698 research outputs found
Amplification, Saturation, and Q Thresholds for Runaway: Growth of Self-Gravitating Structures in Models of Magnetized Galactic Gas Disks
We investigate the susceptibility of gaseous, magnetized galactic disks to
formation of self-gravitating condensations using two-dimensional, local
models. We focus on two issues: (1) determining the threshold condition for
gravitational runaway, taking into account nonlinear effects, and (2)
distinguishing the magneto-Jeans instability (MJI) that arises under
inner-galaxy rotation curves from the modified swing amplification (MSA) that
arises under outer-galaxy rotation curves. For axisymmetric density
fluctuations, instability is known to require a Toomre parameter Q<1. For
nonaxisymmetric fluctuations, any nonzero shear winds up wavefronts such that in linear theory amplification saturates. Any
Q threshold for nonaxisymmetric gravitational runaway must originate from
nonlinear effects. We use numerical magnetohydrodynamic simulations to
demonstrate the anticipated threshold phenomenon, to analyze the nonlinear
processes involved, and to evaluate the critical value for stabilization.
We find for a wide variety of conditions, with the largest
values corresponding to nonzero but subthermal mean magnetic fields. Our
findings for are similar to those inferred from thresholds for active
star formation in the outer regions of spiral galaxies. MJI is distinct from
MSA in that opposition to Coriolis forces by magnetic tension, rather than
cooperation of epicyclic motion with kinematic shear, enables nonaxisymmetric
density perturbations to grow. We suggest that under low-shear inner-disk
conditions, will be larger than that in outer disks by a factor , where and are the respective Alfven and sound
speeds.Comment: 45 pages, 15 figures, Accepted for publication in ApJ; better
postscript figures available from http://www.astro.umd.edu/~kimwt/FIGURES/ ;
for associated Animated GIF movies, see
http://www.astro.umd.edu/~kimwt/MOVIES
Effect of melt conditioning on heat treatment and mechanical properties of AZ31 alloy strips produced by twin roll casting
In the present investigation, magnesium strips were produced by twin roll casting (TRC) and melt conditioned twin roll casting (MC-TRC) processes. Detailed optical microscopy studies were carried out on as-cast and homogenized TRC and MC-TRC strips. The results showed uniform, fine and equiaxed grain structure was observed for MC-TRC samples in as-cast condition. Whereas, coarse columnar grains with centreline segregation were observed in the case of as-cast TRC samples. The solidification mechanisms for TRC and MC-TRC have been found completely divergent. The homogenized TRC and MC-TRC samples were subjected to tensile test at elevated temperature (250-400 °C). At 250 °C, MC-TRC sample showed significant improvement in strength and ductility. However, at higher temperatures the tensile properties were almost comparable, despite of TRC samples having larger grains compared to MC-TRC samples. The mechanism of deformation has been explained by detailed fractures surface and sub-surface analysis carried out by scanning electron and optical microscopy. Homogenized MC-TRC samples were formed (hot stamping) into engineering component without any trace of crack on its surface. Whereas, TRC samples cracked in several places during hot stamping process.EPSRC â LiME, UK and Towards Affordable, Closed-Loop Recyclable Future Low Carbon Vehicle Structures â TARF-LCV(EP/I038616/1), Department of Mechanical Engineering, Imperial College London, UK, Mr. Steve Cook, Mr. Peter Lloyd, Mr. Graham Mitchell and Mr. Carmelo and BCAST, Brunel University London
The Growth of Black Holes and Their Host Spheroids in (Sub)mm-loud QSOs at High Redshift
We study the growth of black holes and stellar population in spheroids at
high redshift using several (sub)mm-loud QSO samples. Applying the same
criteria established in an earlier work, we find that, similar to IR QSOs at
low redshift, the far-infrared emission of these (sub)mm-loud QSOs mainly
originates from dust heated by starbursts. By combining low-z IR QSOs and
high-z (sub)mm-loud QSOs, we find a trend that the star formation rate
(\Mstardot) increases with the accretion rate (\Mdot). We compare the
values of \Mstardot/\Mdot for submm emitting galaxies (SMGs), far-infrared
ultraluminous/hyperluminous QSOs and typical QSOs, and construct a likely
evolution scenario for these objects. The (sub)mm-loud QSO transition phase has
both high \Mdot and \Mstardot and hence is important for establishing the
correlation between the masses of black holes and spheroids.Comment: 19 pages,3 figures,submitted to Chin. J. Astron. Astrophys. This
paper was first prepared for publication on August 10th, 200
Synthesis of Mesoporous Silica@CoâAl Layered Double Hydroxide Spheres: Layer-by-Layer Method and Their Effects on the Flame Retardancy of Epoxy Resins
Hierarchical mesoporous silica@CoâAl layered double hydroxide (m-SiO2@CoâAl LDH) spheres were prepared through a layer-by-layer assembly process, in order to integrate their excellent physical and chemical functionalities. TEM results depicted that, due to the electrostatic potential difference between m-SiO2 and CoâAl LDH, the synthetic m-SiO2@CoâAl LDH hybrids exhibited that m-SiO2 spheres were packaged by the CoâAl LDH nanosheets. Subsequently, the m-SiO2@CoâAl LDH spheres were incorporated into epoxy resin (EP) to prepare specimens for investigation of their flame-retardant performance. Cone results indicated that m-SiO2@CoâAl LDH incorporated obviously improved fire retardant of EP. A plausible mechanism of fire retardant was hypothesized based on the analyses of thermal conductivity, char residues, and pyrolysis fragments. Labyrinth effect of m-SiO2 and formation of graphitized carbon char catalyzed by CoâAl LDH play pivotal roles in the flame retardance enhancement
The Formation of the First Massive Black Holes
Supermassive black holes (SMBHs) are common in local galactic nuclei, and
SMBHs as massive as several billion solar masses already exist at redshift z=6.
