Rayleigh–Taylor and Richtmyer-Meshkov instability induced flow, turbulence, and mixing. II

Mon. Not. R. Astron. Soc. 000, 1–15 (2015) Printed 31 July 2015 (MN L A TEX style file v2.2) The First Population II Stars Formed in Externally Enriched Mini-halos Britton D. Smith 1? , John H. Wise 2 , Brian W. O’Shea 3 †, Michael L. Norman 4 , and Sadegh Khochfar 1 Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK for Relativistic Astrophysics, Georgia Institute of Technology, 837 State Street Atlanta, GA 30332, USA 3 Department of Physics & Astronomy, Michigan State University, East Lansing, MI 48824, USA 4 Center for Astrophysics and Space Sciences, University of California at San Diego, La Jolla, CA 92093, USA arXiv:1504.07639v2 [astro-ph.GA] 30 Jul 2015 2 Center 31 July 2015 ABSTRACT We present a simulation of the formation of the earliest Population II stars, starting from cosmological initial conditions and ending when metals created in the first supernovae are in- corporated into a collapsing gas-cloud. This occurs after a supernova blast-wave collides with a nearby mini-halo, inducing further turbulence that efficiently mixes metals into the dense gas in the center of the halo. The gas that first collapses has been enriched to a metallicity of Z ∼ 2 × 10 −5 Z . Due to the extremely low metallicity, collapse proceeds similarly to metal- free gas until dust cooling becomes efficient at high densities, causing the cloud to fragment into a large number of low mass objects. This external enrichment mechanism provides a plau- sible origin for the most metal-poor stars observed, such as SMSS J031300.36-670839.3, that appear to have formed out of gas enriched by a single supernova. This mechanism operates on shorter timescales than the time for low-mass mini-halos (M 6 5 × 10 5 M ) to recover their gas after experiencing a supernova. As such, metal-enriched stars will likely form first via this channel if the conditions are right for it to occur. We identify a number of other externally enriched halos that may form stars in this manner. These halos have metallicities as high as 0.01 Z , suggesting that some members of the first generation of metal-enriched stars may be hiding in plain sight in current stellar surveys. Key words: cosmology, hydrodynamics, radiative transfer, methods: numerical, galaxies: star formation INTRODUCTION The physical processes relevant to star formation have conspired to produce stars of predominantly low mass for most of the history of the Universe. The stellar initial mass function (IMF) appears to be so robust to variations in environment that debate over its univer- sality primarily concerns whether the statement “the IMF is uni- versal” should include the word almost. Excellent reviews of this subject have been provided by Kroupa (2002) and more recently, Bastian et al. (2010). The one notable exception to the universal- ity of the IMF is the case of metal-free (Population III or Pop III) stars. The mid 2000s was a boom for quality reviews of this subject (Barkana & Loeb 2001; Bromm & Larson 2004; Ciardi & Ferrara 2005; Glover 2005; Ripamonti & Abel 2006; Bromm et al. 2009). In short, neutral metal-free gas cools very inefficiently as it col- e-mail:brs@roe.ac.uk † Department of Computational Mathematics, Science, and Engineering; Lyman Briggs College; and the Joint Institute for Nuclear Astrophysics, Michigan State University, East Lansing, MI 48824, USA c 2015 RAS lapses, resulting in Jeans mass-scale fragments that are of the order of thousands of M . Early simulations found that this resulted in the formation of only one (Abel et al. 2002; Bromm et al. 2002; Yoshida et al. 2006; O’Shea & Norman 2007) or two (Turk et al. 2009; Stacy et al. 2010) dense cores that were free to accrete from this massive envelope and grow uncontested. More recent simu- lations have found that the disks surrounding the massive, central core can be unstable to fragmentation, potentially providing a chan- nel for lower mass Pop III stars (Clark et al. 2011; Greif et al. 2011; Stacy & Bromm 2014). However, simulations that follow the late- time accretion onto the central object find that they will grow to be 10s to 1000s of M in final size (Hosokawa et al. 2011; Hirano et al. 2014, 2015; Susa et al. 2014), making it quite clear that the Pop III star formation mode is, at a bare minimum, unquestionably distinct. Even on the low mass end of the Pop III IMF, Hartwig et al. (2015) claim that the existing sample size of low-metallicity star surveys in the Milky Way can already rule out the existence of metal-free stars below 0.65 M with 95% certainty, barring enrich- ment via accretion of metals from the interstellar medium (Johnson

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The first Population II stars formed in externally enriched mini-haloes

