分类: 天文学 >> 天文学 提交时间: 2024-05-10 合作期刊: 《Research in Astronomy and Astrophysics》
摘要: Infrared dust bubbles play an important role in the study of star formation and the evolution of the interstellar medium. In this work, we study the infrared dust bubble N75 and the infrared dark cloud G38.93 mainly using the tracers C18O, HCO+, HNC and N2H+ observed by the 30 m IRAM telescope. We also study the targets using data from large-scale surveys: GLIMPSE, MIPSGAL, GRS, NRAO VLA Sky Survey and Bolocam Galactic Plane Survey. We found that the C18O emission is morphologically similar to the Spitzer IRAC 8.0 μm emission. The 1.1 mm cold dust emission of G38.93 shows an elongated structure from southwest to northeast. The ionized gas from G38.93 is surrounded by polycyclic aromatic hydrocarbon emission, which may be excited by radiation from G38.93. We found that the identified young stellar objects tend to cluster around G38.93 and are mostly in class II, with several class I cases distributed around N75, but no class II examples. We also found evidence of expanding feedback, which could have triggered star formation.
分类: 天文学 >> 天文学 提交时间: 2024-05-10 合作期刊: 《Research in Astronomy and Astrophysics》
摘要: The initial condition of high-mass star formation is a complex area of study because of the high densities () and low temperatures (Tdust < 18 K) involved. Under such conditions, many molecules become depleted from the gas phase by freezing out onto dust grains. However, the N-bearing and deuterated species could remain gaseous under these extreme conditions, suggesting that they may serve as ideal tracers. In this paper, using the Plateau de Bure Interferometer and Very Large Array observations at 1.3 mm, 3.5 mm, and 1.3 cm, we investigate the possible habitats for NH3, NH2D, H13CN, HC15N, SO, and C18O in eight massive precluster and protocluster clumps G18.17, G18.21, G23.97N, G23.98, G23.44, G23.97S, G25.38, and G25.71. We found that the NH3 cores are in good agreement with the 3.5 mm peak emission, but the NH3 is much more extended than the 3.5 mm emission structure. The SO distributions agree well with the 3.5 mm peaks for the evolved star formation stage, but we did not detect any SO emission in the four earliest star formation sources. C18O is a poor tracer in conditions of the cold (≲18 K) and dense (≳104 cm−3) cores, e.g., the prestellar cores. We also found that the NH2D cores are mainly located in the temperature range of 13.0–20.0 K, and the NH2D lines may be strongly depleted above 20 K.
分类: 光学 >> 量子光学 提交时间: 2023-02-19
摘要: We propose and demonstrate the effectual generation and control of nonparaxial self-accelerating beams by using UV-resin pendant droplets. We show that the geometrical shape of the hanging droplets formed as a result of the interplay between surface tension and gravity offers a natural curvature enabling the generation of nonparaxial self-accelerating beams. By simply adjusting the tilt angle of the surface where the droplets reside, a passing light beam is set to propagate along different curved trajectories, bending into large angles with non-diffracting features superior to a conventional Airy beam. Such self-accelerating beams are directly traced experimentally through the scattered light in yeast-cell suspensions, along with extensive ray tracing and numerical simulations. Furthermore, by modifying the shape of uncured pendant resin droplets in real time, we showcase the dynamical trajectory control of the self-accelerating beams. Our scheme and experimental method may be adopted for droplet-based shaping of other waves such as microfluidic jets and surface acoustic waves.
分类: 物理学 >> 核物理学 提交时间: 2016-09-02
摘要: Reactor neutrino experiments build large-scale detector systems to trap ghost-like neutrinos. In liquid scintillator, a neutral bound state of a positron and an electron named positronium could be formed. The spin triplet state was called ortho-positronium (o-Ps). In this script, an experiment was designed to measure the lifetime of o-Ps, which turned out to be 3.1 ns. A PSD parameter based on photon emission time distribution (PETD) was constructed to discriminate e+/e−. Finally the application of e+/e− discrimination in JUNO experiment was shown. It helps suppression of 8He/9Li backgrounds and improves the sensitivity by 0.6 in χ2 analysis with assumption of σ=1 ns PMT Transit Time Spread, which will bring smearing effect to PETD.
