Search

Context. Regularly spaced star-forming regions along the spiral arms of nearby galaxies provide insight into the early stages and initial conditions of star formation. The regular separation of these star-forming regions suggests spiral arm instability as their origin. Aims. We explore the effects of magnetic fields on the spiral arm instability. Methods. We use 3D global magnetohydrodynamical simulations of isolated spiral galaxies, comparing three different initial plasma β values (ratios of the thermal to magnetic pressure) of β = ∞, 50, and 10. We perform a Fourier analysis to calculate the separation of the over-dense regions that formed as a result of the spiral instability. We then compare the separations with observations. Results. We find that the spiral arms in the hydro case (β = ∞) are unstable. The fragments are initially connected by gas streams that are reminiscent of the Kelvin-Helmholtz instability. For β = 50, the spiral arms also fragment, but the fragments separate earlier and tend to be slightly elongated in the direction perpendicular to the spiral arms. However, in the β = 10 run, the arms are stabilised against fragmentation by magnetic pressure. Despite the difference in the initial magnetic field strengths of the β = 50 and 10 runs, the magnetic field is amplified to βarm ∼ 1 inside the spiral arms for both runs. The spiral arms in the unstable cases (hydro and β = 50) fragment into regularly spaced over-dense regions. We determine their separation to be ∼0.5 kpc in the hydro and ∼0.65 kpc in the β = 50 case. These two values agree with the observed values found in nearby galaxies. We find a smaller median characteristic wavelength of the over-densities along the spiral arms of 0.73−0.36+0.31 kpc in the hydro case compared to 0.98−0.46+0.49 kpc in the β = 50 case. Moreover, we find a higher growth rate of the over-densities in the β = 50 run compared to the hydro run. We observe magnetic hills and valleys along the fragmented arms in the β = 50 run, which is characteristic of the Parker instability. [ABSTRACT FROM AUTHOR]

The streaming instability is a leading candidate mechanism to explain the formation of planetesimals. However, the role of this instability in the driving of turbulence in protoplanetary disks, given its fundamental nature as a linear hydrodynamical instability, has so far not been investigated in detail. We study the turbulence that is induced by the streaming instability as well as its interaction with the vertical shear instability. For this purpose, we employ the FLASH Code to conduct two-dimensional axisymmetric global disk simulations spanning radii from 1 to 100 au, including the mutual drag between gas and dust as well as the radial and vertical stellar gravity. If the streaming instability and the vertical shear instability start their growth at the same time, we find the turbulence in the dust midplane layer to be primarily driven by the streaming instability. The streaming instability gives rise to vertical gas motions with a Mach number of up to ~10-2. The dust scale height is set in a self-regulatory manner to about 1% of the gas scale height. In contrast, if the vertical shear instability is allowed to saturate before the dust is introduced into our simulations, then it continues to be the main source of the turbulence in the dust layer. The vertical shear instability induces turbulence with a Mach number of ~10-1 and thus impedes dust sedimentation. Nonetheless, we find the vertical shear instability and the streaming instability in combination to lead to radial dust concentration in long-lived accumulations that are significantly denser than those formed by the streaming instability alone. Therefore, the vertical shear instability may promote planetesimal formation by creating weak overdensities that act as seeds for the streaming instability. [ABSTRACT FROM AUTHOR]

Magnetic fields are an elemental part of the interstellar medium in galaxies. However, their impact on gas dynamics and star formation in galaxies remains controversial. We use a suite of global magnetohydrodynamic simulations of isolated disc galaxies to study the influence of magnetic fields on the diffuse and dense gas in the discs. We find that the magnetic field acts in multiple ways. Stronger magnetized discs fragment earlier due to the shorter growth time of the Parker instability. Due to the Parker instability in the magnetized discs, we also find cold (⁠|$T \lt 50\, \mathrm{K}$|⁠) and dense (⁠|$n\sim 10^3 {--}10^4\, \mathrm{cm}^{-3}$|⁠) gas several hundred pc above/below the mid-plane without any form of stellar feedback. In addition, magnetic fields change the fragmentation pattern. While in the hydrodynamic case, the disc breaks up into ring-like structures, magnetized discs show the formation of filamentary entities that extent both in the azimuthal and radial direction. These kpc scale filaments become magnetically (super-)critical very quickly and allow for the rapid formation of massive giant molecular clouds. Our simulations suggest that major differences in the behaviour of star formation – due to a varying magnetization – in galaxies could arise. [ABSTRACT FROM AUTHOR]

