Supercritical Collapse Research Articles

Stable three-dimensional optical solitons supported by competing quadratic and self-focusing cubic nonlinearities

We show that the quadratic (chi(2)) interaction of fundamental and second harmonics in a bulk dispersive medium, combined with self-focusing cubic (chi(3)) nonlinearity, give rise to stable three-dimensional spatiotemporal solitons (STSs), despite the possibility of the supercritical collapse, induced by the chi(3) nonlinearity. At exact phase matching (beta = 0) , the STSs are stable for energies from zero up to a certain maximum value, while for beta not equal 0 the solitons are stable in energy intervals between finite limits.

Critical collapse in (2+1)-dimensional AdS spacetime: quasi-CSS solutions and linear perturbations

We construct a one-parameter family of exact time-dependent solutions to 2+1 gravity with a negative cosmological constant and a massless minimally coupled scalar field as source. These solutions present a continuously self-similar (CSS) behavior near the central singularity, as observed in critical collapse, and an asymptotically AdS behavior at spatial infinity. We consider the linear perturbation analysis in this background, and discuss the crucial question of boundary conditions. These are tested in the special case where the scalar field decouples and the linear perturbations describe exactly the small-mass static BTZ black hole. In the case of genuine scalar perturbations, we find a growing mode with a behavior characteristic of supercritical collapse, the spacelike singularity and apparent horizon appearing simultaneously and evolving towards the AdS boundary. Our boundary conditions lead to the value of the critical exponent γ=0.4.

Singularity in supercritical collapse of a spherical scalar field

We study the singularity created in the supercritical collapse of a spherical massless scalar field. We first model the geometry and the scalar field to be homogeneous, and find a generic solution (in terms of a formal series expansion) describing a spacelike singularity which is monotonic, scalar polynomial and strong. We confront the predictions of this analytical model with the pointwise behavior of fully-nonlinear and inhomogeneous numerical simulations, and find full compliance. We also study the phenomenology of the spatial structure of the singularity numerically. At asymptotically late advanced time the singularity approaches the Schwarzschild singularity, in addition to discrete points at finite advanced times, where the singularity is Schwarzschild-like. At other points the singularity is different from Schwarzschild due to the nonlinear scalar field.

Collapse and Fragmentation of Molecular Cloud Cores. V. Loss of Magnetic Field Support

The fragmentation mechanism has been quite successful at providing an explanation for the formation of binary stars during the collapse phase of dense cloud cores. However, nearly all fragmentation calculations to date have ignored the effects of magnetic fields, whereas magnetic fields are generally regarded as the dominant force in molecular clouds. Here, we present the first three-dimensional, radiative hydrodynamical models of the collapse and fragmentation of dense molecular cloud cores, including the effects of magnetic fields and ambipolar diffusion. Starting from a prolate, Gaussian cloud that would collapse and fragment in the absence of magnetic fields (a thermally supercritical cloud), we introduce sufficient magnetic field support [through the magnetic field pressure, B2/8π, with B = B0(ρ/ρ0)κ] to ensure a magnetically subcritical (stable) cloud. The effects of ambipolar diffusion are then simulated by reducing the magnetic pressure scaling factor (B0) over a specified time interval (=tAD), which leads to a magnetically supercritical cloud and collapse. The estimated timescale for ambipolar diffusion in these dense clouds is about 10 free-fall times. The numerical models show that when tAD is as long as 10 or even 20 free-fall times, fragmentation into a binary can still occur. The main effect of the magnetic field support is to delay somewhat the formation of the binary protostar. Once the dynamic collapse phase begins, a rapidly rotating, (βi = Erot/| Egrav | = 0.12) cloud fragments into a binary protostar. While it remains to be seen if magnetic fields can stifle fragmentation in slowly rotating clouds, rapidly rotating, magnetically supported clouds appear to be quite capable of forming binary stars.

The Magnetic Fields in the Ophiuchus and Taurus Molecular Clouds

We discuss the significance of previously published OH Zeeman effect measurements of the magnetic field in the ρ Oph cloud (L1688) and also a new OH Zeeman effect measurement for the dark cloud core TMC-1C, the latter obtained with the Arecibo telescope. Results for both of these clouds, like previously published OH Zeeman results for other dark clouds, show that the line-of-sight field strength |B||| ≤ 10 μG. The ρ Oph cloud is significantly higher in column density than other clouds studied, and recent H I Zeeman effect observations have revealed B|| ≈ +10 μG in the H I self-absorption region in front of the cloud. The low values of B derived from the OH Zeeman effect may indicate that OH poorly samples the dense gas in the ρ Oph cloud, or else that the cloud is in a state of supercritical collapse. The TMC-1C cloud core may have formed via ambipolar diffusion in a manner similar to recent modeling of dark cloud Barnard 1.