Andrew J. Cunningham

The formation of massive star systems by accretion.

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.

Radiation Feedback, Fragmentation, and the Environmental Dependence of the Initial Mass Function

The fragmentation of star-forming interstellar clouds, and the resulting stellar initial mass function (IMF), is strongly affected by the temperature structure of the collapsing gas. Since radiation feedback from embedded stars can modify this as collapse proceeds, feedback plays an important role in determining the IMF. However, the effects and importance of radiative heating are likely to depend strongly on the surface density of the collapsing clouds, which determines both their effectiveness at trapping radiation and the accretion luminosities of the stars forming within them. In this paper, we report a suite of adaptive mesh refinement radiation-hydrodynamic simulations using the ORION code in which we isolate the effect of column density on fragmentation by following the collapse of clouds of varying column density while holding the mass, initial density and velocity structure, and initial virial ratio fixed. We find that radiation does not significantly modify the overall star formation rate or efficiency, but that it suppresses fragmentation more and more as cloud surface densities increase from those typical of low-mass star-forming regions like Taurus, through the typical surface density of massive star-forming clouds in the Galaxy, up to conditions found only in super-star clusters. In regions of low surface density, fragmentation during collapse leads to the formation of small clusters rather than individual massive star systems, greatly reducing the fraction of the stellar population with masses 10 M ☉. Our simulations have important implications for the formation of massive stars and the universality of the IMF.

Radiation-Hydrodynamic Simulations of Massive Star Formation with Protostellar Outflows

We report the results of a series of adaptive mesh refinement 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 1.7-15 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 the collapse. As a result, outflow effects are strongest in low surface density cores and weakest in high surface density ones. 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.

The turbulent origin of spin–orbit misalignment in planetary systems

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.

RADIATIVELY EFFICIENT MAGNETIZED BONDI ACCRETION

We have carried out a numerical study of the effect of large-scale magnetic fields on the rate of accretion from a uniform, isothermal gas onto a resistive, stationary point mass. Only mass, not magnetic flux, accretes onto the point mass. The simulations for this study avoid complications arising from boundary conditions by keeping the boundaries far from the accreting object. Our simulations leverage adaptive refinement methodology to attain high spatial fidelity close to the accreting object. Our results are particularly relevant to the problem of star formation from a magnetized molecular cloud in which thermal energy is radiated away on timescales much shorter than the dynamical timescale. Contrary to the adiabatic case, our simulations show convergence toward a finite accretion rate in the limit in which the radius of the accreting object vanishes, regardless of magnetic field strength. For very weak magnetic fields, the accretion rate first approaches the Bondi value and then drops by a factor of ~2 as magnetic flux builds up near the point mass. For strong magnetic fields, the steady-state accretion rate is reduced by a factor of ~0.2 β1/2 compared to the Bondi value, where β is the ratio of the gas pressure to the magnetic pressure. We give a simple expression for the accretion rate as a function of the magnetic field strength. Approximate analytic results are given in the Appendices for both time-dependent accretion in the limit of weak magnetic fields and steady-state accretion for the case of strong magnetic fields.

The effects of magnetic fields and protostellar feedback on low-mass cluster formation

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.