128 research outputs found
NICIL: A stand alone library to self-consistently calculate non-ideal magnetohydrodynamic coefficients in molecular cloud cores
In this paper, we introduce Nicil: Non-Ideal magnetohydrodynamics
Coefficients and Ionisation Library. Nicil is a stand-alone Fortran90 module
that calculates the ionisation values and the coefficients of the non-ideal
magnetohydrodynamics terms of Ohmic resistivity, the Hall effect, and ambipolar
diffusion. The module is fully parameterised such that the user can decide
which processes to include and decide upon the values of the free parameters,
making this a versatile and customisable code. The module includes both cosmic
ray and thermal ionisation; the former includes two ion species and three
species of dust grains (positively charged, negatively charged and neutral),
and the latter includes five elements which can be doubly ionised. We
demonstrate tests of the module, and then describe how to implement it into an
existing numerical code.Comment: 13 pgs; accepted to PASA; source code for NICIL:
https://bitbucket.org/jameswurster/nicil ; Corrected typos in eqn 47 and in
initial conditions of section
Accretion Disc Particle Accretion in Major Merger Simulations
A recent approach to simulating localized feedback from active galactic
nuclei by Power et al. (2011) uses an accretion disc particle to represent both
the black hole and its accretion disc. We have extrapolated and adapted this
approach to simulations of Milky Way-sized galaxy mergers containing black
holes and explored the impact of the various parameters in this model as well
as its resolution dependence. The two key parameters in the model are an
effective accretion radius, which determines the radius within which gas
particles are added to the accretion disc, and a viscous time-scale which
determines how long it takes for material in the accretion disc to accrete on
to the black hole itself. We find that there is a limited range of permitted
accretion radii and viscous time-scales, with unphysical results produced
outside this range. For permitted model parameters, the nuclear regions of
simulations with the same resolution follow similar evolutionary paths,
producing final black hole masses that are consistent within a factor of two.
When comparing the resolution dependence of the model, there is a trend towards
higher resolution producing slightly lower mass black holes, but values for the
two resolutions studied again agree within a factor of two. We also compare
these results to two other AGN feedback algorithms found in the literature.
While the evolution of the systems vary, most notably the intermediate total
black hole mass, the final black hole masses differ by less than a factor of
five amongst all of our models, and the remnants exhibit similar structural
parameters. The implication of this accretion model is that, unlike most
accretion algorithms, a decoupling of the accretion rate on to the black hole
and the local gas properties is permitted and obtained; this allows for black
hole growth even after feedback has prevented additional accretion events on to
the disc.Comment: 17 pages, accepted to MNRA
Ambipolar diffusion in smoothed particle magnetohydrodynamics
In partially ionised plasmas, the magnetic field can become decoupled from
the neutral gas and diffuse through it in a process known as ambipolar
diffusion. Although ambipolar diffusion has been implemented in several grid
codes, we here provide an implementation in smoothed particle
magnetohydrodynamics (SPMHD). We use the strong coupling approximation in which
the ion density is negligible, allowing a single fluid approach. The equations
are derived to conserve energy, and to provide a positive definite contribution
to the entropy. We test the implementation in both a simple 1D SPMHD code and
the fully 3D code PHANTOM. The wave damping test yields agreement within 0.03-2
per cent of the analytical result, depending on the value of the collisional
coupling constant. The oblique C-shocks test yields results that typically
agree within 4 per cent of the semi-analytical result. Our algorithm is
therefore suitable for exploring the effect ambipolar diffusion has on physical
processes, such as the formation of stars from molecular clouds.Comment: Accepted for publication by MNRA
AGN Feedback models: Correlations with star formation and observational implications of time evolution
We examine the correlation between the star formation rate (SFR) and black
hole accretion rate (BHAR) across a suite of different AGN feedback models,
using the time evolution of a merger simulation. By considering three different
stages of evolution, and a distinction between the nuclear and outer regions of
star formation, we consider 63 different cases. Despite many of the feedback
models fitting the M-\sigma\ relationship well, there are often distinct
differences in the SFR-BHAR correlations, with close to linear trends only
being present after the merger. Some of the models also show evolution in the
SFR-BHAR parameter space that is at times directly across the long-term
averaged SFR-BHAR correlation. This suggests that the observational SFR-BHAR
correlation found for ensembles of galaxies is an approximate statistical
trend, as suggested by Hickox et al. Decomposing the SFR into nuclear and outer
components also highlights notable differences between models and there is only
modest agreement with observational studies examining this in Seyfert galaxies.
