bluuebunny/crossref_metadata_2025_split huggingface dataset:
Modeling High-Mass Star Formation and Ultracompact H ii Regions
Authors: Klessen Ralf S. , Peters Thomas , Banerjee Robi , Mac Low Mordecai-Mark , Galván-Madrid Roberto , Keto Eric R. 
Published: April 2011

AbstractMassive stars influence the surrounding universe far out of proportion to their numbers through ionizing radiation, supernova explosions, and heavy element production. Their formation requires the collapse of massive interstellar gas clouds with very high accretion rates. We discuss results from the first three-dimensional simulations of the gravitational collapse of a massive, rotating molecular cloud core that include heating by both non-ionizing and ionizing radiation. Local gravitational instabilities in the accretion flow lead to the build-up of a small cluster of stars. These lower-mass companions subsequently compete with the high-mass star for the same common gas reservoir and limit its overall mass growth. This process is called fragmentation-induced starvation, and explains why massive stars are usually found as members of high-order stellar systems. These simulations also show that the H ii regions forming around massive stars are initially trapped by the infalling gas, but soon begin to fluctuate rapidly. Over time, the same ultracompact H ii region can expand anisotropically, contract again, and take on any of the observed morphological classes. The total lifetime of H ii regions is given by the global accretion timescale, rather than their short internal sound-crossing time. This solves the so-called lifetime problem of ultracompact H ii region. We conclude that the 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 flow.
Magnetic Diffusion in Star Formation
Authors: Basu Shantanu , Dapp Wolf B. 
Published: April 2011

AbstractMagnetic diffusion plays a vital role in star formation. We trace its influence from interstellar cloud scales down to star-disk scales. On both scales, we find that magnetic diffusion can be significantly enhanced by the buildup of strong gradients in magnetic field structure. Large scale nonlinear flows can create compressed cloud layers within which ambipolar diffusion occurs rapidly. However, in the flux-freezing limit that may be applicable to photoionized molecular cloud envelopes, supersonic motions can persist for long times if driven by an externally generated magnetic field that corresponds to a subcritical mass-to-flux ratio. In the case of protostellar accretion, rapid magnetic diffusion (through Ohmic dissipation with additional support from ambipolar diffusion) near the protostar causes dramatic magnetic flux loss. By doing so, it also allows the formation of a centrifugal disk, thereby avoiding the magnetic braking catastrophe.
Dependence of star formation on initial conditions and molecular cloud structure
Authors: Bate Matthew R. 
Published: April 2011

AbstractI review what has been learnt so far regarding the origin of stellar properties from numerical simulations of the formation of groups and clusters of stars. In agreement with observations, stellar properties are found to be relatively robust to variations of initial conditions in terms of molecular cloud structure and kinetics, as long as extreme initial conditions (e.g. strong central condensation, weak or no turbulence) and small-scale driving are avoided, but properties may differ between bound and unbound clouds. Radiative feedback appears crucial for setting stellar masses, even for low-mass stars, while magnetic fields can provide low star formation rates.
Simulations of the IMF in Clusters
Authors: Pudritz Ralph E. 
Published: April 2011

AbstractWe review computational approaches to understanding the origin of the Initial Mass Function (IMF) during the formation of star clusters. We examine the role of turbulence, gravity and accretion, equations of state, and magnetic fields in producing the distribution of core masses - the Core Mass Function (CMF). Observations show that the CMF is similar in form to the IMF. We focus on feedback processes such as stellar dynamics, radiation, and outflows can reduce the accreted mass to give rise to the IMF. Numerical work suggests that filamentary accretion may play a key role in the origin of the IMF.
Theories of the initial mass function
Authors: Hennebelle Patrick , Chabrier Gilles 
Published: April 2011

AbstractWe review the various theories which have been proposed along the years to explain the origin of the stellar initial mass function. We pay particular attention to four models, namely the competitive accretion and the theories based respectively on stopped accretion, MHD shocks and turbulent dispersion. In each case, we derive the main assumptions and calculations that support each theory and stress their respective successes and failures or difficulties.
Star Formation with Adaptive Mesh Refinement Radiation Hydrodynamics
Authors: Krumholz Mark R. 
Published: April 2011

