3,100 research outputs found
The Astrochemical Evolution of Turbulent Giant Molecular Clouds : I - Physical Processes and Method of Solution for Hydrodynamic, Embedded Starless Clouds
Contemporary galactic star formation occurs predominantly within
gravitationally unstable, cold, dense molecular gas within supersonic,
turbulent, magnetized giant molecular clouds (GMCs). Significantly, because the
chemical evolution timescale and the turbulent eddy-turnover timescale are
comparable at typical GMC conditions, molecules evolve via inherently
non-equilibrium chemistry which is strongly coupled to the dynamical evolution
of the cloud.
Current numerical simulation techniques, which include at most three decades
in length scale, can just begin to bridge the divide between the global
dynamical time of supersonic turbulent GMCs, and the thermal and chemical
evolution within the thin post-shock cooling layers of their background
turbulence. We address this GMC astrochemical scales problem using a solution
methodology, which permits both complex three-dimensional turbulent dynamics as
well as accurate treatment of non-equilibrium post-shock thermodynamics and
chemistry.
We present the current methodology in the context of the larger scope of
physical processes important in understanding the chemical evolution of GMCs,
including gas-phase chemistry, dust grains and surface chemistry, and turbulent
heating. We present results of a new Lagrangian verification test for
supersonic turbulence. We characterize the evolution of these species according
to the dimensionless local post-shock Damk\"{o}hler number, which quantifies
the ratio of the dynamical time in the post-shock cooling flow to the chemical
reaction time of a given species.
Lastly, we discuss implications of this work to the selection of GMC
molecular tracers, and the zeroing of chemical clocks of GMC cores.Comment: 35 pages, 7 figures, 16 tables. Accepted to MNRAS. Revised to correct
some typographic error
Intermittency and Universality in Fully-Developed Inviscid and Weakly-Compressible Turbulent Flows
We performed high resolution numerical simulations of homogenous and
isotropic compressible turbulence, with an average 3D Mach number close to 0.3.
We study the statistical properties of intermittency for velocity, density and
entropy. For the velocity field, which is the primary quantity that can be
compared to the isotropic incompressible case, we find no statistical
differences in its behavior in the inertial range due either to the slight
compressibility or to the different dissipative mechanism. For the density
field, we find evidence of ``front-like'' structures, although no shocks are
produced by the simulation.Comment: Submitted to Phys. Rev. Let
Chromosome Numbers and Observations in the Genus Silphium
Author Institution: Department of Botany and Plant Pathology, The Ohio State University, Columbus 1
Sub-Alfvenic Non-Ideal MHD Turbulence Simulations with Ambipolar Diffusion: I. Turbulence Statistics
Most numerical investigations on the role of magnetic fields in turbulent
molecular clouds (MCs) are based on ideal magneto-hydrodynamics (MHD). However,
MCs are weakly ionized, so that the time scale required for the magnetic field
to diffuse through the neutral component of the plasma by ambipolar diffusion
(AD) can be comparable to the dynamical time scale. We have performed a series
of 256^3 and 512^3 simulations on supersonic but sub-Alfvenic turbulent systems
with AD using the Heavy-Ion Approximation developed in Li, McKee, & Klein
(2006). Our calculations are based on the assumption that the number of ions is
conserved, but we show that these results approximately apply to the case of
time-dependent ionization in molecular clouds as well. Convergence studies
allow us to determine the optimal value of the ionization mass fraction when
using the heavy-ion approximation for low Mach number, sub-Alfvenic turbulent
systems. We find that ambipolar diffusion steepens the velocity and magnetic
power spectra compared to the ideal MHD case. Changes in the density PDF, total
magnetic energy, and ionization fraction are determined as a function of the AD
Reynolds number. The power spectra for the neutral gas properties of a strongly
magnetized medium with a low AD Reynolds number are similar to those for a
weakly magnetized medium; in particular, the power spectrum of the neutral
velocity is close to that for Burgers turbulence.Comment: 37 pages, 11 figures, 4 table
The Post-Merger Magnetized Evolution of White Dwarf Binaries: The Double-Degenerate Channel of Sub-Chandrasekhar Type Ia Supernovae and the Formation of Magnetized White Dwarfs
Type Ia supernovae (SNe Ia) play a crucial role as standardizable
cosmological candles, though the nature of their progenitors is a subject of
active investigation. Recent observational and theoretical work has pointed to
merging white dwarf binaries, referred to as the double-degenerate channel, as
the possible progenitor systems for some SNe Ia. Additionally, recent
theoretical work suggests that mergers which fail to detonate may produce
magnetized, rapidly-rotating white dwarfs. In this paper, we present the first
multidimensional simulations of the post-merger evolution of white dwarf
binaries to include the effect of the magnetic field. In these systems, the two
white dwarfs complete a final merger on a dynamical timescale, and are tidally
disrupted, producing a rapidly-rotating white dwarf merger surrounded by a hot
corona and a thick, differentially-rotating disk. The disk is strongly
susceptible to the magnetorotational instability (MRI), and we demonstrate that
this leads to the rapid growth of an initially dynamically weak magnetic field
in the disk, the spin-down of the white dwarf merger, and to the subsequent
central ignition of the white dwarf merger. Additionally, these magnetized
models exhibit new features not present in prior hydrodynamic studies of white
dwarf mergers, including the development of MRI turbulence in the hot disk,
magnetized outflows carrying a significant fraction of the disk mass, and the
magnetization of the white dwarf merger to field strengths
G. We discuss the impact of our findings on the origins, circumstellar media,
and observed properties of SNe Ia and magnetized white dwarfs.Comment: Accepted ApJ version published on 8/20/13, with significant
additional text added discussing the nature of the magnetized outflows, and
possible CSM observational features relevant to NaID detection
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