arXiv subjects

C. Li

Publications and source records attributed to C. Li.

At least 145 records · Page 8Linked to original sources

arXiv2015

Controversial Age Spreads from the Main Sequence Turn-Off and Red Clump in Intermediate-Age Clusters in the LMC

Most star clusters at an intermediate age (1-2 Gyr) in the Large and Small Magellanic Clouds show a puzzling feature in their color-magnitude diagrams (CMD) that is not in agreement with a simple stellar population. The main sequence turn-off of these clusters is much broader than would be expected from photometric uncertainties. One interpretation of this feature is that age spreads of the order 200-500 Myr exist within individual clusters, although this interpretation is highly debated. Such large age spreads should affect other parts of the CMD, which are sensitive to age, as well. In this study, we analyze the CMDs of a sample of 12 intermediate-age clusters in the Large Magellanic Cloud that all show an extended turn-off using archival optical data taken with the Hubble Space Telescope. We fit the star formation history of the turn-off region and the red clump region independently with two different theoretical isochrone models. We find that in most of the cases, the age spreads inferred from the red clumps are smaller than the ones resulting from the turn-off region. However, the age ranges resulting from the red clump region are broader than would be expected for a single age. Only two out of 12 clusters in our sample show a red clump which seems to be consistent with a single age. As our results are not unambiguous, we can not ultimately tell if the extended main sequence turn-off feature is due to an age spread, or not, by fitting the star formation histories to the red clump regions. However, we find that the width of the extended main sequence turn-off feature is correlated with the age of the clusters in a way which would be unexplained in the "age spread" interpretation, but which may be expected if stellar rotation is the cause of the spread at the turn-off.

astro-ph.SR↗

arXiv2015

Ultrafast Surface Plasmonic Switch in Non-Plasmonic Metals

We demonstrate that ultrafast carrier excitation can drastically affect electronic structures and induce brief surface plasmonic response in non-plasmonic metals, potentially creating a plasmonic switch. Using first-principles molecular dynamics and Kubo-Greenwood formalism for laser-excited tungsten we show that carrier heating mobilizes d electrons into collective inter and intraband transitions leading to a sign flip in the imaginary optical conductivity, activating plasmonic properties for the initial non-plasmonic phase. The drive for the optical evolution can be visualized as an increasingly damped quasi-resonance at visible frequencies for pumping carriers across a chemical potential located in a d-band pseudo-gap with energy-dependent degree of occupation. The subsequent evolution of optical indices for the excited material is confirmed by time-resolved ultrafast ellipsometry. The large optical tunability extends the existence spectral domain of surface plasmons in ranges typically claimed in laser self-organized nanostructuring. Non-equilibrium heating is thus a strong factor for engineering optical control of evanescent excitation waves, particularly important in laser nanostructuring strategies.

physics.optics↗

arXiv2015

New measurement of the excited states in $^{11}$B via the elastic resonance scattering of $^{10}$Be + p

The elastic resonance scattering protons decayed from $^{11}$B to the ground state of $^{10}$Be were measured using the thick-target technique in inverse kinematics at the Heavy Ion Research Facility in Lanzhou (HIRFL). The obtained excitation functions were well described by a multichannel R-matrix procedure under the kinematics process assumption of resonant elastic scattering. The excitation energy of the resonant states ranges from 13.0 to 17.0 MeV, and their resonant parameters such as the resonant energy E$_{x}$, the spin-parity J$^π$, and the proton-decay partial width $Γ_p$ were determined from R-matrix fits to the data. Two of these states around E$_{x}$ = 14.55 MeV [J$^π$ = (3/2$^+$, 5/2$^+$), $Γ_p$ = 475 $\pm$ 80 keV] and E$_{x}$ = 14.74 MeV [J$^π$ = 3/2$^-$, $Γ_p$ = 830 $\pm$ 145 keV], and a probably populated state at E$_x$ = 16.18 MeV [J$^π$ =(1/2$^-$, 3/2$^-$), $Γ_p$ $<$ 60 keV], are respectively assigned to the well-known states in $^{11}$B at 14.34 MeV, 15.29 MeV, and 16.43 MeV. The isospin of these three states were previously determined to be T = 3/2, but discrepancies exist in widths and energies due to the current counting statistics and energy resolution. We have compared these states with previous measurements, and the observation of the possibly populated resonance is discussed.

