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We investigate the properties of circumstellar discs (CDs) produced in hydrodynamical simulations of gravoturbulent core collapse considering Kolmogorov and Burger-type turbulence. We report that massive discs are more prevalent in the Kolmogorov regime than for Burger-type turbulence. A significant number of discs are formed with a radius of ∼15 au in both cases. However, the number of extended discs with radii >15 au is significantly larger in case of Kolmogorov turbulence. The two regimes of turbulence, in general, yield disc radii in the ranges of 7−30 au and 13−39 au, respectively. The corresponding ranges of the disc masses are 30.37 M Jup−0.92 M⊙ and 2.09  M Jup−0.13 M⊙, respectively. Moreover, the ratio M disc/ M star is higher in models of Kolmogorov-type turbulence than in models of Burgers-type turbulence. We do not find any correlation between R disc and M disc over the explored range of initial temperatures (8−14 K) and the type of turbulence. Also, for these initial thermal variations, the turbulent CD structures do not exhibit signs of turbulent diffusion. Nonetheless, both sub- and supersonic velocity dispersions cause variations in the specific angular momentum of infalling gas, especially for CDs with radii ∼ 16−21 au. The radial profiles of CDs do not correlate with the initial conditions. [ABSTRACT FROM AUTHOR]

Past studies have long emphasised the key role played by galactic stellar bars in the context of disc secular evolution, via the redistribution of gas and stars, the triggering of star formation, and the formation of prominent structures such as rings and central mass concentrations. However, the exact physical processes acting on those structures, as well as the timescales associated with the building and consumption of central gas reservoirs are still not well understood. We are building a suite of hydro-dynamical RAMSES simulations of isolated, low-redshift galaxies that mimic the properties of the PHANGS sample. The initial conditions of the models reproduce the observed stellar mass, disc scale length, or gas fraction, and this paper presents a first subset of these models. Most of our simulated galaxies develop a prominent bar structure, which itself triggers central gas fuelling and the building of an over-density with a typical scale of 100−1000 pc. We confirm that if the host galaxy features an ellipsoidal component, the formation of the bar and gas fuelling are delayed. We show that most of our simulations follow a common time evolution, when accounting for mass scaling and the bar formation time. In our simulations, the stellar mass of 1010 M⊙ seems to mark a change in the phases describing the time evolution of the bar and its impact on the interstellar medium. In massive discs (M⋆ ≥ 1010 M⊙), we observe the formation of a central gas reservoir with star formation mostly occurring within a restricted starburst region, leading to a gas depletion phase. Lower-mass systems (M⋆ < 1010 M⊙) do not exhibit such a depletion phase, and show a more homogeneous spread of star-forming regions along the bar structure, and do not appear to host inner bar-driven discs or rings. Our results seem to be supported by observations, and we briefly discuss how this new suite of simulations can help our understanding of the secular evolution of main sequence disc galaxies. [ABSTRACT FROM AUTHOR]

Molecular hydrogen allows cooling in primordial gas, facilitating its collapse into Population III stars within primordial halos. Lyman–Werner (LW) radiation from these stars can escape the halo and delay further star formation by destroying H2 in other halos. As cosmological simulations show that increasing the background LW field strength increases the average halo mass required for star formation, we perform follow-up simulations of selected halos to investigate the knock-on effects this has on the Population III IMF. We follow 5 halos for each of the J 21  = 0, 0.01, and 0.1 LW field strengths, resolving the pre-stellar core density of 10−6 g cm−3 (1018 cm−3) before inserting sink particles and following the fragmentation behaviour for hundreds of years further. We find that the mass accreted onto sinks by the end of the simulations is proportional to the mass within the ∼10−2 pc molecular core, which is not correlated to the initial mass of the halo. As such, the IMFs for masses above the brown dwarf limit show little dependence on the LW strength, although they do show variance in the number of low-mass clumps formed. As the range of background LW field strengths tested here covers the most likely values from literature, we conclude that the IMF for so-called Pop III.2 stars is not significantly different from the initial population of Pop III.1 stars. The primordial IMF therefore likely remains unchanged until the formation of the next generation of Population II stars. [ABSTRACT FROM AUTHOR]

Many studies concluded that magnetic fields suppress star formation in molecular clouds and Milky Way like galaxies. However, most of these studies are based on fully developed fields that have reached the saturation level, with little work on investigating how an initial weak primordial field affects star formation in low metallicity environments. In this paper, we investigate the impact of a weak initial field on low metallicity dwarf galaxies. We perform high-resolution arepo simulations of five isolated dwarf galaxies. Two models are hydrodynamical, two start with a primordial magnetic field of 10 |$^{-6} \, \mu$| G and different sub-solar metallicities, and one starts with a saturated field of 10 |$^{-2} \, \mu$| G. All models include a non-equilibrium, time-dependent chemical network that includes the effects of gas shielding from the ambient ultraviolet field. Sink particles form directly from the gravitational collapse of gas and are treated as star-forming clumps that can accrete gas. We vary the ambient uniform far ultraviolet field, and cosmic ray ionization rate between 1 per cent and 10 per cent of solar values. We find that the magnetic field has little impact on the global star formation rate (SFR), which is in tension with some previously published results. We further find that the initial field strength has little impact on the global SFR. We show that an increase in the mass fractions of both molecular hydrogen and cold gas, along with changes in the perpendicular gas velocity dispersion and the magnetic field acting in the weak-field model, overcome the expected suppression in star formation. [ABSTRACT FROM AUTHOR]

During primordial star formation, the main cooling channel is provided by H2 and super-molecules, such as H2 or H2, at sufficiently high densities. When the latter form at |$n_{\rm H}\, \ge \, 10^{14}$|  cm−3, collision-induced emission (CIE) provides efficient gas cooling. We investigate how CIE cooling affects the formation of metal-free binaries comparing simulations with and without this process. Irrespective of the cooling mechanism, we find a typical protostellar mass range between 0.01 and 100 M⊙. However, models with only H2 line cooling produce a greater number of low-mass protostars that exhibit stronger variations in their radial velocities than the high-mass protostars. Similarly, in models with both H2 cooling and CIE cooling, significant variations in the radial velocities are found for protostars in the intermediate-mass range. The initial number of fragments N max decreases with increasing strength of turbulence. Cooling via super-molecules lets the most massive protobinaries (MMPBs) efficiently accrete mass. The maximum mass accretion rate |$\dot{M}_{\rm max}$| for the MMPBs is more than an order of magnitude higher in the presence of CIE cooling than for pure H2 line cooling. As a result, compact binaries with a semimajor axis as small as 3.57 au may form through the H2– H2 cooling channel. Our results indicate that, in addition to the MMPBs, most population III (Pop. III) binaries should be in eccentric i.e. non-circular orbits. This provides an important connection to the eccentric binaries reported in previous studies, which were found to exhibit rich temporal accretion signals during their evolution. [ABSTRACT FROM AUTHOR]