AbstractI provide a pedagogic review of adaptive mesh refinement (AMR) radiation hydrodynamics (RHD) methods and codes used in simulations of star formation, at a level suitable for researchers who are not computational experts. I begin with a brief overview of the types of RHD processes that are most important to star formation, and then I formally introduce the equations of RHD and the approximations one uses to render them computationally tractable. I discuss strategies for solving these approximate equations on adaptive grids, with particular emphasis on identifying the main advantages and disadvantages of various approximations and numerical approaches. Finally, I conclude by discussing areas ripe for improvement.
The role of accretion disks in the formation of massive stars
Authors: Kuiper R. , Klahr H. , Beuther H. , Henning Th. 
Published: April 2011

AbstractWe present radiation hydrodynamics simulations of the collapse of massive pre-stellar cores. We treat frequency dependent radiative feedback from stellar evolution and accretion luminosity at a numerical resolution down to 1.27 AU. In the 2D approximation of axially symmetric simulations, it is possible for the first time to simulate the whole accretion phase of several 105 yr for the forming massive star and to perform a comprehensive scan of the parameter space. Our simulation series show evidently the necessity to incorporate the dust sublimation front to preserve the high shielding property of massive accretion disks. Our disk accretion models show a persistent high anisotropy of the corresponding thermal radiation field, yielding to the growth of the highest-mass stars ever formed in multi-dimensional radiation hydrodynamics simulations. Non-axially symmetric effects are not necessary to sustain accretion. The radiation pressure launches a stable bipolar outflow, which grows in angle with time as presumed from observations. For an initial mass of the pre-stellar host core of 60, 120, 240, and 480⊙ the masses of the final stars formed in our simulations add up to 28.2, 56.5, 92.6, and at least 137.2⊙ respectively.
Embedded disks around low-mass protostars
Authors: Vorobyov Eduard I. , Basu Shantanu 
Published: April 2011

AbstractThe time evolution of protostellar disks in the embedded phase of star formation (EPSF) is reviewed based on numerical hydrodynamics simulations of the gravitational collapse of two cloud cores with distinct initial masses. Special emphasis is given to disk, stellar, and envelope masses and also mass accretion rates onto the star. It is shown that accretion is highly variable in the EPSF, in agreement with recent theoretical and observational expectations. Protostellar disks quickly accumulate mass upon formation and may reach a sizeable fraction of the envelope mass (~35%) by the end of the Class 0 phase. Systems with disk-to-star mass ratio ξ≈0.5 are common but systems with ξ≥1.0 are rare because the latter quickly evolve into binary or multiple systems. Embedded disks are characterized by radial pulsations, the amplitude of which increases with growing core mass.
The Effects of Radiation Feedback on Early Fragmentation and Stellar Multiplicity
Authors: Offner Stella S. R. 
Published: April 2011

AbstractForming stars emit a significant amount of radiation into their natal environment. While the importance of radiation feedback from high-mass stars is widely accepted, radiation has generally been ignored in simulations of low-mass star formation. I use ORION, an adaptive mesh refinement (AMR) three-dimensional gravito-radiation-hydrodynamics code, to model low-mass star formation in a turbulent molecular cloud. I demonstrate that including radiation feedback has a profound effect on fragmentation and protostellar multiplicity. Although heating is mainly confined within the core envelope, it is sufficient to suppress disk fragmentation that would otherwise result in low-mass companions or brown dwarfs. As a consequence, turbulent fragmentation, not disk fragmentation, is likely the origin of low-mass binaries.
Simulations of Massive Star Cluster Formation and Feedback in Turbulent Giant Molecular Clouds
Authors: Harper-Clark Elizabeth , Murray Norman 
Published: April 2011

AbstractUsing the AMR code ENZO we are simulating the formation of massive star clusters within turbulent Giant Molecular Clouds (GMCs). Here we discuss the simulations from the first stages of building realistic turbulent GMCs, to accurate star formation, and ultimately comprehensive feedback. These simulations aim to build a better understanding of how stars affect GMCs, helping to answer the questions of how long GMCs live and why only a small fraction of the GMC gas becomes stars.
Radiation Driven Implosion and Triggered Star Formation
Authors: Bisbas T. G. , Whitworth A. P. , Wünsch R. , Hubber D. A. , Walch S. 
Published: April 2011

