1,061 research outputs found
A scheme for radiation pressure and photon diffusion with the M1 closure in RAMSES-RT
We describe and test an updated version of radiation-hydrodynamics (RHD) in
the RAMSES code, that includes three new features: i) radiation pressure on
gas, ii) accurate treatment of radiation diffusion in an unresolved optically
thick medium, and iii) relativistic corrections that account for Doppler
effects and work done by the radiation to first order in v/c. We validate the
implementation in a series of tests, which include a morphological assessment
of the M1 closure for the Eddington tensor in an astronomically relevant
setting, dust absorption in a optically semi-thick medium, direct pressure on
gas from ionising radiation, convergence of our radiation diffusion scheme
towards resolved optical depths, correct diffusion of a radiation flash and a
constant luminosity radiation, and finally, an experiment from Davis et al. of
the competition between gravity and radiation pressure in a dusty atmosphere,
and the formation of radiative Rayleigh-Taylor instabilities. With the new
features, RAMSES-RT can be used for state-of-the-art simulations of radiation
feedback from first principles, on galactic and cosmological scales, including
not only direct radiation pressure from ionising photons, but also indirect
pressure via dust from multi-scattered IR photons reprocessed from
higher-energy radiation, both in the optically thin and thick limits.Comment: 25 pages, 13 figures, accepted for publication in MNRAS. Revised to
match published versio
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
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