147 research outputs found
TreeCol: a novel approach to estimating column densities in astrophysical simulations
We present TreeCol, a new and efficient tree-based scheme to calculate column
densities in numerical simulations. Knowing the column density in any direction
at any location in space is a prerequisite for modelling the propagation of
radiation through the computational domain. TreeCol therefore forms the basis
for a fast, approximate method for modelling the attenuation of radiation
within large numerical simulations. It constructs a HEALPix sphere at any
desired location and accumulates the column density by walking the tree and by
adding up the contributions from all tree nodes whose line of sight contributes
to the pixel under consideration. In particular when combined with widely-used
tree-based gravity solvers the new scheme requires little additional
computational cost. In a simulation with resolution elements, the
computational cost of TreeCol scales as , instead of the
scaling of most other radiative transfer schemes. TreeCol is naturally
adaptable to arbitrary density distributions and is easy to implement and to
parallelize. We discuss its accuracy and performance characteristics for the
examples of a spherical protostellar core and for the turbulent interstellar
medium. We find that the column density estimates provided by TreeCol are on
average accurate to better than 10 percent. In another application, we compute
the dust temperatures for solar neighborhood conditions and compare with the
result of a full-fledged Monte Carlo radiation-transfer calculation. We find
that both methods give very similar answers. We conclude that TreeCol provides
a fast, easy to use, and sufficiently accurate method of calculating column
densities that comes with little additional computational cost when combined
with an existing tree-based gravity solver.Comment: 11 pages, 10 figures, submitted to MNRA
The Origin and Universality of the Stellar Initial Mass Function
We review current theories for the origin of the Stellar Initial Mass
Function (IMF) with particular focus on the extent to which the IMF can be
considered universal across various environments. To place the issue in an
observational context, we summarize the techniques used to determine the IMF
for different stellar populations, the uncertainties affecting the results, and
the evidence for systematic departures from universality under extreme
circumstances. We next consider theories for the formation of prestellar cores
by turbulent fragmentation and the possible impact of various thermal,
hydrodynamic and magneto-hydrodynamic instabilities. We address the conversion
of prestellar cores into stars and evaluate the roles played by different
processes: competitive accretion, dynamical fragmentation, ejection and
starvation, filament fragmentation and filamentary accretion flows, disk
formation and fragmentation, critical scales imposed by thermodynamics, and
magnetic braking. We present explanations for the characteristic shapes of the
Present-Day Prestellar Core Mass Function and the IMF and consider what
significance can be attached to their apparent similarity. Substantial
computational advances have occurred in recent years, and we review the
numerical simulations that have been performed to predict the IMF directly and
discuss the influence of dynamics, time-dependent phenomena, and initial
conditions.Comment: 24 pages, 6 figures. Accepted for publication as a chapter in
Protostars and Planets VI, University of Arizona Press (2014), eds. H.
Beuther, R. S. Klessen, C. P. Dullemond, Th. Hennin
Fragmentation and Evolution of Molecular Clouds. I: Algorithm and First Results
We present a series of simulations of the fragmentation of a molecular cloud,
leading to the formation of a cluster of protostellar cores. The purpose of
these simulations is to address a specific numerical problem called artificial
fragmentation, that plagues SPH simulations of cloud fragmentation. We argue
that this is a serious problem that needs to be addressed, and that the only
reasonable and practical way to address it is to use a relatively new technique
called particle splitting. Our largest simulation has an effective resolution
of 256^3 particles (much higher than most previous SPH simulations of cloud
fragmentation) and results in the formation of a dense cluster containing ~3000
protostellar cores. It is the first simulation of this kind to properly resolve
the Jeans mass throughout the entire system, at all times, thus preventing
artificial fragmentation.Comment: 47 pages, 15 figures (2 grayscale, one color), ApJ Suppl, in pres
Where are the drought tolerant crops? An assessment of more than two decades of plant biotechnology effort in crop improvement
Since the dawn of modern biotechnology public and private enterprise have pursued the development of a new breed of drought tolerant crop products. After more than 20 years of research and investment only a few such products have reached the market. This is due to several technical and market constraints. The technical challenges include the difficulty in defining tractable single-gene trait development strategies, the logistics of moving traits from initial to commercial genetic backgrounds, and the disconnect between conditions in farmer’s fields and controlled environments. Market constraints include the significant difficulty, and associated costs, in obtaining access to markets around the world. Advances in the biology of plant water management, including response to water deficit reveal new opportunities to improve crop response to water deficit and new genome-based tools promise to usher in the next era of crop improvement. As biotechnology looks to improve crop productivity under drought conditions, the environmental and food security advantages will influence public perception and shift the debate toward benefits rather than risks
The Effects of Accretion Luminosity upon Fragmentation in the Early Universe