nucl-ex↗

arXiv2015

Cosmic Ray Test of Mini-drift Thick Gas Electron Multiplier Chamber for Transition Radiation Detector

A thick gas electron multiplier (THGEM) chamber with an effective readout area of 10$\times$10 cm$^{2}$ and a 11.3 mm ionization gap has been tested along with two regular gas electron multiplier (GEM) chambers in a cosmic ray test system. The thick ionization gap makes the THGEM chamber a mini-drift chamber. This kind mini-drift THGEM chamber is proposed as part of a transition radiation detector (TRD) for identifying electrons at an Electron Ion Collider (EIC) experiment. Through this cosmic ray test, an efficiency larger than 94$\%$ and a spatial resolution $\sim$220 $μ$m are achieved for the THGEM chamber at -3.65 kV. Thanks to its outstanding spatial resolution and thick ionization gap, the THGEM chamber shows excellent track reconstruction capability. The gain uniformity and stability of the THGEM chamber are also presented.

physics.ins-det↗

arXiv2014

An accurate measurement of PMT TTS based on the photoelectron spectrum

The water Cherenkov detector array (WCDA) for the large high altitude air shower observatory(LHAASO) will employ more than 3600 hemisphere 8 inch photomultiplier tubes (PMT). The good time performance of PMT, especially the transit time spread (TTS), is required for WCDA. TTS is usually defined as the TTS of single photoelectron, and usually determined by using single photoelectron counting technique. A method using the photoelectron spectrum is researched for the measurement of TTS. The method is appropriate for multi-photoelectrons and makes it possible to measure the TTS of different photoelectrons at the same time. The TTS of different photoelectrons is measured for Hamamatsu R5912 with the divider circuit designed in specifically. The TTS of single photoelectron is determined to 3.3 ns and satisfies the requirement of WCDA.

physics.ins-det↗

arXiv2014

Test of High Time Resolution MRPC with Different Readout Modes

In order to further enhance the particle identification capability of the Beijing Spectrometer (BESIII), it is proposed to upgrade the current end-cap time-of-flight (eTOF) detector with multi-gap resistive plate chamber (MRPC). The prototypes, together with the front end electronics (FEE) and time digitizer (TDIG) module have been tested at the E3 line of Beijing Electron Positron Collider (BEPCII) to study the difference between the single and double-end readout MRPC designs. The time resolutions (sigma) of the single-end readout MRPC are 47/53 ps obtained by 600 MeV/c proton/pion beam, while that of the double-end readout MRPC is 40 ps (proton beam). The efficiencies of three MRPC modules tested by both proton and pion beam are better than 98%. For the double-end readout MRPC, no incident position dependence is observed.

physics.ins-det↗

arXiv2014

Conventional quantum phase transition driven by complex parameter in non-Hermitian PT-symmetric Ising model

A conventional quantum phase transition (QPT) can be accessed by varying a real parameter at absolute zero temperature. Motivated by the discovery of the pseudo-Hermiticity of non-Hermitian systems, we explore the QPT in non-Hermitian PT-symmetric Ising model, which is driven by a staggered complex transverse field. Exact solution shows that the Laplacian of the groundstate energy density, with respect to real and imaginary components of the transverse field, diverges on the boundary in the complex plane. The phase diagram indicate that the imaginary transverse field has the effect of shrinking the paramagnet phase. In addition, we also investigate the connection between the geometric phase and the QPT.

quant-ph↗

arXiv2014

Calibration and performance of the STAR Muon Telescope Detector using cosmic rays

We report the timing and spatial resolution from the Muon Telescope Detector (MTD) installed in the STAR experiment at RHIC. Cosmic ray muons traversing the STAR detector have an average transverse momentum of 6 GeV/c. Due to their very small multiple scattering, these cosmic muons provide an ideal tool to calibrate the detectors and measure their timing and spatial resolution. The values obtained were ~100 ps and ~1-2 cm, respectively. These values are comparable to those obtained from cosmic-ray bench tests and test beams.

