distances, the formation of binaries occurs within higher hierarchical systems. The size distribution is top-heavy and can be described with a Gaussian distribution of planetesimal mass. For the asteroid belt, we can infer a most likely size of 125 km, all stemming from pebble clouds of equivalent size 152 km.

Numerical simulation of the collapse of a bubble cloud near a wall

Numerical simulation of the collapse of a bubble cloud near a wall

Modeling disks and magnetic outflows around a forming massive star

Context. Similar to their lower mass siblings, massive protostars can be expected to: (a) be surrounded by circumstellar disks, and (b) launch magnetically driven jets and outflows. The disk formation and global evolution is thereby controlled by advection of angular momentum from large scales, the efficiency of magnetic braking and the resistivity of the medium, and the internal thermal and magnetic pressures of the disk. Aims. We determine the dominant physical mechanisms that shape the appearance of these circumstellar disks, their sizes, and aspect ratios. Methods. We performed a series of 30 simulations of a massive star forming from the gravitational collapse of a molecular cloud threaded by an initially uniform magnetic field, starting from different values for the mass of the cloud, its initial density and rotation profiles, its rotational energy content, the magnetic field strength, and the resistivity of the material. The gas and dust was modeled with the methods of resistive magnetohydrodynamics, also considering radiation transport of thermal emission and self-gravity. We checked for the impact of spatial resolution in a dedicated convergence study. Results. After the initial infall phase dominated by the gravitational collapse, an accretion disk was formed, shortly followed by the launching of magnetically driven outflows. Two layers can be distinguished in the accretion disk: a thin layer, vertically supported by thermal pressure, and a thick layer, vertically supported by magnetic pressure. Both regions exhibit Keplerian-like rotation and grow outward over time. We observed the effects of magnetic braking in the inner ~50 au of the disk at late times in our fiducial case. The parameter study reveals that the size of the disk is mostly determined by the density and rotation profiles of the initial mass reservoir and not by the magnetic field strength. We find that the disk size and protostellar mass gain scale with the initial mass of the cloud. Magnetic pressure can slightly increase the size of the accretion disk, while magnetic braking is more relevant in the innermost parts of the disk as opposed to the outer disk. From the parameter study, we infer that multiple initial conditions for the onset of gravitational collapse are able to produce a given disk size and protostellar mass.

Structure and evolution of interstellar carbonaceous dust. Insights from the laboratory

A large fraction of interstellar carbon is locked up in solid grains. The nature, origin and evolution of these grains have been investigated for decades. A combination of observations, models and experiments indicates that carbonaceous dust is mostly made of a mixture of grains composed almost exclusively of carbon and hydrogen. They have different proportions of aliphatic and aromatic structures, and a variable H/C ratio. Their sizes can vary typically between the nm and the hundreds of nm. Carbonaceous grains are largely formed in the envelopes of carbon rich asymptotic giant branch (AGB) stars and evolve in the interstellar medium, where they can be transformed or destroyed by the effects of hydrogen atoms, UV radiation, cosmic rays or shock waves from supernovae. Surviving grains eventually enter dense clouds and participate in the cloud collapse leading to star formation, closing thus their lifecycle. Within this general picture, there are doubts and issues that cannot be solved just by observation and modeling and require laboratory work. In this article we provide an overview of the development and present state of the field indicating open problems and debated questions. We stress recent experimental progress in the understanding of dust formation, both in circumstellar envelopes and the cold interstellar medium, and also in the energetic processing of dust analogs, that points to a possible top down chemistry in the diffuse medium, and especially in photon irradiated regions.

Where are the Eddington-limited starbursts? Gravitational lensing provides a way forward for sub-kiloparsec views of star formation

AbstractIn the past decade, submillimeter surveys have been employed to define samples of gravitationally-lensed dusty star-forming galaxies (DSFGs) at z ∼ 1 − 4. These extreme objects () appear to form stars prodigiously at rates of . Using all-sky Planck and WISE surveys, and wide-area Herschel surveys, we have identified the PASSAGES sample, with some of the rarest hyper-luminous IR galaxies ever discovered. We have found that their globally-averaged star formation surface densities are always sub-Eddington, typically by an order of magnitude. This may suggest that our understanding of how radiation pressure from massive stars disrupts the collapse of molecular clouds (thereby quenching star formation) is flawed—or simply that smaller physical resolutions are necessary. With the aid of lensing, we can now capture the source-plane distribution of star formation at ∼ 100pc scales, letting us identify isolated super-Eddington regions where quenching is occurring.