分类: 物理学 >> 地球物理学、天文学和天体物理学 分类: 天文学 >> 星系和宇宙学 提交时间: 2020-09-09
摘要: Motivated by recent work on rotating black hole shadow [Phys. Rev. D101, 084029 (2020)], we investigate the shadow behaviors of rotating Hayward-de Sitter black hole for static observers at a finite distance in terms of astronomical observables. This paper uses the newly introduced distortion parameter in [arXiv:2006.00685] to describe the shadow's shape quantitatively. We show that the spin parameter would distort shadows and the magnetic monopole charge would increase the degree of deformation. At the same time, the distortion could be relieved because of the cosmological constant and the distortion would increase with the distance from the black hole. Besides, the spin parameter, magnetic monopole charge and cosmological constant increase will cause the shadow to shrink.
分类: 物理学 >> 凝聚态:结构、力学和热性能 提交时间: 2017-03-26
摘要: The correlation between Cu precipitates and vacancy-like defects in three Fe-Cu model alloys, Fe-0.15%Cu, Fe-0.3%Cu and Fe-0.6%Cu, irradiated at 400 °C with hydrogen ions at doses from 0.045 to 0.45 dpa, were revealed by positron annihilation spectroscopy. The formation of vacancy-like defects surrounded by tiny Cu precipitates induced the increment of the S and W parameters after irradiation. The peak value of the relative W parameter (ΔW/W), as the high momentum information on Cu precipitates, located in the track region and decreased with irradiation dose increased from 0.045 dpa to 0.45 dpa. The information on high momentum depended on the coverage fraction of the defects by Cu atoms. The effect of the irradiation dose, the irradiation depth and the Cu content on the Cu coverage fraction of the defects has been investigated in the present work
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Ultra-low-frequency gravitational waves (GWs) generated by individual inspiraling supermassive black hole binaries (SMBHBs) in the centers of galaxies may be detected by pulsar timing arrays (PTAs) in the future. These GW signals encoding absolute cosmic distances can serve as bright and dark sirens, having potential to be developed into a precise cosmological probe. Here we show that an SKA-era PTA consisting of 100 millisecond pulsars may observe about 20 bright sirens and 90 dark sirens during a 10-year observation. The bright sirens, together with the CMB data, have comparable capabilities to current mainstream data for measuring the equation of state of dark energy. The dark sirens could make the measurement precision of the Hubble constant far beyond the standard of precision cosmology. Our results indicate that ultra-low-frequency GWs from individual SMBHBs are of great significance in exploring the nature of dark energy and measuring the Hubble constant.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Primordial black holes (PBHs) could be formed if large perturbations are generated on small scales in inflation. We study a toy inflation model with a local minimum. The curvature perturbations are enhanced when the inflaton passes through the local minimum, with more efficient amplification rate than that of quasi-inflection point inflation, leading to the production of PBHs on small scales. For parameter spaces provided in this paper, the PBHs could comprise a fraction of the total dark matter around $0.1\%$--$1\%$.
分类: 物理学 >> 基本粒子与场物理学 提交时间: 2020-09-06
摘要: We regard the background of space-time as a physical system composed of discrete volume elements at the Planck scale and get the internal energy of space-time by Debye model. A temperature-dependent minimum energy limit of the particles is proposed from the thermal motion part of the internal energy. As decreases of the temperature caused by the expansion of the universe, more and more particles would be "released" because of the change of the energy limit, we regard these new particles as a source of dark energy. The minimum energy limit also leads to a corrected number of particles in universe and a modified conservation equation. According to the modified conservation equation, an effective cosmological constant consistent with its observed value is obtained.
分类: 物理学 >> 凝聚态:结构、力学和热性能 提交时间: 2017-03-26
摘要: Dislocations would be induced after plastic deformation, which might change the mechanical properties of solids. FeCrNi austenitic model alloy and its Mo-diluted alloy were cold rolled with different degree of thickness reduction. Positrons are sensitive to point defects, which are easily trapped and annihilated around the trapping sites. The mean positron lifetimes have been used to estimate the average dislocation concentration in solids. Meanwhile, the trapping efficiency μ was calculated from the lifetime results. The trapping efficiency value is estimated about 3.31×10-7 cm3s-1 for FeCrNi alloy and 3.31×10-7 cm3s-1 for Mo-diluted alloy, respectively. The increment of the hardness value during plastic deformation is related to the increase of the dislocation density and dislocation pile up in solids.