AbstractWe present simulations of stable isothermal clouds exposed to ionizing radiation from a discrete external source, and identify the conditions that lead to Radiatively Driven Implosion and Star Formation. We use the Smoothed Particle Hydrodynamics code SEREN (Hubber et al. 2010) and the HEALPix-based photoionization algorithm described in Bisbas et al. (2009). We find that the incident ionizing flux is the critical parameter determining the evolution; high fluxes disperse the cloud, whereas low fluxes trigger star formation. We find a clear connection between the intensity of the incident flux and the parameters of star formation.
Action of Winds Inside and Outside of Star Clusters
Authors: Palouš Jan , Dale Jim , Wünsch Richard , Silich Sergiy , Tenorio-Tagle Guillermo , Whitworth Anthony 
Published: April 2011

AbstractThe feedback form pre-main sequence and young stars influences their vicinity. The stars are formed in clusters, which implies that the winds of individual stars collide with each other. Inside of a star cluster, winds thermalize a fraction of their kinetic energy, forming a very hot medium able to escape from the cluster in the form of a large-scale wind. Outside of the cluster, the cluster wind forms a shock front as it interacts with the ambient medium which is accreted onto the expanding shell. A variety of instabilities may develop in such shells, and in some cases they fragment, triggering second generation of star formation. However, if the cluster surpasses a certain mass (depending on the radius and other parameters) the hot medium starts to be thermally unstable even inside of the cluster, forming dense warm clumps. The formation of next generations of stars may start if the clumps are big enough to self-shield against stellar radiation creating cold dense cores.
Theory of Feedback in Clusters and Molecular Cloud Turbulence
Authors: Vázquez-Semadeni Enrique 
Published: April 2011

AbstractI review recent numerical and analytical work on the feedback from both low- and high-mass cluster stars into their gaseous environment. The main conclusions are that i) outflow driving appears capable of maintaing the turbulence in parsec-sized clumps and retarding their collapse from the free-fall rate, although there exist regions within molecular clouds, and even some examples of whole clouds, which are not actively forming stars, yet are just as turbulent, so that a more universal turbulence-driving mechanism is needed; ii) outflow-driven turbulence exhibits specific spectral features that can be tested observationally; iii) feedback plays an important role in reducing the SFR; iv) nevertheless, numerical simulations suggest feedback cannot completely prevent a net contracting motion of clouds and clumps. Therefore, an appealing source for driving the turbulence everywhere in GMCs is the accretion from the environment, at all scales. In this case, feedback's most important role may be to prevent a fraction of the gas nearest to newly formed stars from actually reaching them, thus reducing the SFE.
The theory of young cluster disruption
Authors: Goodwin Simon P. 
Published: April 2011

AbstractMost stars seem to form in clusters, but the vast majority of these clusters do not seem to survive much beyond their embedded phase. The most favoured mechanism for the early destruction of star clusters is the effect of the removal of residual gas by feedback which dramatically changes the cluster potential. The effects of feedback depend on the ratio of the masses of stars and gas, and the velocity dispersion of the stars at the onset of gas removal. As gas removal is delayed by a few Myr from star formation these crucial parameters can change significantly from their initial values. In particular, in dynamically cool and clumpy clusters, the stars will collapse to a far denser state and if they decouple from the gas then gas removal may be far less destructive than previously thought. This might well help explain the survival of very massive clusters, such as globular clusters, without the need for extremely high star formation efficiencies or initial masses far greater than their current masses.
Ionisation Feedback in Star and Cluster Formation Simulations
Authors: Ercolano Barbara , Gritschneder Matthias 
Published: April 2011

