| 1. |
- Li, Wenxiong, et al.
(författare)
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Can the Helium-detonation Model Explain the Observed Diversity of Type Ia Supernovae?
- 2021
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 906:2
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Tidskriftsartikel (refereegranskat)abstract
- We study a sample of 16 Type Ia supernovae (SNe Ia) having both spectroscopic and photometric observations within 2-3 days after the first light. The early B - V colors of such a sample tend to show a continuous distribution. For objects with normal ejecta velocity (NV), the C ii lambda 6580 feature is always visible in the early spectra, while it is absent or very weak in the high-velocity (HV) counterpart. Moreover, the velocities of the detached high-velocity features (HVFs) of the Ca II near-IR triplet (CaIR3) above the photosphere are found to be much higher in HV objects than in NV objects, with typical values exceeding 30,000 km s(-1) at 2-3 days. We further analyze the relation between the velocity shift of late-time [Fe II] lines (v([Fe II])) and host galaxy mass. We find that all HV objects have redshifted v([Fe II]), while NV objects have both blue- and redshifted v([Fe II]). It is interesting to point out that the objects with redshifted v([Fe II]) are all located in massive galaxies, implying that HV and a portion of NV objects may have similar progenitor metallicities and explosion mechanisms. We propose that, with a geometric/projected effect, the He-detonation model may account for the similarity in birthplace environment and the differences seen in some SNe Ia, including B - V colors, C II features, CaIR3 HVFs at early times, and v([Fe II]) in the nebular phase. Nevertheless, some features predicted by He-detonation simulation, such as the rapidly decreasing light curve, deviate from the observations, and some NV objects with blueshifted nebular v([Fe II]) may involve other explosion mechanisms.
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| 2. |
- Sai, Hanna, et al.
(författare)
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Observations of the very young Type Ia Supernova 2019np with early-excess emission
- 2022
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 514:3, s. 3541-3558
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Tidskriftsartikel (refereegranskat)abstract
- Early-time radiative signals from Type Ia supernovae (SNe Ia) can provide important constraints on the explosion mechanism and the progenitor system. We present observations and analysis of SN 2019np, a nearby SN Ia discovered within 1–2 days after the explosion. Follow-up observations were conducted in optical, ultraviolet, and near-infrared bands, covering the phases from ∼−16.7 d to ∼+ 367.8 d relative to its B-band peak luminosity. The photometric and spectral evolutions of SN 2019np resemble the average behaviour of normal SNe Ia. The absolute B-band peak magnitude and the post-peak decline rate are Mmax(B) = −19.52 ± 0.47 mag and Δm15(B) = 1.04 ± 0.04 mag, respectively. No Hydrogen line has been detected in the nebular-phase spectra of SN 2019np. Assuming that the 56Ni powering the light curve is centrally located, we find that the bolometric light curve of SN 2019np shows a flux excess up to 5.0 per cent in the early phase compared to the radiative diffusion model. Such an extra radiation perhaps suggests the presence of an additional energy source beyond the radioactive decay of central nickel. Comparing the observed colour evolution with that predicted by different models, such as interactions of SN ejecta with circumstellar matter (CSM)/companion star, a double-detonation explosion from a sub-Chandrasekhar mass white dwarf (WD) and surface 56Ni mixing, we propose that the nickel mixing is more favoured for SN 2019np.
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| 3. |
- Lin, Weili, et al.
(författare)
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A superluminous supernova lightened by collisions with pulsational pair-instability shells
- 2023
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Ingår i: Nature Astronomy. - 2397-3366. ; 7:7, s. 779-789
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Tidskriftsartikel (refereegranskat)abstract
- Superluminous supernovae are among the most energetic stellar explosions in the Universe, but their energy sources remain an open question. Here we present long-term observations of one of the closest examples of the hydrogen-poor superluminous supernovae subclass SLSNe-I, supernova SN 2017egm, revealing the most complicated known luminosity evolution of SLSNe-I. Three distinct post-peak bumps were recorded in its light curve collected at about 100–350 days after maximum brightness, challenging current popular power models such as magnetar, fallback accretion, and interaction between ejecta and a circumstellar shell. However, the complex light curve can be well modelled by successive interactions with multiple circumstellar shells with a total mass of about 6.8–7.7 M⊙. In this scenario, large energy deposition from interaction-induced reverse shocks results in ionization of neutral oxygen in the supernova ejecta and hence a much lower nebular-phase line ratio of [O I] λ6,300/([Ca II] + [O II]) λ7,300 (~0.2) compared with that derived for other superluminous and normal stripped-envelope supernovae. The pre-existing multiple shells indicate that the progenitor of SN 2017egm experienced pulsational mass ejections triggered by pair instability within 2 years before explosion, in robust agreement with theoretical predictions for a pre-pulsation helium-core mass of 48–51 M⊙.
