3,048 research outputs found
SPH Simulations of Counterrotating Disk Formation in Spiral Galaxies
We present the results of Smoothed Particle Hydrodynamics (SPH) simulations
of the formation of a massive counterrotating disk in a spiral galaxy. The
current study revisits and extends (with SPH) previous work carried out with
sticky particle gas dynamics, in which adiabatic gas infall and a retrograde
gas-rich dwarf merger were tested as the two most likely processes for
producing such a counterrotating disk. We report on experiments with a cold
primary similar to our Galaxy, as well as a hot, compact primary modeled after
NGC 4138. We have also conducted numerical experiments with varying amounts of
prograde gas in the primary disk, and an alternative infall model (a spherical
shell with retrograde angular momentum). The structure of the resulting
counterrotating disks is dramatically different with SPH. The disks we produce
are considerably thinner than the primary disks and those produced with sticky
particles. The time-scales for counterrotating disk formation are shorter with
SPH because the gas loses kinetic energy and angular momentum more rapidly.
Spiral structure is evident in most of the disks, but an exponential radial
profile is not a natural byproduct of these processes. The infalling gas shells
that we tested produce counterrotating bulges and rings rather than disks. The
presence of a considerable amount of preexisting prograde gas in the primary
causes, at least in the absence of star formation, a rapid inflow of gas to the
center and a subsequent hole in the counterrotating disk. In general, our SPH
experiments yield stronger evidence to suggest that the accretion of massive
counterrotating disks drives the evolution of the host galaxies towards earlier
(S0/Sa) Hubble types.Comment: To appear in ApJ. 20 pages LaTex 2-column with 3 tables, 23 figures
(GIF) available at this site. Complete gzipped postscript preprint with
embedded figures available from http://tarkus.pha.jhu.edu/~thakar/cr3.html (3
Mb
Angular Signatures of Annihilating Dark Matter in the Cosmic Gamma-Ray Background
The extragalactic cosmic gamma-ray background (CGB) is an interesting channel
to look for signatures of dark matter annihilation. In particular, besides the
imprint in the energy spectrum, peculiar anisotropy patterns are expected
compared to the case of a pure astrophysical origin of the CGB. We take into
account the uncertainties in the dark matter clustering properties on
sub-galactic scales, deriving two possible anisotropy scenarios. A clear dark
matter angular signature is achieved when the annihilation signal receives only
a moderate contribution from sub-galactic clumps and/or cuspy haloes.
Experimentally, if galactic foregrounds systematics are efficiently kept under
control, the angular differences are detectable with the forthcoming GLAST
observatory, provided that the annihilation signal contributes to the CGB for a
fraction >10-20%. If, instead, sub-galactic structures have a more prominent
role, the astrophysical and dark matter anisotropies become degenerate,
correspondingly diluting the DM signature. As complementary observables we also
introduce the cross-correlation between surveys of galaxies and the CGB and the
cross-correlation between different energy bands of the CGB and we find that
they provide a further sensitive tool to detect the dark matter angular
signatures.Comment: 13 pages, 8 figures; improved discussion; matches published versio
Maximum gravitational-wave energy emissible in magnetar flares
Recent searches of gravitational-wave (GW) data raise the question of what
maximum GW energies could be emitted during gamma-ray flares of highly
magnetized neutron stars (magnetars). The highest energies (\sim 10^{49} erg)
predicted so far come from a model [K. Ioka, Mon. Not. Roy. Astron. Soc. 327,
639 (2001)] in which the internal magnetic field of a magnetar experiences a
global reconfiguration, changing the hydromagnetic equilibrium structure of the
star and tapping the gravitational potential energy without changing the
magnetic potential energy. The largest energies in this model assume very
special conditions, including a large change in moment of inertia (which was
observed in at most one flare), a very high internal magnetic field, and a very
soft equation of state. Here we show that energies of 10^{48}-10^{49} erg are
possible under more generic conditions by tapping the magnetic energy, and we
note that similar energies may also be available through cracking of exotic
solid cores. Current observational limits on gravitational waves from magnetar
fundamental modes are just reaching these energies and will beat them in the
era of advanced interferometers.Comment: 16 pages, 5 figures, 1 tabl
Equilibrium Configurations of Strongly Magnetized Neutron Stars with Realistic Equations of State
We investigate equilibrium sequences of magnetized rotating stars with four
kinds of realistic equations of state (EOSs) of SLy (Douchin et al.), FPS
(Pandharipande et al.), Shen (Shen et al.), and LS (Lattimer & Swesty).