分类: 物理学 >> 凝聚态:结构、力学和热性能 提交时间: 2017-03-26
摘要: Chitosan, CS, cross-linked with bivalent palladium has shown enhanced mechanical and thermal properties depending on the transformation of the structure at a microscopic scale. In the present study, CS directly cross-linked by palladium cation membranes (CS-cr-PM) were prepared through solution-casting method. The motion of chitosan chains was great suppressed after crosslinking, making a great reduce of swelling ratio by a water-swelling degree measurement, which led to molecular chain rigidity to be improvement. In order to investigate the chain packing at the molecular level in the ionic cross-linked CS system, the structure of chemically-crosslinked CS is investigated by means of the combined use of wide angle X-ray diffraction (WAXD), Infrared measurements, and a combination of positron annihilation lifetime spectroscopy (PALS) and simultaneous coincidence Doppler broadening (CDB) spectroscopy offers coherent information on both the free-volume related sub-nano level molecular packing and the chemical surrounding of free volume nanoholes in CS-cr-PM as a function of palladium salt loading. The variations in free volume size and size distribution have been determined through the ortho-positroium (o-Ps) lifetime and its lifetime distribution. The studies showed that strong interaction between CS molecules and palladium cations results in the change of crystallinity in formed CS-cr-PM leading to variational chain packing density. Meanwhile, significant inhibition effects on positronium formation due to doping are observed, which could be interpreted in terms of the existence of chlorid ion. Applications of positron annihilation spectroscopy to study the microstructure and correlated positron annihilation characteristics in ionic cross-linked CS system are systematic discussed.
分类: 物理学 >> 凝聚态:结构、力学和热性能 提交时间: 2017-03-26
摘要: Fe–0.6%Cu alloy was irradiated with H ions to 0.1 dpa, and then annealed for 30 min from 150 oC to 500 oC. We focused the evolution of Cu precipitates in irradiated Fe–0.6%Cu alloy after the isochronal annealing from the perspective of positron annihilation. The ΔW parameters after thermal annealing (400 oC and 500 oC) were much larger than that induced by 0.1 dpa H irradiation. Annealing could promote the aggregation of the Cu-vacancy complexes, and form the Cu cluster–vacancies complexes. When the vacancy-like defects recovered around 500 oC, it meant the formation and growing of the defect-free Cu precipitates.
分类: 物理学 >> 凝聚态:结构、力学和热性能 提交时间: 2017-03-26
摘要: The formation of Cu precipitates in Fe-0.3%Cu binary model alloy after hydrogen ion irradiation at 400 was investigated by positron annihilation spectroscopy based on slow positron beam. In order to study the effect of elevated temperature for the Cu precipitates, the annealing treatment at 400 in Fe-0.3%Cu alloy was also investigated. The S-parameters of specimens increased with the irradiation dose increased, especially in the damage peak region. H+ implantation produced a large number of vacancy-type defects in Fe-0.3%Cu alloy. Compared to the unirradiated samples, the irradiated specimens show an overall major increase in W parameter values. Experimental results indicate that Cu precipitates formed easily under lower irradiation dose at elevated temperature. No obvious Cu precipitates formed when Fe-0.3%Cu alloy was annealed for 2h at 400 .
分类: 物理学 >> 凝聚态:结构、力学和热性能 提交时间: 2017-03-23
摘要: The effect of annealing on VmHn complexes and Cu precipitate behaviors in hydrogen ion irradiated Fe and Fe-0.3%Cu alloys was investigated by positron annihilation spectroscopy using a slow positron beam. The results of S parameters indicated that the room temperature irradiation was benefit for the formation of the VmHn complex compared to the elevated temperature irradiation. The S-W results confirmed the formation of Cu precipitates in Fe-0.3%Cu even at the irradiation dose of 0.1 dpa. The formation of the evident S value peaks in the damage region after annealing treatment suggested that the VmHn complexes were broken and a larger of hydrogen atoms were escaping. The residual vacancy defects would migrate towards both the surface region and the opposite direction with the increasing annealing temperature.
分类: 物理学 >> 凝聚态:结构、力学和热性能 提交时间: 2017-03-26
摘要: Doppler broadening and Coincidence Doppler broadening of annihilation radiation experiments have been performed in three kinds of polyethylene glycol (PEG) membrane formed with different average molecular weight using the tunable monoenergy slow positron probe as a function of implantion energy. The obtained positron annihilation parameters are interpreted from two aspects: surface effect and differences in micro-structure or chemical environment of positron annihilation. The experimental results show that the regulation of densification of PEG molecular packing and distribution uniformity from the near surface layer to bulk region in film forming process can be well realized by changing its molecular weight. Combining a variable monoenergetic slow positron beam and these two positron annihilation spectroscopy methods are powerful tool to study positron annihilation characteristics and for polymeric thin-film fine structure analysis.
