231 research outputs found
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
Prestellar Core Formation, Evolution, and Accretion from Gravitational Fragmentation in Turbulent Converging Flows
We investigate prestellar core formation and accretion based on
three-dimensional hydrodynamic simulations. Our simulations represent local
pc regions within giant molecular clouds where a supersonic turbulent
flow converges, triggering star formation in the post-shock layer. We include
turbulence and self-gravity, applying sink particle techniques, and explore a
range of inflow Mach number . Two sets of cores are identified
and compared: -cores are identified of a time snapshot in each simulation,
representing dense structures in a single cloud map; -cores
are identified at their individual time of collapse, representing the initial
mass reservoir for accretion. We find that cores and filaments form and evolve
at the same time. At the stage of core collapse, there is a well-defined,
converged characteristic mass for isothermal fragmentation that is comparable
to the critical Bonner-Ebert mass at the post-shock pressure. The core mass
functions (CMFs) of -cores show a deficit of high-mass cores
() compared to the observed stellar initial mass function
(IMF). However, the CMFs of -cores are similar to the observed CMFs and
include many low-mass cores that are gravitationally stable. The difference
between -cores and -cores suggests that the full sample
from observed CMFs may not evolve into protostars. Individual sink particles
accrete at a roughly constant rate throughout the simulations, gaining one
-core mass per free-fall time even after the initial mass
reservoir is accreted. High-mass sinks gain proportionally more mass at late
times than low-mass sinks. There are outbursts in accretion rates, resulting
from clumpy density structures falling into the sinks
Maximally Star-Forming Galactic Disks I. Starburst Regulation Via Feedback-Driven Turbulence
Star formation rates in the centers of disk galaxies often vastly exceed
those at larger radii. We investigate the idea that these central starbursts
are self-regulated, with the momentum flux injected to the ISM by star
formation balancing the gravitational force confining the gas. For most
starbursts, supernovae are the largest contributor to the momentum flux, and
turbulence provides the main pressure support for the predominantly-molecular
ISM. If the momentum feedback per stellar mass formed is p_*/m_* ~ 3000 km/s,
the predicted star formation rate is Sigma_SFR=2 pi G Sigma^2 m_*/p_*
~0.1(Sigma/100Msun/pc^2)^2 Msun/kpc^2/yr in regions where gas dominates the
vertical gravity. We compare this prediction with numerical simulations of
vertically-resolved disks that model star formation including feedback, finding
good agreement for gas surface densities Sigma ~ 10^2-10^3 Msun/pc^2. We also
compare to a compilation of star formation rates and gas contents from local
and high-redshift galaxies (both mergers and normal galaxies), finding good
agreement provided that X_CO decreases weakly as Sigma and Sigma_SFR increase.
Star formation rates in dense, turbulent gas are also expected to depend on the
gravitational free-fall time; if the efficiency per free-fall time is
epsilon_ff ~ 0.01, the turbulent velocity dispersion driven by feedback is
expected to be v_z = 0.4 epsilon_ff p_*/m_* ~ 10 km/s, relatively independent
of Sigma or Sigma_SFR. Turbulence-regulated starbursts (controlled by kinetic
momentum feedback) are part of the larger scheme of self-regulation;
primarily-atomic low-Sigma outer disks may have star formation regulated by UV
heating feedback, whereas regions at extremely high Sigma may be regulated by
feedback of radiation that is reprocessed into trapped IR.Comment: 35 pages, 5 figures; accepted by the Ap
A Two-moment Radiation Hydrodynamics Module in Athena Using a Time-explicit Godunov Method
We describe a module for the Athena code that solves the gray equations of
radiation hydrodynamics (RHD), based on the first two moments of the radiative
transfer equation. We use a combination of explicit Godunov methods to advance
the gas and radiation variables including the non-stiff source terms, and a
local implicit method to integrate the stiff source terms. We adopt the M1
closure relation and include all leading source terms. We employ the reduced
speed of light approximation (RSLA) with subcycling of the radiation variables
in order to reduce computational costs. Our code is dimensionally unsplit in
one, two, and three space dimensions and is parallelized using MPI. The
streaming and diffusion limits are well-described by the M1 closure model, and
our implementation shows excellent behavior for a problem with a concentrated
radiation source containing both regimes simultaneously. Our operator-split
method is ideally suited for problems with a slowly varying radiation field and
dynamical gas flows, in which the effect of the RSLA is minimal. We present an
analysis of the dispersion relation of RHD linear waves highlighting the
conditions of applicability for the RSLA. To demonstrate the accuracy of our
method, we utilize a suite of radiation and RHD tests covering a broad range of
regimes, including RHD waves, shocks, and equilibria, which show second-order
convergence in most cases. As an application, we investigate radiation-driven
ejection of a dusty, optically thick shell in the interstellar medium (ISM).
