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Differential evolution based small signal modeling of GAN HEMT
摘要： In this article differential evolution based method of small signal modeling of GAN HEMT has been investigated. The method uses a unique search space exploration strategy to obtain optimized values of intrinsic and extrinsic elements pertaining to compact small signal model from extracted equivalent circuit elements and measured S-parameter data. Effectiveness of the method has been illustrated by comparing the measured S-parameter data of a 4 × 0.1 × 75 μm2 GaN/SiC HEMT in the frequency range of 1 to 30 GHz wherein modeled and measured data are in good agreement.
关键词： GaN HEMT,small signal model,differential evolution
更新于2025-09-19 17:15:36
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Solar jet-like features rooted in flare ribbons
摘要： Employing the high spatio-temporal Interface Region Imaging Spectrograph 1330 ?A observations, we investigated the jet-like features that occurred during the X8.2 class ?are in NOAA active region (AR) 12673 on 2017 September 10. These jet-like features were rooted in the ?are ribbons. We examined 15 features, and the mean values of the lifetimes, projected widths, lengths, and velocities of these features were 87 s, 890 km, 2.7 Mm, and 70 km s?1, respectively. We also observed many jet-like features which happened during the X1.0 class ?are on 2014 October 25. We studied the spectra at the base of a jet-like feature during its development. The Fe XXI 1354.08 ?A line in the corona displays blueshift, while the Si IV 1402.77 ?A line in the transition region exhibits redshift, which indicates the chromospheric evaporation. This is the ?rst time that the jet-like features are reported to be rooted in the ?are ribbons, and we suggest that these jet-like features were driven by the mechanism of chromospheric evaporation.
关键词： Sun: atmosphere,Sun: ?ares,Sun: evolution,Sun: activity
更新于2025-09-19 17:15:36
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Metal-ion bridged high conductive RGO-M-MoS2 (M = Fe3+, Co2+, Ni2+, Cu2+ and Zn2+) composite electrocatalysts for photo-assisted hydrogen evolution
摘要： Efficient photo-electrocatalysts for hydrogen evolution reaction (HER) are synthesized using a facile one-step hydrothermal method. With metal-ion bridges, highly dispersed molybdenum disulfide (MoS2) nanolayers are vertically grown on the reduced graphene oxide (RGO) to form RGO-M-MoS2 photocatalysts for HER, where M = Fe3+, Co2+, Ni2+, Cu2+ and Zn2+. The results show that the cross-bridging ions can modulate the MoS2 growth priority and act as efficient charge transfer channels between RGO and MoS2, and combine the advantages of the high conductivity of graphene with the photo-electrochemical activity of MoS2. The metal-ion bridged MoS2-M-RGO heterostructures demonstrate superior catalytic activity toward hydrogen evolution reaction (HER) in acid medium, evidenced by the remarkable higher catalytic current density at low overpotential compared with that of MoS2-RGO without metal-ion bridge. This study provides a novel and facile route for establishing efficient composite photo-electrocatalysts for water splitting to generate hydrogen.
关键词： hydrogen evolution reaction,MoS2 nanosheets,reduced graphene oxide,metal-ion bridge
更新于2025-09-19 17:15:36
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Aligned Heterointerface-Induced 1T-MoS <sub/>2</sub> Monolayer with Near-Ideal Gibbs Free for Stable Hydrogen Evolution Reaction
摘要： 1T-phase molybdenum disulfide (1T-MoS2) exhibits superior hydrogen evolution reaction (HER) over 2H-phase MoS2 (2H-MoS2). However, its thermodynamic instability is the main drawback impeding its practical application. In this work, a stable 1T-MoS2 monolayer formed at edge-aligned 2H-MoS2 and a reduced graphene oxide heterointerface (EA-2H/1T/RGO) using a precursor-in-solvent synthesis strategy are reported. Theoretical prediction indicates that the edge-aligned layer stacking can induce heterointerfacial charge transfer, which results in a phase transition of the interfacial monolayer from 2H to 1T that realizes thermodynamic stability based on the adhesion energy between MoS2 and graphene. As an electrocatalyst for HER, EA-2H/1T/RGO displays an onset potential of ?103 mV versus RHE, a Tafel slope of 46 mV dec?1 and 10 h stability in acidic electrolyte. The unexpected activity of EA-2H/1T/RGO beyond 1T-MoS2 is due to an inherent defect caused by the gliding of S atoms during the phase transition from 2H to 1T, leading the Gibbs free energy of hydrogen adsorption (ΔGH*) to decrease from 0.13 to 0.07 eV, which is closest to the ideal value (0.06 eV) of 2H-MoS2. The presented work provides fundamental insights into the impressive electrochemical properties of HER and opens new avenues for phase transitions at 2D/2D hybrid interfaces.
