6,683 research outputs found
Dark matter halo response to the disk growth
We consider the sensitivity of the circular-orbit adiabatic contraction
approximation to the baryon condensation rate and the orbital structure of dark
matter halos in the CDM paradigm. Using one-dimensional hydrodynamic
simulations including the dark matter halo mass accretion history and gas
cooling, we demonstrate that the adiabatic approximation is approximately valid
even though halos and disks may assemble simultaneously. We further demonstrate
the validity of the simple approximation for CDM halos with isotropic
velocity distributions using three-dimensional N-body simulations. This result
is easily understood: an isotropic velocity distribution in a cuspy halo
requires more circular orbits than radial orbits. Conversely, the approximation
is poor in the extreme case of a radial orbit halo. It overestimates the
response a core dark matter halo, where radial orbit fraction is larger.
Because no astronomically relevant models are dominated by low-angular momentum
orbits in the vicinity of the disk and the growth time scale is never shorter
than a dynamical time, we conclude that the adiabatic contraction approximation
is useful in modeling the response of dark matter halos to the growth of a
disk.Comment: 7 pages, 6 figures, accepted for publication in MNRA
Does Exam-targeted Training Help Village Doctors Pass the Certified (Assistant) Physician Exam and Improve Their Practical Skills? A Cross-sectional Analysis of Village Doctors\u27 Perspectives in Changzhou in Eastern China
Background Quality of health care needs to be improved in rural China. The Chinese government, based on the 1999 Law on Physicians, started implementing the Rural Doctor Practice Regulation in 2004 to increase the percentage of certified physicians among village doctors. Special exam-targeted training for rural doctors therefore was launched as a national initiative. This study examined these rural doctorsâ perceptions of whether that training helps them pass the exam and whether it improves their skills. Methods Three counties were selected from the 4 counties in Changzhou City in eastern China, and 844 village doctors were surveyed by a questionnaire in July 2012. Chi-square test and Fisher exact test were used to identify differences of attitudes about the exam and training between the rural doctors and certified (assistant) doctors. Longitudinal annual statistics (1980â2014) of village doctors were further analyzed. Results Eight hundred and forty-four village doctors were asked to participate, and 837 (99.17%) responded. Only 14.93% of the respondents had received physician (assistant) certification. Only 49.45% of the village doctors thought that the areas tested by the certification exam were closely related to the healthcare needs of rural populations. The majority (86.19%) felt that the training program was âvery helpfulâ or âhelpfulâ for preparing for the exam. More than half the village doctors (61.46%) attended the âweekly schoolâ. The village doctors considered the most effective method of learning was âcontinuous training (40.36%)â . The majority of the rural doctors (89.91%) said they would be willing to participate in the training and 96.87% stated that they could afford to pay up to 2000 yuan for it. Conclusions The majority of village doctors in Changzhou City perceived that neither the certification exam nor the training for it are closely related to the actual healthcare needs of rural residents. Policies and programs should focus on providing exam-preparation training for selected rural doctors, reducing training expenditures, and utilizing web-based methods. The training focused on rural practice should be provided to all village doctors, even certified physicians. The government should also adjust the local licensing requirements to attract and recruit new village doctors
Neutron Scattering Measurements of Spatially Anisotropic Magnetic Exchange Interactions in Semiconducting K0.85Fe1.54Se2 (TN=280 K)
We use neutron scattering to study the spin excitations associated with the
stripe antiferromagnetic (AFM) order in semiconducting
KFeSe (= K). We show that the spin wave spectra
can be accurately described by an effective Heisenberg Hamiltonian with highly
anisotropic in-plane couplings at = K. At high temperature (=
K) above , short range magnetic correlation with anisotropic correlation
lengths are observed. Our results suggest that, despite the dramatic difference
in the Fermi surface topology, the in-plane anisotropic magnetic couplings are
a fundamental property of the iron based compounds; this implies that their
antiferromagnetism may originate from local strong correlation effects rather
than weak coupling Fermi surface nesting.Comment: 5 pages, 4 figure
Influence of coastal groundwater salinization on the distribution and risks of heavy metals