Employing the Tomimura-Eriguchi scheme to construct the equilibrium
configurations. we study the basic physical properties of the sequences in the
framework of Newton gravity. In addition we newly take into account a general
relativistic effect to the magnetized rotating configurations. With these
computations, we find that the properties of the Newtonian magnetized stars,
e.g., structure of magnetic field, highly depends on the EOSs.
The toroidal magnetic fields concentrate rather near the surface for Shen and
LS EOSs than those for SLy and FPS EOSs. The poloidal fields are also affected
by the toroidal configurations. Paying attention to the stiffness of the EOSs,
we analyze this tendency in detail. In the general relativistic stars, we find
that the difference due to the EOSs becomes small because all the employed EOSs
become sufficiently stiff for the large maximum density, typically greater than
. The maximum baryon mass of the magnetized stars
with axis ratio increases about up to twenty percents for that of
spherical stars. We furthermore compute equilibrium sequences at finite
temperature, which should serve as an initial condition for the hydrodynamic
study of newly-born magnetars. Our results suggest that we may obtain
information about the EOSs from the observation of the masses of magnetars.Comment: submitted to MNRA
SPH simulations of accretion disks and narrow rings
We model a massless viscous disk using Smoothed Particle Hydrodynamics (SPH) and note that it evolves according to the Lynden-Bell \& Pringle theory (1974) until a non-axisymmetric instability develops at the inner edge of the disk. This instability may have the same origin as the instability of initially axisymmetric viscous disks discussed by Lyubarskij et al. (1994). To clarify the evolution we evolved single and double rings of particles. It is actually inconsistent with the SPH scheme to set up a single ring as an initial condition because SPH assumes a smoothed initial state. As would be expected from an SPH simulation, the ring rapidly breaks up into a band. We analyse the stability of the ring and show that the predictions are confirmed by the simulation
Non-adjacent dependency learning in infancy, and its link to language development
To acquire language, infants must learn how to identify words and linguistic structure in speech. Statistical learning has been suggested to assist both of these tasks. However, infants’ capacity to use statistics to discover words and structure together remains unclear. Further, it is not yet known how infants’ statistical learning ability relates to their language development. We trained 17-month-old infants on an artificial language comprising non-adjacent dependencies, and examined their looking times on tasks assessing sensitivity to words and structure using an eye-tracked head-turn-preference paradigm. We measured infants’ vocabulary size using a Communicative Development Inventory (CDI) concurrently and at 19, 21, 24, 25, 27, and 30 months to relate performance to language development. Infants could segment the words from speech, demonstrated by a significant difference in looking times to words versus part-words. Infants’ segmentation performance was significantly related to their vocabulary size (receptive and expressive) both currently, and over time (receptive until 24 months, expressive until 30 months), but was not related to the rate of vocabulary growth. The data also suggest infants may have developed sensitivity to generalised structure, indicating similar statistical learning mechanisms may contribute to the discovery of words and structure in speech, but this was not related to vocabulary size
The effect of calcium aluminates on the coke analogue gasification
The gasification rate in CO2 of the coke analogue containing calcium oxide was studied using analogues doped with alumina (corundum), calcium aluminates (CA6, CA, C3A) and lime minerals. The coke analogue is a laboratory material with simplified carbon structure that has a mineral component with a particle size, size distribution and mineral dispersion that can be controlled. The main focus of this study was to quantitatively assess the effect of calcium in the mineral on the analogue’s reactivity. The analogues were reacted with CO2 isothermally in the temperature range of 1173–1623 K. It was found that the reaction rate increased with increasing CaO activity/number of moles of Ca in the mineral. The relative reaction rates (from lowest to highest) of the analogues doped were alumina, CA6, CA, C3A and lime. The relative apparent activation energies of the gasification from lowest to highest was lime, C3A, CA, CA6 and alumina
De‐novo abnormal uteroplacental circulation in third trimester: pregnancy outcome and pathological implications
Objective
Hypertensive disorders of pregnancy (HDP) are associated with impaired placentation, as evidenced by abnormal uterine artery (UtA) Doppler. In normal pregnancy, mean UtA pulsatility index (PI) shows a progressive decline with gestational age (GA). However, previous studies have reported that a proportion of pregnancies demonstrate worsening UtA Doppler in later pregnancy. The aim of this study was to investigate the incidence of HDP according to the change in mean UtA‐PI between the second and third trimesters.