We revisit the conditions present in supermassive discs (SMDs) formed by the merger of gas-rich, metal-enriched galaxies at redshift z ∼ 10. We find that SMDs naturally form hydrostatic cores which go through a rapidly accreting supermassive star phase, before directly collapsing into massive black holes via the general relativistic instability. The growth and collapse of the cores occurs within ∼5 × 105 yr from the formation of the SMD, producing bright electromagnetic, neutrino and gravitational wave transients with a typical duration of a few minutes and, respectively, a typical flux and a typical strain amplitude at Earth of ∼10−8 erg s−1 cm−2 and ∼4 × 10−21. We provide a simple fitting formula for the resulting black hole masses, which range from a few 106 to 108 M⊙ depending on the initial SMD configuration. Crucially, our analysis does not require any specific assumption on the thermal properties of the gas, nor on the angular momentum loss mechanisms within the SMD. Led by these findings, we argue that the merger-driven scenario provides a robust pathway for the rapid formation of supermassive black holes at z > 6. It provides an explanation for the origin of the brightest and oldest quasars without the need of a sustained growth phase from a much smaller seed. Its smoking gun signatures can be tested directly via multimessenger observations. [ABSTRACT FROM AUTHOR]

Population III (Pop III) stars form in groups due to the fragmentation of primordial gas. While uniform magnetic fields have been shown to support against fragmentation in present-day star formation, it is unclear whether realistic k 3/2 primordial fields can have the same effect. We bypass the issues associated with simulating the turbulent dynamo by introducing a saturated magnetic field at equipartition with the velocity field when the central densities reach 10−13 g cm−3. We test a range of sink particle creation densities from 10−10 to 10−8 g cm−3. Within the range tested, the fields did not suppress fragmentation of the gas and hence could not prevent the degree of fragmentation from increasing with increased resolution. The number of sink particles formed and total mass in sink particles was unaffected by the magnetic field across all seed fields and resolutions. The magnetic pressure remained sub-dominant to the gas pressure except in the highest density regions of the simulation box, where it became equal to but never exceeded gas pressure. Our results suggest that the inclusion of magnetic fields in numerical simulations of Pop III star formation is largely unimportant. [ABSTRACT FROM AUTHOR]

Gaseous substructures such as feathers and spurs dot the landscape of spiral arms in disc galaxies. One of the candidates to explain their formation is the wiggle instability of galactic spiral shocks. We study the wiggle instability using local 2D hydrodynamical isothermal non-self-gravitating simulations. We find that: (1) simulations agree with analytic linear stability analysis only under stringent conditions. They display surprisingly strong non-linear coupling between the different modes, even for small mode amplitudes (⁠|${\sim}1{{\ \rm per\ cent}}$|⁠). (2) We demonstrate that the wiggle instability originates from a combination of two physically distinct mechanisms: the first is the Kelvin–Helmholtz instability, and the second is the amplification of infinitesimal perturbations from repeated shock passages. These two mechanisms can operate simultaneously, and which mechanism dominates depends on the underlying parameters. (3) We explore the parameter space and study the properties of spurs/feathers generated by the wiggle instability. The wiggle instability is highly sensitive to the underlying parameters. The feather separation decreases, and the growth rate increases, with decreasing sound speed, increasing potential strength and decreasing interarm distance. (4) We compare our simulations with a sample of 20 galaxies in the HST Archival Survey of Spiral Arm Substructure of La Vigne et al. and find that the wiggle instability is able to reproduce the typical range of feather separations seen in observations. It remains unclear how the wiggle instability relates to competing mechanisms for spur/feather formation such as the magneto-jeans instability and the stochastic accumulation of gas due to correlated supernova feedback. [ABSTRACT FROM AUTHOR]

In recent years, it has been speculated that in extreme low-metallicity galactic environments, stars form in regions that lack H2. In this paper, we investigate how changing the metallicity and ultraviolet (UV) field strength of a galaxy affects the star formation within, and the molecular gas Kennicutt–Schmidt (KS) relation. Using extremely high-resolution arepo simulations of isolated dwarf galaxies, we independently vary the metallicity and UV field to between 1 per cent and 10 per cent solar neighbourhood values. We include a non-equilibrium, time-dependent chemical network to model the molecular composition of the interstellar medium and include the effects of gas shielding from an ambient UV field. Crucially, our simulations directly model the gravitational collapse of gas into star-forming clumps and cores and their subsequent accretion using sink particles. In this first publication, we find that reducing the metallicity and UV field by a factor of 10 has no effect on star formation and minimal effect on the cold, dense star-forming gas. The cold gas depletion times are almost an order of magnitude longer than the molecular gas depletion time due to the presence of star formation in H  i dominated cold gas. We study the H2 KS relationship that arises naturally within the simulations and find a near-linear power-law index of N  = 1.09 ± 0.014 in our fiducial |$10{{\ \rm per\ cent}}$| solar metallicity model. As the metallicity and UV field are reduced, this becomes moderately steeper, with a slope of N  = 1.24 ± 0.022 for our |$1{{\ \rm per\ cent}}$| solar metallicity and |$1{{\ \rm per\ cent}}$| solar UV field model. [ABSTRACT FROM AUTHOR]

To investigate how molecular clouds react to different environmental conditions at a galactic scale, we present a catalogue of giant molecular clouds (GMCs) resolved down to masses of ∼10 M⊙ from a simulation of the entire disc of an interacting M51-like galaxy and a comparable isolated galaxy. Our model includes time-dependent gas chemistry, sink particles for star formation, and supernova feedback, meaning we are not reliant on star formation recipes based on threshold densities and can follow the physics of the cold molecular phase. We extract GMCs from the simulations and analyse their properties. In the disc of our simulated galaxies, spiral arms seem to act merely as snowplows, gathering gas, and clouds without dramatically affecting their properties. In the centre of the galaxy, on the other hand, environmental conditions lead to larger, more massive clouds. While the galaxy interaction has little effect on cloud masses and sizes, it does promote the formation of counter-rotating clouds. We find that the identified clouds seem to be largely gravitationally unbound at first glance, but a closer analysis of the hierarchical structure of the molecular interstellar medium shows that there is a large range of virial parameters with a smooth transition from unbound to mostly bound for the densest structures. The common observation that clouds appear to be virialized entities may therefore be due to CO bright emission highlighting a specific level in this hierarchical binding sequence. The small fraction of gravitationally bound structures found suggests that low galactic star formation efficiencies may be set by the process of cloud formation and initial collapse. [ABSTRACT FROM AUTHOR]