Rapid formation of exoplanetesimals revealed by white dwarfs

The timing of formation for the first planetesimals determines the mode of planetary accretion and their geophysical and compositional evolution. Astronomical observations of circumstellar discs and Solar System geochronology provide evidence for planetesimal formation during molecular cloud collapse, much earlier than previously estimated. Here, we present distinct observational evidence from white dwarf planetary systems for planetesimal formation occurring during the first few hundred thousand years after cloud collapse in exoplanetary systems. A significant fraction of white dwarfs have accreted planetary material rich in iron core or mantle material. In order for the exo-asteroids accreted by white dwarfs to form iron cores, substantial heating is required. By simulating planetesimal evolution and collisional evolution we show that the most likely heat source is short-lived radioactive nuclides such as Al-2 (half life of approximately 0.7 Myr). Core-rich materials in the atmospheres of white dwarfs, therefore, provide independent evidence for rapid planetesimal formation, concurrent with star formation.

Saturation Level of Turbulence in Collapsing Gas Clouds

We investigate the physical mechanism that determines the saturation level of the turbulence in collapsing gas clouds. We perform a suite of high-resolution numerical simulations that follow the collapse of turbulent gas clouds, with various effective polytropic exponents γ eff, initial Mach numbers , and initial turbulent seeds. Equating the energy injection rate from gravitational contraction and the dissipation rate of the turbulence, we obtain an analytic expression for the saturation level of the turbulence, and compare it with the numerical results. Consequently, the numerical results are well described by the analytic model, given that the turbulent driving scale in collapsing gas clouds is one-third the Jeans length of the collapsing core. These results indicate that the strength of the turbulence at first-core formations in the early universe/the present-day star formation process can be estimated solely by γ eff.

SIMULATIONS OF UNEQUAL BINARY COLLISIONS EMBEDDED IN A TURBULENT GAS CLOUD FAR FROM A MASSIVE GRAVITATIONAL CENTER

We simulate the collapse of a turbulent gas cloud, in which we choose two sub-clouds. A translational velocity $\vec{r}_L$ or $\vec{r}_R$ is added, so that the sub-clouds move towards each other to collide. The radius and pre-collision velocity of the sub-clouds are chosen to be unequal, and both head-on and oblique collisions are considered. The simulations are all calibrated to have the same total mass and initial energy ratio α = 0.16, which is defined as the ratio of thermal energy to gravitational energy. We compare low-β models to a high-β models, where β is defined as the ratio of kinetic energy to gravitational energy. Finally, we consider the turbulent cloud to be under the gravitational influence of an object located far enough, in order to approximate the tidal effects by means of an azimuthal velocity Vcir added to the cloud particles apart from the translational and turbulent velocities mentioned above. We compare a low-Vcir model with a high-Vcir one.

The Internal Line-of-Sight Kinematics of NGC 346: The Rotation of the Core Region

We present the stellar radial velocity analysis of the central of the young massive Small Magellanic Cloud star cluster NGC 346. Using VLT/MUSE integral field spectroscopy in combination with Hubble Space Telescope photometry, we extract 103 spectra of cluster member stars suited to measure accurate line-of-sight kinematics. The cluster member stars show two distinct velocity groups at and , relative to the systemic velocity of (165.5 ± 0.2) km s−1, and hint at a third group at . We show that there is neither a correlation between the velocity groups and the spatial location of the stars, nor their locus on optical color–magnitude diagrams, which makes the stellar velocity a key parameter to separate individual stellar components in such a young star cluster. Velocity group 2 shows clear rotation with Ω2 =(−0.4 ± 0.1) Myr−1, corresponding to (−4.9 ± 0.7) km s−1 at radial distance of 10 pc from the center, a possible remnant of the formation process of NGC 346 through the hierarchical collapse of the giant molecular cloud. The ionizing gas has lost any natal kinematic imprint and shows clear expansion, driven by far-ultraviolet fluxes and stellar winds of the numerous OB stars in the cluster center. The size of this expanding bubble and its expansion velocity of 7.9 km s−1 are in excellent agreement with the estimate that the latest star formation episode occurred about two million years ago.

The formation of early-type galaxies through monolithic collapse of gas clouds in Milgromian gravity

ABSTRACT Studies of stellar populations in early-type galaxies (ETGs) show that the more massive galaxies form earlier and have a shorter star formation history. In this study, we investigate the initial conditions of ETG formation. The study begins with the collapse of non-rotating post-Big-Bang gas clouds in Milgromian (MOND) gravitation. These produce ETGs with star-forming time-scales (SFT) comparable to those observed in the real Universe. Comparing these collapse models with observations, we set constraints on the initial size and density of the post-Big-Bang gas clouds in order to form ETGs. The effective-radius–mass relation of the model galaxies falls short of the observed relation. Possible mechanisms for later radius expansion are discussed. Using hydrodynamic MOND simulations, this work thus for the first time shows that the SFTs observed for ETGs may be a natural occurrence in the MOND paradigm. We show that different feedback algorithms change the evolution of the galaxies only to a very minor degree in MOND. The first stars have, however, formed more rapidly in the real Universe than possible just from the here studied gravitational collapse mechanism. Dark-matter-based cosmological structure formation simulations disagree with the observed SFTs at more than 5σ confidence.