We present radiation-magnetohydrodynamic simulations aimed at studying evolutionary properties of H  ii regions in turbulent, magnetized, and collapsing molecular clouds formed by converging flows in the warm neutral medium. We focus on the structure, dynamics, and expansion laws of these regions. Once a massive star forms in our highly structured clouds, its ionizing radiation eventually stops the accretion (through filaments) towards the massive star-forming regions. The new overpressured H  ii regions push away the dense gas, thus disrupting the more massive collapse centres. Also, because of the complex density structure in the cloud, the H  ii regions expand in a hybrid manner: they virtually do not expand towards the densest regions (cores), while they expand according to the classical analytical result towards the rest of the cloud, and in an accelerated way, as a blister region, towards the diffuse medium. Thus, the ionized regions grow anisotropically, and the ionizing stars generally appear off-centre of the regions. Finally, we find that the hypotheses assumed in standard H  ii -region expansion models (fully embedded region, blister-type, or expansion in a density gradient) apply simultaneously in different parts of our simulated H  ii regions, producing a net expansion law (R ∝ t α, with α in the range of 0.93–1.47 and a mean value of 1.2 ± 0.17) that differs from any of those of the standard models. [ABSTRACT FROM AUTHOR]

Both observational and theoretical research over the past decade has demonstrated that the probability distribution function (PDF) of the gas density in turbulent molecular clouds is a key ingredient for understanding star formation. It has recently been argued that the PDF of molecular clouds is a pure power-law distribution. It has been claimed that the log-normal part is ruled out when using only the part of the PDF up/down to which it is complete, that is where the column density contours are still closed. By using the results from high-resolution magnetohydrodynamical simulations of molecular cloud formation and evolution, we find that the column density PDF is indeed composed of a log-normal and, if including self-gravity, a power-law part. We show that insufficient sampling of a molecular cloud results in closed contours that cut-off the log-normal part. In contrast, systematically increasing the field of view and sampling the entire cloud yields a completeness limit at the lower column densities, which also recovers the log-normal part. This demonstrates that the field of view must be sufficiently large for the PDF to be complete down to its log-normal part, which has important implications for predictions of star formation activity based on the PDF. [ABSTRACT FROM AUTHOR]

The origin of cosmic magnetic fields is an unsolved problem and magnetogenesis could have occurred in the early Universe. We study the evolution of such primordial magnetic fields across the cosmological recombination epoch via 3D magnetohydrodynamic numerical simulations. We compute the effective or net heating rate of baryons due to decaying magnetic fields and its dependence on the magnetic field strength and spectral index. In the drag-dominated regime (⁠|$z$| ≳ 1500), prior to recombination, we find no real heating is produced. Our simulations allow us to smoothly trace a new transition regime (600 ≲ |$z$| ≲ 1500), where magnetic energy decays, at first, into the kinetic energy of baryons. A turbulent velocity field is built up until it saturates, as the net heating rate rises from a low value at recombination to its peak towards the end of the transition regime. This is followed by a turbulent decay regime (⁠|$z$| ≲ 600) where magnetic energy dissipates via turbulent decay of both magnetic and velocity fields while net heating remains appreciable and declines slowly. Both the peak of the net heating rate and the onset of turbulent decay are delayed significantly beyond recombination, by up to 0.5 Myr (until |$z$| ≃ 600–700), for scale-invariant magnetic fields. We concentrate on low magnetic field strength to avoid confusion with magnetic field-generated density fluctuations. Analytic approximations are provided and we present numerical results for a range of field strengths (≃10−3− 2 × 10−2nG) and spectral indices, illustrating the redshift-dependence of dissipation and net heating rates. These can be used to study cosmic microwave background constraints on primordial magnetic fields. [ABSTRACT FROM AUTHOR]

