2857756
Comment regarding the functional form of the Schmidt law
Abstract Star formation rates on the galactic scale are described phenomenologically by two distinct relationships, as emphasized recently by Elmegreen [Elmegreen, B.G., 2002. ApJ, 577, 206. astro-ph/0207114 .]. The first of these is the Schmidt law, which is a power-law relation between the star formation rate SFR and the column density Σ. The other relationship is that there is a cutoff in the gas density below which star formation shuts off. The purpose of this paper is to argue that: (1) these two relationships can be accommodated by a single functional form of the Schmidt law, (2) this functional form is motivated by the hypothesis that star formation is a critical phenomenon, and that as a corollary, (3) the existence of a sharp cutoff may thus be an emergent property of galaxies, as was argued by Seiden [Seiden, P.E.,1983. ApJ, 266, 555.], as opposed to the classical view that this cutoff is due to an instability criterion.
14148736
Viscoelastic Analogy for the Acceleration and Collimation of Astrophysical Jets
Jets are ubiquitous in astronomy. It has been conjectured that the existence of jets is intimately connected with the spin of the central object and the viscous angular momentum transport of the inner disk. Bipolar jet-like structures propelled by the viscous torque on a spinning central object are also known in a completely different context, namely the flow in the laboratory of a viscoelastic fluid. On the basis of an analogy of the tangled magnetic field lines of magnetohydrodynamic (MHD) turbulence to the tangled polymers of viscoelastic polymer solutions, we propose a viscoelastic description of the dynamics of highly turbulent conductive fluid. We argue that the same mechanism that forms jets in viscoelastic fluids in the laboratory may be responsible for collimating and powering astrophysical jets by the angular momentum of the central object.
7486677
The Allen Telescope Array: The First Widefield, Panchromatic, Snapshot Radio Camera for Radio Astronomy and SETI
The first 42 elements of the Allen Telescope Array (ATA-42) are beginning to deliver data at the Hat Creek Radio Observatory in northern California. Scientists and engineers are actively exploiting all of the flexibility designed into this innovative instrument for simultaneously conducting surveys of the astrophysical sky and conducting searches for distant technological civilizations. This paper summarizes the design elements of the ATA, the cost savings made possible by the use of commercial off-the-shelf components, and the cost/performance tradeoffs that eventually enabled this first snapshot radio camera. The fundamental scientific program of this new telescope is varied and exciting; some of the first astronomical results will be discussed.
54654783
Collective Effects in a Short-Pulse FEL Driver
We explore a design for a linac driver which utilises 1.3 GHz CW accelerating structures to deliver short electron bunches with repetition rates potentially up to 1 MHz. A 200 pC bunch is compressed to deliver significant charge into a time width less than 10 fs, delivering a peak current in excess of 10 kA with a reasonable energy spread. We discuss start-to-end modelling of this system, including the limitations from coherent radiation emission.
55719243
Study of some Design Concepts and Collective Effects in the MAX IV Linac
The MAX IV linac will be used both for injection and top up into two storage rings, and as a high brightness injector for a Short Pulse Facility (SPF) and an FEL (phase 2) [1]. Compression is done in two double achromats with positive R56. The natural second order momentum compaction, T566, from the achromats is used together with weak sextupoles to linearise longitudinal phase space, leaving no need for a harmonic cavity for linearisation of longitudinal phase space. In this proceeding we present results from particle tracking through the MAX IV linac in high brightness mode. We also investigate emittance dilution due to CSR, in the achromat compressors, and transverse wakefields in a high beta function lattice. From the light-source point of view, we present preliminary simulation of the expected spontaneous emission in the x-ray range of the SPF electron beam.
112783302
Design of the MAX IV Ring Injector and SPF/FEL Driver