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
We introduce a new suite of simulations, ‘The Cloud Factory’, which self-consistently forms molecular cloud complexes at high enough resolution to resolve internal substructure (up to 0.25 ...M⊙ in mass) all while including galactic-scale forces. We use a version of the arepo code modified to include a detailed treatment of the physics of the cold molecular ISM, and an analytical galactic gravitational potential for computational efficiency. The simulations have nested levels of resolution, with the lowest layer tied to tracer particles injected into individual cloud complexes. These tracer refinement regions are embedded in the larger simulation so continue to experience forces from outside the cloud. This allows the simulations to act as a laboratory for testing the effect of galactic environment on star formation. Here we introduce our method and investigate the effect of galactic environment on filamentary clouds. We find that cloud complexes formed after a clustered burst of feedback have shorter lengths and are less likely to fragment compared to quiescent clouds (e.g. the Musca filament) or those dominated by the galactic potential (e.g. Nessie). Spiral arms and differential rotation preferentially align filaments, but strong feedback randomizes them. Long filaments formed within the cloud complexes are necessarily coherent with low internal velocity gradients, which has implications for the formation of filamentary star-clusters. Cloud complexes formed in regions dominated by supernova feedback have fewer star-forming cores, and these are more widely distributed. These differences show galactic-scale forces can have a significant impact on star formation within molecular clouds.
We present results from high-resolution three-dimensional simulations of turbulent interstellar gas that self-consistently follow its coupled thermal, chemical and dynamical evolution, with a ...particular focus on the formation and destruction of H2 and CO. We quantify the formation time-scales for H2 and CO in physical conditions corresponding to those found in nearby giant molecular clouds, and show that both species form rapidly, with chemical time-scales that are comparable to the dynamical time-scale of the gas. We also investigate the spatial distributions of H2 and CO, and how they relate to the underlying gas distribution. We show that H2 is a good tracer of the gas distribution, but that the relationship between CO abundance and gas density is more complex. The CO abundance is not well-correlated with either the gas number density n or the visual extinction AV: both have a large influence on the CO abundance, but the inhomogeneous nature of the density field produced by the turbulence means that n and AV are only poorly correlated. There is a large scatter in AV, and hence CO abundance, for gas with any particular density, and similarly a large scatter in density and CO abundance for gas with any particular visual extinction. This will have important consequences for the interpretation of the CO emission observed from real molecular clouds. Finally, we also examine the temperature structure of the simulated gas. We show that the molecular gas is not isothermal. Most of it has a temperature in the range of 10–20 K, but there is also a significant fraction of warmer gas, located in low-extinction regions where photoelectric heating remains effective.
Abstract
We study the influence of a high baryonic streaming velocity on the formation of direct collapse black holes (DCBHs) with the help of cosmological simulations carried out using the moving ...mesh code arepo. We show that a streaming velocity that is as large as three times the root-mean-squared value is effective at suppressing the formation of H2-cooled minihaloes, while still allowing larger atomic cooling haloes (ACHs) to form. We find that enough H2 forms in the centre of these ACHs to effectively cool the gas, demonstrating that a high streaming velocity by itself cannot produce the conditions required for DCBH formation. However, we argue that high streaming velocity regions do provide an ideal environment for the formation of DCBHs in close pairs of ACHs (the ‘synchronized halo’ model). Due to the absence of star formation in minihaloes, the gas remains chemically pristine until the ACHs form. If two such haloes form with only a small separation in time and space, then the one forming stars earlier can provide enough ultraviolet radiation to suppress H2 cooling in the other, allowing it to collapse to form a DCBH. Baryonic streaming may therefore play a crucial role in the formation of the seeds of the highest redshift quasars.
The IMF as a function of supersonic turbulence Bertelli Motta, C; Clark, P. C; Glover, S. C. O ...
Monthly notices of the Royal Astronomical Society,
11/2016, Letnik:
462, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Recent studies seem to suggest that the stellar initial mass function (IMF) in early-type galaxies might be different from a classical Kroupa or Chabrier IMF, i.e. contain a larger fraction of the ...total mass in low-mass stars. From a theoretical point of view, supersonic turbulence has been the subject of interest in many analytical theories proposing a strong correlation with the characteristic mass of the core mass function (CMF) in star-forming regions, and as a consequence with the stellar IMF. Performing two suites of smoothed particle hydrodynamics (SPH) simulations with different mass resolutions, we aim at testing the effects of variations in the turbulent properties of a dense, star-forming molecular cloud on the shape of the system mass function in different density regimes. While analytical theories predict a shift of the peak of the CMF towards lower masses with increasing velocity dispersion of the cloud, we observe in the low-density regime the opposite trend, with high Mach numbers giving rise to a top-heavy mass distribution. For the high-density regime we do not find any trend correlating the Mach number with the characteristic mass of the resulting IMF, implying that the dynamics of protostellar accretion discs and fragmentation on small scales is not strongly affected by turbulence driven at the scale of the cloud. Furthermore, we suggest that a significant fraction of dense cores are disrupted by turbulence before stars can be formed in their interior through gravitational collapse. Although this particular study has limitations in its numerical resolution, we suggest that our results, along with those from other studies, cast doubt on the turbulent fragmentation models on the IMF that simply map the CMF to the IMF.