Observations show that galaxies and their interstellar media are pervaded by strong magnetic fields with energies in the diffuse component being at least comparable to the thermal and even as large or larger than the turbulent energy. Such strong magnetic fields prevent the formation of stars because patches of the interstellar medium are magnetically subcritical. Here we present the results from global numerical simulations of strongly magnetized and self-gravitating galactic discs, which show that the buoyancy of the magnetic field due to the Parker instability leads at first to the formation of giant filamentary regions. These filamentary structures become gravitationally unstable and fragment into $${\sim }10^5 \,\mathrm{M}_{\odot }$$ clouds that attract kpc long coherent filamentary flows that build them into giant molecular clouds (GMCs). Our results thus provide a solution to the long-standing problem of how the transition from sub- to supercritical regions in the interstellar medium proceeds. [ABSTRACT FROM AUTHOR]

We study different manifestations of the speed of light in theories of gravity where metric and connection are regarded as independent fields. We find that for a generic gravity theory in a frame with locally vanishing affine connection, the usual degeneracy between different manifestations of the speed of light is broken. In particular, the space-time causal structure constant () may become variable in that local frame. For theories of the form , this variation in has an impact on the definition of the luminosity distance (and distance modulus), which can be used to confront the predictions of particular models against Supernovae type Ia (SN Ia) data. We carry out this test for a quadratic gravity model without cosmological constant assuming (i) a constant speed of light and (ii) a varying speed of light (VSL), and find that the latter scenario is favored by the data. [ABSTRACT FROM AUTHOR]

We present magnetohydrodynamic simulations aimed at studying the effect of the magnetic suppression of turbulence (generated through various instabilities during the formation of molecular clouds by converging) on the subsequent star formation (SF) activity. We study four magnetically supercritical models with magnetic field strengths B = 0, 1, 2, and 3 μG (corresponding to mass–to–flux ratios of ∞, 4.76, 2.38, and 1.59 times the critical value), with the magnetic field, initially being aligned with the flows. We find that, for increasing magnetic field strength, the clouds formed tend to be more massive, denser, less turbulent, and with higher SF activity. This causes the onset of SF activity in the non-magnetic or more weakly magnetized cases to be delayed by a few Myr in comparison to the more strongly magnetized cases. We attribute this behaviour to the suppression of the non-linear thin shell instability (NTSI) by the magnetic field, previously found by Heitsch and coworkers. This result is contrary to the standard notion that the magnetic field provides support to the clouds, thus reducing their star formation rate. However, our result is a completely non-linear one, and could not be foreseen from simple linear considerations. [ABSTRACT FROM AUTHOR]

Molecular clouds are to a great extent influenced by turbulent motions in the gas. Numerical and observational studies indicate that the star formation rate and efficiency crucially depend on the mixture of solenoidal and compressive modes in the turbulent acceleration field, which can be quantified by the turbulent driving parameter b. For purely solenoidal (divergencefree) driving previous studies showed that b = 1/3 and for entirely compressive (curl-free) driving b = 1. In this study, we determine the evolution of the turbulent driving parameter b in magnetohydrodynamical simulations of molecular cloud formation and evolution. The clouds form due to the convergence of two flows of warm neutral gas. We explore different scenarios by varying the magnitude of the initial turbulent perturbations in the flows. We show that the driving mode of the turbulence within the cloud strongly fluctuates with time and exhibits no clear correlation with typical cloud properties, such as the cloud mass and the (Alfvén) Mach number. We specifically find that b strongly varies from b ~ 0.3 to b ~ 0.8 on time-scales t ≲ 5Myr, where the time-scale and range of variation can change from cloud to cloud. This rapid change of b is primarily associated with gravity making the acceleration field more compressive as a result of global contraction of the cloud and subsequent onset of star formation. We conclude that the effective turbulence driving parameter should be treated as a free parameter that can vary from solenoidal to compressive in both time and space. [ABSTRACT FROM AUTHOR]