AbstractFeedback from photoionisation may dominate on parsec scales in massive star-forming regions. Such feedback may inhibit or enhance the star formation efficiency and sustain or even drive turbulence in the parent molecular cloud. Photoionisation feedback may also provide a mechanism for the rapid expulsion of gas from young clusters' potentials, often invoked as the main cause of ‘infant mortality’. There is currently no agreement, however, with regards to the efficiency of this process and how environment may affect the direction (positive or negative) in which it proceeds. The study of the photoionisation process as part of hydrodynamical simulations is key to understanding these issues, however, due to the computational demand of the problem, crude approximations for the radiation transfer are often employed.We will briefly review some of the most commonly used approximations and discuss their major drawbacks. We will then present the results of detailed tests carried out using the detailed photoionisation code mocassin and the SPH+ionisation code iVINE code, aimed at understanding the error introduced by the simplified photoionisation algorithms. This is particularly relevant as a number of new codes have recently been developed along those lines.We will finally propose a new approach that should allow to efficiently and self-consistently treat the photoionisation problem for complex radiation and density fields.
Pillars, Jets and Dynamical Features
Authors: Gritschneder Matthias , Burkert Andreas , Naab Thorsten , Walch Stefanie 
Published: April 2011

AbstractWe present high resolution simulations on the impact of ionizing radiation on turbulent molecular clouds. The combination of hydrodynamics, gravitational forces and ionization in the tree-SPH code iVINE naturally leads to the formation of elongated filaments and clumps, which are in excellent agreement with the pillars observed around HII regions. Including gravity the formation of a second generation of low-mass stars with surrounding protostellar disks is triggered at the tips of the pillars, as also observed. A parameter study allows us to determine the physical conditions under which irregular structures form and whether they resemble large pillars or a system of small, isolated globules.
Scaling Relations between Gas and Star Formation in Nearby Galaxies
Authors: Bigiel Frank , Leroy Adam , Walter Fabian 
Published: April 2011

AbstractHigh resolution, multi-wavelength maps of a sizeable set of nearby galaxies have made it possible to study how the surface densities of H i, H2 and star formation rate (ΣHI, ΣH2, ΣSFR) relate on scales of a few hundred parsecs. At these scales, individual galaxy disks are comfortably resolved, making it possible to assess gas-SFR relations with respect to environment within galaxies. ΣH2, traced by CO intensity, shows a strong correlation with ΣSFR and the ratio between these two quantities, the molecular gas depletion time, appears to be constant at about 2 Gyr in large spiral galaxies. Within the star-forming disks of galaxies, ΣSFR shows almost no correlation with ΣHI. In the outer parts of galaxies, however, ΣSFR does scale with ΣHI, though with large scatter. Combining data from these different environments yields a distribution with multiple regimes in Σgas – ΣSFR space. If the underlying assumptions to convert observables to physical quantities are matched, even combined datasets based on different SFR tracers, methodologies and spatial scales occupy a well define locus in Σgas – ΣSFR space.
Theory of the Star Formation Rate
Authors: Padoan Paolo , Nordlund Åke 
Published: April 2011

AbstractThis work presents a new physical model of the star formation rate (SFR), tested with a large set of numerical simulations of driven, supersonic, self-gravitating, magneto-hydrodynamic (MHD) turbulence, where collapsing cores are captured with accreting sink particles. The model depends on the relative importance of gravitational, turbulent, magnetic, and thermal energies, expressed through the virial parameter, αvir, the rms sonic Mach number, S,0, and the ratio of mean gas pressure to mean magnetic pressure, β0. The SFR is predicted to decrease with increasing αvir (stronger turbulence relative to gravity), and to depend weakly on S,0 and β0, for values typical of star forming regions (S,0≈4-20 and β0≈1-20). The star-formation simulations used to test the model result in an approximately constant SFR, after an initial transient phase. Both the value of the SFR and its dependence on the virial parameter found in the simulations agree very well with the theoretical predictions.
Collisions of supersonic clouds
Authors: McLeod Andrew , Palouš Jan , Whitworth Anthony 
Published: April 2011