These earliest SMBHs may grow by the combination of radiation-pressure-limited
accretion and mergers of stellar-mass seed BHs, left behind by the first
generation of metal-free stars, or may be formed by more rapid direct collapse
of gas in rare special environments where dense gas can accumulate without
first fragmenting into stars. This chapter offers a review of these two
competing scenarios, as well as some more exotic alternative ideas. It also
briefly discusses how the different models may be distinguished in the future
by observations with JWST, (e)LISA and other instruments.Comment: 47 pages with 306 references; this review is a chapter in "The First
Galaxies - Theoretical Predictions and Observational Clues", Springer
Astrophysics and Space Science Library, Eds. T. Wiklind, V. Bromm & B.
Mobasher, in pres
New determination of the 13C(a, n)16O reaction rate and its influence on the s-process nucleosynthesis in AGB stars
We present a new measurement of the -spectroscopic factor
() and the asymptotic normalization coefficient (ANC) for the 6.356
MeV 1/2 subthreshold state of O through the C(B,
Li)O transfer reaction and we determine the -width of this
state. This is believed to have a strong effect on the rate of the
C(, )O reaction, the main neutron source for {\it
slow} neutron captures (the -process) in asymptotic giant branch (AGB)
stars. Based on the new width we derive the astrophysical S-factor and the
stellar rate of the C(, )O reaction. At a temperature
of 100 MK our rate is roughly two times larger than that by \citet{cau88} and
two times smaller than that recommended by the NACRE compilation. We use the
new rate and different rates available in the literature as input in
simulations of AGB stars to study their influence on the abundances of selected
-process elements and isotopic ratios. There are no changes in the final
results using the different rates for the C(, )O
reaction when the C burns completely in radiative conditions. When the
C burns in convective conditions, as in stars of initial mass lower than
2 M_\sun and in post-AGB stars, some changes are to be expected, e.g.,
of up to 25% for Pb in our models. These variations will have to be carefully
analyzed when more accurate stellar mixing models and more precise
observational constraints are available
In situ edge engineering in two-dimensional transition metal dichalcogenides
Exerting synthetic control over the edge structure and chemistry of two-dimensional (2D) materials is of critical importance to direct the magnetic, optical, electrical, and catalytic properties for specific applications. Here, we directly image the edge evolution of pores in Mo1-xWxSe2 monolayers via atomic-resolution in situ scanning transmission electron microscopy (STEM) and demonstrate that these edges can be structurally transformed to theoretically predicted metastable atomic configurations by thermal and chemical driving forces. Density functional theory calculations and ab initio molecular dynamics simulations explain the observed thermally induced structural evolution and exceptional stability of the four most commonly observed edges based on changing chemical potential during thermal annealing. The coupling of modeling and in situ STEM imaging in changing chemical environments demonstrated here provides a pathway for the predictive and controlled atomic scale manipulation of matter for the directed synthesis of edge configurations in Mo-1_xWxSe2 to achieve desired functionality
Dengue Virus Infection-Enhancing Activity in Serum Samples with Neutralizing Activity as Determined by Using FcÎłR-Expressing Cells
Dengue has become a major international public health concern in recent decades. There are four dengue virus serotypes. Recovery from infection with one serotype confers life-long protection to the homologous serotype but only partial protection to subsequent infection with other serotypes. Secondary infection with a serotype different from that in primary infection increases the risk of development of severe complications. Antibodies may play two competing roles during infection: virus neutralization that leads to protection and recovery, or infection-enhancement that may cause severe complications. Progress in vaccine development has been hampered by limited understanding on protective immunity against dengue virus infection. We report the neutralization activity and infection-enhancement activity in individuals with dengue in Malaysia. We show that infection-enhancement activity is present when neutralizing activity is absent or low, and cross-reactive neutralizing activity may be hampered by infection-enhancing activity. Conventional assays for titration of neutralizing antibody do not consider infection-enhancement activity. We used an alternative assay that determines the sum of neutralizing and infection-enhancement activity in sera from dengue patients. In addition to providing insights into antibody responses during infection, the alternative assay provides a new platform for the study of immune responses to vaccine
Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much of modern
astrophysics. For several decades it has been thought that stellar birth is
primarily controlled by the interplay between gravity and magnetostatic
support, modulated by ambipolar diffusion. Recently, however, both
observational and numerical work has begun to suggest that support by
supersonic turbulence rather than magnetic fields controls star formation. In
this review we outline a new theory of star formation relying on the control by
turbulence. We demonstrate that although supersonic turbulence can provide
global support, it nevertheless produces density enhancements that allow local
collapse. Inefficient, isolated star formation is a hallmark of turbulent
support, while efficient, clustered star formation occurs in its absence. The
consequences of this theory are then explored for both local star formation and
galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28
figures, in pres
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