High-mass stars are expected to form from dense prestellar cores. Their precise formation conditions are widely discussed, including their virial condition, which results in slow collapse for supervirial cores with strong support by turbulence or magnetic fields, or fast collapse for subvirial sources. To disentangle their formation processes, measurements of the deuterium fractions are frequently employed to approximately estimate the ages of these cores and to obtain constraints on their dynamical evolution. We here present 3D magnetohydrodynamical simulations including for the first time an accurate non-equilibrium chemical network with 21 gas-phase species plus dust grains and 213 reactions. With this network we model the deuteration process in fully depleted prestellar cores in great detail and determine its response to variations in the initial conditions. We explore the dependence on the initial gas column density, the turbulent Mach number, the mass-to-magnetic flux ratio and the distribution of the magnetic field, as well as the initial ortho-to-para ratio (OPR) of H2. We find qualitatively good agreement with recent observations of deuterium fractions in quiescent sources. Our results show that deuteration is rather efficient, even when assuming a conservative OPR of 3 and highly subvirial initial conditions, leading to large deuterium fractions already within roughly a free-fall time. We discuss the implications of our results and give an outlook to relevant future investigations. [ABSTRACT FROM AUTHOR]

We use magnetohydrodynamical simulations of converging warm neutral medium flows to analyse the formation and global evolution of magnetized and turbulent molecular clouds subject to supernova feedback from massive stars. We show that supernova feedback alone fails to disrupt entire, gravitationally bound, molecular clouds, but is able to disperse smallsized (~10 pc) regions on time-scales of less than 1 Myr. Efficient radiative cooling of the supernova remnant as well as strong compression of the surrounding gas result in non-persistent energy and momentum input from the supernovae. However, if the time between subsequent supernovae is short and they are clustered, large hot bubbles form that disperse larger regions of the parental cloud. On longer time-scales, supernova feedback increases the amount of gas with moderate temperatures (T ≈ 300-3000 K). Despite its inability to disrupt molecular clouds, supernova feedback leaves a strong imprint on the star formation process. We find an overall reduction of the star formation efficiency by a factor of 2 and of the star formation rate by roughly factors of 2-4. [ABSTRACT FROM AUTHOR]

Stars are born in turbulent, magnetized filamentary molecular clouds, typically as members of star clusters. Several remarkable technical advances enable observations of magnetic structure and field strengths across many physical scales, from galactic scales on which giant molecular clouds (GMCs) are assembled, down to the surfaces of magnetized accreting young stars. These are shedding new light on the role of magnetic fields in star formation. Magnetic fields affect the gravitational fragmentation and formation of filamentary molecular clouds, the formation and fragmentation of magnetized disks, and finally to the shedding of excess angular momentum in jets and outflows from both the disks and young stars. Magnetic fields play a particularly important role in angular momentum transport on all of these scales. Numerical simulations have provided an important tool for tracking the complex process of the collapse and evolution of protostellar gas since several competing physical processes are at play - turbulence, gravity, MHD, and radiation fields. This paper focuses on the role of magnetic fields in three crucial regimes of star formation: the formation of star clusters emphasizing fragmentation, disk formation and the origin of early jets and outflows, to processes that control the spin evolution of young stars. [ABSTRACT FROM PUBLISHER]

We study the influence of the choice of cooling function on the formation of molecular clouds in high-resolution three-dimensional simulations of converging flows. We directly compare the results obtained using the simple, parametrized cooling function introduced by Koyama and Inutsuka and used by a number of converging flow studies with the results of the detailed calculation of the non-equilibrium chemistry and thermal balance of the gas. We find that a number of the cloud properties, such as the mass and volume filling fractions of cold gas, are relatively insensitive to the choice of cooling function. On the other hand, the cloud morphology and the large-scale velocity distribution of the gas do strongly depend on the cooling function. We show that the differences that we see can largely be explained by differences in the way that Lyman α cooling is treated in the two complementary approaches, and that a proper non-equilibrium treatment of the ionization and recombination of the gas is necessary in order to model the high-temperature cooling correctly.We also investigate the properties of the dense clumps formed within the cloud. In agreement with previous models, we find that the majority of these clumps are not self-gravitating, suggesting that some form of large-scale collapse of the cloud may be required in order to produce gravitationally unstable clumps and hence stars. Overall, the physical properties of the dense clumps are similar in both simulations, suggesting that they do not depend strongly on the choice of cooling function. However, we do find a systematic difference of around 10 K in the mean temperatures of the clumps produced by the two models. [ABSTRACT FROM AUTHOR]