The stellar Initial Mass Function (IMF) seems to be close to universal in the local star‐forming regions. However, this quantity of a newborn stellar population responds differently at gas metallicities Z ∼ Z⊙ than Z = 0. A view on the cosmic star formation history suggests that the cooling agents in the gas vary both in their types and molecular abundances. For instance, in the primordial gas environment, the gas temperature can be higher by a factor of 30 as compared to the present day. Stellar radiation feedback and cosmic microwave background (CMB) radiation may even contribute toward increasing the floor temperature of the star‐forming gas which subsequently can leave profound impacts on the IMF. We present the contribution of the radiation sources toward the thermodynamical evolution of the Jeans unstable gas cloud which experiences fragmentation and mass accretion. We find evidence which suggests that the latter becomes the dominant process for star formation efficiencies (SFE) above 5–7%, thus increasing the average mass of the stars. We focus on the isolated and binary stellar configurations emerging during the gas collapse. The binary fraction on average remains 0.476 and contributes significantly toward the total SFE of 15%. [ABSTRACT FROM AUTHOR]

We present here the first of a series of papers aimed at better understanding the evolution and properties of giant molecular clouds (GMCs) in a galactic context. We perform high-resolution, three-dimensional arepo simulations of an interacting galaxy inspired by the well-observed M51 galaxy. Our fiducial simulations include a non-equilibrium, time-dependent, chemical network that follows the evolution of atomic and molecular hydrogen as well as carbon and oxygen self-consistently. Our calculations also treat gas self-gravity and subsequent star formation (described by sink particles), and coupled supernova feedback. In the densest parts of the simulated interstellar medium (ISM), we reach sub-parsec resolution, granting us the ability to resolve individual GMCs and their formation and destruction self-consistently throughout the galaxy. In this initial work, we focus on the general properties of the ISM with a particular focus on the cold star-forming gas. We discuss the role of the interaction with the companion galaxy in generating cold molecular gas and controlling stellar birth. We find that while the interaction drives large-scale gas flows and induces spiral arms in the galaxy, it is of secondary importance in determining gas fractions in the different ISM phases and the overall star formation rate. The behaviour of the gas on small GMC scales instead is mostly controlled by the self-regulating property of the ISM driven by coupled feedback. [ABSTRACT FROM AUTHOR]

Supersonic turbulence is a key player in controlling the structure and star formation potential of molecular clouds (MCs). The three-dimensional (3D) turbulent Mach number, |$\operatorname{\mathcal {M}}$|⁠ , allows us to predict the rate of star formation. However, determining Mach numbers in observations is challenging because it requires accurate measurements of the velocity dispersion. Moreover, observations are limited to two-dimensional (2D) projections of the MCs and velocity information can usually only be obtained for the line-of-sight component. Here we present a new method that allows us to estimate |$\operatorname{\mathcal {M}}$| from the 2D column density, Σ, by analysing the fractal dimension, |$\mathcal {D}$|⁠. We do this by computing |$\mathcal {D}$| for six simulations, ranging between 1 and 100 in |$\operatorname{\mathcal {M}}$|⁠. From this data we are able to construct an empirical relation, |$\log \operatorname{\mathcal {M}}(\mathcal {D}) = \xi _1(\operatorname{erfc}^{-1} [(\mathcal {D}-\operatorname{\mathcal {D}_\text{min}})/\Omega ] + \xi _2),$| where |$\operatorname{erfc}^{-1}$| is the inverse complimentary error function, |$\operatorname{\mathcal {D}_\text{min}}= 1.55 \pm 0.13$| is the minimum fractal dimension of Σ, Ω = 0.22 ± 0.07, ξ1 = 0.9 ± 0.1, and ξ2 = 0.2 ± 0.2. We test the accuracy of this new relation on column density maps from Herschel observations of two quiescent subregions in the Polaris Flare MC, 'saxophone' and 'quiet'. We measure |$\operatorname{\mathcal {M}}\sim 10$| and |$\operatorname{\mathcal {M}}\sim 2$| for the subregions, respectively, which are similar to previous estimates based on measuring the velocity dispersion from molecular line data. These results show that this new empirical relation can provide useful estimates of the cloud kinematics, solely based upon the geometry from the column density of the cloud. [ABSTRACT FROM AUTHOR]

Observations of interstellar gas clouds are typically limited to two-dimensional (2D) projections of the intrinsically three-dimensional (3D) structure of the clouds. In this study, we present a novel method for relating the 2D projected fractal dimension (⁠|$\mathcal {D}_{\text{p}}$|⁠) to the 3D fractal dimension (⁠|$\mathcal {D}_{\text{3D}}$|⁠) of turbulent clouds. We do this by computing the fractal dimension of clouds over two orders of magnitude in turbulent Mach number |$(\mathcal {M} = 1\rm{-}100)$|⁠ , corresponding to seven orders of magnitude in spatial scales within the clouds. This provides us with the data to create a new empirical relation between |$\mathcal {D}_{\text{p}}$| and |$\mathcal {D}_{\text{3D}}$|⁠. The proposed relation is |$\mathcal {D}_{\text{3D}}(\mathcal {D}_{\text{p}}) = \Omega _1 \operatorname{erfc}(\xi _1 \operatorname{erfc}^{-1}[ (\mathcal {D}_{\text{p}} - \mathcal {D}_{\text{p,min}})/\Omega _2 ] + \xi _2) + \mathcal {D}_{\text{3D,min}}$|⁠ , where the minimum 3D fractal dimension, |$\mathcal {D}_{\text{3D,min}} = 2.06 \pm 0.35$|⁠ , the minimum projected fractal dimension, |$\mathcal {D}_{\text{p,min}} = 1.55 \pm 0.13$|⁠ , Ω1 = 0.47 ± 0.18, Ω2 = 0.22 ± 0.07, ξ1 = 0.80 ± 0.18, and ξ2 = 0.26 ± 0.19. The minimum 3D fractal dimension, |$\mathcal {D}_{\text{3D,min}} = 2.06 \pm 0.35$|⁠ , indicates that in the high |$\mathcal {M}$| limit the 3D clouds are dominated by planar shocks. The relation between |$\mathcal {D}_{\text{p}}$| and |$\mathcal {D}_{\text{3D}}$| of molecular clouds may be a useful tool for those who are seeking to understand the 3D structures of molecular clouds, purely based upon 2D projected data and shows promise for relating the physics of the turbulent clouds to the fractal dimension. [ABSTRACT FROM AUTHOR]