physics.ins-det↗

arXiv2014

Type-Ia Supernova Rates to Redshift 2.4 from CLASH: the Cluster Lensing And Supernova survey with Hubble

We present the supernova (SN) sample and Type-Ia SN (SN Ia) rates from the Cluster Lensing And Supernova survey with Hubble (CLASH). Using the Advanced Camera for Surveys and the Wide Field Camera 3 on the Hubble Space Telescope (HST), we have imaged 25 galaxy-cluster fields and parallel fields of non-cluster galaxies. We report a sample of 27 SNe discovered in the parallel fields. Of these SNe, ~13 are classified as SN Ia candidates, including four SN Ia candidates at redshifts z > 1.2. We measure volumetric SN Ia rates to redshift 1.8 and add the first upper limit on the SN Ia rate in the range 1.8 < z < 2.4. The results are consistent with the rates measured by the HST/GOODS and Subaru Deep Field SN surveys. We model these results together with previous measurements at z < 1 from the literature. The best-fitting SN Ia delay-time distribution (DTD; the distribution of times that elapse between a short burst of star formation and subsequent SN Ia explosions) is a power law with an index of -1.00 +0.06(0.09) -0.06(0.10) (statistical) +0.12 -0.08 (systematic), where the statistical uncertainty is a result of the 68% and 95% (in parentheses) statistical uncertainties reported for the various SN Ia rates (from this work and from the literature), and the systematic uncertainty reflects the range of possible cosmic star-formation histories. We also test DTD models produced by an assortment of published binary population synthesis (BPS) simulations. The shapes of all BPS double-degenerate DTDs are consistent with the volumetric SN Ia measurements, when the DTD models are scaled up by factors of 3-9. In contrast, all BPS single-degenerate DTDs are ruled out by the measurements at a >99% significance level.

astro-ph.CO↗

arXiv2013

Up to 700k GPU cores, Kepler, and the Exascale future for simulations of star clusters around black holes

We present direct astrophysical N-body simulations with up to a few million bodies using our parallel MPI/CUDA code on large GPU clusters in China, Ukraine and Germany, with different kinds of GPU hardware. These clusters are directly linked under the Chinese Academy of Sciences special GPU cluster program in the cooperation of ICCS (International Center for Computational Science). We reach about the half the peak Kepler K20 GPU performance for our phi-GPU code [2], in a real application scenario with individual hierarchically block time-steps with the high (4th, 6th and 8th) order Hermite integration schemes and a real core-halo density structure of the modeled stellar systems. The code and hardware are mainly used to simulate star clusters [23, 24] and galactic nuclei with supermassive black holes [20], in which correlations between distant particles cannot be neglected.

astro-ph.IM↗

arXiv2013

Start-to-end simulation of the injector for a compact THz source

Terahertz radiation has broad application prospect due to its ability to penetrate deep into many organic materials without damage caused by ionizing radiations. A FEL-based THz source is the best choice to produce high-power radiation. In this paper, a 14 MeV injector is introduced for generating high-quality beam for FEL, which is composed of an EC-ITC RF gun, compensating coils and a travelling-wave structure. Start-to-end simulation has been done with ASTRA code to verify the design and to optimize parameters. Simulation of operating mode at 6 MeV is also executed.

physics.acc-ph↗

arXiv2013

Asteroseismology of the ZZ Ceti star KUV 11370+4222

KUV 11370+4222 is a ZZ Ceti star discovered in 1996, which has not been observed since then. We performed observations for KUV 11370+4222 in 2010 January. From the Fourier transform spectrum of the light curves, ten independent modes are detected. We searched for the best-fitting model by using the observed periods to match the model periods and get it with the total mass of 0.625 ${\rm M_{\odot}}$, the effective temperature of 10950 K, and the helium mass fraction and hydrogen mass fraction of $10^{-2.2}$ and $10^{-4.0}$, respectively. We have found a triplet of frequency split by rotation, which are $l=1$ modes. Using the frequency shifts we estimate a rotation period of $5.56\pm0.08$ h. Besides it, another $l=1$ mode is identified. The other observed periods are identified as $l=2$ modes. At last we investigated the property of the mode trapping.