The first galaxies form due to gravitational collapse of primordial halos. During this collapse, weak magnetic seed fields get amplified exponentially by the small-scale dynamo - a process converting kinetic energy from turbulence into magnetic energy. We use the Kazantsev theory, which describes the small-scale dynamo analytically, to study magnetic field amplification for different turbulent velocity correlation functions. For incompressible turbulence (Kolmogorov turbulence), we find that the growth rate is proportional to the square root of the hydrodynamic Reynolds number, Re1/2. In the case of highly compressible turbulence (Burgers turbulence) the growth rate increases proportional to Re1/3. With a detailed chemical network we are able to follow the chemical evolution and determine the kinetic and magnetic viscosities (due to Ohmic and ambipolar diffusion) during the collapse of the halo. This way, we can calculate the growth rate of the small-scale dynamo quantitatively and predict the evolution of the small-scale magnetic field. As the magnetic energy is transported to larger scales on the local eddy-timescale, we obtain an estimate for the magnetic field on the Jeans scale. Even there, we find that equipartition with the kinetic energy is reached on small timescales. Dynamically relevant field structures can thus be expected already during the formation of the first objects in the Universe. [ABSTRACT FROM PUBLISHER]

ABSTRACT We study the influence of initial conditions on the magnetic field amplification during the collapse of a magnetized gas cloud. We focus on the dependence of the growth and saturation level of the dynamo-generated field on the turbulent properties of the collapsing cloud. In particular, we explore the effect of varying the initial strength and injection scale of turbulence and the initial uniform rotation of the collapsing magnetized cloud. In order to follow the evolution of the magnetic field in both the kinematic and the non-linear regime, we choose an initial field strength of with the magnetic to kinetic energy ratio, Em/ Ek∼ 10−4. Both gravitational compression and the small-scale dynamo initially amplify the magnetic field. Further into the evolution, the dynamo-generated magnetic field saturates but the total magnetic field continues to grow because of compression. The saturation of the small-scale dynamo is marked by a change in the slope of B/ρ2/3 and by a shift in the peak of the magnetic energy spectrum from small scales to larger scales. For the range of initial Mach numbers explored in this study, the dynamo growth rate increases as the Mach number increases from vrms/ cs∼ 0.2 to 0.4 and then starts decreasing from vrms/ cs∼ 1.0. We obtain saturation values of Em/ Ek= 0.2-0.3 for these runs. Simulations with different initial injection scales of turbulence also show saturation at similar levels. For runs with different initial rotation of the cloud, the magnetic energy saturates at Em/ Ek∼ 0.2-0.4 of the equipartition value. The overall saturation level of the magnetic energy, obtained by varying the initial conditions, is in agreement with previous analytical and numerical studies of small-scale dynamo action where turbulence is driven by an external forcing instead of gravitational collapse. [ABSTRACT FROM AUTHOR]

ABSTRACT We investigate the formation of protostellar clusters during the collapse of dense molecular cloud cores with a focus on the evolution of potential and kinetic energy, the degree of substructure and the early phase of mass segregation. Our study is based on a series of hydrodynamic simulations of dense cores, where we vary the initial density profile and the initial turbulent velocity. In the three-dimensional adaptive mesh refinement simulations, we follow the dynamical formation of filaments and protostars until a star formation efficiency of 20 per cent. Despite the different initial configurations, the global ensemble of all protostars in a setup shows a similar energy evolution and forms sub-virial clusters with an energy ratio Ekin/| Epot|∼ 0.2. Concentrating on the innermost central region, the clusters show a roughly virialized energy balance. However, the region of virial balance only covers the innermost ∼10-30 per cent of all the protostars. In all simulations with multiple protostars, the total kinetic energy of the protostars is higher than the kinetic energy of the gas cloud, although the protostars only contain 20 per cent of the total mass. The clusters vary significantly in size, mass and number of protostars, and show different degrees of substructure and mass segregation. Flat density profiles and compressive turbulent modes produce more subclusters than centrally concentrated profiles and solenoidal turbulence. We find that dynamical relaxation and hence dynamical mass segregation is very efficient in all cases from the very beginning of the nascent cluster, i.e. during a phase when protostars constantly form and accrete. [ABSTRACT FROM AUTHOR]

