81 research outputs found
Test particle acceleration in a numerical MHD experiment of an anemone jet
To use a 3D numerical MHD experiment representing magnetic flux emerging into
an open field region as a background field for tracing charged particles. The
interaction between the two flux systems generates a localised current sheet
where MHD reconnection takes place. We investigate how efficiently the
reconnection region accelerates charged particles and what kind of energy
distribution they acquire. The particle tracing is done numerically using the
Guiding Center Approximation on individual data sets from the numerical MHD
experiment. We derive particle and implied photon distribution functions having
power law forms, and look at the impact patterns of particles hitting the
photosphere. We find that particles reach energies far in excess of those seen
in observations of solar flares. However the structure of the impact region in
the photosphere gives a good representation of the topological structure of the
magnetic field.Comment: 9 pages, 7 figures, accepted for publication in A&
A simple model for molecular hydrogen chemistry coupled to radiation hydrodynamics
We introduce non-equilibrium molecular hydrogen chemistry into the radiation
hydrodynamics code Ramses-RT. This is an adaptive mesh refinement grid code
with radiation hydrodynamics that couples the thermal chemistry of hydrogen and
helium to moment-based radiative transfer with the Eddington tensor closure
model. The H2 physics that we include are formation on dust grains, gas phase
formation, formation by three-body collisions, collisional destruction,
photodissociation, photoionization, cosmic ray ionization, and self-shielding.
In particular, we implement the first model for H2 self-shielding that is tied
locally to moment-based radiative transfer by enhancing photodestruction. This
self-shielding from Lyman-Werner line overlap is critical to H2 formation and
gas cooling. We can now track the non-equilibrium evolution of molecular,
atomic, and ionized hydrogen species with their corresponding dissociating and
ionizing photon groups. Over a series of tests we show that our model works
well compared to specialized photodissociation region codes. We successfully
reproduce the transition depth between molecular and atomic hydrogen, molecular
cooling of the gas, and a realistic Stromgren sphere embedded in a molecular
medium. In this paper we focus on test cases to demonstrate the validity of our
model on small scales. Our ultimate goal is to implement this in large-scale
galactic simulations.Comment: 21 pages, 12 figures, printed in MNRA
Feedback in Clouds II: UV Photoionisation and the first supernova in a massive cloud
Molecular cloud structure is regulated by stellar feedback in various forms.
Two of the most important feedback processes are UV photoionisation and
supernovae from massive stars. However, the precise response of the cloud to
these processes, and the interaction between them, remains an open question. In
particular, we wish to know under which conditions the cloud can be dispersed
by feedback, which in turn can give us hints as to how feedback regulates the
star formation inside the cloud. We perform a suite of radiative
magnetohydrodynamic simulations of a 10^5 solar mass cloud with embedded
sources of ionising radiation and supernovae, including multiple supernovae and
a hypernova model. A UV source corresponding to 10% of the mass of the cloud is
required to disperse the cloud, suggesting that the star formation efficiency
should be on the order of 10%. A single supernova is unable to significantly
affect the evolution of the cloud. However, energetic hypernovae and multiple
supernovae are able to add significant quantities of momentum to the cloud,
approximately 10^{43} g cm/s of momentum per 10^{51} ergs of supernova energy.
This is on the lower range of estimates in other works, since dense gas clumps
that remain embedded inside the HII region cause rapid cooling in the supernova
blast. We argue that supernovae alone are unable to regulate star formation in
molecular clouds, and that strong pre-supernova feedback is required to allow
supernova blastwaves to propagate efficiently into the interstellar mediumComment: 15 pages, 10 figures, submitted to MNRA
Galaxies that Shine: radiation-hydrodynamical simulations of disk galaxies
Radiation feedback is typically implemented using subgrid recipes in
hydrodynamical simulations of galaxies. Very little work has so far been
performed using radiation-hydrodynamics (RHD), and there is no consensus on the
importance of radiation feedback in galaxy evolution. We present RHD
simulations of isolated galaxy disks of different masses with a resolution of
18 pc. Besides accounting for supernova feedback, our simulations are the first
galaxy-scale simulations to include RHD treatments of photo-ionisation heating
and radiation pressure, from both direct optical/UV radiation and
multi-scattered, re-processed infrared (IR) radiation. Photo-heating smooths
and thickens the disks and suppresses star formation about as much as the
inclusion of ("thermal dump") supernova feedback does. These effects decrease
with galaxy mass and are mainly due to the prevention of the formation of dense
clouds, as opposed to their destruction. Radiation pressure, whether from
direct or IR radiation, has little effect, but for the IR radiation we show
that its impact is limited by our inability to resolve the high optical depths
for which multi-scattering becomes important. While artificially boosting the
IR optical depths does reduce the star formation, it does so by smoothing the
gas rather than by generating stronger outflows. We conclude that although
higher-resolution simulations, and potentially also different supernova
implementations, are needed for confirmation, our findings suggest that
radiation feedback is more gentle and less effective than is often assumed in
subgrid prescriptions.Comment: 28 pages, 26 figures, accepted for publication in MNRAS. Revised to
match published versio
Snap, Crackle, Pop: sub-grid supernova feedback in AMR simulations of disk galaxies
We compare 5 sub-grid models for supernova (SN) feedback in adaptive mesh
refinement (AMR) simulations of isolated dwarf and L-star disk galaxies with
20-40 pc resolution. The models are thermal dump, stochastic thermal,
'mechanical' (injecting energy or momentum depending on the resolution),
kinetic, and delayed cooling feedback. We focus on the ability of each model to
suppress star formation and generate outflows. Our highest-resolution runs
marginally resolve the adiabatic phase of the feedback events, which correspond
to 40 SN explosions, and the first three models yield nearly identical results,
possibly indicating that kinetic and delayed cooling feedback converge to wrong
results. At lower resolution all models differ, with thermal dump feedback
becoming inefficient. Thermal dump, stochastic, and mechanical feedback
generate multiphase outflows with mass loading factors , which is
much lower than observed. For the case of stochastic feedback we compare to
published SPH simulations, and find much lower outflow rates. Kinetic feedback
yields fast, hot outflows with , but only if the wind is in effect
hydrodynamically decoupled from the disk by using a large bubble radius.