分类: 物理学 >> 核物理学 提交时间: 2023-12-13
摘要: Simulating the total ionizing dose (TID) of an electrical system using transistor-level models can be difficult and expensive, particularly for digital integrated circuits (ICs). In this study, a method for modeling TID effects in complementary metal-oxide semiconductor (CMOS) digital ICs based on the input/output buffer information specification (IBIS) was proposed. The digital IC was first divided into three parts based on its internal structure: the input buffer, output buffer, and functional area. Each of these three parts was separately modeled. Using the IBIS model, the transistor V-I characteristic curves of the buffers were processed, and the physical parameters were extracted and modeled using VHDL-AMS. In the functional area, logic functions were modeled in VHDL according to the data sheet. A golden digital IC model was developed by combining the input buffer, output buffer, and functional area models. Furthermore, the golden ratio was reconstructed based on TID experimental data, enabling the assessment of TID effects on the threshold voltage, carrier mobility, and time series of the digital IC. TID experiments were conducted using a CMOS noninverting multiplexer, NC7SZ157, and the results were compared with the simulation results, which showed that the relative errors were less than 2% at each dose point. This confirms the practicality and accuracy of the proposed modeling method. The TID effect model for digital ICs developed using this modeling technique includes both the logical function of the IC and changes in electrical properties and functional degradation impacted by TID, which has potential applications in the design of radiation-hardening tolerance in digital ICs.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Ultra-low-frequency gravitational waves (GWs) generated by individual inspiraling supermassive black hole binaries (SMBHBs) in the centers of galaxies may be detected by pulsar timing arrays (PTAs) in the future. These GW signals encoding absolute cosmic distances can serve as bright and dark sirens, having potential to be developed into a precise cosmological probe. Here we show that an SKA-era PTA consisting of 100 millisecond pulsars may observe about 20 bright sirens and 90 dark sirens during a 10-year observation. The bright sirens, together with the CMB data, have comparable capabilities to current mainstream data for measuring the equation of state of dark energy. The dark sirens could make the measurement precision of the Hubble constant far beyond the standard of precision cosmology. Our results indicate that ultra-low-frequency GWs from individual SMBHBs are of great significance in exploring the nature of dark energy and measuring the Hubble constant.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: In radio astronomy, radio frequency interference (RFI) becomes more and more serious for radio observational facilities. The RFI always influences the search and study of the interesting astronomical objects. Mitigating the RFI becomes an essential procedure in any survey data processing. Five-hundred-meter Aperture Spherical radio Telescope (FAST) is an extremely sensitive radio telescope. It is necessary to find out an effective and precise RFI mitigation method for FAST data processing. In this work, we introduce a method to mitigate the RFI in FAST spectral observation and make a statistics for the RFI using around 300 hours FAST data. The details are as follows. Firstly, according to the characteristics of FAST spectra, we propose to use the ArPLS algorithm for baseline fitting. Our test results show that it has a good performance. Secondly, we flag the RFI with four strategies, which are to flag extremely strong RFI, flag long-lasting RFI, flag polarized RFI, and flag beam-combined RFI, respectively. The test results show that all the RFI above a preset threshold could be flagged. Thirdly, we make a statistics for the probabilities of polarized XX and YY RFI in FAST observations. The statistical results could tell us which frequencies are relatively quiescent. With such statistical data, we are able to avoid using such frequencies in our spectral observations. Finally, based on the around 300 hours FAST data, we got an RFI table, which is the most complete database currently for FAST.
分类: 物理学 >> 凝聚态:结构、力学和热性能 提交时间: 2017-03-26
摘要: The contribution of positron source for the results of a positron annihilation lifetime spectrum (PALS) is simulated using Geant4 code. The geometrical structure of PALS measurement system is a sandwich structure: the 22Na radiation source is encapsulated by Kapton films, and the specimens are attached on the outside of the films. The probabilities of a positron being annihilated in the films, annihilated in the targets, and the effect of positrons reflected back from the specimen surface, are simulated. The probability of a positron annihilated in the film is related to the species of targets and the source film thickness. The simulation result is in reasonable agreement with the available experimental data. Thus, modification of the source contribution calculated by Geant4 is viable, and it beneficial for the analysis of the results of PALS.
分类: 物理学 >> 凝聚态:结构、力学和热性能 提交时间: 2017-03-26
摘要: Solution annealed type 316L austenitic stainless steels were irradiated using 2 MeV Fe ions at room temperature. The implanted fluences were 2012 ions/cm2 and 1013 ions/cm2, respectively. Variable mono-energetic positron beam was performed to characterize the evolution of microstructure and irradiation induced defects. Results show that large amount of vacancy defects formed after heavy ion irradiation. In which, some of mono-vacancies might migrate to form small-sized clusters at room temperature. After irradiation, implanted Fe atoms mainly be interstitials atoms, but some Fe atoms might recombine with vacancies due to their high mobility, which could decrease the defect concentration, effectively.