We develop a dynamical theory for the origin of nuclear rings in barred galaxies. In analogy with the standard theory of accretion discs, our theory is based on shear viscous forces among nested annuli of gas. However, the fact that gas follows non-circular orbits in an external barred potential has profound consequences: it creates a region of reverse shear in which it is energetically favourable to form a stable ring that does not spread despite dissipation. Our theory allows us to approximately predict the size of the ring given the underlying gravitational potential. The size of the ring is loosely related to the location of the Inner Lindblad Resonance in the epicyclic approximation, but the predicted location is more accurate and is also valid for strongly barred potentials. By comparing analytical predictions with the results of hydrodynamical simulations, we find that our theory provides a viable mechanism for ring formation if the effective sound speed of the gas is low (⁠|$c_{\rm s}\lesssim 1\, {\rm km\, s^{-1}}$|⁠), but that nuclear spirals/shocks created by pressure destroy the ring when the sound speed is high (⁠|$c_{\rm s}\simeq 10\, {\rm km\, s^{-1}}$|⁠). We conclude that whether this mechanism for ring formation is relevant for real galaxies ultimately depends on the effective equation of state of the interstellar medium (ISM). Promising confirmation comes from simulations in which the ISM is modelled using state-of-the-art cooling functions coupled to live chemical networks, but more tests are needed regarding the role of turbulence driven by stellar feedback. If the mechanism is relevant in real galaxies, it could provide a powerful tool to constrain the gravitational potential, in particular the bar pattern speed. [ABSTRACT FROM AUTHOR]

The escape of ultraviolet photons from the densest regions of the interstellar medium (ISM) - giant molecular clouds (GMCs) - is a poorly constrained parameter which is vital to understanding the ionization of the ISM and the intergalactic medium. We characterize the escape fraction, fesc,GMC, from a suite of individual GMC simulations with masses in the range 104-6 M☉ using the adaptive-mesh refinement code FLASH. We find significantly different fesc,GMC depending on the GMC mass that can reach >90 per cent in the evolution of 5 × 104 and 105 M☉ clouds or remain low at ∼5 per cent for most of the lifetime of more massive GMCs. All clouds show fluctuations over short, sub-Myr time-scales produced by flickering HII regions. We combine our results to calculate the total escape fraction (fesc,tot) from GMC populations in dwarf starburst and spiral galaxies by randomly drawing clouds from a GMC mass distribution (dN/dM α Mα, where α is either -1.5 or -2.5) over fixed time intervals.We find typical fesc,tot values of 8 per cent for both the dwarf and spiral models. The fluctuations of fesc,tot, however, are much larger for the dwarf models with values as high as 90 per cent. The photons escaping from the 5 × 104 and 105 M☉ GMCs are the dominant contributors to fesc,tot in all cases. We also show that the accompanying star formation rates (SFRs) of our model (∼2 × 10-2 and 0.73 M☉yr-1) are consistent with observations of SFRs in dwarf starburst and spiral galaxies, respectively. [ABSTRACT FROM AUTHOR]

Line emission is strongly dependent on the local environmental conditions in which the emitting tracers reside. In this work, we focus on modelling the CO emission from simulated giant molecular clouds (GMCs), and study the variations in the resulting line ratios arising from the emission from the J = 1-0, J = 2-1, and J = 3-2 transitions. We perform a set of smoothed particle hydrodynamics simulations with time-dependent chemistry, in which environmental conditions - including total cloud mass, density, size, velocity dispersion, metallicity, interstellar radiation field (ISRF), and the cosmic ray ionization rate (CRIR) - were systematically varied. The simulations were then post-processed using radiative transfer to produce synthetic emission maps in the three transitions quoted above. We find that the cloud-averaged values of the line ratios can vary by up to±0.3 dex, triggered by changes in the environmental conditions. Changes in the ISRF and/or in the CRIR have the largest impact on line ratios since they directly affect the abundance, temperature, and distribution of CO-rich gas within the clouds. We show that the standard methods used to convert CO emission to H2 column density can underestimate the total H2 molecular gas in GMCs by factors of 2 or 3, depending on the environmental conditions in the clouds. [ABSTRACT FROM AUTHOR]

We use hydrodynamical simulations to construct a new coherent picture for the gas flow in the Central Molecular Zone (CMZ), the region of our Galaxy within R ≲ 500 pc. We relate connected structures observed in (l, b, v) data cubes of molecular tracers to nuclear spiral arms. These arise naturally in hydrodynamical simulations of barred galaxies, and are similar to those that can be seen in external galaxies such as NGC 4303 or NGC 1097. We discuss a face-on view of the CMZ, including the positions of several prominent molecular clouds, such as Sgr B2, the 20 and 50 km s-1 clouds, the polar arc, Bania Clump 2 and Sgr C. Our model is also consistent with the larger scale gas flow, up to R ≃ 3 kpc, thus providing a consistent picture of the entire Galactic bar region. [ABSTRACT FROM AUTHOR]

Direct collapse black holes forming in pristine, atomically cooling haloes at z ≈ 10-20 may act as the seeds of supermassive black holes (BHs) at high redshifts. In order to create a massive BH seed, the host halo needs to be prevented from forming stars. H2 therefore needs to be irradiated by a large flux of Lyman-Werner (LW) UV photons in order to suppress H2 cooling. A key uncertainty in this scenario is the escape fraction of LW radiation from first galaxies, which is the dominant source of UV photons at this epoch. To better constrain this escape fraction, we have performed radiation-hydrodynamical simulations of the growth of H II regions and their associated photodissociation regions in the first galaxies using the ZEUS-MP code.We find that the LWescape fraction crucially depends on the propagation of the ionization front (I-front). For an R-type I-front overrunning the halo, the LW escape fraction is always larger than 95 per cent. If the halo recombines later from the outside-in, due to a softened and weakened spectrum, the LW escape fraction in the rest frame of the halo (the near-field) drops to zero. A detailed and careful analysis is required to analyse slowly moving, D-type I-fronts, where the escape fraction depends on the microphysics and can be as small as 3 per cent in the near-field and 61 per cent in the far-field or as large as 100 per cent in both the near-field and the far-field. [ABSTRACT FROM AUTHOR]

