A WENO Algorithm of the Temperature and Ionization Profiles around a Point Source

We develop a numerical solver for radiative transfer problems based on the weighted essentially nonoscillatory (WENO) scheme modified with anti-diffusive flux corrections, in order to solve the temperature and ionization profiles around a point source of photons in the reionization epoch. Algorithms for such simulation must be able to handle the following two features: 1. the sharp profiles of ionization and temperature at the ionizing front (I-front) and the heating front (T-front), and 2. the fraction of neutral hydrogen within the ionized sphere is extremely small due to the stiffness of the rate equations of atom processes. The WENO scheme can properly handle these two features, as it has been shown to have high order of accuracy and good convergence in capturing discontinuities and complicated structures in fluid as well as to be significantly superior over piecewise smooth solutions containing discontinuities. With this algorithm, we show the time-dependence of the preheated shell around a UV photon source. In the first stage the I-front and T-front are coincident, and propagate with almost the speed of light. In later stage, when the frequency spectrum of UV photons is hardened, the speeds of propagation of the ionizing and heating fronts are both significantly less than the speed of light, and the heating front is always beyond the ionizing front. In the spherical shell between the I- and T-fronts, the IGM is heated, while atoms keep almost neutral. The time scale of the preheated shell evolution is dependent on the intensity of the photon source. We also find that the details of the pre-heated shell and the distribution of neutral hydrogen remained in the ionized sphere are actually sensitive to the parameters used. The WENO algorithm can provide stable and robust solutions to study these details.Comment: 24 pages, 7 figures, accepted in New Astronom

A WENO algorithm for radiative transfer with resonant scattering: the time scale of the Wouthuysen-Field Coupling

We develop a numerical solver for the integral-differential equations, which describes the radiative transfer of photon distribution in the frequency space with resonant scattering of Lyalpha photons by hydrogen gas in the early universe. The time-dependent solutions of this equation is crucial to the estimation of the effect of the Wouthuysen-Field (WF) coupling in relation to the 21 cm emission and absorption at the epoch of reionization. The resonant scattering leads to the photon distribution in the frequency space to be piecewise smooth containing sharp changes. The weighted essentially nonoscillatory (WENO) scheme is suitable to handle this problem, as this algorithm has been found to be highly stable and robust for solving Boltzmann equation. We test this numerical solver against analytic solutions of the evolution of the photon distribution in rest background, analytic solution in expanding background without resonant scattering and formation of local Boltzmann distribution around the resonant frequency with the temperature same as that of atom for recoil. We find that evolution of photon distribution undergoes three phases; profile is similar to the initial one, a flat plateau (without recoil) or local Boltzmann distribution (with recoil) forms around the resonant frequency, and finally the distribution around the resonant frequency is saturated when the photons from the source is balanced by the redshift of the expansion. This result indicates that the onset of the W-F coupling should not be determined by the third phase, but by the time scale of the second phase. We found that the time scale of the W-F coupling is equal to about a few hundreds of the mean free flight time of photons with resonant frequency, and is independent of the Sobolev parameter if this parameter is much less than 1.Comment: 20 pages, 13 figure

A WENO Algorithm for the Radiative Transfer and Ionized Sphere at Reionization

We show that the algorithm based on the weighted essentially nonoscillatory (WENO) scheme with anti-diffusive flux corrections can be used as a solver of the radiative transfer equations. This algorithm is highly stable and robust for solving problems with both discontinuities and smooth solution structures. We test this code with the ionized sphere around point sources. It shows that the WENO scheme can reveal the discontinuity of the radiative or ionizing fronts as well as the evolution of photon frequency spectrum with high accuracy on coarse meshes and for a very wide parameter space. This method would be useful to study the details of the ionized patch given by individual source in the epoch of reionization. We demonstrate this method by calculating the evolution of the ionized sphere around point sources in physical and frequency spaces. It shows that the profile of the fraction of neutral hydrogen and the ionized radius are sensitively dependent on the intensity of the source.Comment: Elsart Latex file, 20 pages, 8 figures included, accepted for publication in New Astronom

Colorectal cancer screening with fecal occult blood test: A 22-year cohort study.

