65 research outputs found
The Abundance of Molecular Hydrogen and its Correlation with Midplane Pressure in Galaxies: Non-Equilibrium, Turbulent, Chemical Models
Observations of spiral galaxies show a strong linear correlation between the
ratio of molecular to atomic hydrogen surface density R_mol and midplane
pressure. To explain this, we simulate three-dimensional, magnetized
turbulence, including simplified treatments of non-equilibrium chemistry and
the propagation of dissociating radiation, to follow the formation of H_2 from
cold atomic gas. The formation time scale for H_2 is sufficiently long that
equilibrium is not reached within the 20-30 Myr lifetimes of molecular clouds.
The equilibrium balance between radiative dissociation and H_2 formation on
dust grains fails to predict the time-dependent molecular fractions we find. A
simple, time-dependent model of H_2 formation can reproduce the gross behavior,
although turbulent density perturbations increase molecular fractions by a
factor of few above it. In contradiction to equilibrium models, radiative
dissociation of molecules plays little role in our model for diffuse radiation
fields with strengths less than ten times that of the solar neighborhood,
because of the effective self-shielding of H_2. The observed correlation of
R_mol with pressure corresponds to a correlation with local gas density if the
effective temperature in the cold neutral medium of galactic disks is roughly
constant. We indeed find such a correlation of R_mol with density. If we
examine the value of R_mol in our local models after a free-fall time at their
average density, as expected for models of molecular cloud formation by
large-scale gravitational instability, our models reproduce the observed
correlation over more than an order of magnitude range in density.Comment: 24 pages, 4 figures, accepted for publication in Astrophys. J,
changes include addition of models with higher radiation fields and
substantial clarification of the narrativ
Magnetorotational turbulence transports angular momentum in stratified disks with low magnetic Prandtl number but magnetic Reynolds number above a critical value
The magnetorotational instability (MRI) may dominate outward transport of
angular momentum in accretion disks, allowing material to fall onto the central
object. Previous work has established that the MRI can drive a mean-field
dynamo, possibly leading to a self-sustaining accretion system. Recently,
however, simulations of the scaling of the angular momentum transport parameter
\alphaSS with the magnetic Prandtl number \Prandtl have cast doubt on the
ability of the MRI to transport astrophysically relevant amounts of angular
momentum in real disk systems. Here, we use simulations including explicit
physical viscosity and resistivity to show that when vertical stratification is
included, mean field dynamo action operates, driving the system to a
configuration in which the magnetic field is not fully helical. This relaxes
the constraints on the generated field provided by magnetic helicity
conservation, allowing the generation of a mean field on timescales independent
of the resistivity. Our models demonstrate the existence of a critical magnetic
Reynolds number \Rmagc, below which transport becomes strongly
\Prandtl-dependent and chaotic, but above which the transport is steady and
\Prandtl-independent. Prior simulations showing \Prandtl-dependence had
\Rmag < \Rmagc. We conjecture that this steady regime is possible because the
mean field dynamo is not helicity-limited and thus does not depend on the
details of the helicity ejection process. Scaling to realistic astrophysical
parameters suggests that disks around both protostars and stellar mass black
holes have \Rmag >> \Rmagc. Thus, we suggest that the strong \Prandtl
dependence seen in recent simulations does not occur in real systems.Comment: 17 pages, 9 figures. as accepted to Ap
Centre of rotation of the human subtalar joint using weight-bearing clinical computed tomography
Accurate in vivo quantification of subtalar joint kinematics can provide important information for the clinical evaluation of subtalar joint function; the analysis of outcome of surgical procedures of the hindfoot; and the design of a replacement subtalar joint prosthesis. The objective of the current study was to explore the potential of full weight-bearing clinical computed tomography (CT) to evaluate the helical axis and centre of rotation of the subtalar joint during inversion and eversion motion. A subject specific methodology was proposed for the definition of the subtalar joint motion combining three-dimensional (3D) weight-bearing imaging at different joint positions with digital volume correlation (DVC). The computed subtalar joint helical axis parameters showed consistency across all healthy subjects and in line with previous data under simulated loads. A sphere fitting approach was introduced for the computation of subtalar joint centre of rotation, which allows to demonstrate that this centre of rotation is located in the middle facet of the subtalar joint. Some translation along the helical axis was also observed, reflecting the elasticity of the soft-tissue restraints. This study showed a novel technique for non-invasive quantitative analysis of bone-to-bone motion under full weight-bearing of the hindfoot. Identifying different joint kinematics in patients with ligamentous laxity and instability, or in the presence of stiffness and arthritis, could help clinicians to define optimal patient-specific treatments