Coastal groundwater quality significantly affects the regional sustainable development, due to the water resource shortage in coastal zone. Current studies on coastal groundwater have mainly focused on saline water intrusion and over-extraction. Information on the heavy metal pollution of coastal groundwater with salinization trend is limited. This study investigated heavy metals in groundwater from a typical coastal region with intensive anthropogenic activities and saline water intrusion. The southwestern coastal plains of the study area showed significant groundwater salinization trend. Heavy metals in the coastal groundwater mainly originated from anthropogenic activities and groundwater salinization according to principal component analysis. Relative high concentrations of As, Co, Cr, Cu, Fe, Mn, and Ni occurred in the southwestern coast of the study area with high TDS level, indicating that the concentrations of these heavy metals in shallow aquifer of the study area might be influenced by the groundwater salinization. Average concentrations of heavy metals in groundwater of the study area ranged from 0.03 (Cd) to 686.92 (Fe) mu g/L. Fe was the dominant heavy metal in groundwater with the maximal concentration of 2333.76 mu g/L and exceeding-standard rate reaching 98.23%. Approximately 13.27% of sampling sites showed moderate or higher heavy metal pollution of groundwater based on heavy metal pollution index. Heavy metals in groundwater exerted low ecological risks. Elements Fe, Ni, and As were the main contributors for ecological risks. Cancer risks of heavy metals for both adults and children were high at all sampling sites. Non-cancer risks for adults and children were unacceptable at 4.42% and 17.70% of sampling sites, respectively. The salinization of the coastal groundwater could also lead to the increase in the ecological and health risks of heavy metals in coastal groundwater. These findings provide initial and important information on heavy metals pollution in coastal aquifer with saline water intrusion. (C) 2018 Elsevier B.V. All rights reserved
Liquid-Gas Coexistence and Critical Behavior in Boxed Pseudo-Fermi Matter
A schematic model is presented that allows one to study the behavior of
interacting pseudo-Fermi matter locked in a thermostatic box. As a function of
the box volume and temperature, the matter is seen to show all of the familiar
charactersitics of a Van der Waals gas, which include the coexistence of two
phases under certain circumstances and the presence of a critical point
Non-Thermal X-ray Properties of Rotation Powered Pulsars and Their Wind Nebulae
We present a statistical study of the non-thermal X-ray emission of 27 young
rotation powered pulsars (RPPs) and 24 pulsar wind nebulae (PWNe) by using the
Chandra and the XMM-Newton observations, which with the high spatial
resolutions enable us to spatially resolve pulsars from their surrounding PWNe.
We obtain the X-ray luminosities and spectra separately for RPPs and PWNe, and
then investigate their distribution and relation to each other as well as the
relation with the pulsar rotational parameters. In the pair-correlation
analysis we find that: (1) the X-ray (2-10 keV) luminosities of both pulsar and
PWN (L_{psr} and L_{pwn}) display a strong correlation with pulsar spin down
power Edot and characteristic age, and the scalings resulting from a simple
linear fit to the data are L_{psr} \propto Edot^{0.92 \pm 0.04} and L_{pwn}
\propto Edot^{1.45 \pm 0.08} (68% confidence level), respectively, however,
both the fits are not statistically acceptable; (2) L_{psr} also shows a
possible weak correlation with pulsar period P and period derivative Pdot,
whereas L_{pwn} manifests a similar weak correlation with Pdot only; (3) The
PWN photon index Gamma_{pwn} is positively correlated with L_{pwn} and
L_{pwn}/Edot. We also found that the PWN X-ray luminosity is typically 1 to 10
times larger than that from the underlying pulsar, and the PWN photon indices
span a range of ~1.5 to ~2. The statistic study of PWN spectral properties
supports the particle wind model in which the X-ray emitting electrons are
accelerated by the termination shock of the wind.Comment: 15 pages, 9 figures, 3 Tables, ApJ accepted version. Substantial
revision, especially luminosity uncertainty taken into accounted and one fig
added. Main conclusions unchange
Witness of unsatisfiability for a random 3-satisfiability formula
The random 3-satisfiability (3-SAT) problem is in the unsatisfiable (UNSAT) phase when the clause density α exceeds a critical value αsâ4.267. Rigorously proving the unsatisfiability of a given large 3-SAT instance is, however, extremely difficult. In this paper we apply the mean-field theory of statistical physics to the unsatisfiability problem, and show that a reduction to 3-XORSAT, which permits the construction of a specific type of UNSAT witnesses (Feige-Kim-Ofek witnesses), is possible when the clause density α>19. We then construct Feige-Kim-Ofek witnesses for single 3-SAT instances through a simple random sampling algorithm and a focused local search algorithm. The random sampling algorithm works only when α scales at least linearly with the variable number N, but the focused local search algorithm works for clause density α>cNb with bâ0.59 and prefactor câ8. The exponent b can be further decreased by enlarging the single parameter S of the focused local search algorithm.Peer reviewe
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