ABSTRACT We investigate the impact of different initial conditions for the initial density profile and the initial turbulence on the formation process of protostellar clusters. We study the collapse of dense molecular cloud cores with three-dimensional adaptive mesh refinement simulations. We focus our discussion on the distribution of the gas among the protostellar objects in the turbulent dynamical cores. Despite the large variations in the initial configurations and the resulting gas and cluster morphology we find that all stellar clusters follow a very similar gas accretion behaviour. Once secondary protostars begin to form, the central region of a cluster is efficiently shielded from further accretion. Hence, objects located close to the centre are starved of material, as indicated by a strong decrease of the central accretion rate. This fragmentation-induced starvation occurs not only in rotationally supported discs and filaments, but also in more spherically symmetric clusters with complex chaotic motions. [ABSTRACT FROM AUTHOR]

ABSTRACT Ultracompact and hypercompact H ii regions appear when a star with a mass larger than about 15 M⊙ starts to ionize its own environment. Recent observations of time variability in these objects are one of the pieces of evidence that suggest that at least some of them harbour stars that are still accreting from an infalling neutral accretion flow that becomes ionized in its innermost part. We present an analysis of the properties of the H ii regions formed in the three-dimensional radiation-hydrodynamic simulations presented by Peters et al. as a function of time. Flickering of the H ii regions is a natural outcome of this model. The radio-continuum fluxes of the simulated H ii regions as well as their flux and size variations are in agreement with the available observations. From the simulations, we estimate that a small but non-negligible fraction (∼10 per cent) of observed H ii regions should have detectable flux variations (larger than 10 per cent) on time-scales of ∼10 yr, with positive variations being more likely to happen than negative variations. A novel result of these simulations is that negative flux changes do happen, in contrast to the simple expectation of ever growing H ii regions. We also explore the temporal correlations between properties that are directly observed (flux and size) and other quantities like density and ionization rates. [ABSTRACT FROM AUTHOR]

We investigate the formation and evolution of giant molecular clouds (GMCs) by the collision of convergent warm neutral medium (WNM) streams in the interstellar medium, in the presence of magnetic fields and ambipolar diffusion (AD), focusing on the evolution of the star formation rate and efficiency (SFE), as well as of the mass-to-magnetic-flux ratio (M2FR) in the forming clouds. We find that (1) clouds formed by supercritical inflow streams proceed directly to collapse, while clouds formed by subcritical streams first contract and then re-expand, oscillating on the scale of tens of Myr; (2) our suite of simulations with the initial magnetic field strength of 2, 3 and show that only supercritical or marginal critical streams lead to reasonable star forming rates. This result is not altered by the inclusion of AD; (3) the GMC's M2FR is a generally increasing function of time, whose growth rate depends on the details of how mass is added to the GMC from the WNM; (4) the M2FR is a highly fluctuating function of position in the clouds. This implies that a significant fraction of a cloud's mass may remain magnetically supported, while SF occurs in the supercritical regions that are not supported; (5) in our simulations, the SFE approaches stationarity, because mass is added to the GMC at a similar rate to which it converts mass to stars. In such an approximately stationary regime, we find that the SFE provides a proxy of the supercritical mass fraction in the cloud; and (6) the low-M2FR regions exhibit buoyancy within the gravitationally contracting GMCs, so that the latter naturally segregate into a high-density, high-M2FR 'core' and a low-density, low-M2FR 'envelope', without the intervention of AD. [ABSTRACT FROM AUTHOR]

