We present the results of a series of numerical simulations of compressible, self‐gravitating hydrodynamic turbulence of cluster‐forming clumps in molecular clouds. We examine the role that turbulence has in the formation of gravitationally bound cores, studying the dynamical state, internal structure and bulk properties of these cores. Complex structure in turbulent clumps is formed provided that the damping time of the turbulence, tdamp, is longer than the gravitational free‐fall time tff in a region. We find a variety of density and infall velocity structures among the cores in the simulation, including cores that resemble the Larson–Penston collapse of an isothermal sphere (ρ∝r−2) and cores that resemble the McLaughlin–Pudritz collapse of logatropic spheres (ρ∝r−1). The specific angular momentum profiles range between j∝r1−r2. The masses of the bound cores that form fit the initial mass function, while the specific angular momentum distribution can be fit by a broken power law. While our hydrodynamic simulations reproduce many of the observed properties of cores, we find an upper limit for the star formation efficiency in clusters of 40–50 per cent.

In this paper, we have used the RIEMANN code for computational astrophysics to study the interaction of a realistic distribution of dust grains with gas at specific radial locations in a vertically stratified protostellar accretion disc. The disc was modelled to have the density and temperature of a minimum mass solar nebula, and shearing box simulations at radii of 0.3 and 10 au are reported here. The disc was driven to a fully developed turbulence via the magnetorotational instability (MRI). The simulations span three gas scaleheights about the disc's midplane. We find that the inclusion of standard dust-to-gas ratios does not have any significant effect on the MRI even when the dust sediments to the midplane of the accretion disc. The density distribution of the dust of all sizes reached a Gaussian profile within two scaleheights of the disc's midplane. The vertical scaleheights of these Gaussian profiles are shown to be proportional to the reciprocal of the square root of the dust radius when large spherical dust grains are considered. This result is consistent with theoretical expectation.The largest two families of dust in one of our simulations show a strong tendency to settle to the midplane of the accretion disc. The large dust tends to organize itself into elongated clumps of high density. The dynamics of these clumps is shown to be consistent with a streaming instability. The streaming instability is seen to be very vigorous and persistent once it forms. Each stream of high-density dust displays a reduced rms velocity dispersion. The velocity directions within the streams are also aligned relative to the mean shear, providing further evidence that we are witnessing a streaming instability. The densest clumpings of large dust are shown to form where the streams intersect.We have also shown that the mean free path and collision time for dust that participates in the streaming instability are reduced by almost two orders of magnitude relative to the average mean free paths and collision times. The rms velocities between the grains also need to fall below a minimum threshold in order for the grains to stick and we show that a small amount of the large dust in our 10 au simulation should have a propensity for grain coalescence. The results of our simulations are likely to be useful for those who model spectral energy distributions of protostellar discs and also for those who model dust coagulation and growth.

We develop models for the self-similar collapse of magnetized isothermal cylinders. We find solutions for the case of a fluid with a constant toroidal flux-to-mass ratio (Gamma_phi=constant) and the case of a fluid with a constant gas to magnetic pressure ratio (beta=constant). In both cases, we find that a low magnetization results in density profiles that behave as rho ~ r^{-4} at large radii, and at high magnetization we find density profiles that behave as rho ~ r^{-2}. This density behaviour is the same as for hydrostatic filamentary structures, suggesting that density measurements alone cannot distinguish between hydrostatic and collapsing filaments--velocity measurements are required. Our solutions show that the self-similar radial velocity behaves as v_r ~ r during the collapse phase, and that unlike collapsing self-similar spheres, there is no subsequent accretion (i.e. expansion-wave) phase. We also examine the fragmentation properties of these cylinders, and find that in both cases, the presence of a toroidal field acts to strengthen the cylinder against fragmentation. Finally, the collapse time scales in our models are shorter than the fragmentation time scales. Thus, we anticipate that highly collapsed filaments can form before they are broken into pieces by gravitational fragmentation.Comment: 20 pages, 4 figures, accepted to Ap

We present a series of decaying turbulence simulations that represent a cluster‐forming clump within a molecular cloud, investigating the role of magnetic fields on the formation of potential star‐forming cores. We present an exhaustive analysis of numerical data from these simulations that include a compilation of all of the distributions of physical properties that characterize bound cores – including their masses, radii, mean densities, angular momenta, spins, magnetizations and mass‐to‐flux ratios. We also present line maps of our models that can be compared with observations. Our simulations range between 5 and 30 Jeans masses of gas, and are representative of molecular cloud clumps with masses between 100 and 1000 M⊙. The field strengths in the bound cores that form tend to have the same ratio of gas pressure to magnetic pressure, β, as the mean β of the simulation. The cores have mass‐to‐flux ratios that are generally less than that of the original cloud, and so a cloud that is initially highly supercritical can produce cores that are slightly supercritical, similar to that seen by Zeeman measurements of molecular cloud cores. Clouds that are initially only slightly supercritical will instead collapse along the field lines into sheets, and the cores that form as these sheets fragment have a different distribution of masses than what is observed. The spin rates of these cores (wherein 20–40 per cent of cores have Ωtff≥ 0.2) suggests that subsequent fragmentation into multiple systems is likely. The sizes of the bound cores that are produced are typically 0.02–0.2 pc and have densities in the range 104–105 cm−3 in agreement with observational surveys. Finally, our numerical data allow us to test theoretical models of the mass spectrum of cores, such as the turbulent fragmentation picture of Padoan & Nordlund. We find that while this model gets the shape of the core mass spectrum reasonably well, it fails to predict the peak mass in the core mass spectrum.

We present high-resolution infrared spectra of four YSOs (T Tau N, T Tau S, RNO 91, and HL Tau). The spectra exhibit narrow absorption lines of 12CO, 13CO, and C18O as well as broad emission lines of gas phase12CO. The narrow absorption lines of CO are shown to originate from the colder circumstellar gas. We find that the line of sight gas column densities resulting from the CO absorption lines are much higher than expected for the measured extinction for each source and suggest the gas to dust ratio is measuring the dust settling and/or grain coagulation in these extended disks. We provide a model of turbulence, dust settling and grain growth to explain the results. The techniques presented here allow us to provide some observationally-motivated bounds on accretion disk alpha in protostellar systems