We introduce a prescription for the luminosity from accreting protostars into
smoothed particle hydrodynamics simulation, and apply the method to simulations
of five primordial minihalos generated from cosmological initial conditions. We
find that accretion luminosity delays fragmentation within the halos, but does
not prevent it. In halos that slowly form a low number of protostars, the
accretion luminosity can reduce the number of fragments that are formed before
the protostars start ionising their surroundings. However, halos that rapidly
form many protostars become dominated by dynamical processes, and the effect of
accretion luminosity becomes negligible. Generally the fragmentation found in
the halos is highly dependent on the initial conditions. Accretion luminosity
does not substantially affect the accretion rates experienced by the
protostars, and is far less important than dynamical interactions, which can
lead to ejections that effectively terminate the accretion. We find that the
accretion rates onto the inner regions of the disks (20 AU) around the
protostars are highly variable, in contrast to the constant or smoothly
decreasing accretion rates currently used in models of the pre-main sequence
evolution of Population III stars.Comment: 12 pages, 10 figures and 3 tables. Accepted by MNRA
Modeling Collapse and Accretion in Turbulent Gas Clouds: Implementation and Comparison of Sink Particles in AMR and SPH
We implemented sink particles in the adaptive mesh refinement (AMR)
hydrodynamics code FLASH. Sink particles are created in regions of local
gravitational collapse, and their trajectories and accretion can be followed
over many dynamical times. We perform a series of tests including the time
integration of circular and elliptical orbits, the collapse of a Bonnor-Ebert
sphere and a rotating, fragmenting cloud core. We compare the collapse of a
highly unstable singular isothermal sphere to the theory by Shu (1977), and
show that the sink particle accretion rate is in excellent agreement with the
theoretical prediction.
To model eccentric orbits and close encounters of sink particles accurately,
we show that a very small timestep is often required, for which we implemented
subcycling of the N-body system. We emphasize that a sole density threshold for
sink particle creation is insufficient in supersonic flows, if the density
threshold is below the opacity limit. In that case, the density can exceed the
threshold in strong shocks that do not necessarily lead to local collapse.
Additional checks for bound state, gravitational potential minimum, Jeans
instability and converging flows are absolutely necessary for a meaningful
creation of sink particles.
We apply our new sink particle module for FLASH to the formation of a stellar
cluster, and compare to a smoothed particle hydrodynamics (SPH) code with sink
particles. Our comparison shows encouraging agreement of gas properties,
indicated by column density distributions and radial profiles, and of sink
particle formation times and positions. We find excellent agreement in the
number of sink particles formed, and in their accretion and mass distributions.Comment: 30 pages, 17 figures, ApJ accepted, simulation movies available at
http://www.ita.uni-heidelberg.de/~chfeder/videos.shtml?lang=e
How to humiliate and shame: A reporter's guide to the power of the mugshot
This is an Author's Accepted Manuscript of an article published in Social Semiotics, 24(1), 56-87, 2014, copyright Taylor & Francis, available online at: http://www.tandfonline.com/The judicial photograph – the “mugshot” – is a ubiquitous and instantly recognisable form, appearing in the news media, on the internet, on book covers, law enforcement noticeboards and in many other mediums. This essay attempts to situate the mugshot in a historical and theoretical context to explain the explicit and implicit meaning of the genre as it has developed, focussing in particular on their use in the UK media in late modernity. The analysis is based on the author's reflexive practice as a journalist covering crime in the national news media for 30 years and who has used mugshots to illustrate stories for their explicit and specific content. The author argues that the visual limitations of the standardised “head and shoulders” format of the mugshot make it a robust subject for analysing the changing meaning of images over time. With little variation in the image format, arguments for certain accreted layers of signification are easier to make. Within a few years of the first appearance of the mugshot form in the mid-19th century, it was adopted and adapted as a research tool by scientists and criminologists. While the positivist scientists claimed empirical objectivity we can now see that mugshots played a part in the construction of subjective notions of “the other”, “the lesser” or “sub-human” on the grounds of class, race and religion. These dehumanising ideas later informed the theorists and bureaucrats of National Socialist ideology from the 1920s to 1940s. The author concludes that once again the mugshot has become, in certain parts of the media, a signifier widely used to exclude or deride certain groups. In late modernity, the part of the media that most use mugshots – the tabloid press and increasingly tabloid TV – is part of a neo-liberal process that, in a conscious commercial appeal to the paying audience, seeks to separate rather than unify wider society
Measurement of Trace I-129 Concentrations in CsI Powder and Organic Liquid Scintillator with Accelerator Mass Spectrometry
Levels of trace radiopurity in active detector materials is a subject of
major concern in low-background experiments. Procedures were devised to measure
trace concentrations of I-129 in the inorganic salt CsI as well as in organic
liquid scintillator with Accelerator Mass Spectrometry (AMS) which leads to
improvement in sensitivities by several orders of magnitude over other methods.