The aim of the present study was to investigate the efficacy of colorectal cancer (CRC) screening with a three-tier fecal occult blood test (FOBT) in the Chinese population. The study was performed between 1987 and 2008 at the Beijing Military General Hospital, in a cohort of army service males and females aged >50 years. Between 1987 and 2005, a three-tier screening program, comprising guaiac-based FOBTs (gFOBTs), followed by immunochemical FOBTs for positive guaiac test samples and then colonoscopy for positive immunochemical test subjects, was performed annually. The cohort was followed up until 2008. The cohort included 5,104 subjects, of which, 3,863 subjects participated in screening (screening group) and 1,241 did not (non-screening group). The two groups did not differ in age, gender or other major risk factors for colon cancer. Overall, 36 CRCs occurred in the screening group and 21 in the non-screening group. Compared with the non-screening group, the relative risk for the incidence and mortality of CRC was 0.51 [95% confidence interval (CI), 0.30-0.87] and 0.36 (95% CI, 0.18-0.71), respectively, in the screening group. The general sensitivity of this three-tier FOBT was 80.6% (95% CI, 65.3-91.1). Thus, annual screening using the three-tier FOBT program may reduce the CRC incidence and mortality rate

Geochemical characteristics of surface dune sand in the Mu Us Desert, Inner Mongolia, and implications for reconstructing the paleoenvironment

We present a new dataset from 28 active and 13 stabilizedesemi stabilized dunefields in the Mu Us Desert to report the geochemical macroscale properties as well as reveal the relationship between various chemical ratios and modern climate conditions among different types of Mu Us sandy landscapes. We find that several chemical-weathering indexes, such as CIA, CIW, CPA, and WIP, can be used for the reconstruction of the paleoclimate and servel conditions. One should be cautious in interpreting the weathering intensity using these chemical ratios at a given deposition site when the geochemical background is unknown. This preliminary geochemistry study shows that stabilizedesemi stabilized dunefields, which are influenced by Asian summer monsoon (ASM) precipitation, are analogous to buried paleosols, whereas active dunefields, which are controlled by Asian winter monsoon (AWM) wind, resemble paleo-dune sand. The comparison with the geochemical results from an excellent dune-paleosol succession implies that stronger ASM and AWM periods could have recurred 8-9 times in the Mu Us Desert during the early Holocene. (C) 2017 Published by Elsevier Ltd

21 cm Signals from Early Ionizing Sources

We investigate the 21 cm signals from the UV ionizing sources in the reionization epoch. The formation and evolution of 21 cm emission and absorption regions depend essentially on the kinetics of photons in the physical and frequency spaces. To solve the radiative transfer equation, we use the WENO algorithm, which is effective to capture the sharp ionization profile and the cut-off at the front of light $(r=ct)$ and to handle the small fraction of neutral hydrogen and helium in the ionized sphere. We show that a spherical shell of 21 cm emission and absorption will develop around a point source once the speed of the ionization front (I-front) is significantly lower than the speed of light. The 21 cm shell extends from the I-front to the front of light; its inner part is the emission region and its outer part is the absorption region. The 21 cm emission region depends strongly on the intensity, frequency-spectrum and life-time of the UV ionizing source. For a source of short life-time, no 21 cm emission region can be formed if the source dies out before the I-front speed is significantly lower than the speed of light. Yet, a 21 cm absorption region can form and develop even after the emission of the source ceases.Comment: 25 pages, 9 figures, accepted by Ap

Rapid environmental changes in the Lake Qinghai basin during the late Holocene

The Lake Qinghai Basin is sensitive to global and regional climate change because of its unique geographical location. It is the hotspot for paleoclimate research in East Asia. In this study, we reconstructed the environmental evolution of the Lake Qinghai since ∼9 ka by using a high-resolution peat and fluvial-lacustrine record (Laoyinggou profile) obtained at the foot of Nanshan Mountain. Based on 8 AMS14C dates and lithology, loss on ignition (LOI), total organic matter (TOC), X-ray fluorescence (XRF) core-scanning measurements, ratio of total organic carbon to nitrogen (TOC/TN), and sediment particle sorting coefficients, we show that during the Middle Holocene (∼9–4.4 ka BP) this region was primarily dominated by the Asian summer monsoon, with a consistent, warm, and humid environment. By contrast, during the late Holocene (4.4 ka to present), the climatic context in this area fluctuated dramatically at the millennial scales. The low TOC content, lower TOC/TN ration and strong hydroclimatic indicate six rapid climate change events, which occurred at ∼4.0 ka, ∼3.6 ka, ∼3.2 ka, ∼2.8 ka, ∼2.1 ka, and ∼1.4 ka, all of which coincided to cold episodes in the North Atlantic Ocean