Methods
This was a retrospective cohort study of singleton pregnancies between March 1997 and March 2016 that underwent longitudinal UtA Doppler assessment in the second and third trimesters. All parameters were converted into centiles or multiples of the median (MoM), adjusting for GA. The study cohort was divided into two groups, according to change in mean UtA‐PI between the second and third trimesters (those with a decrease or no change and those with an increase). HDP included women who developed pre‐eclampsia and/or gestational hypertension. Logistic regression analysis was used to adjust for potential confounders.
Results
The analysis included 5887 pregnancies. The incidence of HDP was significantly higher in the group with increasing mean UtA‐PI compared with those without (7.9% vs 5.8%; P = 0.002). Logistic regression analysis demonstrated that both third‐trimester UtA‐PI MoM (odds ratio (OR), 7.35; 95% CI, 4.66–11.6; P < 0.001) and stable or decrease in UtA‐PI MoM between the second and third trimesters (OR, 0.43; 95% CI, 0.31–0.60; P < 0.001) were significant independent predictors for the development of HDP.
Conclusion
Worsening of UtA Doppler is associated with HDP, independent of the value recorded in the second trimester
Dust distribution in protoplanetary disks - Vertical settling and radial migration
We present the results of a three dimensional, locally isothermal,
non-self-gravitating SPH code which models protoplanetary disks with two
fluids: gas and dust. We ran simulations of a 1 Msun star surrounded by a 0.01
Msun disk comprising 99% gas and 1% dust in mass and extending from 0.5 to ~300
AU. The grain size ranges from 0.001 mm to 10 m for the low resolution (~25 000
SPH particles) simulations and from 0.1 mm to 10 cm for the high resolution
(~160 000 SPH particles) simulations. Dust grains are slowed down by the
sub-Keplerian gas and lose angular momentum, forcing them to migrate towards
the central star and settle to the midplane. The gas drag efficiency varies
according to the grain size, with the larger bodies being weakly influenced and
following marginally perturbed Keplerian orbits, while smaller grains are
strongly coupled to the gas. For intermediate sized grains, the drag force
decouples the dust and gas, allowing the dust to preferentially migrate
radially and efficiently settle to the midplane. The resulting dust
distributions for each grain size will indicate, when grain growth is added,
the regions when planets are likely to form.Comment: Accepted for publication in Astronomy & Astrophysics. 11 pages, 6
figure
Modeling the Geographic Distribution of \u3ci\u3eIxodes scapularis\u3c/i\u3e and \u3ci\u3eIxodes pacificus\u3c/i\u3e (Acari: Ixodidae) in the Contiguous United States
In addition to serving as vectors of several other human pathogens, the black-legged tick, Ixodes scapularis Say, and western black-legged tick, Ixodes pacificus Cooley and Kohls, are the primary vectors of the spirochete (Borrelia burgdorferi) that causes Lyme disease, the most common vector-borne disease in the United States. Over the past two decades, the geographic range of I. pacificus has changed modestly while, in contrast, the I. scapularis range has expanded substantially, which likely contributes to the concurrent expansion in the distribution of human Lyme disease cases in the Northeastern, North-Central and Mid-Atlantic states. Identifying counties that contain suitable habitat for these ticks that have not yet reported established vector populations can aid in targeting limited vector surveillance resources to areas where tick invasion and potential human risk are likely to occur. We used county-level vector distribution information and ensemble modeling to map the potential distribution of I. scapularis and I. pacificus in the contiguous United States as a function of climate, elevation, and forest cover. Results show that I. pacificus is currently present within much of the range classified by our model as suitable for establishment. In contrast, environmental conditions are suitable for I. scapularis to continue expanding its range into northwestern Minnesota, central and northern Michigan, within the Ohio River Valley, and inland from the southeastern and Gulf coasts. Overall, our ensemble models show suitable habitat for I. scapularis in 441 eastern counties and for I. pacificus in 11 western counties where surveillance records have not yet supported classification of the counties as established
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