Waves on the surface of the Orion molecular cloud

Massive stars influence their parental molecular cloud, and it has long been suspected that the development of hydrodynamical instabilities can compress or fragment the cloud. Identifying such instabilities has proved difficult. It has been suggested that elongated structures (such as the `pillars of creation') and other shapes arise because of instabilities, but alternative explanations are available. One key signature of an instability is a wave-like structure in the gas, which has hitherto not been seen. Here we report the presence of `waves' at the surface of the Orion molecular cloud near where massive stars are forming. The waves seem to be a Kelvin-Helmholtz instability that arises during the expansion of the nebula as gas heated and ionized by massive stars is blown over pre-existing molecular gas.Comment: Preprint of publication in Natur

On Landau damping

Going beyond the linearized study has been a longstanding problem in the theory of Landau damping. In this paper we establish exponential Landau damping in analytic regularity. The damping phenomenon is reinterpreted in terms of transfer of regularity between kinetic and spatial variables, rather than exchanges of energy; phase mixing is the driving mechanism. The analysis involves new families of analytic norms, measuring regularity by comparison with solutions of the free transport equation; new functional inequalities; a control of nonlinear echoes; sharp scattering estimates; and a Newton approximation scheme. Our results hold for any potential no more singular than Coulomb or Newton interaction; the limit cases are included with specific technical effort. As a side result, the stability of homogeneous equilibria of the nonlinear Vlasov equation is established under sharp assumptions. We point out the strong analogy with the KAM theory, and discuss physical implications.Comment: News: (1) the main result now covers Coulomb and Newton potentials, and (2) some classes of Gevrey data; (3) as a corollary this implies new results of stability of homogeneous nonmonotone equilibria for the gravitational Vlasov-Poisson equatio

Exploring astrophysics-relevant magnetohydrodynamics with pulsed-power laboratory facilities

Laboratory facilities employing high pulsed currents and voltages, and called generally “pulsedpower facilities,” allow experimenters to produce a variety of hydrodynamical structures replicating, often in a scalable fashion, a broad range of dynamical astrophysical phenomena. Among these are astrophysical jets and outflows, astrophysical blast waves, magnetized radiatively dominated flows, and, more recently, aspects of simulated accretion disks. The magnetic field thought to play a significant role in most of the aforementioned objects is naturally present and controllable in pulsedpower environments. The size of the objects produced in pulsed-power experiments ranges from a centimeter to tens of centimeters, thereby allowing the use of a variety of diagnostic techniques. In a number of situations astrophysical morphologies can be replicated down to the finest structures. The configurations and their parameters are highly reproducible; one can vary them to isolate the most important phenomena and thereby help in developing astrophysical models. This approach has emerged as a useful tool in the quest to better understand magnetohydrodynamical effects in astronomical environments. The present review summarizes the progress made during the last decade and is designed to help readers identify and, perhaps, implement new experiments in this growing research area. Techniques used for the generation and characterization of the flows are described

Taming the Heat Flux Problem: Advanced Divertors Towards Fusion Power

The next generation fusion machines are likely to face enormous heat exhaust problems. In addition to summarizing major issues and physical processes connected with these problems, we discuss how advanced divertors, obtained by modifying the local geometry, may yield workable solutions. We also point out that: (1) the initial interpretation of recent experiments show that the advantages, predicted, for instance, for the X-divertor (in particular, being able to run a detached operation at high pedestal pressure) correlate very well with observations, and (2) the X-D geometry could be implemented on ITER (and DEMOS) respecting all the relevant constraints. A roadmap for future research efforts is proposed

Laboratory Study on Disconnection Events in Comets

Abstract When comets interacting with solar wind, straight and narrow plasma tails will be often formed. The most remarkable phenomenon of the plasma tails is the disconnection event, in which a plasma tail is uprooted from the comet’s head and moves away from the comet. In this paper, the interaction process between a comet and solar wind is simulated by using a laser-driven plasma cloud to hit a cylinder obstacle. A disconnected plasma tail is observed behind the obstacle by optical shadowgraphy and interferometry. Our particle-in-cell simulations show that the difference in thermal velocity between ions and electrons induces an electrostatic field behind the obstacle. This field can lead to the convergence of ions to the central region, resulting in a disconnected plasma tail. This electrostatic-field-induced model may be a possible explanation for the disconnection events of cometary tails