Non-local Thermal Equilibrium Model Research Articles

3D NLTE modelling of Y and Eu

Context. Abundances of s- and r-process elements in Sun-like stars constrain nucleosynthesis in extreme astrophysical events, such as compact binary mergers and explosions of highly magnetised rapidly rotating massive stars. Aims. We measure solar abundances of yttrium (Y) and europium (Eu) using 3D non-local thermal equilibrium (NLTE) models. We use the model to determine the abundance of Y, and also explore the model’s ability to reproduce the solar centre-to-limb variation of its lines. In addition, we determine the Eu abundance using solar disc-centre and integrated flux spectra. Methods. We developed an NLTE model of Eu and updated our model of Y with collisional data from detailed quantum-mechanical calculations. We used the IAG spatially resolved high-resolution solar spectra to derive the solar abundances of Y across the solar disc and of Eu for integrated flux and at disc centre using a set of carefully selected lines and a 3D radiation-hydrodynamics model of the solar atmosphere. Results. We find 3D NLTE solar abundances of A(Y)3D NLTE = 2.30 ± 0.03stat ± 0.07syst dex based on observations at all angles and A(Eu) = 0.57 ± 0.01stat ± 0.06syst dex based on the integrated flux and disc-centre intensity. 3D NLTE modelling offers the most consistent abundances across the solar disc, and resolves the problem of severe systematic bias in Y and Eu abundances inherent to 1D LTE, 1D NLTE, and 3D LTE modelling.

A novel bionic packed bed latent heat storage system filled with encapsulated PCM for thermal energy collection

• A bionic structure brings a more homogeneous temperature difference and a smaller pressure drop compared with conventional systems. • The parameters related to heat transfer and phase transition acquired from numerical study were validated by experiments. • A non-local thermal equilibrium model was employed to characterize the temperature distribution of PCM and HTF in porous media. • The enhancement of flow resistance and heat exchange performance was evaluated. The main content of this work is to propose a novel bionic solution to overcome the nonuniformity of flow and temperature distribution, which is an inherent problem and restriction for conventional latent heat storage devices. By learning from animal circulatory systems, the inner space and flow channel network are distributed hierarchically as arteries, veins, capillaries and ventricles. A conceptual configuration of the bionic system is presented, and its numerical model is established to demonstrate the flow and heat transfer phenomena. The encapsulated PCM that is used in this study is fabricated and parameters related have been measured by experiments. A numerical model of a 3D continuous PCM bed is established to help research the flow of HTF through the surfaces of PCM particles and the heat transfer between them. Then, a model of a simplified bionic device is developed where the PCM region is set as a porous domain. The results show the flow and temperature fields are distributed uniformly, along with a much smaller global pressure drop. By allocating the thermal load on cascaded layers with stepwise PCMs, a more homogeneous global temperature difference can be achieved.

Stellar wind properties of the nearly complete sample of O stars in the low metallicity young star cluster NGC 346 in the SMC galaxy

Context. Massive stars are among the main cosmic engines driving the evolution of star-forming galaxies. Their powerful ionising radiation and stellar winds inject a large amount of energy in the interstellar medium. Furthermore, mass-loss (Ṁ) through radiatively driven winds plays a key role in the evolution of massive stars. Even so, the wind mass-loss prescriptions used in stellar evolution models, population synthesis, and stellar feedback models often disagree with mass-loss rates empirically measured from the UV spectra of low metallicity massive stars. Aims. The most massive young star cluster in the low metallicity Small Magellanic Cloud galaxy is NGC 346. This cluster contains more than half of all O stars discovered in this galaxy so far. A similar age, metallicity (Z), and extinction, the O stars in the NGC 346 cluster are uniquely suited for a comparative study of stellar winds in O stars of different subtypes. We aim to use a sample of O stars within NGC 346 to study stellar winds at low metallicity. Methods. We mapped the central 1′ of NGC 346 with the long-slit UV observations performed by the Space Telescope Imaging Spectrograph (STIS) on board of the Hubble Space Telescope and complemented these new datasets with archival observations. Multi-epoch observations allowed for the detection of wind variability. The UV dataset was supplemented by optical spectroscopy and photometry. The resulting spectra were analysed using a non-local thermal equilibrium model atmosphere code (PoWR) to determine wind parameters and ionising fluxes. Results. The effective mapping technique allowed us to obtain a mosaic of almost the full extent of the cluster and resolve stars in its core. Among hundreds of extracted stellar spectra, 21 belong to O stars. Nine of them are classified as O stars for the first time. We analyse, in detail, the UV spectra of 19 O stars (with a further two needing to be analysed in a later paper due to the complexity of the wind lines as a result of multiplicity). This more than triples the number of O stars in the core of NGC 346 with constrained wind properties. We show that the most commonly used theoretical mass-loss recipes for O stars over-predict mass-loss rates. We find that the empirical scaling between mass-loss rates (Ṁ) and luminosity (L), Ṁ ∝ L2.4, is steeper than theoretically expected by the most commonly used recipes. In agreement with the most recent theoretical predictions, we find within Ṁ ∝ Zα that α is dependent upon L. Only the most luminous stars dominate the ionisation feedback, while the weak stellar winds of O stars in NGC 346 and the lack of previous supernova explosions in this cluster restrict the kinetic energy input.

