15,715 research outputs found
Identification of the Sequence of Steps Intrinsic to Spheromak Formation
A planar coaxial electrostatic helicity source is used for studying the relaxation process intrinsic to spheromak formation Experimental observations reveal that spheromak formation involves: (1) breakdown and creation of a number of distinct, arched, filamentary, plasma-filled flux loops that span from cathode to anode gas nozzles, (2) merging of these loops to form a central column, (3) jet-like expansion of the central column, (4) kink instability of the central column, (5) conversion of toroidal flux to poloidal flux by the kink instability. Steps 1 and 3 indicate that spheromak formation involves an MHD pumping of plasma from the gas nozzles into the magnetic flux tube linking the nozzles. In order to measure this pumping, the gas puffing system has been modified to permit simultaneous injection of different gas species into the two ends of the flux tube linking the wall. Gated CCD cameras with narrow-band optical filters are used to track the pumped flows
Distortion of genealogical properties when the sample is very large
Study sample sizes in human genetics are growing rapidly, and in due course
it will become routine to analyze samples with hundreds of thousands if not
millions of individuals. In addition to posing computational challenges, such
large sample sizes call for carefully re-examining the theoretical foundation
underlying commonly-used analytical tools. Here, we study the accuracy of the
coalescent, a central model for studying the ancestry of a sample of
individuals. The coalescent arises as a limit of a large class of random mating
models and it is an accurate approximation to the original model provided that
the population size is sufficiently larger than the sample size. We develop a
method for performing exact computation in the discrete-time Wright-Fisher
(DTWF) model and compare several key genealogical quantities of interest with
the coalescent predictions. For realistic demographic scenarios, we find that
there are a significant number of multiple- and simultaneous-merger events
under the DTWF model, which are absent in the coalescent by construction.
Furthermore, for large sample sizes, there are noticeable differences in the
expected number of rare variants between the coalescent and the DTWF model. To
balance the tradeoff between accuracy and computational efficiency, we propose
a hybrid algorithm that utilizes the DTWF model for the recent past and the
coalescent for the more distant past. Our results demonstrate that the hybrid
method with only a handful of generations of the DTWF model leads to a
frequency spectrum that is quite close to the prediction of the full DTWF
model.Comment: 27 pages, 2 tables, 14 figure
Stable dilute supersolid of two-dimensional dipolar bosons
We consider two-dimensional bosonic dipoles oriented perpendicularly to the
plane. On top of the usual two-body contact and long-range dipolar interactions
we add a contact three-body repulsion as expected, in particular, for dipoles
in the bilayer geometry with tunneling. The three-body repulsion is crucial for
stabilizing the system, and we show that our model allows for stable continuous
space supersolid states in the dilute regime and calculate the zero-temperature
phase diagram.Comment: revised version, 5 pages, 2 figures, with 3 pages supplementary
materia
Cluster Formation in Protostellar Outflow-Driven Turbulence
Most, perhaps all, stars go through a phase of vigorous outflow during
formation. We examine, through 3D MHD simulation, the effects of protostellar
outflows on cluster formation. We find that the initial turbulence in the
cluster-forming region is quickly replaced by motions generated by outflows.
The protostellar outflow-driven turbulence (``protostellar turbulence'' for
short) can keep the region close to a virial equilibrium long after the initial
turbulence has decayed away. We argue that there exist two types of turbulence
in star-forming clouds: a primordial (or ``interstellar'') turbulence and a
protostellar turbulence, with the former transformed into the latter mostly in
embedded clusters such as NGC 1333. Since the majority of stars are thought to
form in clusters, an implication is that the stellar initial mass function is
determined to a large extent by the stars themselves, through outflows which
individually limit the mass accretion onto forming stars and collectively shape
the environments (density structure and velocity field) in which most cluster
members form. We speculate that massive cluster-forming clumps supported by
protostellar turbulence gradually evolve towards a highly centrally condensed
``pivotal'' state, culminating in rapid formation of massive stars in the
densest part through accretion.Comment: 11 pages (aastex format), 2 figures submitted to ApJ
Molecular Gas Content of HI Monsters and Implications to Cold Gas Content Evolution in Galaxies
We present 12CO (J=1-0) observations of a sample of local galaxies
(0.04<z<0.08) with a large neutral hydrogen reservoir, or "HI monsters". The
data were obtained using the Redshift Search Receiver on the FCRAO 14 m
telescope. The sample consists of 20 HI-massive galaxies with M(HI)>3e10Msun
from the ALFALFA survey and 8 LSBs with a comparable M(HI) (>1.5e10Msun). Our
sample selection is purely based on the amount of neutral hydrogen, thereby
providing a chance to study how atomic and molecular gas relate to each other
in these HI-massive systems. We have detected CO in 15 out of 20 ALFALFA
selected galaxies and 4 out of 8 LSBs with molecular gas mass M(H2) of
(1-11)e9Msun. Their total cold gas masses of (2-7e10Msun make them some of the
most gas-massive galaxies identified to date in the Local Universe. Observed
trends associated with HI, H2, and stellar properties of the HI massive
galaxies and the field comparison sample are analyzed in the context of
theoretical models of galaxy cold gas content and evolution, and the importance
of total gas content and improved recipes for handling spatially differentiated
behaviors of disk and halo gas are identified as potential areas of improvement
for the modeling.Comment: 18 pages, 11 figures, 2 tables; Accepted for publication in MNRA
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