Collisional N-Body Dynamics Coupled to Self-Gravitating Magnetohydrodynamics Reveals Dynamical Binary Formation
We describe a star cluster formation model that includes individual star
formation from self-gravitating, magnetized gas, coupled to collisional stellar
dynamics. The model uses the Astrophysical Multi-purpose Software Environment
(AMUSE) to integrate an adaptive-mesh magnetohydrodynamics code (FLASH) with a
fourth order Hermite N-body code (ph4), a stellar evolution code (SeBa), and a
method for resolving binary evolution (multiples). This combination yields
unique star formation simulations that allow us to study binaries formed
dynamically from interactions with both other stars and dense, magnetized gas
subject to stellar feedback during the birth and early evolution of stellar
clusters. We find that for massive stars, our simulations are consistent with
the observed dynamical binary fractions and mass ratios. However, our binary
fraction drops well below observed values for lower mass stars, presumably due
to unincluded binary formation during initial star formation. Further, we
observe a build up of binaries near the hard-soft boundary that may be an
important mechanism driving early cluster contraction.Comment: 14 pages, 12 figures. Submitted to Ap
Modelling of the Effects of Stellar Feedback during Star Cluster Formation Using a Hybrid Gas and N-Body Method
Understanding the formation of stellar clusters requires following the
interplay between gas and newly formed stars accurately. We therefore couple
the magnetohydrodynamics code FLASH to the N-body code ph4 and the stellar
evolution code SeBa using the Astrophysical Multipurpose Software Environment
(AMUSE) to model stellar dynamics, evolution, and collisional N-body dynamics
and the formation of binary and higher-order multiple systems, while
implementing stellar feedback in the form of radiation, stellar winds and
supernovae in FLASH. We here describe the algorithms used for each of these
processes. We denote this integrated package Torch. We then use this novel
numerical method to simulate the formation and early evolution of several
examples of open clusters of ~1000 stars formed from clouds with a mass range
of 10^3-10^5 M_sun. Analyzing the effects of stellar feedback on the gas and
stars of the natal clusters, we find that in these examples, the stellar
clusters are resilient to disruption, even in the presence of intense feedback.
This can even slightly increase the amount of dense, Jeans unstable gas by
sweeping up shells; thus, a stellar wind strong enough to trap its own H II
region shows modest triggering of star formation. Our clusters are born
moderately mass segregated, an effect enhanced by feedback, and retained after
the ejection of their natal gas, in agreement with observations.Comment: Accepted to ApJ. 29 pages, 18 figures. Source code at
https://bitbucket.org/torch-sf/torch/ and documentation at
https://torch-sf.bitbucket.io
Planetesimal and Protoplanet Dynamics in a Turbulent Protoplanetary Disk: Ideal Stratified Disks
Due to the gravitational influence of density fluctuations driven by
magneto-rotational instability in the gas disk, planetesimals and protoplanets
undergo diffusive radial migration as well as changes in other orbital
properties. The magnitude of the effect on particle orbits can have important
consequences for planet formation scenarios. We use the local-shearing-box
approximation to simulate an ideal, isothermal, magnetized gas disk with
vertical density stratification and simultaneously evolve numerous massless
particles moving under the gravitational field of the gas and the host star. We
measure the evolution of the particle orbital properties, including mean
radius, eccentricity, inclination, and velocity dispersion, and its dependence
on the disk properties and the particle initial conditions. Although the
results converge with resolution for fixed box dimensions, we find the response
of the particles to the gravity of the turbulent gas correlates with the
horizontal box size, up to 16 disk scale heights. This correlation indicates
that caution should be exercised when interpreting local-shearing-box models
involving gravitational physics of magneto-rotational turbulence. Based on
heuristic arguments, nevertheless, the criterion L_h / R ~ O(1), where L_h is
the horizontal box size and R is the distance to the host star, is proposed to
possibly circumvent this conundrum. If this criterion holds, we can still
conclude that magneto-rotational turbulence seems likely to be ineffective at
driving either diffusive migration or collisional erosion under most
circumstances.Comment: Accepted to ApJ. Major expansion in Secs. 2.1 & 2.2 and new Sec. 4.