AbstractWe present simulations of supersonic collisions between molecular clouds of mass 500 M⊙ and radius 2.24 pc. The simulations are performed with the SEREN SPH code. The code treats the energy equation and the associated transport of heating and cooling radiation. The formation of protostars is captured by introducing sink particles. Low velocity collisions form a shock-compressed layer which fragments to form stars. For high-velocity collisions, υrel ⪆ 5 km s−1, the non-linear thin shell instability strongly disrupts the shock-compressed layer, and may inhibit the formation of stars.
Galactic scale star formation: Interplay between stellar spirals and the ISM
Authors: Wada Keiichi , Baba Junichi , Fujii Michiko , Saitoh Takayuki R. 
Published: April 2011

AbstractSpiral structures in the disk galaxies have been extensively studied by many theoretical papers, but conventional steady-state models are not consistent with what we observe in time-dependent, multi-dimensional numerical simulations and also in real galaxies. Here we review recent progress in numerical modeling of stellar and gas spirals in disk galaxies. The spiral arms excited in a stellar disk can last for 10 Gyrs without the ISM, but each spiral arm is short-lived and is recurrently formed. The stellar spirals are not waves propagating with a single pattern speed. The ISM is concentrated in local potential minima, which roughly follow the galactic rotation together with the stellar arms, therefore galactic dust lanes are not the classic ‘galactic shocks’.
GRAPE Accelerators
Authors: Makino Junichiro 
Published: April 2011

AbstractI'll overview the past, present, and future of the GRAPE project, which started as the effort to design and develop specialized hardware for gravitational N-body problem. The current hardware, GRAPE-DR, has an architecture quite different from previous GRAPEs, in the sense that it is a collection of small, but programmable processors, while previous GRAPEs had hardwired pipelines. I'll discuss pros and cons of these two approaches, comparisons with other accelerators and future directions.
Future Trends in Computing
Authors: Elmegreen Bruce G. 
Published: April 2011

AbstractAccording to a Top500.org compilation, large computer systems have been doubling in sustained speed every 1.14 years for the last 17 years. If this rapid growth continues, we will have computers by 2020 that can execute an Exaflop (1018) per second. Storage is also improving in cost and density at an exponential rate. Several innovations that will accompany this growth are reviewed here, including shrinkage of basic circuit components on Silicon, three-dimensional integration, and Phase Change Memory. Further growth will require new technologies, most notably those surrounding the basic building block of computers, the Field Effect Transistor. Implications of these changes for the types of problems that can be solved are briefly discussed.
Implementing and comparing sink particles in AMR and SPH
Authors: Federrath Christoph , Banerjee Robi , Seifried Daniel , Clark Paul C. , Klessen Ralf S. 
Published: April 2011

AbstractWe implemented sink particles in the Adaptive Mesh Refinement (AMR) code FLASH to model the gravitational collapse and accretion in turbulent molecular clouds and cores. Sink particles are frequently used to measure properties of star formation in numerical simulations, such as the star formation rate and efficiency, and the mass distribution of stars. We show that only using a density threshold for sink particle creation is insufficient in case of supersonic flows, because the density can exceed the threshold in strong shocks that do not necessarily lead to local collapse. Additional physical collapse indicators have to be considered. We apply our AMR sink particle module to the formation of a star cluster, and compare it to a Smoothed Particle Hydrodynamics (SPH) code with sink particles. Our comparison shows encouraging agreement of gas and sink particle properties between the AMR and SPH code.
Convergence of SPH and AMR simulations
Authors: Hubber David A. , Falle Sam A. E. G. , Goodwin Simon P. 
Published: April 2011

AbstractWe present the first results of a large suite of convergence tests between Adaptive Mesh Refinement (AMR) Finite Difference Hydrodynamics and Smoothed Particle Hydrodynamics (SPH) simulations of the non-linear thin shell instability and the Kelvin-Helmholtz instability. We find that the two methods converge in the limit of high resolution and accuracy. AMR and SPH simulations of the non-linear thin shell instability converge with each other with standard algorithms and parameters. The Kelvin-Helmholtz instability in SPH requires both an artificial conductivity term and a kernel with larger compact support and more neighbours (e.g. the quintic kernel) in order converge with AMR. For purely hydrodynamical problems, SPH simulations take an order of magnitude longer than the grid code when converged.