The carbon monoxide (CO) rotational transition lines are the most common tracers of molecular gas within giant molecular clouds (MCs).We study the ratio (R2-1/1-0) between CO's first two emission lines and examine what information it provides about the physical properties of the cloud. To study R2-1/1-0, we perform smooth particle hydrodynamic simulations with timedependent chemistry (using GADGET-2), along with post-process radiative transfer calculations on an adaptive grid (using RADMC-3D) to create synthetic emission maps of a MC. R2-1/1-0 has a bimodal distribution that is a consequence of the excitation properties of each line, given that J = 1 reaches local thermal equilibrium while J = 2 is still sub-thermally excited in the considered clouds. The bimodality of R2-1/1-0 serves as a tracer of the physical properties of different regions of the cloud, and it helps constrain local temperatures, densities and opacities. Additionally, this bimodal structure shows an important portion of the CO emission comes from diffuse regions of the cloud, suggesting that the commonly used conversion factor of R2-1/1-0 ~ 0.7 between both lines may need to be studied further. [ABSTRACT FROM AUTHOR]

The SILCC project (SImulating the Life-Cycle of molecular Clouds) aims at a more selfconsistent understanding of the interstellar medium (ISM) on small scales and its link to galaxy evolution. We present three-dimensional (magneto)hydrodynamic simulations of the ISM in a vertically stratified box including self-gravity, an external potential due to the stellar component of the galactic disc, and stellar feedback in the form of an interstellar radiation field and supernovae (SNe). The cooling of the gas is based on a chemical network that follows the abundances of H+, H, H2, C+, and CO and takes shielding into account consistently. We vary the SN feedback by comparing different SN rates, clustering and different positioning, in particular SNe in density peaks and at random positions, which has a major impact on the dynamics. Only for random SN positions the energy is injected in sufficiently low-density environments to reduce energy losses and enhance the effective kinetic coupling of the SNe with the gas. This leads to more realistic velocity dispersions (sHI 0.8s300-8000K ~ 10-20 km s-1, sHa 0.6s8000-3 105 K ~ 20-30 km s-1), and strong outflows with mass loading factors (ratio of outflow to star formation rate) of up to 10 even for solar neighbourhood conditions. Clustered SNe abet the onset of outflows compared to individual SNe but do not influence the net outflow rate. The outflows do not contain any molecular gas and are mainly composed of atomic hydrogen. The bulk of the outflowing mass is dense (~ 10-25-10-24 g cm-3) and slow (v ~ 20-40 km s-1) but there is a high-velocity tail of up to v ~ 500 km s-1 with ~ 10-28-10-27 g cm-3. [ABSTRACT FROM AUTHOR]

Population III (Pop III) stars can regulate star formation in the primordial Universe in several ways. They can ionize nearby haloes, and even if their ionizing photons are trapped by their own haloes, their Lyman-Werner (LW) photons can still escape and destroy H2 in other haloes, preventing them from cooling and forming stars. LWescape fractions are thus a key parameter in cosmological simulations of early reionization and star formation but have not yet been parametrized for realistic haloes by halo or stellar mass. To do so, we perform radiation hydrodynamical simulations of LW UV escape from 9-120MΘ Pop III stars in 105-107 MΘ haloes with ZEUS-MP.We find that photons in the LWlines (i.e. those responsible for destroying H2 in nearby systems) have escape fractions ranging from 0 to 85 per cent. No LW photons escape the most massive halo in our sample, even from the most massive star. Escape fractions for photons elsewhere in the 11.18-13.6 eV energy range, which can be redshifted into the LWlines at cosmological distances, are generally much higher, being above 60 per cent for all but the least massive stars in the most massive haloes. We find that shielding of H2 by neutral hydrogen, which has been neglected in most studies to date, produces escape fractions that are up to a factor of 3 smaller than those predicted by H2 self-shielding alone. [ABSTRACT FROM AUTHOR]

It has been shown that the behaviour of primordial gas collapsing in a dark matter minihalo can depend on the adopted choice of three-body H2 formation rate. The uncertainties in this rate span two orders of magnitude in the current literature, and so it remains a source of uncertainty in our knowledge of Population III star formation. Here, we investigate how the amount of fragmentation in primordial gas depends on the adopted three-body rate. We present the results of calculations that follow the chemical and thermal evolution of primordial gas as it collapses in two dark matter minihaloes. Our results on the effect of three-body rate on the evolution until the first protostar forms agree well with previous studies. However, our modified version of GADGET-2 smoothed particle hydrodynamics also includes sink particles, which allows us to follow the initial evolution of the accretion disc that builds up on the centre of each halo, and capture the fragmentation in gas as well as its dependence on the adopted three-body H2 formation rate. We find that the fragmentation behaviour of the gas is only marginally affected by the choice of three-body rate co-efficient, and that halo-to-halo differences are of equal importance in affecting the final mass distribution of stars. [ABSTRACT FROM AUTHOR]

The 13CO molecule is often used as a column density tracer in regions where the 12CO emission saturates. The 13CO column density is then related to that of 12CO by a uniform isotopic ratio. A similar approximation is frequently used when deriving 13CO emission maps from numerical simulations of molecular clouds. To test this assumption, we calculate the 12CO/13CO ratio self-consistently, taking the isotope-selective photodissociation and the chemical fractionation of CO into account. We model the coupled chemical, thermal and dynamical evolution and the emergent 13CO emission of isolated, starless molecular clouds in various environments. Selective photodissociation has a minimal effect on the ratio, while the chemical fractionation causes a factor of 2–3 decrease at intermediate cloud depths. The variation correlates with both the 12CO and the 13CO column densities. Neglecting the depth dependence results in ≤60 per cent error in 12CO column densities derived from 13CO. The same assumption causes ≤50 per cent disparity in the 13CO emission derived from simulated clouds. We show that the discrepancies can be corrected by a fitting formula. The formula is consistent with millimetre-wavelength isotopic ratio measurements of dense molecular clouds, but underestimates the ratios from the ultraviolet absorption of diffuse regions. [ABSTRACT FROM AUTHOR]

We use a suite of high-resolution molecular cloud simulations carried out with the moving mesh code arepo to explore the nature of star-forming filaments. The simulated filaments are identified and categorized from column density maps in the same manner as for recent Herschel observations. When fitted with a Plummer-like profile, the filaments are in excellent agreement with observations, and have shallow power-law profiles of p ∼ 2.2 without the need for magnetic support. When data within 1 pc of the filament centre are fitted with a Gaussian function, the average full width at half-maximum (FWHM) is ∼0.3 pc, in agreement with predictions for accreting filaments. However, if the fit is constructed using only the inner regions, as in Herschel observations, the resulting FWHM is only ∼0.2 pc. This value is larger than that measured in IC 5146 and Taurus, but is similar to that found in the Planck Galactic cold cores and in Cygnus X. The simulated filaments have a range of widths rather than a constant value. When the column density maps are compared to the 3D gas densities, the filaments seen in column density do not belong to a single structure. Instead, they are made up of a network of short ribbon-like sub-filaments reminiscent of those seen in Taurus. The sub-filaments are pre-existing within the simulated clouds, have radii similar to their Jeans radius, and are not primarily formed through fragmentation of the larger filament seen in column density. Instead, small filamentary clumps are swept together into a single column density structure by the large-scale collapse of the cloud. [ABSTRACT FROM AUTHOR]