Planetesimal and Protoplanet Dynamics in a Turbulent Protoplanetary Disk: Ideal Unstratified Disks
The dynamics of planetesimals and planetary cores may be strongly influenced
by density perturbations driven by magneto-rotational turbulence in their natal
protoplanetary gas disks. Using the local shearing box approximation, we
perform numerical simulations of planetesimals moving as massless particles in
a turbulent, magnetized, unstratified gas disk. Our fiducial disk model shows
turbulent accretion characterized by a Shakura-Sunyaev viscosity parameter of
, with root-mean-square density perturbations of
10%. We measure the statistical evolution of particle orbital properties
in our simulations including mean radius, eccentricity, and velocity
dispersion. We confirm random walk growth in time of all three properties, the
first time that this has been done with direct orbital integration in a local
model. We find that the growth rate increases with the box size used at least
up to boxes of eight scale heights in horizontal size. However, even our
largest boxes show velocity dispersions sufficiently low that collisional
destruction of planetesimals should be unimportant in the inner disk throughout
its lifetime. Our direct integrations agree with earlier torque measurements
showing that type I migration dominates over diffusive migration by stochastic
torques for most objects in the planetary core and terrestrial planet mass
range. Diffusive migration remains important for objects in the mass range of
kilometer-sized planetesimals. Discrepancies in the derived magnitude of
turbulence between local and global simulations of magneto-rotationally
unstable disks remains an open issue, with important consequences for planet
formation scenarios.Comment: Accepted for publication in ApJ. Major revision. 39 pages, 15 figure
Radiation shielding of protoplanetary discs in young star-forming regions
Protoplanetary discs spend their lives in the dense environment of a star
forming region. While there, they can be affected by nearby stars through
external photoevaporation and dynamic truncations. We present simulations that
use the AMUSE framework to couple the Torch model for star cluster formation
from a molecular cloud with a model for the evolution of protoplanetary discs
under these two environmental processes. We compare simulations with and
without extinction of photoevaporation-driving radiation. We find that the
majority of discs in our simulations are considerably shielded from
photoevaporation-driving radiation for at least 0.5 Myr after the formation of
the first massive stars. Radiation shielding increases disc lifetimes by an
order of magnitude and can let a disc retain more solid material for planet
formation. The reduction in external photoevaporation leaves discs larger and
more easily dynamically truncated, although external photoevaporation remains
the dominant mass loss process. Finally, we find that the correlation between
disc mass and projected distance to the most massive nearby star (often
interpreted as a sign of external photoevaporation) can be erased by the
presence of less massive stars that dominate their local radiation field.
Overall, we find that the presence and dynamics of gas in embedded clusters
with massive stars is important for the evolution of protoplanetary discs.Comment: 23 pages, 22 figures, 1 table, accepted for publication in MNRA
Magnetic fields do not suppress global star formation in low metallicity dwarf galaxies
Funding: DJW is grateful for support through a STFC Doctoral Training Partnership. RJS gratefully acknowledges an STFC Ernest Rutherford fellowship (grant ST/N00485X/1). DJW, SCOG, RT, JDS, and RSK acknowledge funding from the European Research Council via the ERC Synergy Grant ‘ECOGAL – Understanding our Galactic ecosystem: from the disc of the Milky Way to the formation sites of stars and planets’ (project ID 855130).Many studies concluded that magnetic fields suppress star formation in molecular clouds and Milky Way like galaxies. However, most of these studies are based on fully developed fields that have reached the saturation level, with little work on investigating how an initial weak primordial field affects star formation in low metallicity environments. In this paper, we investigate the impact of a weak initial field on low metallicity dwarf galaxies. We perform high-resolution AREPO simulations of five isolated dwarf galaxies. Two models are hydrodynamical, two start with a primordial magnetic field of 10-6 μG and different sub-solar metallicities, and one starts with a saturated field of 10-2 μG. All models include a non-equilibrium, time-dependent chemical network that includes the effects of gas shielding from the ambient ultraviolet field. Sink particles form directly from the gravitational collapse of gas and are treated as star-forming clumps that can accrete gas. We vary the ambient uniform far ultraviolet field, and cosmic ray ionization rate between 1 per cent and 10 per cent of solar values. We find that the magnetic field has little impact on the global star formation rate (SFR), which is in tension with some previously published results. We further find that the initial field strength has little impact on the global SFR. We show that an increase in the mass fractions of both molecular hydrogen and cold gas, along with changes in the perpendicular gas velocity dispersion and the magnetic field acting in the weak-field model, overcome the expected suppression in star formation.Peer reviewe
On the distribution of the CNM in spiral galaxies
The Cold Neutral Medium (CNM) is an important part of the galactic gas cycle
and a precondition for the formation of molecular and star forming gas, yet its
distribution is still not fully understood. In this work we present extremely
high resolution simulations of spiral galaxies with time-dependent chemistry
such that we can track the formation of the CNM, its distribution within the
galaxy, and its correlation with star formation. We find no strong radial
dependence between the CNM fraction and total HI due to the decreasing
interstellar radiation field counterbalancing the decreasing gas column density
at larger galactic radii.However, the CNM fraction does increase in spiral arms
where the CNM distribution is clumpy, rather than continuous, overlapping more
closely with H2. The CNM doesn't extend out radially as far as HI, and the
vertical scale height is smaller in the outer galaxy compared to HI with no
flaring. The CNM column density scales with total midplane pressure and
disappears from the gas phase below values of PT/kB =1000 K/cm3. We find that
the star formation rate density follows a similar scaling law with CNM column
density to the total gas Kennicutt-Schmidt law. In the outer galaxy we produce
realistic vertical velocity dispersions in the HI purely from galactic dynamics
but our models do not predict CNM at the extremely large radii observed in HI
absorption studies of the Milky Way. We suggest that extended spiral arms might
produce isolated clumps of CNM at these radii.Comment: 13 pages, 19 figures, submitted to MNRA
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