We present a detailed parameter study of collapsing turbulent cloud cores, varying the initial density profile and the initial turbulent velocity field. We systematically investigate the influence of different initial conditions on the star formation process, mainly focusing on the fragmentation, the number of formed stars and the resulting mass distributions. Our study compares four different density profiles (uniform, Bonnor-Ebert type, and ), combined with six different supersonic turbulent velocity fields (compressive, mixed and solenoidal, initialized with two different random seeds each) in three-dimensional simulations using the adaptive-mesh refinement, hydrodynamics code . The simulations show that density profiles with flat cores produce hundreds of low-mass stars, either distributed throughout the entire cloud or found in subclusters, depending on the initial turbulence. Concentrated density profiles always lead to the formation of one high-mass star in the centre of the cloud and, if at all, low-mass stars surrounding the central one. In uniform and Bonnor-Ebert type density distributions, compressive initial turbulence leads to local collapse about 25 per cent earlier than solenoidal turbulence. However, central collapse in the steep power-law profiles is too fast for the turbulence to have any significant influence. We conclude that (i) the initial density profile and turbulence mainly determine the cloud evolution and the formation of clusters, (ii) the initial mass function (IMF) is not universal for all setups and (iii) that massive stars are much less likely to form in flat density distributions. The IMFs obtained in the uniform and Bonnor-Ebert type density profiles are more consistent with the observed IMF, but shifted to lower masses. [ABSTRACT FROM AUTHOR]

We present a detailed computational study of the assembly of protostellar discs and massive stars in molecular clouds with supersonic turbulence. We follow the evolution of large-scale filamentary structures in a cluster-forming clump down to protostellar length-scales by means of very highly resolved, 3D adaptive mesh refined (AMR) simulations, and show how accretion discs and massive stars form in such environments. We find that an initially elongated cloud core which has a slight spin from oblique shocks collapses first to a filament and later develops a turbulent disc close to the centre of the filament. The continued large-scale flow that shocks with the filament maintains the high density and pressure within it. Material within the cooling filament undergoes gravitational collapse and an outside-in assembly of a massive protostar. Our simulations show that very high mass accretion rates of up to and high, supersonic, infall velocities result from such filamentary accretion. Accretion at these rates is higher by an order of magnitude than those found in semi-analytic studies, and can quench the radiation field of a growing massive young star. Our simulations include a comprehensive set of the important chemical and radiative processes such as cooling by molecular line emission, gas–dust interaction and radiative diffusion in the optically thick regime, as well as H2 formation and dissociation. Therefore, we are able to probe, for the first time, the relevant physical phenomena on all scales from those characterizing the clump down to protostellar core. [ABSTRACT FROM AUTHOR]

A large body of theoretical and computational work shows that jets - modelled as magnetized disk winds - exert an external torque on their underlying disks that can efficiently remove angular momentum and act as major drivers of disk accretion. These predictions have recently been confirmed in direct HST measurements of the jet rotation and angular momentum transport in low mass protostellar systems. We review the theory of disc winds and show that their physics is universal and scales to jets from both low and high mass star forming regions. This explains the observed properties of outflows in massive star forming regions, before the central massive star generates an ultracompact HII region. We also discuss the recent numerical studies on the formation of massive accretion disks and outflows through gravitational collapse, including our own work on 3D Adaptive Mesh simulations (using the FLASH code) of the hydromagnetic collapse of an initial rotating, and cooling Bonner-Ebert sphere. Magnetized collapse gives rise to outflows. Our own simulations show that both a jet-like disk wind on sub AU scales, and a larger scale molecular outflow occur (Banerjee & Pudritz 2005). [ABSTRACT FROM PUBLISHER]