The first stars to form in the universe are believed to have distribution of masses biased toward massive stars. This contrasts with the present-day initial mass function, which has a predominance of stars with masses lower than 1M☉. Therefore, the mode of star formation must have changed as the universe evolved. Such a transition is attributed to a more efficient cooling provided by increasing metallicity. Especially dust cooling can overcome the compressional heating, which lowers the gas temperature thus increasing its instability to fragmentation. The purpose of this paper is to verify if dust cooling can efficiently cool the gas, and enhance the fragmentation of gas clouds at the early stages of the universe. To confirm that, we calculate a set of hydrodynamic simulations that include sink particles, which represent contracting protostars. The thermal evolution of the gas during the collapse is followed by making use of a primordial chemical network and also a recipe for dust cooling. We model four clouds with different amounts of metals (10-4, 10-5, 10-6 Z☉, and 0), and analyze how this property affect the fragmentation of star-forming clouds. We find evidence for fragmentation in all four cases, and hence conclude that there is no critical metallicity below which fragmentation is impossible. Nevertheless, there is a clear change in the behavior of the clouds at Z ≲ 10-5 Z☉, caused by the fact that at this metallicity, fragmentation takes longer to occur than accretion, leading to a flat mass function at lower metallicities. [ABSTRACT FROM AUTHOR]

ABSTRACT We combine the output of hydrodynamical simulations of Population III star cluster formation with stellar evolution models, and calculate the evolution of protostars experiencing variable mass accretion rates due to interactions within a massive disc. We find that the primordial protostars are extended 'fluffy' objects for the bulk of their pre-main-sequence lifetimes. Accretion luminosity feedback from such objects is high, but, as shown in previous work, it has a minimal effect on the star cluster. The extended radii of the protostars, combined with the observation of close encounters in the simulations, suggest that mergers will occur in such systems. Furthermore, mass transfer between close protostellar binaries with extended radii could lead to massive tight binaries, which are a possible progenitor of gamma-ray bursts. [ABSTRACT FROM AUTHOR]

ABSTRACT We investigate the formation of the first stars at the end of the cosmic dark ages with a suite of three-dimensional, moving-mesh simulations that directly resolve the collapse of the gas beyond the formation of the first protostar at the centre of a dark matter minihalo. The simulations cover more than 25 orders of magnitude in density and have a maximum spatial resolution of 0.05 R⊙, which extends well below the radius of individual protostars and captures their interaction with the surrounding gas. In analogy to previous studies that employed sink particles, we find that the Keplerian disc around the primary protostar fragments into a number of secondary protostars, which is facilitated by H2 collisional dissociation cooling and collision-induced emission. The further evolution of the protostellar system is characterized by strong gravitational torques that transfer angular momentum between the secondary protostars formed in the disc and the surrounding gas. This leads to the migration of about half of the secondary protostars to the centre of the cloud in a free-fall time, where they merge with the primary protostar and enhance its growth to about five times the mass of the second most massive protostar. By the same token, a fraction of the protostars obtain angular momentum from other protostars via N-body interactions and migrate to higher orbits. On average, only every third protostar survives until the end of the simulation. However, the number of protostars present at any given time increases monotonically, suggesting that the system will continue to grow beyond the limited period of time simulated here. [ABSTRACT FROM AUTHOR]

ABSTRACT We present TreeCol, a new and efficient tree-based scheme to calculate column densities in numerical simulations. Knowing the column density in any direction at any location in space is a prerequisite for modelling the propagation of radiation through the computational domain. TreeCol therefore forms the basis for a fast, approximate method for modelling the attenuation of radiation within large numerical simulations. It constructs a healpix sphere at any desired location and accumulates the column density by walking the tree and by adding up the contributions from all tree nodes whose line of sight contributes to the pixel under consideration. In particular, when combined with widely-used tree-based gravity solvers, the new scheme requires little additional computational cost. In a simulation with N resolution elements, the computational cost of TreeCol scales as Nlog N, instead of the N5/3 scaling of most other radiative transfer schemes. TreeCol is naturally adaptable to arbitrary density distributions and is easy to implement and to parallelize, particularly if a tree structure is already in place for calculating the gravitational forces. We describe our new method and its implementation into the smoothed particle hydrodynamics (SPH) code gadget2 (although note that the scheme is not limited to particle-based fluid dynamics). We discuss its accuracy and performance characteristics for the examples of a spherical protostellar core and for the turbulent interstellar medium. We find that the column density estimates provided by TreeCol are on average accurate to better than 10 per cent. In another application, we compute the dust temperatures for solar neighbourhood conditions and compare with the result of a full-fledged Monte Carlo radiation-transfer calculation. We find that both methods give similar answers. We conclude that TreeCol provides a fast, easy to use and sufficiently accurate method of calculating column densities that comes with little additional computational cost when combined with an existing tree-based gravity solver. [ABSTRACT FROM AUTHOR]

Turbulence plays a major role in the formation and evolution of molecular clouds. Observationally, turbulent velocities are convolved with the density of an observed region. To correct for this convolution, we investigate the relation between the turbulence spectrum of model clouds, and the statistics of their synthetic observations obtained from principal component analysis (PCA). We apply PCA to spectral maps generated from simulated density and velocity fields, obtained from hydrodynamic simulations of supersonic turbulence, and from fractional Brownian motion (fBm) fields with varying velocity, density spectra, and density dispersion. We examine the dependence of the slope of the PCA pseudo-structure function, αPCA on intermittency, on the turbulence velocity (βv) and density (βn) spectral indexes, and on density dispersion. We find that PCA is insensitive to β n and to the log-density dispersion σ s, provided σs ≤ 2. For σs > 2, αPCA increases with σs due to the intermittent sampling of the velocity field by the density field. The PCA calibration also depends on intermittency. We derive a PCA calibration based on fBm structures with σs ≤ 2 and apply it to 367 13CO spectral maps of molecular clouds in the Galactic Ring Survey. The average slope of the PCA structure function, <αPCA> = 0.62 ± 0.2, is consistent with the hydrodynamic simulations and leads to a turbulence velocity exponent of (βv = 2.06 ± 0.6 for a non-intermittent, low density dispersion flow. Accounting for intermittency and density dispersion, the coincidence between the PCA slope of the GRS clouds and the hydrodynamic simulations suggests βv ≃ 1.9, consistent with both Burgers and compressible intermittent turbulence. [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]