Delayed cooling generates cold, dense and slow winds with , but large
amounts of gas occupy regions of temperature-density space with short cooling
times. We conclude that either our resolution is too low to warrant physically
motivated models for SN feedback, that feedback mechanisms other than SNe are
important, or that other aspects of galaxy evolution, such as star formation,
require better treatment.Comment: 22 pages, 15 figures. Accepted for publication in MNRAS with minor
revision
Galaxies that shine: radiation-hydrodynamical simulations of disc galaxies
Radiation feedback is typically implemented using subgrid recipes in hydrodynamical simulations of galaxies. Very little work has so far been performed using radiation-hydrodynamics (RHD), and there is no consensus on the importance of radiation feedback in galaxy evolution. We present RHD simulations of isolated galaxy discs of different masses with a resolution of 18pc. Besides accounting for supernova feedback, our simulations are the first galaxy-scale simulations to include RHD treatments of photoionization heating and radiation pressure, from both direct optical/UV radiation and multiscattered, re-processed infrared (IR) radiation. Photoheating smooths and thickens the discs and suppresses star formation about as much as the inclusion of (‘thermal dump') supernova feedback does. These effects decrease with galaxy mass and are mainly due to the prevention of the formation of dense clouds, as opposed to their destruction. Radiation pressure, whether from direct or IR radiation, has little effect, but for the IR radiation we show that its impact is limited by our inability to resolve the high optical depths for which multiscattering becomes important. While artificially boosting the IR optical depths does reduce the star formation, it does so by smoothing the gas rather than by generating stronger outflows. We conclude that although higher resolution simulations, and potentially also different supernova implementations, are needed for confirmation, our findings suggest that radiation feedback is more gentle and less effective than is often assumed in subgrid prescription
On the Indeterministic Nature of Star Formation on the Cloud Scale
Molecular clouds are turbulent structures whose star formation efficiency
(SFE) is strongly affected by internal stellar feedback processes. In this
paper we determine how sensitive the SFE of molecular clouds is to randomised
inputs in the star formation feedback loop, and to what extent relationships
between emergent cloud properties and the SFE can be recovered. We introduce
the yule suite of 26 radiative magnetohydrodynamic (RMHD) simulations of a
10,000 solar mass cloud similar to those in the solar neighbourhood. We use the
same initial global properties in every simulation but vary the initial mass
function (IMF) sampling and initial cloud velocity structure. The final SFE
lies between 6 and 23 percent when either of these parameters are changed. We
use Bayesian mixed-effects models to uncover trends in the SFE. The number of
photons emitted early in the cluster's life and the length of the cloud provide
are the strongest predictors of the SFE. The HII regions evolve following an
analytic model of expansion into a roughly isothermal density field. The more
efficient feedback is at evaporating the cloud, the less the star cluster is
dispersed. We argue that this is because if the gas is evaporated slowly, the
stars are dragged outwards towards surviving gas clumps due to the
gravitational attraction between the stars and gas. While star formation and
feedback efficiencies are dependent on nonlinear processes, statistical models
describing cloud-scale processes can be constructed.Comment: 24 pages, 16 figures, 6 tables. Accepted to MNRAS, version updated
with published titl
FORMATION OF GLOBULAR CLUSTERS IN ATOMIC-COOLING HALOS VIA RAPID GAS CONDENSATION AND FRAGMENTATION DURING THE EPOCH OF REIONIZATION
We investigate the formation of metal-poor globular clusters (GCs) at the center of two dark matter halos with ~ 4 10 at > 10 using cosmological radiation-hydrodynamics simulations. We find that very compact (1 pc) and massive (~ 6 10 ) clusters form rapidly when pristine gas collapses isothermally with the aid of efficient Ly emission during the transition from molecular-cooling halos to atomic-cooling halos. Because the local free-fall time of dense star-forming gas is very short (1 Myr), a large fraction of the collapsed gas is turned into stars before stellar feedback processes blow out the gas and shut down star formation. Although the early stage of star formation is limited to a small region of the central star-forming disk, we find that the disk quickly fragments due to metal enrichment from supernovae. Sub-clusters formed in the fragmented clouds eventually merge with the main cluster at the center. The simulated clusters closely resemble the local GCs in mass and size but show a metallicity spread that is much wider than found in the local GCs. We discuss a role of pre-enrichment by Pop III and II stars as a potential solution to the latter issue. Although not without shortcomings, it is encouraging that a naive blind (not tuned) cosmological simulation presents a possible channel for the formation of at least some massive GCs.The research is supported in part by NSF grant AST-1108700 and NASA grant NNX12AF91G and in part by the ERC Advanced Grant 320596 “The Emergence of Structure during the epoch of Reionization.” JR was funded by the European Research Council under the European Unions Seventh Framework Programme (FP7/2007- 2013)/ERC Grant agreement 278594-GasAroundGalaxies, and the Marie Curie Training Network CosmoComp (PITN-GA- 2009-238356). SKY acknowledges support from the Korean National Research Foundation (Doyak 2014003730).This is the final version of the article. It first appeared from the Institute of Physics via http://dx.doi.org/10.3847/0004-637X/823/1/5
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