Finally, we compare the timing of our method with other well-known iterative
schemes for the RHD equations. Our code implementation, Hyperion, is suitable
for a wide variety of astrophysical applications and will be made freely
available on the Web.Comment: 30 pages, 29 figures, accepted for publication in ApJ
Feedback Regulated Turbulence, Magnetic Fields, and Star Formation Rates in Galactic Disks
We use three-dimensional magnetohydrodynamic (MHD) simulations to investigate
the quasi-equilibrium states of galactic disks regulated by star formation
feedback. We incorporate effects from massive-star feedback via time-varying
heating rates and supernova (SN) explosions. We find that the disks in our
simulations rapidly approach a quasi-steady state that satisfies vertical
dynamical equilibrium. The star formation rate (SFR) surface density
self-adjusts to provide the total momentum flux (pressure) in the vertical
direction that matches the weight of the gas. We quantify feedback efficiency
by measuring feedback yields, \eta_c\equiv P_c/\Sigma_SFR (in suitable units),
for each pressure component. The turbulent and thermal feedback yields are the
same for HD and MHD simulations, \eta_th~1 and \eta_ turb~4, consistent with
the theoretical expectations. In MHD simulations, turbulent magnetic fields are
rapidly generated by turbulence, and saturate at a level corresponding to
\eta_mag,t~1. The presence of magnetic fields enhances the total feedback yield
and therefore reduces the SFR, since the same vertical support can be supplied
at a smaller SFR. We suggest further numerical calibrations and observational
tests in terms of the feedback yields.Comment: To appear in Proceedings of IAU Symposium 315, From interstellar
clouds to star-forming galaxies: universal processes?, P. Jablonka, P. Andre,
and F.. van der Tak, ed
Vertical Equilibrium, Energetics, and Star Formation Rates in Magnetized Galactic Disks Regulated by Momentum Feedback from Supernovae
Recent hydrodynamic (HD) simulations have shown that galactic disks evolve to
reach well-defined statistical equilibrium states. The star formation rate
(SFR) self-regulates until energy injection by star formation feedback balances
dissipation and cooling in the interstellar medium (ISM), and provides vertical
pressure support to balance gravity. In this paper, we extend our previous
models to allow for a range of initial magnetic field strengths and
configurations, utilizing three-dimensional, magnetohydrodynamic (MHD)
simulations. We show that a quasi-steady equilibrium state is established as
rapidly for MHD as for HD models unless the initial magnetic field is very
strong or very weak, which requires more time to reach saturation. Remarkably,
models with initial magnetic energy varying by two orders of magnitude approach
the same asymptotic state. In the fully saturated state of the fiducial model,
the integrated energy proportions E_kin:E_th:E_mag,t:E_mag,o are
0.35:0.39:0.15:0.11, while the proportions of midplane support
P_turb:P_th:\Pi_mag,t:\Pi_mag,o are 0.49:0.18:0.18:0.15. Vertical profiles of
total effective pressure satisfy vertical dynamical equilibrium with the total
gas weight at all heights. We measure the "feedback yields"
\eta_c=P_c/\Sigma_SFR (in suitable units) for each pressure component, finding
that \eta_turb~4 and \eta_th~1 are the same for MHD as in previous HD
simulations, and \eta_mag,t~1. These yields can be used to predict the
equilibrium SFR for a local region in a galaxy based on its observed gas and
stellar surface densities and velocity dispersions. As the ISM weight (or
dynamical equilibrium pressure) is fixed, an increase in from turbulent
magnetic fields reduces the predicted \Sigma_SFR by ~25% relative to the HD
case.Comment: To appear in Ap
Can Nonlinear Hydromagnetic Waves Support a Self-Gravitating Cloud?