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| 4. |
- Meng, Qingpeng, et al.
(författare)
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Geochemical speciation and risk assessment of metals in the river sediments from Dan River Drainage, China
- 2016
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Ingår i: Chemistry in ecology. - : Informa UK Limited. - 0275-7540 .- 1029-0370. ; 32:3, s. 221-237
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Tidskriftsartikel (refereegranskat)abstract
- This study investigates the partitioning of metals in surface sediments from the Dan River drainage basin, the source of drinking water for the South to North Water Transfer Project, China, to determine their bioavailability and associated levels of risk. Sediment samples were collected from 99 sites along the Dan River, and the concentration of each element fraction was determined using sequential extraction and inductively coupled plasma-mass spectrometry. The residual fraction was the major phase for most metals sampled. Among the non-residual fractions, greater proportions of Zn, Mn and Cd were transported in in the available phase, whereas Ba, Sb, Pb and As were mainly found in the reducible phase. The oxidisable phase was important for the transport of Ni, Co, Cr and Cu in sediments. This analysis of sediments from the Dan River basin indicates a very high risk of pollution from Cd, Co, Mn, Sb and Zn. Combined with the sediments with extremely high concentrations, the Dan, Laoguan and Yinhua rivers, which have been affected by ore-mining activities, pose a very high risk to the surrounding areas and should be the subject of future studies.
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| 5. |
- Zhang, Hongwei, et al.
(författare)
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Influences of Flow Intensity, Cooling Rate and Nucleation Density at Ingot Surface on Deflective Growth of Dendrites for Al-based Alloy
- 2009
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Ingår i: ISIJ International. - : Iron and Steel Institute of Japan. - 0915-1559 .- 1347-5460. ; 49:7, s. 1010-1018
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Tidskriftsartikel (refereegranskat)abstract
- The dendrite tip growth kinetics in the flow field and the decentred quadrilateral growth algorithm for describing the evolution of grain growth are combined in Cellular Automaton model to predict the deflective growth of dendrites inclined toward upstream direction. The influences of flow intensity, cooling rate (or solidification rate), nucleation density at ingot surface on the deflective growth of dendrites are discussed. The increase of flow intensity dominantly forces the dendrites to grow in upstream direction; on the contrary, the increases of nucleation density at ingot surface and cooling rate suppress slightly this deflective growth. The relations predicted among deflection angle, flow intensity and solidification rate for AI-Si alloy and Al-Cu alloy show the same tendency as that in Okano et al.'s empirical expression deduced from experiments on steel. The deflection angle predicted for Al-Cu alloy fits well with previous experimental results.
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| 6. |
- Zhang, Hongwei, et al.
(författare)
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Partial Equilibrium Prediction of Solidification and Carbide Precipitation in Ti-added High Cr Cast Irons
- 2014
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Ingår i: ISIJ International. - : Iron and Steel Institute of Japan. - 0915-1559 .- 1347-5460. ; 54:2, s. 374-383
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Tidskriftsartikel (refereegranskat)abstract
- Carbide precipitation and eutectic phase transformation during solidification of Fe-C-Cr-Ti-Mn-Mo-Ni-Si Ti-added high-chromium cast irons (HCCIs) were studied numerically and experimentally by the help of Partial Equilibrium approximation, DSC thermal analyses and EDX analyses. The main carbides formed during the solidification are distinguished as MC, primary M7C3 and eutectic M7C3 from their distinguished constitution, while other researchers didn't distinguish the primary and eutectic M7C3 carbide. Through comparing the prediction of Partial Equilibrium approximation with DSC thermal analysis measurement, the precipitation sequence of the eutectic structure in HCCIs is clarified to follow the sequence of FCC prior to the eutectic M7C3, although they were usually expected to precipitate simultaneously. The hardness index of the HCCIs is evaluated quantatively by summation of the contributions of the Vickers hardness of MC, primary M7C3 and eutectic M7C3 carbides with predicted precipitation amount and composition / constitution. The effects of C, Ti and Cr contents on the precipitation sequence, the amount and the composition of carbides as well as the hardness of the HCCIs are discussed deeply. Finally, the validity of Partial Equilibrium approximation is shown in prediction of the solidification in multicomponent system with large amount of precipitated carbides.
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| 7. |
- Zhang, Hongwei, et al.
