5,803 research outputs found
Radial penetration of flux surface shaping in tokamaks
Using analytic calculations, the effects of the edge flux surface shape and
the toroidal current profile on the penetration of flux surface shaping are
investigated in a tokamak. It is shown that the penetration of shaping is
determined by the poloidal variation of the poloidal magnetic field on the
surface. This fact is used to investigate how different flux surface shapes
penetrate from the edge. Then, a technique to separate the effects of magnetic
pressure and tension in the Grad-Shafranov equation is presented and used to
calculate radial profiles of strong elongation for nearly constant current
profiles. Lastly, it is shown that more hollow toroidal current profiles are
significantly better at conveying shaping from the edge to the core.Comment: 11 pages, 13 figure
Measuring the local dark matter density
We examine systematic problems in determining the local matter density from
the vertical motion of stars, i.e. the 'Oort limit'. Using collisionless
simulations and a Monte Carlo Markov Chain technique, we determine the data
quality required to detect local dark matter at its expected density. We find
that systematic errors are more important than observational errors and apply
our technique to Hipparcos data to reassign realistic error bars to the local
dark matter density.Comment: 3 pages, 1 figure, to be published in "Hunting for the Dark: The
Hidden Side of Galaxy Formation", Malta, 19-23 Oct. 2009, eds. V.P.
Debattista & C.C. Popescu, AIP Conf. Se
Fourteen candidate RR Lyrae star streams in the inner Galaxy
We apply the GC3 stream-finding method to RR Lyrae stars (RRLS) in the
Catalina survey. We find two RRLS stream candidates at confidence
and another 12 at confidence over the Galactocentric distance
range . Of these, only two are associated with known
globular clusters (NGC 1261 and Arp2). The remainder are candidate `orphan'
streams, consistent with the idea that globular cluster streams are most
visible close to dissolution. Our detections are likely a lower bound on the
total number of dissolving globulars in the inner galaxy, since many globulars
have few RRLS while only the brightest streams are visible over the Galactic
RRLS background, particularly given the current lack of kinematical
information. We make all of our candidate streams publicly available and
provide a new GALSTREAMS Python library for the footprints of all known streams
and overdensities in the Milky Way.Comment: 18 pages, 4 figures. Accepted for publication at MNRAS. GALSTREAMS
Milky Way Streams Footprint Library are available at
https://github.com/cmateu/galstreams . All RRL data and code used in the
paper are available at
https://cmateu.github.io/Cecilia_Mateu_WebPage/CatalinaGC3_Streams.htm
A low pre-infall mass for the Carina dwarf galaxy from disequilibrium modelling
Dark matter only simulations of galaxy formation predict many more subhalos
around a Milky Way like galaxy than the number of observed satellites. Proposed
solutions require the satellites to inhabit dark matter halos with masses
between one to ten billion solar masses at the time they fell into the Milky
Way. Here we use a modelling approach, independent of cosmological simulations,
to obtain a preinfall mass of 360 (+380,-230) million solar masses for one of
the Milky Way's satellites: Carina. This determination of a low halo mass for
Carina can be accommodated within the standard model only if galaxy formation
becomes stochastic in halos below ten billion solar masses. Otherwise Carina,
the eighth most luminous Milky Way dwarf, would be expected to inhabit a
significantly more massive halo. The implication of this is that a population
of "dark dwarfs" should orbit the Milky Way: halos devoid of stars and yet more
massive than many of their visible counterparts.Comment: 5 pages, 3 figures, 1 table, and supplementary material availabl
The mass distribution of the Fornax dSph: constraints from its globular cluster distribution
Uniquely among the dwarf spheroidal (dSph) satellite galaxies of the Milky
Way, Fornax hosts globular clusters. It remains a puzzle as to why dynamical
friction has not yet dragged any of Fornax's five globular clusters to the
centre, and also why there is no evidence that any similar star cluster has
been in the past (for Fornax or any other dSph). We set up a suite of 2800
N-body simulations that sample the full range of globular-cluster orbits and
mass models consistent with all existing observational constraints for Fornax.
In agreement with previous work, we find that if Fornax has a large dark-matter
core then its globular clusters remain close to their currently observed
locations for long times. Furthermore, we find previously unreported behaviour
for clusters that start inside the core region. These are pushed out of the
core and gain orbital energy, a process we call 'dynamical buoyancy'. Thus a
cored mass distribution in Fornax will naturally lead to a shell-like globular
cluster distribution near the core radius, independent of the initial
conditions. By contrast, CDM-type cusped mass distributions lead to the rapid
infall of at least one cluster within \Delta t = 1-2Gyr, except when picking
unlikely initial conditions for the cluster orbits (\sim 2% probability), and
almost all clusters within \Delta t = 10Gyr. Alternatively, if Fornax has only
a weakly cusped mass distribution, dynamical friction is much reduced. While
over \Delta t = 10Gyr this still leads to the infall of 1-4 clusters from their
present orbits, the infall of any cluster within \Delta t = 1-2Gyr is much less
likely (with probability 0-70%, depending on \Delta t and the strength of the
cusp). Such a solution to the timing problem requires that in the past the
globular clusters were somewhat further from Fornax than today; they most
likely did not form within Fornax, but were accreted.Comment: 12 pages, 8 figures, 3 tables, submitted to MNRA
A Semi-Analytic dynamical friction model that reproduces core stalling
We present a new semi-analytic model for dynamical friction based on
Chandrasekhar's formalism. The key novelty is the introduction of physically
motivated, radially varying, maximum and minimum impact parameters. With these,
our model gives an excellent match to full N-body simulations for isotropic
background density distributions, both cuspy and shallow, without any
fine-tuning of the model parameters. In particular, we are able to reproduce
the dramatic core-stalling effect that occurs in shallow/constant density
cores, for the first time. This gives us new physical insight into the
core-stalling phenomenon. We show that core stalling occurs in the limit in
which the product of the Coulomb logarithm and the local fraction of stars with
velocity lower than the infalling body tends to zero. For cuspy backgrounds,
this occurs when the infalling mass approaches the enclosed background mass.
For cored backgrounds, it occurs at larger distances from the centre, due to a
combination of a rapidly increasing minimum impact parameter and a lack of slow
moving stars in the core. This demonstrates that the physics of core-stalling
is likely the same for both massive infalling objects and low-mass objects
moving in shallow density backgrounds. We implement our prescription for
dynamical friction in the direct summation code NBODY6 as an analytic
correction for stars that remain within the Roche volume of the infalling
object. This approach is computationally efficient, since only stars in the
inspiralling system need to be evolved with direct summation. Our method can be
applied to study a variety of astrophysical systems, including young star
clusters orbiting near the Galactic Centre; globular clusters moving within the
Galaxy; and dwarf galaxies orbiting within dark matter halos.Comment: 16 pages, 21 figures, Accepted for publication in MNRA
Enhancement of field renormalization in scalar theories via functional renormalization group
The flow equations of the Functional Renormalization Group are applied to the
O(N)-symmetric scalar theory, for N=1 and N=4, in four Euclidean dimensions,
d=4, to determine the effective potential and the renormalization function of
the field in the broken phase. In our numerical analysis, the infrared limit,
corresponding to the vanishing of the running momentum scale in the equations,
is approached to obtain the physical values of the parameters by extrapolation.
In the N=4 theory a non-perturbatively large value of the physical
renormalization of the longitudinal component of the field is observed. The
dependence of the field renormalization on the UV cut-off and on the bare
coupling is also investigated.Comment: 20 pages, 7 figures. To appear in Physical Review
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