Using self-consistent magnetohydrodynamic (MHD) simulations, we explore the
hypothesis that nonlinear MHD waves dominate the internal dynamics of galactic
molecular clouds. We employ an isothermal equation of state and allow for
self-gravity. We adopt ``slab-symmetry,'' which permits motions
and fields perpendicular to the mean field, but permits gradients
only parallel to the mean field. The Alfv\'en speed exceeds the sound
speed by a factor . We simulate the free decay of a spectrum of
Alfv\'en waves, with and without self-gravity. We also perform simulations with
and without self-gravity that include small-scale stochastic forcing.
Our major results are as follows: (1) We confirm that fluctuating transverse
fields inhibit the mean-field collapse of clouds when the energy in Alfv\'en-
like disturbances remains comparable to the cloud's gravitational binding
energy. (2) We characterize the turbulent energy spectrum and density structure
in magnetically-dominated clouds. The spectra evolve to approximately
with ,
i.e. approximately consistent with a ``linewidth-size'' relation . The simulations show large density contrasts, with high
density regions confined in part by the fluctuating magnetic fields. (3) We
evaluate the input power required to offset dissipation through shocks, as a
function of , the velocity dispersion , and the scale
of the forcing. In equilibrium, the volume dissipation rate is
, for a cloud of
linear size and density . (4) Somewhat speculatively, we apply our
results to a ``typical'' molecular cloud. The mechanical power input requiredComment: Accepted for publication in Ap.J. 47 pages, 13 postscript figures.
Report also available at http://cfa-www.harvard.edu/~gammie/MHD.p
Numerical Simulations of Turbulent Molecular Clouds Regulated by Reprocessed Radiation Feedback from Nascent Super Star Clusters
Radiation feedback from young star clusters embedded in giant molecular
clouds (GMCs) is believed to be important to the control of star formation. For
the most massive and dense clouds, including those in which super star clusters
(SSCs) are born, pressure from reprocessed radiation exerted on dust grains may
disperse a significant portion of the cloud mass back into the interstellar
medium (ISM). Using our radiaton hydrodynamics (RHD) code, Hyperion, we conduct
a series of numerical simulations to test this idea. Our models follow the
evolution of self-gravitating, strongly turbulent clouds in which collapsing
regions are replaced by radiating sink particles representing stellar clusters.
We evaluate the dependence of the star formation efficiency (SFE) on the size
and mass of the cloud and , the opacity of the gas to infrared (IR)
radiation. We find that the single most important parameter determining the
evolutionary outcome is , with needed to disrupt clouds. For , the resulting SFE=50-70% is similar to empirical estimates for some
SSC-forming clouds. The opacities required for GMC disruption likely apply only
in dust-enriched environments. We find that the subgrid model approach of
boosting the direct radiation force by a "trapping factor" equal to a
cloud's mean IR optical depth can overestimate the true radiation force by
factors of . We conclude that feedback from reprocessed IR radiation
alone is unlikely to significantly reduce star formation within GMCs unless
their dust abundances or cluster light-to-mass ratios are enhanced.Comment: 19 pages, 18 figures, accepted for publication in Ap
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