For the fraction of the black hole mass growth from the merger event relative
to the final black hole mass, we find as much as a factor of three variation
among models. This also translates into a similar variation in the
post-starburst black hole mass growth. Overall, we find that while qualitative
features are often similar amongst models, precise quantitative analysis shows
there can be quite distinct differences.Comment: Accepted to MNRAS. Comments welcom
Investigating prescriptions for artificial resistivity in smoothed particle magnetohydrodynamics
In numerical simulations, artificial terms are applied to the evolution
equations for stability. To prove their validity, these terms are thoroughly
tested in test problems where the results are well known. However, they are
seldom tested in production-quality simulations at high resolution where they
interact with a plethora of physical and numerical algorithms. We test three
artificial resistivities in both the Orszag-Tang vortex and in a star formation
simulation. From the Orszag-Tang vortex, the Price et. al. (2017) artificial
resistivity is the least dissipative thus captures the density and magnetic
features; in the star formation algorithm, each artificial resistivity
algorithm interacts differently with the sink particle to produce various
results, including gas bubbles, dense discs, and migrating sink particles. The
star formation simulations suggest that it is important to rely upon physical
resistivity rather than artificial resistivity for convergence.Comment: 8 pages, 7 figures. Proceedings of the "12th international SPHERIC
workshop", Ourense, Spain, 13-15 June 201
Gas and star kinematics in cloud-cloud collisions
We model the collision of molecular clouds to investigate the role of the
initial properties on the remnants. Our clouds collide and evolve in a
background medium that is approximately ten times less dense than the clouds,
and we show that this relatively dense background is dynamically important for
the evolution of the collision remnants. Given the motion of the clouds and the
remnants through the background, we develop, implement, and introduce dynamic
boundary conditions. We investigate the effect of the initial cloud mass,
velocity, internal turbulence, and impact angle. The initial velocity and its
velocity components have the largest affect on the remnant. This affects the
spatial extent of the remnant, which affects the number of resulting star
clusters and the distribution of their masses. The less extended remnants tend
to have fewer, but more massive, clusters. Unlike the clusters, the gas
distributions are relatively insensitive to the initial conditions, both the
distribution of the bulk gas properties and the gas clumps. In general, cloud
collisions are relatively insensitive to their initial conditions when modelled
hydrodynamically in a dynamically important background medium.Comment: 22 pages, 23 figures. Accepted for publication in MNRA
Windfall Wealth and Shale Development in Appalachian Ohio: Preliminary Results
The response by agriculture/natural resources and community development Extension educators to shale development in Ohio has been proactive. There is a need, however, to understand the impact that shale development is having broadly on families and communities and specifically as it relates to lease payments and the perceptions and realities of resource windfalls or sudden wealth. This article presents the preliminary results of a qualitative study. In the course of data analysis, themes emerged around the topics of money, family and community life, and land. A discussion of the role of Extension professionals is provided
Non-ideal magnetohydrodynamics versus turbulence II : which is the dominant process in stellar core formation?
Funding: JW acknowledges support from the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007- 2013 grant agreement no. 339248), and from the University of St Andrews. BTL acknowledges the support of the National Aeronautics and Space Administration (NASA) through grant NN17AK90G and from the National Science Foundation (NSF) through grants no. 1517488 and PHY-1748958.Non-ideal magnetohydrodynamics (MHD) is the dominant process. We investigate the effect of magnetic fields (ideal and non-ideal) and turbulence (sub- and transsonic) on the formation of protostars by following the gravitational collapse of 1 M☉ gas clouds through the first hydrostatic core to stellar densities. The clouds are imposed with both rotational and turbulent velocities, and are threaded with a magnetic field that is parallel/antiparallel or perpendicular to the rotation axis; we investigate two rotation rates and four Mach numbers. The initial radius and mass of the stellar core are only weakly dependent on the initial parameters. In the models that include ideal MHD, the magnetic field strength implanted in the protostar at birth is much higher than observed, independent of the initial level of turbulence; only non-ideal MHD can reduce this strength to near or below the observed levels. This suggests that not only is ideal MHD an incomplete picture of star formation, but that the magnetic fields in low mass stars are implanted later in life by a dynamo process. Non-ideal MHD suppresses magnetically launched stellar core outflows, but turbulence permits thermally launched outflows to form a few years after stellar core formation.Publisher PDFPeer reviewe
Non-ideal magnetohydrodynamics versus turbulence – I. Which is the dominant process in protostellar disc formation?
Funding: European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013 grant agreement no. 339248); University of St Andrews (JW). National Aeronautics and Space Administration (NASA) through grant NN17AK90G and from the National Science Foundation (NSF) through grants no 1517488 and PHY-1748958 (BTL).Non-ideal magnetohydrodynamics (MHD) is the dominant process. We investigate the effect of magnetic fields (ideal and non-ideal) and turbulence (sub- and transsonic) on the formation of circumstellar discs that form nearly simultaneously with the formation of the protostar. This is done by modelling the gravitational collapse of a 1 M☉ gas cloud that is threaded with a magnetic field and imposed with both rotational and turbulent velocities. We investigate magnetic fields that are parallel/antiparallel and perpendicular to the rotation axis, two rotation rates, and four Mach numbers. Disc formation occurs preferentially in the models that include non-ideal MHD where the magnetic field is antiparallel or perpendicular to the rotation axis. This is independent of the initial rotation rate and level of turbulence, suggesting that subsonic turbulence plays a minimal role in influencing the formation of discs. Aside from first core outflows that are influenced by the initial level of turbulence, non-ideal MHD processes are more important than turbulent processes during the formation of discs around low-mass stars.Publisher PDFPeer reviewe
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