Insights into the collapse and expansion of molecular clouds in outflows from observable pressure gradients

Insights into the collapse and expansion of molecular clouds in outflows from observable pressure gradients

Dynamics of charged string cloud collapse in rainbow gravity

This work will proceed to examine the dynamics of collapsing charged string cloud in a spherically symmetric rainbow metric. This is the framework that allows new stars and compact objects to emerge. The dynamical variables (mass density, string tension) and their physical existence as energy conditions are computed and visually depicted. We also calculated the duration and radius of development of the collapsing string cloud’s apparent horizons. The mass density, string tension, expected time and radius of apparent horizon creation are dependent on the Rainbow parameter [Formula: see text] At the center of the configuration [Formula: see text] the mass density, string tension and electric field intensity are at their maximum leading to the formation of a compact object (black hole). The fluid’s mass density and string tension are reduced by the electric field. Furthermore, the presence of an electric field delays the formation of a compact object (black hole).

Evolution of circumstellar discs in young star-forming regions

ABSTRACT The evolution of circumstellar discs is influenced by their surroundings. The relevant processes include external photoevaporation due to nearby stars and dynamical truncations. The impact of these processes on disc populations depends on the star-formation history and on the dynamical evolution of the region. Since star-formation history and the phase-space characteristics of the stars are important for the evolution of the discs, we start simulating the evolution of the star cluster with the results of molecular cloud collapse simulations. In the simulation, we form stars with circumstellar discs, which can be affected by different processes. Our models account for the viscous evolution of the discs, internal and external photoevaporation of gas, external photoevaporation of dust, and dynamical truncations. All these processes are resolved together with the dynamical evolution of the cluster, and the evolution of the stars. An extended period of star formation, lasting for at least 2 Myr, results in some discs being formed late. These late-formed discs have a better chance of survival because the cluster gradually expands with time, and a lower local stellar density reduces the effects of photoevaporation and dynamical truncation. Late formed discs can then be present in regions of high UV radiation, solving the proplyd lifetime problem. We also find a considerable fraction of discs that lose their gas content but remain sufficiently rich in solids to be able to form a rocky planetary system.

Predicting HCN, HCO+, multi-transition CO, and dust emission of star-forming galaxies

The interstellar medium is a turbulent, multiphase, and multi-scale medium that follows scaling relations that link the surface density, volume density, and velocity dispersion with the cloud size. Galactic clouds range from below 1 pc to about 100 pc in size. Extragalactic clouds appear to follow the same range, although they are only now becoming observable in atomic and molecular lines. Analytical models of galactic gaseous disks need to take the multi-scale and multiphase nature of the interstellar medium into account. They can be described as clumpy star-forming accretion disks in vertical hydrostatic equilibrium, with the mid-plane pressure balancing the gravity of the gaseous and stellar disk. Interstellar medium turbulence is taken into account by applying Galactic scaling relations to the cold atomic and molecular gas phases. Turbulence is maintained through energy injection by supernovae. With the determination of the gas mass fraction at a given spatial scale, the equilibrium gas temperature between turbulent heating and line cooling, the molecular abundances, and the molecular line emission can be calculated. The resulting model radial profiles of infrared, HI, CO, HCN, and HCO+ emission are compared to THINGS, HERACLES, EMPIRE, SINGS, and GALEX observations of 17 local spiral galaxies. The model free parameters were constrained for each galactic radius independently. The Toomre parameter, which measures the stability against star formation (cloud collapse), exceeds unity in the inner disk of a significant number of galaxies. In two galaxies it also exceeds unity in the outer disk. Therefore, in spiral galaxies Qtot = 1 is not ubiquitous. The model gas velocity dispersion is consistent with the observed HI velocity dispersion where available. Within our model, HCN and HCO+ is already detectable in relatively low-density gas (∼1000 cm−3). We derive CO and HCN conversion factors and molecular gas depletion times. Both conversion factors are consistent with values found in the literature. Whereas in the massive galaxies the viscous timescale greatly exceeds the star-formation timescale, the viscous timescale is smaller than the star-formation timescale within R ∼ 2 Rd, the disk scale length, in the low-mass galaxies. We suggest that massive spiral galaxies undergo starvation in the absence of gas accretion from the halo, whereas in low-mass galaxies the fuel for star formation reaches R ∼ 2 Rd from outside via a thick gas disk component with a high radial infall velocity observable in the HI line.