关键词： 1T-MoS2 monolayers,stability,hydrogen evolution reaction,edge-aligned structure
更新于2025-09-19 17:15:36
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POISSON project
摘要： Context. As part of the Protostellar Optical-Infrared Spectral Survey On NTT (POISSON) project, we present the results of the analysis of low-resolution near-IR spectroscopic data (0.9–2.4 μm) of two samples of young stellar objects in the Lupus (52 objects) and Serpens (17 objects) star-forming clouds, with masses in the range of 0.1 to 2.0 M☉ and ages spanning from 10^5 to a few 10^7 yr. Aims. After determining the accretion parameters of the targets by analysing their H i near-IR emission features, we added the results from the Lupus and Serpens clouds to those from previous regions (investigated in POISSON with the same methodology) to obtain a final catalogue (143 objects) of mass accretion rate values (?_acc) derived in a homogeneous and consistent fashion. Our final goal is to analyse how ?_acc correlates with the stellar mass (M_*) and how it evolves in time in the whole POISSON sample. Methods. We derived the accretion luminosity (L_acc) and ?_acc for Lupus and Serpens objects from the Brγ (Paβ in a few cases) line by using relevant empirical relationships available in the literature that connect the H i line luminosity and L_acc. To minimise the biases that arise from adopting literature data that are based on different evolutionary models and also for self-consistency, we re-derived mass and age for each source of the POISSON samples using the same set of evolutionary tracks. Results. We observe a correlation ?_acc ～ M_*^2.2 between mass accretion rate and stellar mass, similarly to what has previously been observed in several star-forming regions. We find that the time variation of ?_acc is roughly consistent with the expected evolution of the accretion rate in viscous disks, with an asymptotic decay that behaves as t^?1.6. However, ?_acc values are characterised by a large scatter at similar ages and are on average higher than the predictions of viscous models. Conclusions. Although part of the scattering may be related to systematics due to the employed empirical relationship and to uncertainties on the single measurements, the general distribution and decay trend of the ?_acc points are real. These findings might be indicative of a large variation in the initial mass of the disks, of fairly different viscous laws among disks, of varying accretion regimes, and of other mechanisms that add to the dissipation of the disks, such as photo-evaporation.
关键词： accretion, accretion disks,infrared: stars,stars: formation,stars: pre-main sequence,stars: evolution
更新于2025-09-19 17:15:36
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Photometric scaling relations of anti-truncated stellar discs in S0-Scd galaxies
摘要： Context. It has been recently found that the characteristic photometric parameters of anti-truncated discs in S0 galaxies follow tight scaling relations. Aims. We investigate whether similar scaling relations are satisfied by galaxies of other morphological types. Methods. We have analysed the trends in several photometric planes relating the characteristic surface brightness and scalelengths of the breaks and the inner and outer discs of local anti-truncated S0–Scd galaxies, using published data and fits performed to the surface brightness profiles of two samples of Type-III galaxies in the R and Spitzer 3.6 μm bands. We have performed linear fits to the correlations followed by different galaxy types in each plane, as well as several statistical tests to determine their significance. Results. We have found that 1) the anti-truncated discs of all galaxy types from Sa to Scd obey tight scaling relations both in R and 3.6 μm, as observed in S0s; 2) the majority of these correlations are significant accounting for the numbers of the available data samples; 3) the trends are clearly linear when the characteristic scalelengths are plotted on a logarithmic scale; and 4) the correlations relating the characteristic surface brightnesses of the inner and outer discs and the breaks with the various characteristic scalelengths significantly improve when the scalelengths are normalized to the optical radius of the galaxy. The observational uncertainties prevent us from discerning robustly whether the trends differ between the different types and bands, but we do not find statistical evidence of significant differences between the distributions of S0s and spirals or of barred and unbarred galaxies. These results suggest that the scaling relations of Type-III discs are independent of the morphological type and the presence (or absence) of bars within the observational uncertainties of the available datasets. However, larger and deeper samples are required to confirm this. Conclusions. The tight structural coupling implied by these scaling relations imposes strong constraints on the mechanisms proposed to explain the formation of anti-truncated stellar discs in the galaxies across the whole Hubble sequence.