The cosmic dark ages ended a few hundred million years after the big bang, when the first stars began to fill the universe with new light. It has generally been argued that these stars formed in isolation and were extremely massive--perhaps 100 times as massive as the Sun. In a recent study, Clark and collaborators showed that this picture requires revision. They demonstrated that the accretion disks that build up around Population III stars are strongly susceptible to fragmentation and that the first stars should therefore form in clusters rather than in isolation. We here use a series of high-resolution hydrodynamical simulations performed with the moving mesh code AREPO to follow up on this proposal and to study the influence of environmental parameters on the level of fragmentation. We model the collapse of five independent minihalos from cosmological initial conditions, through the runaway condensation of their central gas clouds, to the formation of the first protostar, and beyond for a further 1000 years. During this latter accretion phase, we represent the optically thick regions of protostars by sink particles. Gas accumulates rapidly in the circumstellar disk around the first protostar, fragmenting vigorously to produce a small group of protostars. After an initial burst, gravitational instability recurs periodically, forming additional protostars with masses ranging from ~0.1 to 10 M⊙. Although the shape, multiplicity, and normalization of the protostellar mass function depend on the details of the sink-particle algorithm, fragmentation into protostars with diverse masses occurs in all cases, confirming earlier reports of Population III stars forming in clusters. Depending on the efficiency of later accretion and merging, Population III stars may enter the main sequence in clusters and with much more diverse masses than are commonly assumed. [ABSTRACT FROM AUTHOR]

It has recently been demonstrated that coherent relative streaming velocities of order 30km s-1 between dark matter and gas permeated the universe on scales below a few Mpc directly after recombination. Here we use a series of high-resolution moving-mesh calculations to show that these supersonic motions significantly influence the virialization of the gas in minihalos and delay the formation of the first stars. As the gas streams into minihalos with bulk velocities around 1 km s-1 at z ~ 20, the additional momentum and energy input reduces the gas fractions and central densities of the halos, increasing the typical virial mass required for efficient cooling by a factor of three and delaying Population III star formation by Δz ≃ 4. Since the distribution of the magnitude of the streaming velocities is narrowly peaked around a non-negligible value, this effect is important in most regions of the universe. As a consequence, the increased minimum halo mass implies a reduction of the absolute number of minihalos that can be expected to cool and form Population III stars by up to an order of magnitude. We further find that the streaming velocities increase the turbulent velocity dispersion of the minihalo gas, which could affect its ability to fragment and hence alter the mass function of the first stars. [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]

Theoretical and observational investigations have indicated that the abundance of carbon monoxide (CO) is very sensitive to intrinsic properties of the gaseous medium, such as density, metallicity and the background radiation field. CO observations are often employed to study the properties of molecular clouds (MCs), such as mass, morphology and kinematics. It is thus important to understand how well CO traces the total mass, which in MCs is predominantly due to molecular hydrogen (H). Recent hydrodynamic simulations by Glover & Mac Low have explicitly followed the formation and destruction of molecules in model MCs under varying conditions. These models have confirmed that CO formation strongly depends on the cloud properties. Conversely, the formation of H is primarily determined by the amount of time available for its formation. We apply radiative transfer calculations to these MC models in order to investigate the properties of CO line emission. We focus on integrated CO (-0) intensities emerging from individual clouds, including its relationship to the total, H and CO column densities, as well as the ' factor,' the ratio of H column density to CO intensity. Models with high CO abundances have a threshold CO intensity of 65 K km s at sufficiently large extinctions (or column densities). Clouds with low CO abundances show no such intensity thresholds. The distributions of total and H column densities are well described as lognormal functions, though the distributions of CO intensities and column densities are usually not lognormal. In general, the probability distribution functions of the integrated intensity do not follow the distribution functions of CO column densities. In the model with Milky Way-like conditions, the factor is in agreement with the near-constant value determined from observations. In clouds with lower metallicity, lower density or a higher background UV radiation field, the CO abundances are in general lower, and hence the factor can vary appreciably - sometimes by up to 4 orders of magnitude. In models with high densities, the CO line is fully saturated, so that the factor is directly proportional to the molecular column density. [ABSTRACT FROM AUTHOR]

We present a new systematic way of setting up galactic gas discs based on the assumption of detailed hydrodynamic equilibrium. To do this, we need to specify the density distribution and the velocity field which supports the disc. We first show that the required circular velocity has no dependence on the height above or below the mid-plane so long as the gas pressure is a function of density only. The assumption of discs being very thin enables us to decouple the vertical structure from the radial direction. Based on that, the equation of hydrostatic equilibrium together with the reduced Poisson equation leads to two sets of second-order non-linear differential equations, which are easily integrated to set up a stable disc. We call one approach ‘density method’ and the other one ‘potential method’. Gas discs in detailed balance are especially suitable for investigating the onset of the gravitational instability. We revisit the question of global, axisymmetric instability using fully three-dimensional disc simulations. The impact of disc thickness on the disc instability and the formation of spontaneously induced spirals is studied systematically with or without the presence of the stellar potential. In our models, the numerical results show that the threshold value for disc instability is shifted from unity to 0.69 for self-gravitating thick discs and to 0.75 for combined stellar and gas thick discs. The simulations also show that self-induced spirals occur in the correct regions and with the right numbers as predicted by the analytic theory. [ABSTRACT FROM AUTHOR]

We use three-dimensional smoothed particle hydrodynamics simulations together with a dynamical ray-tracing scheme to investigate the build-up of the first H ii regions around massive Population III stars in minihaloes. We trace the highly anisotropic breakout of the ionizing radiation into the intergalactic medium, allowing us to predict the resulting recombination radiation with greatly increased realism. Our simulations, together with Press–Schechter type arguments, allow us to predict the Population III contribution to the radio background at via bremsstrahlung and 21-cm emission. We find a global bremsstrahlung signal of around , and a combined 21-cm signature which is an order of magnitude larger. Both might be within the reach of the planned Square Kilometer Array experiment, although detection of the free–free emission is only marginal. The imprint of the first stars on the cosmic radio background might provide us with one of the few diagnostics to test the otherwise elusive minihalo star formation site. [ABSTRACT FROM AUTHOR]

We introduce a simple and efficient algorithm for diffusion in smoothed particle hydrodynamics (SPH) simulations and apply it to the problem of chemical mixing. Based on the concept of turbulent diffusion, we link the diffusivity of a pollutant to the local physical conditions and can thus resolve mixing in space and time. We apply our prescription to the evolution of an idealized supernova remnant and find that we can model the distribution of heavy elements without having to explicitly resolve hydrodynamic instabilities in the post-shock gas. Instead, the dispersal of the pollutant is implicitly modelled through its dependence on the local velocity dispersion. Our method can thus be used in any SPH simulation that investigates chemical mixing but lacks the necessary resolution on small scales. Potential applications include the enrichment of the interstellar medium in present-day galaxies, as well as the intergalactic medium at high redshifts. [ABSTRACT FROM AUTHOR]