(författare)
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Prediction of Carbide Precipitation Using Partial Equilibrium Approximation in Fe-C-V-W-Cr-Mo High Speed Steels
- 2013
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Ingår i: ISIJ International. - : Iron and Steel Institute of Japan. - 0915-1559 .- 1347-5460. ; 53:3, s. 493-501
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Tidskriftsartikel (refereegranskat)abstract
- In Fe-C-V-W-Cr-Mo high speed steels, the nature of carbides during the solidification are discussed as a function of C, V and W content by the help of Partial Equilibrium (PE) approximation and thermodynamic calculations. The results show that the solidification path and carbide precipitation can be reasonably predicted by the Partial Equilibrium approximation for cooling rate lower than 10-13 K min(-1). From the viewpoint of hardness control by carbides, it is found that among the main carbides MC, M6C and M7C3, the increase of C favours the formation of MC, M7C3 but decreases the hardness of M7C3 by increasing the Fe content in it. Meanwhile, the increase of V only increases the amount of MC and V content therein, and the increase of W largely increases the amount of M6C and W content in it. As a result, the addition of V and W improves the hardness of MC and M6C carbides.
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| 8. |
- Zhang, Hongwei, et al.
(författare)
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Prediction of Solidification Microstructure and Columnar-to-equiaxed Transition of Al-Si Alloy by Two-dimensional Cellular Automaton with "Decentred Square" Growth Algorithm
- 2009
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Ingår i: ISIJ International. - : Iron and Steel Institute of Japan. - 0915-1559 .- 1347-5460. ; 49:7, s. 1000-1009
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Tidskriftsartikel (refereegranskat)abstract
- A Cellular Automaton (CA)-Finite Difference (FD) coupling model was developed to analyze the evolution of solidification microstructure and the columnar-to-equiaxed transition (CET) in AI-Si alloy. Kobayashi's microsegregation equation was adopted to describe the solute diffusion in solid phase, and a "decentred square" growth algorithm with coordinate transformation was performed to describe the grain growth and the entrapment of neighbor cells. Through the examination on the effects of operation parameters and nucleation parameters on solidification morphologies, it was found that the length of columnar grains is controlled by the dendrite tip growth kinetics, and that the width of columnar grains is controlled by the implicit relationship between nucleation density and cooling rate at ingot surface. It was also found that the size of equiaxed grains is controlled by the competition of the nucleation and the grain growth. With the controllability of nucleation density in the bulk of liquid for equiaxed grain size, the nucleant and the nucleation density in actual AI-Si alloy were estimated. Both of the CET criteria based on the solidification path by CA-FD coupling model and the one based on the curves of critical temperature gradient conditions by Hunt's model were strongly dependent on nucleation undercooling and Si concentration. A good agreement was obtained between these two.
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| 9. |
- Zhang, Hongwei, et al.
(författare)
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Prediction of Solidification Paths for Fe-C-Cr Alloys by a Multiphase Segregation Model Coupled to Thermodynamic Equilibrium Calculations
- 2010
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Ingår i: ISIJ International. - : Iron and Steel Institute of Japan. - 0915-1559 .- 1347-5460. ; 50:12, s. 1859-1866
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Tidskriftsartikel (refereegranskat)abstract
- A microsegregation model for the solidification of multicomponent alloys is developed It couples the volume-averaged conservation equations for total mass solute mass and energy assuming a uniform temperature The diffusion in the liquid and solid phases the growth kinetics of the solidifying microstructures and the velocity of the solid/liquid and solid/solid phase interfaces are considered in the model Equilibrium between phases is taken into account and computed using dedicated thermodynamic software The thermodynamic properties and their evolutions during solidification are directly retrieved from a database Illustration is provided by the solidification of a Fe-C-Cr alloy The occurrence of the recalescence due to the growth of the microstructure and the progress of solidification are predicted The solidification behavior near to recalescence is evaluated By adjusting the cooling intensity and the solute diffusivities extreme approximations are retrieved The model shows potentials to be coupled with a macrosegregation model for application to the solidification of multicomponent alloys
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| 10. |
- Zhang, Hongwei, et al.