No evidence of their existence in these materials were observed. Limits of < 6
X 10^{-13} g/g and < 2.6 X 10^{-17} g/g on the contaminations of I-129 in CsI
and liquid scintillator, respectively, were derived.These are the first results
in a research program whose goals are to develop techniques to measure trace
radioactivity in detector materials by AMS.Comment: Proceedings of 10th International Conference on Accelerator Mass
Spectrometr
Protostellar collapse and fragmentation using an MHD GADGET
Although the influence of magnetic fields is regarded as vital in the star
formation process, only a few magnetohydrodynamics (MHD) simulations have been
performed on this subject within the smoothed particle hydrodynamics (SPH)
method. This is largely due to the unsatisfactory treatment of non-vanishing
divergence of the magnetic field. Recently smoothed particle
magnetohydrodynamics (SPMHD) simulations based on Euler potentials have proven
to be successful in treating MHD collapse and fragmentation problems, however
these methods are known to have some intrinsical difficulties. We have
performed SPMHD simulations based on a traditional approach evolving the
magnetic field itself using the induction equation. To account for the
numerical divergence, we have chosen an approach that subtracts the effects of
numerical divergence from the force equation, and additionally we employ
artificial magnetic dissipation as a regularization scheme. We apply this
realization of SPMHD to a widely known setup, a variation of the 'Boss &
Bodenheimer standard isothermal test case', to study the impact of the magnetic
fields on collapse and fragmentation. In our simulations, we concentrate on
setups, where the initial magnetic field is parallel to the rotation axis. We
examine different field strengths and compare our results to other findings
reported in the literature. We are able to confirm specific results found
elsewhere, namely the delayed onset of star formation for strong fields,
accompanied by the tendency to form only single stars. We also find that the
'magnetic cushioning effect', where the magnetic field is wound up to form a
'cushion' between the binary, aids binary fragmentation in a case, where
previously only formation of a single protostar was expected.Comment: 18 pages, 11 figures. Final version (with revisions). Accepted to
MNRA
Protostellar outflows with Smoothed Particle Magnetohydrodynamics (SPMHD)
The protostellar collapse of a molecular cloud core is usually accompanied by
outflow phenomena. The latter are thought to be driven by magnetorotational
processes from the central parts of the protostellar disc. While several 3D
AMR/nested grid studies of outflow phenomena in collapsing magnetically
supercritical dense cores have been reported in the literature, so far no such
simulation has been performed using the Smoothed Particle Hydrodynamics (SPH)
method. This is mainly due to intrinsic numerical difficulties in handling
magnetohydrodynamics within SPH, which only recently were partly resolved. In
this work, we use an approach where we evolve the magnetic field via the
induction equation, augmented with stability correction and divergence cleaning
schemes. We consider the collapse of a rotating core of one solar mass,
threaded by a weak magnetic field initially parallel to the rotation axis so
that the core is magnetically supercritical. We show, that Smoothed Particle
Magnetohydrodynamics (SPMHD) is able to handle the magnetorotational processes
connected with outflow phenomena, and to produce meaningful results which are
in good agreement with findings reported in the literature. Especially, our
numerical scheme allows for a quantitative analysis of the evolution of the
ratio of the toroidal to the poloidal magnetic field, which we performed in
this work.Comment: 5 pages, 4 figures. Accepted to MNRAS Letter
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