We present results of hydrodynamic simulations of star formation triggered by cloud-cloud collisions. During the early stages of star formation, low-mass objects form by gravitational instabilities in protostellar discs. A number of these low-mass objects are in the sub-stellar mass range, including a few objects of planetary mass. The disc instabilities that lead to the formation of low-mass objects in our simulations are the product of disc-disc interactions and/or interactions between the discs and their surrounding gas. [ABSTRACT FROM PUBLISHER]

Understanding the origin of stellar masses is a key problem in astrophysics. In the solar neighbourhood, the mass distribution of stars follows a seemingly universal pattern. In the centre of the Milky Way, however, there are indications for strong deviations and the same may be true for the nuclei of distant starburst galaxies. Here we present the first numerical hydrodynamical calculations of stars formed in a molecular region with chemical and thermodynamic properties similar to those of warm and dusty circumnuclear starburst regions. The resulting initial mass function is top-heavy with a peak at ∼15 M⊙, a sharp turn-down below ∼7 M⊙ and a power-law decline at high masses. We find a natural explanation for our results in terms of the temperature dependence of the Jeans mass, with collapse occurring at a temperature of ∼100 K and an H2 density of a few ×105 cm−3, and discuss possible implications for galaxy formation and evolution. [ABSTRACT FROM AUTHOR]

We present numerical experiments that demonstrate that the nonlinear development of the Toomre instability in disks of isothermal gas, stars, and dark matter reproduces the observed Schmidt Law for star formation. The rate of gas collapse depends exponentially on the (minimum) value of the Toomre parameter in the disk. We demonstrate that spurious fragmentation occurs in the absence of sufficient resolution in our SPH model. Our models also reproduce observed star formation thresholds in disk galaxies. We finally briefly discuss the application of our models to the study of globular cluster formation in merging galaxies. [ABSTRACT FROM PUBLISHER]

The formation of the first stars out of metal-free gas appears to result in stars at least an order of magnitude more massive than in the present-day case. We here consider what controls the transition from a primordial to a modern initial mass function. We study the influence of low levels of metal enrichment and different initial conditions on the cooling and collapse of initially ionized gas in small protogalactic halos using three-dimensional, smoothed particle hydrodynamics simulations. We argue that fragmentation at moderate density depends on the initial conditions for star formation more than on the metal abundances present. [ABSTRACT FROM PUBLISHER]

We present a new method for estimating the H2 cooling rate in the optically thick regime in simulations of primordial star formation. Our new approach is based on the TreeCol algorithm, which projects matter distributions onto a spherical grid to create maps of column densities for each fluid element in the computational domain. We have improved this algorithm by using the relative gas velocities to weight the individual matter contributions with the relative spectral line overlaps, in order to properly account for the Doppler effect. We compare our new method to the widely used Sobolev approximation, which yields an estimate for the column density based on the local velocity gradient and the thermal velocity. This approach generally underestimates the photon escape probability because it neglects the density gradient and the actual shape of the cloud. We present a correction factor for the true line overlap in the Sobolev approximation and a new method based on local quantities, which fits the exact results reasonably well during the collapse of the cloud, with the error in the cooling rates always being less than 10%. Analytical fitting formulae fail at determining the photon escape probability after formation of the first protostar (error of ∼40%) because they are based on the assumption of spherical symmetry and therefore break down once a protostellar accretion disk has formed. Our method yields lower temperatures and hence promotes fragmentation for densities above ∼1010 cm–3 at a distance of ∼200 AU from the first protostar. Since the overall accretion rates are hardly affected by the cooling implementation, we expect Pop III stars to have lower masses in our simulations, compared to the results of previous simulations that used the Sobolev approximation. [ABSTRACT FROM AUTHOR]

In cold dark matter cosmological models, the first stars to form are believed to do so within small protogalaxies. We study the influence of low levels of metal enrichment on the cooling and collapse of ionized gas in these protogalactic halos using three-dimensional, smoothed particle hydrodynamics simulations. [ABSTRACT FROM PUBLISHER]

The present-day Universe is highly magnetized, even though the first magnetic seed fields were most probably extremely weak. To explain the growth of the magnetic field strength over many orders of magnitude, fast amplification processes need to operate. The most efficient mechanism known today is the small-scale dynamo, which converts turbulent kinetic energy into magnetic energy leading to an exponential growth of the magnetic field. The efficiency of the dynamo depends on the type of turbulence indicated by the slope of the turbulence spectrum v(l)∝a lv, where v(l) is the eddy velocity at a scale I. We explore turbulent spectra ranging from incompressible Kolmogorov turbulence with v = 1/3 to highly compressible Burgers turbulence with v = 1/2. In this work, we analyze the properties of the small-scale dynamo for low magnetic Prandtl numbers Pm, which denotes the ratio of the magnetic Reynolds number, Rm, to the hydrodynamical one, Re. We solve the Kazantsev equation, which describes the evolution of the small-scale magnetic field, using the WKB approximation. In the limit of low magnetic Prandtl numbers, the growth rate is proportional to Rm(1-v)(1+v). We furthermore discuss the critical magnetic Reynolds number Rmcrit, which is required for small-scale dynamo action. The value of Rmcrit is roughly 100 for Kolmogorov turbulence and 2700 for Burgers. Furthermore, we discuss that Rmmcrit provides a stronger constraint in the limit of low Pm than it does for large Pm. We conclude that the small-scale dynamo can operate in the regime of low magnetic Prandtl numbers if the magnetic Reynolds number is large enough. Thus, the magnetic field amplification on small scales can take place in a broad range of physical environments and amplify week magnetic seed fields on short time scales. [ABSTRACT FROM AUTHOR]

We summarize the main results from MODEST-1, the first workshop on MOdeling DEnse STellar systems. Our goal is to go beyond traditional population synthesis models, by introducing dynamical interactions between single stars, binaries, and multiple systems. The challenge is to define and develop a software framework to enable us to combine in one simulation existing computer codes in stellar evolution, stellar dynamics, and stellar hydrodynamics. With this objective, the workshop brought together experts in these three fields, as well as other interested astrophysicists and computer scientists. We report here our main conclusions, questions and suggestions for further steps toward integrating stellar evolution and stellar (hydro)dynamics. [Copyright &y& Elsevier]