Massive stars produce so much light that the radiation pressure they exert on the gas and dust around them is stronger than their gravitational attraction, a condition that has long been expected to prevent them from growing by accretion. We present three-dimensional radiation-hydrodynamic simulations of the collapse of a massive prestellar core and find that radiation pressure does not halt accretion. Instead, gravitational and Rayleigh-Taylor instabilities channel gas onto the star system through nonaxisymmetric disks and filaments that self-shield against radiation while allowing radiation to escape through optically thin bubbles. Gravitational instabilities cause the disk to fragment and form a massive companion to the primary star. Radiation pressure does not limit stellar masses, but the instabilities that allow accretion to continue lead to small multiple systems.

We report the results of a series of AMR radiation-hydrodynamic simulations of the collapse of massive star forming clouds using the ORION code. These simulations are the first to include the feedback effects protostellar outflows, as well as protostellar radiative heating and radiation pressure exerted on the infalling, dusty gas. We find that outflows evacuate polar cavities of reduced optical depth through the ambient core. These enhance the radiative flux in the poleward direction so that it is 1.7 to 15 times larger than that in the midplane. As a result the radiative heating and outward radiation force exerted on the protostellar disk and infalling cloud gas in the equatorial direction are greatly diminished. This simultaneously reduces the Eddington radiation pressure barrier to high-mass star formation and increases the minimum threshold surface density for radiative heating to suppress fragmentation compared to models that do not include outflows. The strength of both these effects depends on the initial core surface density. Lower surface density cores have longer free-fall times and thus massive stars formed within them undergo more Kelvin contraction as the core collapses, leading to more powerful outflows. Furthermore, in lower surface density clouds the ratio of the time required for the outflow to break out of the core to the core free-fall time is smaller, so that these clouds are consequently influenced by outflows at earlier stages of collapse. As a result, outflow effects are strongest in low surface density cores and weakest in high surface density one. We also find that radiation focusing in the direction of outflow cavities is sufficient to prevent the formation of radiation pressure-supported circumstellar gas bubbles, in contrast to models which neglect protostellar outflow feedback.

We present a set of 3-dimensional, radiation-magnetohydrodynamic calculations of the gravitational collapse of massive (300 M ), star-forming molecular cloud cores. We show that the combined effects of magnetic fields and radiative feedback strongly suppress core fragmentation, leading to the production of single star systems rather than small clusters. We find that the two processes are efficient at suppressing fragmentation in different regimes, with the feedback most effective in the dense, central region and the magnetic field most effective in more diffuse, outer regions. Thus, the combination of the two is much more effective at suppressing fragmentation than either one considered in isolation. Our work suggests that typical massive cores, which have mass-to-flux ratios of about 2 relative to critical, likely form a single star system, but that cores with weaker fields may form a small star cluster. This result helps us understand why the observed relationship between the core mass function and the stellar initial mass function holds even for ∼ 100M cores with many thermal Jeans masses of material. We also demonstrate that a ∼ 40 AU Keplerian disk is able to form in our simulations, despite the braking effect caused by the strong magnetic field.

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The turbulent environment from which stars form may lead to misalignment between the stellar spin and the remnant protoplanetary disk. By using hydrodynamic and magnetohydrodynamic simulations, we demonstrate that a wide range of stellar obliquities may be produced as a by-product of forming a star within a turbulent environment. We present a simple semi-analytic model that reveals this connection between the turbulent motions and the orientation of a star and its disk. Our results are consistent with the observed obliquity distribution of hot Jupiters. Migration of misaligned hot Jupiters may, therefore, be due to tidal dissipation in the disk, rather than tidal dissipation of the star-planet interaction.

We present a large suite of simulations of the formation of low-mass star clusters. Our simulations include an extensive set of physical processes -magnetohydrodynamics, radiative transfer, and protostellar outflows -and span a wide range of virial parameters and magnetic field strengths. Comparing the outcomes of our simulations to observations, we find that simulations remaining close to virial balance throughout their history produce star formation efficiencies and initial mass function (IMF) peaks that are stable in time and in reasonable agreement with observations. Our results indicate that small-scale dissipation effects near the protostellar surface provide a feedback loop for stabilizing the star formation efficiency. This is true regardless of whether the balance is maintained by input of energy from large scale forcing or by strong magnetic fields that inhibit collapse. In contrast, simulations that leave virial balance and undergo runaway collapse form stars too efficiently and produce an IMF that becomes increasingly top-heavy with time. In all cases we find that the competition between magnetic flux advection toward the protostar and outward advection due to magnetic interchange instabilities, and the competition between turbulent amplification and reconnection close to newly-formed protostars renders the local magnetic field structure insensitive to the strength of the large-scale field, ensuring that radiation is always more important than magnetic support in setting the fragmentation scale and thus the IMF peak mass. The statistics of multiple stellar systems are similarly insensitive to variations in the initial conditions and generally agree with observations within the range of statistical uncertainty.

In this paper we explore the relationship between protostellar outflows and turbulence in molecular clouds. Using 3-D numerical simulations we focus on the hydrodynamics of multiple outflows interacting within a parsec scale volume. We explore the extent to which transient outflows injecting directed energy and momentum into a sub-volume of a molecular cloud can be converted into random turbulent motions. We show that turbulence can readily be sustained by these interactions and show that it is possible to broadly characterize an effective driving scale of the outflows. We compare the velocity spectrum obtained in our studies to that of isotropically forced hydrodynamic turbulence finding that in outflow driven turbulence a power law of the form E(k) ∝ k −β is indeed achieved. However we find a steeper spectrum β ∼ 3 is obtained in outflow driven turbulence models than in isotropically forced simulations β ∼ 2.0. We discuss possible physical mechanisms responsible for these results as well and their implications for turbulence in molecular clouds where outflows will act in concert with other processes such as gravitational collapse.