We present a detailed study of the collapse of molecular cloud cores using high-resolution three-dimensional adaptive mesh refinement (AMR) numerical simulations. In this first in a series of investigations our initial conditions consist of a spherical molecular core obeying the hydrostatic Bonnor–Ebert profile with varying degrees of initial rotation. Our simulations cover both the formation of massive discs, in which massive stars form, and low-mass discs. We use a customized version of theflashcode the AMR technique, which allows us to follow the formation of a protostellar disc and protostellar core(s) through more than 10 orders in density increase, while continuously resolving the local Jeans length (i.e. obeying the Truelove criterion). Our numerical simulations also incorporate the energy loss due to molecular line emission in order to obtain a more realistic picture of the protostellar core and disc formation. Our initial states model systems of mass 168 andthat will form high- and low-mass stars, respectively. We follow many features such as the development complex shock structures, and the fragmentation of the disc. We find that slowly rotating coresproduce discs in which a strong bar develops but does not fragment. Faster initial rotation ratesresult in the formation of a ring, which may fragment into two protostellar cores. The size of the rings found in our simulated discs agree with the observations of similar systems. [ABSTRACT FROM AUTHOR]

We explore the phenomenon of the recently discovered inverse transfer of energy from small to large scales in decaying magnetohydrodynamical turbulence by Brandenburg et al. [Phys. Rev. Lett. 114, 075001 (2015)], even for nonhelical magnetic fields. For this investigation we mainly employ the Pencil Code performing a parameter study, where we vary the Prandtl number, the kinematic viscosity, and the initial spectrum. We find that to get a decay that exhibits this inverse transfer, large Reynolds numbers (O~10³) are needed and low Prandtl numbers of the order unity Pr=1 are preferred. Compared to helical MHD turbulence, though, the inverse transfer is much less efficient in transferring magnetic energy to larger scales than the well-known effect of the inverse cascade. Hence, applying the inverse transfer to the magnetic field evolution in the Early Universe, we question whether the nonhelical inverse transfer is effective enough to explain the observed void magnetic fields if a magnetogenesis scenario during the electroweak phase transition is assumed. [ABSTRACT FROM AUTHOR]

The small-sc de dynamo is a process by which turbulent kinetic energy is converted into magnetic energy, and thus it is expected to depend crucially on the nature of the turbulence. In this paper, we present a model for the small-scale dynamo that takes into account the slope of the turbulent velocity spectrum v(l) &agr; Iv, where I and v(l) are the size of a turbulent fluctuation and the typical velocity on that scale. The time evolution of the fluctuation component of the magnetic field, i.e. the small-scale field, is described by the Kazantsev equation. We solve this linear differential equation for its eigenvalues with the quantum-mechanical WKB approximation. The validity of this method is estimated as a function of the magnetic Prandtl number Pm. We calculate the minimal magnetic Reynolds number for dynamo action, Rmcrit, using our model of the turbulent velocity correlaiion function. For Kolmogorov turbulence (v = 1/3), we find that the critical magnetic Reynolds number is RmKcrt ≈ 110 and for Burgers turbulence (v = 1/2) RmcritB ≈ 2700. Furthermore, we derive that the growth rate of the small-scale magnetic field for a general type of turbulence is T &agr; Re(1-v)/(1+v) in the limit of infinite magnetic Prandtl number. For decreasing magnetic Prandtl number (down to Pm ≳ 10), the growth rate of the small-scale dynamo decreases. The details of this drop depend on the WKB approximation, which becomes invalid for a magnetic Prandtl number of about unity. [ABSTRACT FROM AUTHOR]

We present the first study on the amplification of magnetic fields by the turbulent dynamo in the highly subsonic regime, with Mach numbers ranging from 10-3 to 0.4. We find that for the lower Mach numbers the saturation efficiency of the dynamo (Emag/Ekin)sat increases as the Mach number decreases. Even in the case when injection of energy is purely through longitudinal forcing modes, (Emag/Ekin)sat≳10-2 at a Mach number of 10-3. We apply our results to magnetic field amplification in the early Universe and predict that a turbulent dynamo can amplify primordial magnetic fields to ≳10-16  G on scales up to 0.1 pc and ≳10-13  G on scales up to 100 pc. This produces fields compatible with lower limits of the intergalactic magnetic field inferred from blazar γ-ray observations. [ABSTRACT FROM AUTHOR]