Implementation of Sink Particles in the Athena Code

We describe implementation and tests of sink particle algorithms in the Eulerian grid-based code Athena. Introduction of sink particles enables long-term evolution of systems in which localized collapse occurs, and it is impractical (or unnecessary) to resolve the accretion shocks at the centers of collapsing regions. We discuss similarities and differences of our methods compared to other implementations of sink particles. Our criteria for sink creation are motivated by the properties of the Larson-Penston collapse solution. We use standard particle-mesh methods to compute particle and gas gravity together. Accretion of mass and momenta onto sinks is computed using fluxes returned by the Riemann solver. A series of tests based on previous analytic and numerical collapse solutions is used to validate our method and implementation. We demonstrate use of our code for applications with a simulation of planar converging supersonic turbulent flow, in which multiple cores form and collapse to create sinks; these sinks continue to interact and accrete from their surroundings over several Myr.Comment: 39 pages, 14 figures, Accepted to ApJ

The Nature of Starburst Activity in M82

We present new evolutionary synthesis models of M82 based mainly on observations consisting of near-infrared integral field spectroscopy and mid-infrared spectroscopy. The models incorporate stellar evolution, spectral synthesis, and photoionization modeling, and are optimized for 1-45 micron observations of starburst galaxies. The data allow us to model the starburst regions on scales as small as 25 pc. We investigate the initial mass function (IMF) of the stars and constrain quantitatively the spatial and temporal evolution of starburst activity in M82. We find a typical decay timescale for individual burst sites of a few million years. The data are consistent with the formation of very massive stars (> 50-100 Msun) and require a flattening of the starburst IMF below a few solar masses assuming a Salpeter slope at higher masses. Our results are well matched by a scenario in which the global starburst activity in M82 occurred in two successive episodes each lasting a few million years, peaking about 10 and 5 Myr ago. The first episode took place throughout the central regions of M82 and was particularly intense at the nucleus while the second episode occurred predominantly in a circumnuclear ring and along the stellar bar. We interpret this sequence as resulting from the gravitational interaction M82 and its neighbour M81, and subsequent bar-driven evolution. The short burst duration on all spatial scales indicates strong negative feedback effects of starburst activity, both locally and globally. Simple energetics considerations suggest the collective mechanical energy released by massive stars was able to rapidly inhibit star formation after the onset of each episode.Comment: 48 pages, incl. 16 Postscript figures; accepted for publication in the Astrophysical Journa

Liquid Limits: The Glass Transition and Liquid-Gas Spinodal Boundaries of Metastable Liquids

The liquid-gas spinodal and the glass transition define ultimate boundaries beyond which substances cannot exist as (stable or metastable) liquids. The relation between these limits is analyzed {\it via} computer simulations of a model liquid. The results obtained indicate that the liquid - gas spinodal and the glass transition lines intersect at a finite temperature, implying a glass - gas mechanical instability locus at low temperatures. The glass transition lines obtained by thermodynamic and dynamic criteria agree very well with each other.Comment: 5 pages, 4 figures, to appear in Phys. Rev. Let

The Adam-Gibbs relation for glass-forming liquids in 2, 3 and 4 dimensions

The Adam-Gibbs relation between relaxation times and the configurational entropy has been tested extensively for glass formers using experimental data and computer simulation results. Although the form of the relation contains no dependence on the spatial dimensionality in the original formulation, subsequent derivations of the Adam-Gibbs relation allow for such a possibility. We test the Adam-Gibbs relation in 2, 3, and 4 spatial dimensions using computer simulations of model glass formers. We find that the relation is valid in 3 and 4 dimensions. But in 2 dimensions, the relation does not hold, and interestingly, no single alternate relation describes the results for the different model systems we study.Comment: Submitted to Phys. Rev. Let

A New Method to Derive Low-Lying N-dimensional Quantum Wave Functions by Quadratures Along a Single Trajectory

We present a new method to derive low-lying N-dimensional quantum wave functions by quadrature along a single trajectory. The N-dimensional Schroedinger equation is cast into a series of readily integrable first order ordinary differential equations. Our approach resembles the familiar W.K.B. approximation in one dimension, but is designed to explore the classically forbidden region and has a much wider applicability than W.K.B.. The method also provides a perturbation series expansion and the Green's functions of the wave equation in N-dimension, all by quadratures along a single trajectory. A number of examples are given for illustration, including a simple algorithm to evaluate the Stark effect in closed form to any finite order of the electric field.Comment: 69 pages, Latex, no figure, no table; more ackowledgment added, typos correcte