In diverse areas of science and technology, including inertial confinement fusion (ICF), astrophysics, geophysics, and engineering processes, turbulent mixing induced by hydrodynamic instabilities is of scientific interest as well as practical significance. Because of the fundamental roles they often play in ICF and other applications, three classes of hydrodynamic instability-induced turbulent flows—those arising from the Rayleigh-Taylor, Richtmyer-Meshkov, and Kelvin-Helmholtz instabilities—have attracted much attention. ICF implosions, supernova explosions, and other applications illustrate that these phases of instability growth do not occur in isolation, but instead are connected so that growth in one phase feeds through to initiate growth in a later phase. Essentially, a description of these flows must encompass both the temporal and spatial evolution of the flows from their inception. Hydrodynamic instability will usually start from potentially infinitesimal spatial perturbations, will eventually transition to a turbulent flow, and then will reach a final state of a true multiscale problem. Indeed, this change in the spatial scales can be vast, with hydrodynamic instability evolving from just a few microns to thousands of kilometers in geophysical or astrophysical problems. These instabilities will evolve through different stages before transitioning to turbulence, experiencing linear, weakly, and highly nonlinear states. The challenges confronted by researchers are enormous. The inherent difficulties include characterizing the initial conditions of such flows and accurately predicting the transitional flows. Of course, fully developed turbulence, a focus of many studies because of its major impact on the mixing process, is a notoriously difficult problem in its own right. In this pedagogical review, we will survey challenges and progress, and also discuss outstanding issues and future directions.

Turbulent mixing and transition criteria of flows induced by hydrodynamic instabilities

Arbitrary Lagrangian-Eulerian methods for modeling high-speed compressible multimaterial flows

A direct Arbitrary-Lagrangian-Eulerian ADER-WENO finite volume scheme on unstructured tetrahedral meshes for conservative and non-conservative hyperbolic systems in 3D

Cross-code comparisons of mixing during the implosion of dense cylindrical and spherical shells

A comparative study of multimaterial Lagrangian and Eulerian methods with pressure relaxation

We study planar shock wave structure in a two-temperature model of a fully ionized plasma that includes electron heat conduction and energy exchange between electrons and ions. For steady flow in a reference frame moving with the shock, the model reduces to an autonomous system of ordinary differential equations which can be numerically integrated. A phase space analysis of the differential equations provides an additional insight into the structure of the solutions. For example, below a threshold Mach number, the model produces continuous solutions, while above another threshold Mach number, the solutions contain embedded hydrodynamic shocks. Between the threshold values, the appearance of embedded shocks depends on the electron diffusivity and the electron–ion coupling term. We also find that the ion temperature may achieve a maximum value between the upstream and downstream states and away from the embedded shock. We summarize the methodology for solving for two-temperature shocks and show results for several values of shock strength and plasma parameters in order to quantify the shock structure and explore the range of possible solutions. Such solutions may be used to verify hydrodynamic codes that use similar plasma physics models.

/pdf/shock-wave-structure-for-a-fully-ionized-plasma-y5mgas1xqn.pdf

Shock wave structure for a fully ionized plasma

Transition and turbulence decay with the Taylor–Green vortex have been effectively used to demonstrate emulation of high Reynolds-number ( R e ) physical dissipation through numerical convective effects of various non-oscillatory finite-volume algorithms for implicit large eddy simulation (ILES), e.g. using the Godunov-based Eulerian adaptive mesh refinement code xRAGE. The inverse-chevron shock tube experiment simulations have been also used to assess xRAGE based ILES for shock driven turbulent mixing, compared with available simulation and laboratory data. The previous assessments are extended to evaluate new directionally-unsplit high-order algorithms in xRAGE, including a correction to address the well-known issue of excessive numerical diffusion of shock-capturing (e.g., Godunov-type) schemes for low Mach numbers. The unsplit options for hydrodynamics in xRAGE are discussed in detail, followed by fundamental tests with representative shock problems. Basic issues of transition to turbulence and turbulent mixing are discussed, and results of simulations of high- R e turbulent flow and mixing in canonical test cases are reported. Compared to the directional-split cases, and for each grid resolution considered, unsplit results exhibit transition to turbulence with much higher effective R e —and significantly more so with the low Mach number correction.

Effects of operator splitting and low Mach-number correction in turbulent mixing transition simulations

http://absimage.aps.org/image/DPP09/MWS_DPP09-2009-001348.pdf

Thomas Masser

Papers

Cross-code comparisons of mixing during the implosion of dense cylindrical and spherical shells

A comparative study of multimaterial Lagrangian and Eulerian methods with pressure relaxation

Shock wave structure for a fully ionized plasma

Effects of operator splitting and low Mach-number correction in turbulent mixing transition simulations

Shock Wave Structure in a Fully Ionized Plasma