关键词： galaxies: fundamental parameters,galaxies: spiral,galaxies: photometry,galaxies: structure,galaxies: evolution,galaxies: elliptical and lenticular, cD
更新于2025-09-19 17:15:36
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Frontier Fields: Combining HST, VLT, and <i>Spitzer</i> data to explore the <i>z</i> ~ 8 Universe behind the lensing cluster MACSJ0416.1?2403
摘要： Context. The Hubble Space Telescope (HST) Frontier Fields (HFFs) project started at the end of 2013 with the aim of providing extremely deep images of six massive galaxy clusters. One of the main goals of this program is to push several telescopes to their limits to provide the best current view of the earliest stages of the Universe. The analysis of the initial data has already demonstrated the huge capabilities of the program. Aims. We present a detailed analysis of z ～ 8 objects behind the HFFs lensing cluster, MACSJ0416.1-2403, combining 0.3?1.6 μm imaging from HST, ground-based Ks imaging from VLT HAWK-I, and 3.6 μm and 4.5 μm Spitzer Space Telescope. The images probe to 5σ depths of ≈29 AB for HST, 25.6 AB for HAWK-I, and ≈0.310 and 0.391 μJy at 3.6 and 4.5 μm, respectively. With these datasets, we assess the photometric properties of z ～ 8 galaxies in this ?eld, as well as their distribution in luminosity, to unprecedented sensitivity. Methods. We applied the classical Lyman break (LB) technique, which combines non detection criteria in bands blueward of the Lyman break at z ～ 8 and color-selection in bands redward of the break. To avoid contamination by mid-z interlopers, we required a strong break between optical and near-infrared data. We determined the photometric properties of z ～ 8 selected candidates using spectral energy distribution (SED)-?tting with standard library templates. The luminosity function at z ～ 8 is computed using a Monte-Carlo method taking advantage of the SED-?tting results. A piece of cautionary information is gleaned from new deep optical photometry of a previously identi?ed z ～ 8 galaxy in this cluster, which is now ?rmly detected as a mid-z interloper with a strong ≈1.5 mag Balmer break (between F606W and F125W). Using the SED of this interloper, we estimated the contamination rate of our MACSJ0416.1?2403 sample, and that of previous samples in Abell 2744 that were based on HFF data, we highlight the dangers of pushing the LB technique too close to the photometry limits. Results. Our selection reliably recovers four objects with mF160W ranging from 26.0 to 27.9 AB that are located in modest-ampli?cation regions (μ < 2.4). Two of the objects display a secondary break between the IRAC 3.6 μm and 4.5 μm bands, which could be associated to the Balmer break or emission lines at z ～ 8. The SED-?tting analysis suggests that all of these objects favor high-z solutions with no reliable secondary solutions. The candidates generally have star formation rates around ～10 M(cid:12)/yr and sizes ranging from 0.2 to 0.5 kpc, which agrees well with previous observations and expectations for objects in the early Universe. The sample size and luminosity distribution are consistent with previous ?ndings.