(författare)
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Restrictions of Physical Properties on Solidification Microstructures of Al-based Binary Alloys by Cellular Automaton
- 2010
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Ingår i: ISIJ International. - : Iron and Steel Institute of Japan. - 0915-1559 .- 1347-5460. ; 50:12, s. 1835-1842
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Tidskriftsartikel (refereegranskat)abstract
- The solidification microstructure evolution and the Columnar to Equiaxed Transition (CET) during Al-Si and Al-Cu binary alloy solidification processes are analyzed by the help of Cellular Automaton-Finite Difference (CA-FD) model The effects of the physical properties except the effects of the nucleation parameters and the operation parameters on the cooling curves the dendrite growth the solidification morphologies and the CET of the Al-Si and Al-Cu binary alloys are emphatically discussed Results show that the solidification morphologies are internally influenced by the physical properties related to the dendrite tip growth kinetics Besides the solute diffusivity in liquid D-L and the growth restriction parameter Q=mC(o)(k(S/L)-1) the Gibbs-Thomson coefficient Gamma shows a great effect on dendrite tip growth rate Their effects on the dendrite tip kinetics are ordered as m(k(S/L)-1)>Gamma>D-L The growth rate can be predicted efficiently by the present simplified expression based on GGAN model combining several physical parameters and the local under-cooling which clearly shows the physical meaning of the constant coefficients in the simplified expression based on KGT model The equiaxed ratio related to the solidification morphologies can also be evaluated as a function of those physical properties
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| 11. |
- Zhang, Wen, et al.
(författare)
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Discovery of an adakite-like pluton near Dongqiyishan (Beishan, NW China) - Its age and tectonic significance
- 2012
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Ingår i: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 142, s. 148-160
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Tidskriftsartikel (refereegranskat)abstract
- The Dongqiyishan granitic pluton is exposed in the Eastern Beishan area, in the central part of the Southern Central Asia Orogenic Belt. SIMS U-Pb zircon dating indicates that the Dongqiyishan pluton was emplaced at 356 +/- 2 Ma. Geochemically the pluton is calc-alkaline and characterized by high SiO2 (> 65%), Al2O3 (mostly > 15%), Na2O/K2O ratios (1.3-3.9) and Sr (generally > 400 ppm). Samples display positive to weakly negative Eu anomalies (delta(Eu) = 0.81-2.32). low HREE, Y and Yb (Y < 18 ppm, Yb < 1.9 ppm), are enriched in LREE, LILE (Rb, Ba, Sr, U, and K), Zr, and Hf, and depleted in HFSE. They have relatively low epsilon(Nd)(t) values (-2.9-+ 0.6) and relatively high (Sr-87/Sr-86)(i) ratios (0.70581-0.70749). Together, these geochemical traits suggest an adakite-like composition that reflects melting of subducted oceanic crust and the involvement of older continental material. Assimilation and fractional crystallization (AFC) modeling requires significant crustal assimilation to modify an initial MORB-derived andesitic melt to produce the hybrid granitic melt, e.g. - the Dongqiyishan pluton. A back-arc basin to the south (represented by the Yueyashan-Xichangjing ophiolitic belt) closed before the Early Carboniferous Dongqiyishan adakite-like pluton was emplaced into the Silurian volcanic arc. Closure of the back-arc basin potentially provides the continental component seen in the isotopic signature of the Dongqiyishan pluton and required by the AFC modeling. Post-Ordovician flattening of the subducting slab together with crustal thickening from closure of the back-arc basin, led to the formation of a tightly coupled subducting slab and lower crust which resulted in the formation of Dongqiyishan adakite-like pluton. The Dongqiyishan pluton is consistent with southward subduction (represented by the Xiaohuangshan ophiolitic belt) and indicates that oceanic crust was still being produced in the Paleo-Asian Ocean during the Early Carboniferous. Adakite-like granitic rocks may be more abundant than previously thought in the Beishan region.
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| 12. |
- Zhang, Wen, et al.
(författare)
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Post-collisional Southeastern Beishan granites : Geochemistry, geochronology, Sr-Nd-Hf isotopes and their implications for tectonic evolution
- 2012
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Ingår i: Journal of Asian Earth Sciences. - : Elsevier BV. - 1367-9120 .- 1878-5786. ; 58, s. 51-63
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Tidskriftsartikel (refereegranskat)abstract
- Bandaoshan granites are exposed in the Southeastern Beishan area, in the central part of the Central Asia Orogenic Belt (CAOB). LA-ICP-MS U-Pb zircon dating indicates that Bandaoshan granites were emplaced into Precambrian basement at 285 +/- 4 Ma and their geochemistry indicates that they are alkali-rich potassium-high granites. Initial epsilon(Nd) values (-4.3 to -2.7) and epsilon(Hf) values (-2.7 to +0.7) suggest that Bandaoshan granites were derived from mantle-derived melt and an upper continental crustal or sedimentary component. The latter plays a significant role in their genesis. In combination with regional geology, the Early Permian Bandaoshan pluton is interpreted to form in a post-collisional environment. In the Southeastern Beishan area Late Carboniferous Qiaowan granites, Early Permian Yin'aoxia granites and Middle Permian Xijianquanzi granites are also considered as post-collisional granites, and together with Bandaoshan granites indicate that the region was in a post-collisional stage from the end of Late Carboniferous to Middle Permian.
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