astro-ph.SR↗

arXiv2013

Tracking the Evolution of A Coherent Magnetic Flux Rope Continuously from the Inner to the Outer Corona

The magnetic flux rope (MFR) is believed to be the underlying magnetic structure of coronal mass ejections (CMEs). However, it remains unclear how an MFR evolves into and forms the multi-component structure of a CME. In this paper, we perform a comprehensive study of an extreme-ultraviolet (EUV) MFR eruption on 2013 May 22 by tracking its morphological evolution, studying its kinematics, and quantifying its thermal property. As EUV brightenings begin, the MFR starts to rise slowly and shows helical threads winding around an axis. Meanwhile, cool filamentary materials descend spirally down to the chromosphere. These features provide direct observational evidence of intrinsically helical structure of the MFR. Through detailed kinematical analysis, we find that the MFR evolution experiences two distinct phases: a slow rise phase and an impulsive acceleration phase. We attribute the first phase to the magnetic reconnection within the quasi-separatrix-layers surrounding the MFR, and the much more energetic second phase to the fast magnetic reconnection underneath the MFR. We suggest that the transition between these two phases be caused by the torus instability. Moreover, we identify that the MFR evolves smoothly into the outer corona and appears as a coherent structure within the white light CME volume. The MFR in the outer corona was enveloped by bright fronts that originated from plasma pile-up in front of the expanding MFR. The fronts are also associated with the preceding sheath region followed the outmost MFR-driven shock.

astro-ph.SR↗

arXiv2013

Coronal magnetic topology and the production of solar impulsive energetic electrons

We investigate two candidate solar sources or active regions (ARs) in association with a solar impulsive energetic electron (SIEE) event on 2002 October 20. The solar particle release (SPR) times of SIEEs are derived by using their velocity dispersion with consideration of the instrumental effect. It is found that there are double electron injections at the Sun. The low-energy (<13 keV) electron injection coincides with a C6.6 flare in AR10154 and is accompanied with prominent type III radio bursts rather than a stronger M1.8 flare in AR10160. The M1.8 flare produces, however, faint type III radio bursts. Whereas electrons of 25 to 300 keV are released 9 min later when a jet-like CME travels to 2.6 solar radii. We further examine the coronal magnetic configurations above the two ARs based on the potential field source surface (PFSS) model. It is found that open field lines, rooted in AR10154 and well connected to the Earth, provide escaping channels for energetic electrons. Only a small portion of magnetic fields are opened above AR10160, being responsible for the faint type III radio bursts. These lines are, however, not well connected, making it impossible for SIEEs detection by near-Earth spacecraft. The results appear to establish a physical link between coronal magnetic topology, formation of type III radio bursts, and production of SIEEs.

astro-ph.SR↗

arXiv2013

Throughput and Energy Efficiency Analysis of Small Cell Networks with Multi-antenna Base Stations

Small cell networks have recently been proposed as an important evolution path for the next-generation cellular networks. However, with more and more irregularly deployed base stations (BSs), it is becoming increasingly difficult to quantify the achievable network throughput or energy efficiency. In this paper, we develop an analytical framework for downlink performance evaluation of small cell networks, based on a random spatial network model, where BSs and users are modeled as two independent spatial Poisson point processes. A new simple expression of the outage probability is derived, which is analytically tractable and is especially useful with multi-antenna transmissions. This new result is then applied to evaluate the network throughput and energy efficiency. It is analytically shown that deploying more BSs or more BS antennas can always increase the network throughput, but the performance gain critically depends on the BS-user density ratio and the number of BS antennas. On the other hand, increasing the BS density or the number of transmit antennas will first increase and then decrease the energy efficiency if different components of BS power consumption satisfy certain conditions, and the optimal BS density and the optimal number of BS antennas can be found. Otherwise, the energy efficiency will always decrease. Simulation results shall demonstrate that our conclusions based on the random network model are general and also hold in a regular grid-based model.

cs.IT↗

arXiv2013

Solar source of energetic particles in interplanetary space during the 2006 December 13 event