关键词： galaxies: formation,galaxies: photometry,galaxies: distances and redshifts,galaxies: evolution,galaxies: star formation,galaxies: high-redshift
更新于2025-09-19 17:15:36
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One-pot, solid-state loading of Zn into g-C3N4 for increasing the population of photoexcited electrons and the rate of photocatalytic hydrogen evolution
摘要： In this research, we load g-C3N4 with Zn through a simple, one-pot solid-state route that results in appreciably improved photocatalytic activity for H2 evolution under visible light. X-ray photoelectron spectroscopy and synchrotron X-ray absorption near edge structure spectroscopy confirm that the Zn species are present in the +2 oxidation state, coordinated to O atoms in the form of nanoclusters. Following the Zn loading, the host g-C3N4 absorbs more light in the UV and visible regions, while its physical features are almost unaltered. In the absence of any co-catalysts, a H2 evolution rate as high as 26.3 μmol/g·h can be achieved by g-C3N4 loaded with 0.2 atomic% of Zn, more than 8 times higher than that of g-C3N4 (3 μmol/g·h). Our results then provide strong evidences that the photocatalytic activity of Zn-loaded g-C3N4 for H2 evolution is directly controlled by the population of photoexcited electrons.
关键词： Zinc loading,Carbon nitride,Hydrogen evolution,Photocatalysis
更新于2025-09-19 17:15:36
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HIDES spectroscopy of bright detached eclipsing binaries from the <i>Kepler</i> field – III. Spectral analysis, updated parameters and new systems
摘要： We present the latest results of our spectroscopic observations and refined modelling of a sample of detached eclipsing binaries, selected from the Kepler Eclipsing Binary Catalog, that are also double-lined spectroscopic binaries (SB2s). New high-resolution spectra obtained with the HIgh-Dispersion Echelle Spectrograph (HIDES), attached to the 1.88-m telescope of the Okayama Astrophysical Observatory, have supplemented the previous observations. This has allowed us to improve the physical parameters of some systems, and to characterize three previously omitted objects. All the obtained radial velocities were combined with Kepler photometry, in order to derive a full set of orbital and physical parameters. Ten out of eleven SB2s have their component spectra tomographically disentangled, and spectral analysis was performed with ISPEC, in order to derive the effective temperatures of components and the metallicities of the systems. By comparing our results with theoretical models, we assess the age and evolutionary status of the studied objects. We find a good match to all systems but one. We have derived distances from the parameters determined in this way, and we compare them with those from the Gaia Data Release 2. For systems previously studied by other authors, our new results lead to better consistency between observations and models.
关键词： binaries: eclipsing,stars: fundamental parameters,stars: late-type,binaries: spectroscopic,stars: evolution
更新于2025-09-19 17:15:36
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Distinguishing circumstellar from stellar photometric variability in Eta Carinae
摘要： The interacting binary Eta Carinae remains one of the most enigmatic massive stars in our Galaxy despite over four centuries of observations. In this work, its light curve from the ultraviolet to the near-infrared is analysed using spatially resolved HST observations and intense monitoring at the La Plata Observatory, combined with previously published photometry. We have developed a method to separate the central stellar object in the ground-based images using HST photometry and applying it to the more numerous ground-based data, which supports the hypothesis that the central source is brightening faster than the almost-constant Homunculus. After detrending from long-term brightening, the light curve shows periodic orbital modulation (?V ～ 0.6 mag) attributed to the wind-wind collision cavity as it sweeps around the primary star and it shows variable projected area to our line-of-sight. Two quasi-periodic components with time scales of 2-3 and 8-10 yr and low amplitude, ?V < 0.2 mag, are superimposed on the brightening light-curve, being the only stellar component of variability found, which indicates minimal stellar instability. Moreover, the light curve analysis shows no evidence of 'shell ejections' at periastron. We propose that the long-term brightening of the stellar core is due to the dissipation of a dusty clump in front of the central star, which works like a natural coronagraph. Thus, the central stars appear to be more stable than previously thought since the dominant variability originates from a changing circumstellar medium. We predict that the brightening phase, due mainly to dust dissipation, will be completed around 2032 ± 4 yr, when the star will be brighter than in the 1600's by up to ?V ～ 1 mag.
关键词： stars: individual (η Carinae),stars: winds, outflows,(stars:) binaries: general,stars: evolution,(ISM:) dust, extinction
更新于2025-09-19 17:15:36