ABSTRACT
The first stars in the Universe, the so-called Population III stars, form in small dark matter minihaloes with virial temperatures Tvir < 104 K. Cooling in these minihaloes is dominated by ...molecular hydrogen (H2), and so Population III star formation is only possible in those minihaloes that form enough H2 to cool on a short timescale. As H2 cooling is more effective in more massive minihaloes, there is therefore a critical halo mass scale Mmin above which Population III star formation first becomes possible. Two important processes can alter this minimum mass scale: streaming of baryons relative to the dark matter and the photodissociation of H2 by a high redshift Lyman–Werner (LW) background. In this paper, we present results from a set of high resolution cosmological simulations that examine the impact of these processes on Mmin and on Mave (the average minihalo mass for star formation), both individually and in combination. We show that streaming has a bigger impact on Mmin than the LW background, but also that both effects are additive. We also provide a fitting functions quantifying the dependence of Mave and Mmin on the streaming velocity and the strength of the LW background.
Abstract
Hydrodynamical simulations of star formation often do not possess the dynamic range needed to fully resolve the build-up of individual stars and star clusters, and thus have to resort to ...sub-grid models. A popular way to do this is by introducing Lagrangian sink particles, which replace contracting high-density regions at the point where the resolution limit is reached. A common problem then is how to assign fundamental stellar properties to sink particles, such as the distribution of stellar masses. We present a new and simple statistical method to assign stellar contents to sink particles. Once the stellar content is specified, it can be used to determine a sink particle's radiative output, supernovae rate or other feedback parameters that may be required in the calculations. Advantages of our method are: (i) it is simple to implement; (ii) it guarantees that the obtained stellar populations are good samples of the initial mass function; (iii) it can easily deal with infalling mass accreted at later times; and (iv) it does not put restrictions on the sink particles’ masses in order to be used. The method works very well for sink particles that represent large star clusters and for which the stellar mass function is well sampled, but can also handle the transition to sink particles that represent a small number of stars.
Abstract
Using the PHANGS–ALMA CO(2–1) survey, we characterize molecular gas properties on ∼100 pc scales across 102,778 independent sightlines in 70 nearby galaxies. This yields the best synthetic ...view of molecular gas properties on cloud scales across the local star-forming galaxy population obtained to date. Consistent with previous studies, we observe a wide range of molecular gas surface densities (3.4 dex), velocity dispersions (1.7 dex), and turbulent pressures (6.5 dex) across the galaxies in our sample. Under simplifying assumptions about subresolution gas structure, the inferred virial parameters suggest that the kinetic energy of the molecular gas typically exceeds its self-gravitational binding energy at ∼100 pc scales by a modest factor (1.3 on average). We find that the cloud-scale surface density, velocity dispersion, and turbulent pressure (1) increase toward the inner parts of galaxies, (2) are exceptionally high in the centers of barred galaxies (where the gas also appears less gravitationally bound), and (3) are moderately higher in spiral arms than in inter-arm regions. The galaxy-wide averages of these gas properties also correlate with the integrated stellar mass, star formation rate, and offset from the star-forming main sequence of the host galaxies. These correlations persist even when we exclude regions with extraordinary gas properties in galaxy centers, which contribute significantly to the inter-galaxy variations. Our results provide key empirical constraints on the physical link between molecular cloud populations and their galactic environment.
The first generation of stars, often called Population III (or Pop III), form from metal-free primordial gas at redshifts
z
∼ 30 and below. They dominate the cosmic star-formation history until
z
∼ ...15-20, at which point the formation of metal-enriched Population II stars takes over. We review current theoretical models for the formation, properties, and impact of Pop III stars and discuss existing and future observational constraints. Key takeaways from this review include the following:
Primordial gas is highly susceptible to fragmentation and Pop III stars form as members of small clusters with a logarithmically flat mass function.
Feedback from massive Pop III stars plays a central role in regulating subsequent star formation, but major uncertainties remain regarding its immediate impact.
In extreme conditions, supermassive Pop III stars can form, reaching masses of several 10
5
M
. Their remnants may be the seeds of the supermassive black holes observed in high-redshift quasars.
Direct observations of Pop III stars in the early Universe remain extremely challenging. Indirect constraints from the global 21-cm signal or gravitational waves are more promising.
Stellar archeological surveys allow us to constrain both the low-mass and the high-mass ends of the Pop III mass distribution. Observations suggest that most massive Pop III stars end their lives as core-collapse supernovae rather than as pair-instability supernovae.
We introduce a radiative transfer code module for the magnetohydrodynamical adaptive mesh refinement code flash 4. It is coupled to an efficient chemical network which explicitly tracks the three ...hydrogen species H, H2, H+ as well as C+ and CO. The module is geared towards modelling all relevant thermal feedback processes of massive stars, and is able to follow the non-equilibrium time-dependent thermal and chemical state of the present-day interstellar medium as well as that of dense molecular clouds. We describe in detail the implementation of all relevant thermal stellar feedback mechanisms, i.e. photoelectric, photoionization and H2 dissociation heating as well as pumping of molecular hydrogen by UV photons. All included radiative feedback processes are extensively tested. We also compare our module to dedicated photodissociation region (PDR) codes and find good agreement in our modelled hydrogen species once our radiative transfer solution reaches equilibrium. In addition, we show that the implemented radiative feedback physics is insensitive to the spatial resolution of the code and show under which conditions it is possible to obtain well-converged evolution in time. Finally, we briefly explore the robustness of our scheme for treating combined ionizing and non-ionizing radiation.
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