An X3.4 solar flare and a fast halo coronal mass ejection (CME) occurred on 2006 December 13, accompanied by a high flux of energetic particles recorded both in near-Earth space and at ground level. Our purpose is to provide evidence of flare acceleration in a major solar energetic particle (SEP) event. We first present observations from ACE/EPAM, GOES, and the Apatity neutron monitor. It is found that the initial particle release time coincides with the flare emission and that the spectrum becomes softer and the anisotropy becomes weaker during particle injection, indicating that the acceleration source changes from a confined coronal site to a widespread interplanetary CME-driven shock. We then describe a comprehensive study of the associated flare active region. By use of imaging data from HINODE/SOT and SOHO/MDI magnetogram, we infer the flare magnetic reconnection rate in the form of the magnetic flux change rate. This correlates in time with the microwave emission, indicating a physical link between the flare magnetic reconnection and the acceleration of nonthermal particles. Combining radio spectrograph data from Huairou/NOAC, Culgoora/IPS, Learmonth/RSTN, and WAVES/WIND leads to a continuous and longlasting radio burst extending from a few GHz down to several kHz. Based on the photospheric vector magnetogram from Huairou/NOAC and the nonlinear force free field (NFFF) reconstruction method, we derive the 3D magnetic field configuration shortly after the eruption. Furthermore, we also compute coronal field lines extending to a few solar radii using a potential-field source-surface (PFSS) model. Both the so-called type III-l burst and the magnetic field configuration suggest that open-field lines extend from the flare active region into interplanetary space, allowing the accelerated and charged particles escape.

astro-ph.SR↗

arXiv2013

Coronal jets, magnetic topologies, and the production of interplanetary electron streams

We investigate the acceleration source of the impulsive solar energetic particle (SEP) events on 2007 January 24. Combining the in situ electron measurements and remote-sensing solar observations, as well as the calculated magnetic fields obtained from a potential-field source-surface model, we demonstrate that the jets associated with the hard X-ray flares and type-III radio bursts, rather than the slow and partial coronal mass ejections, are closely related to the production of interplanetary electron streams. The jets, originated from the well-connected active region (AR 10939) whose magnetic polarity structure favors the eruption, are observed to be forming in a coronal site, extending to a few solar radii, and having a good temporal correlation with the electron solar release. The open-field lines near the jet site are rooted in a negative polarity, along which energetic particles escape from the flaring AR to the near-Earth space, consistent with the in situ electron pitch angle distribution. The analysis enables us to propose a coronal magnetic topology relating the impulsive SEP events to their solar source.

astro-ph.SR↗

arXiv2013

Electron and proton acceleration during the first ground level enhancement of solar cycle 24

High-energy particles were recorded by near-Earth spacecraft and ground-based neutron monitors (NMs) on 2012 May 17. This event was the first ground level enhancement (GLE) of solar cycle 24. In this study, we try to identify the acceleration source(s) of solar energetic particles by combining in situ particle measurements from the WIND/3DP, GOES 13, and solar cosmic rays registered by several NMs, as well as remote-sensing solar observations from SDO/AIA, SOHO/LASCO, and RHESSI. We derive the interplanetary magnetic field (IMF) path length (1.25 +/- 0.05 AU) and solar particle release time (01:29 +/- 00:01 UT) of the first arriving electrons by using their velocity dispersion and taking into account contamination effects. We found that the electron impulsive injection phase, indicated by the dramatic change in the spectral index, is consistent with flare non-thermal emission and type III radio bursts. Based on the potential field source surface concept, modeling of the open-field lines rooted in the active region has been performed to provide escape channels for flare-accelerated electrons.Meanwhile, relativistic protons are found to be released 10 minutes later than the electrons, assuming their scatter-free travel along the same IMF path length. Combining multi-wavelength imaging data of the prominence eruption and coronal mass ejection (CME), we obtain evidence that GLE protons, with an estimated kinetic energy of 1.12 GeV, are probably accelerated by the CME-driven shock when it travels to 3.07 solar radii. The time-of-maximum spectrum of protons is typical for shock wave acceleration.

astro-ph.SR↗