AstroSat soft X-ray observations of the symbiotic recurrent nova V3890 Sgr during its 2019 outburst

ABSTRACT Two long AstroSat Soft X-ray Telescope observations were taken of the third recorded outburst of the symbiotic recurrent nova V3890 Sgr. The first observing run, 8.1–9.9 d after the outburst, initially showed a stable intensity level with a hard X-ray spectrum that we attribute to shocks between the nova ejecta and the pre-existing stellar companion. On day 8.57, the first, weak, signs appeared of supersoft source (SSS) emission powered by residual burning on the surface of the white dwarf. The SSS emission was observed to be highly variable on time-scales of hours. After day 8.9, the SSS component was more stable and brighter. In the second observing run, on days 15.9–19.6 after the outburst, the SSS component was even brighter but still highly variable. The SSS emission was observed to fade significantly during days 16.8–17.8 followed by re-brightening. Meanwhile, the shock component was stable, leading to increase in hardness ratio during the period of fading. AstroSat and XMM–Newton observations have been used to study the spectral properties of V3890 Sgr to draw quantitative conclusions even if their drawback is model dependent. We used the xspec to fit spectral models of plasma emission, and the best fits are consistent with the elemental abundances being lower during the second observing run compared to the first for spectra ≥1 keV. The SSS emission is well fitted by non-local thermal equilibrium model atmosphere used for white dwarfs. The resulting spectral parameters, however, are subject to systematic uncertainties such as completeness of atomic data.

Global nighttime atomic oxygen abundances from GOMOS hydroxyl airglow measurements in the mesopause region

Abstract. This paper presents a new dataset of nighttime atomic oxygen density [O], derived from OH(8–4) ro-vibrational band emissions, using a non-local thermal equilibrium model, with the aim of offering new insight into the atomic oxygen abundances in the mesopause region. The dataset is derived from the level-1 atmospheric background measurements observed by the Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument aboard Envisat, with the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) measurements for the atmospheric background. Raw data are reprocessed into monthly zonal mean values in 10∘ latitude bins with a fixed altitude grid of 3 km. The dataset spans from 70∘ S to 70∘ N in latitude and from 80 to 100 km in altitude, covering a time period from May 2002 to December 2011 at local times from 22:00 to 00:00 LT. The atomic oxygen density peaks at about 95 km and the highest values are in the range of 3–8 × 1011 atoms cm−3, depending on latitude and season. There is a rapid decrease of [O] below the peak. The annual oscillation (AO), semiannual oscillation (SAO) and the solar cycle impact are distinguished from the [O] longtime series variations. This new GOMOS [O] dataset conforms to other published datasets and is consistent with the [O] datasets obtained from the Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY (SCIAMACHY) OH airglow measurements within about ±20 %.

Bias characterization of CrIS shortwave temperature sounding channels using fast NLTE model and GFS forecast field

AbstractThe strong nonlocal thermal equilibrium (NLTE) emission in the upper atmosphere impedes the usage of Cross‐track Infrared Sounder (CrIS) temperature channels near 4.3 µm in the operational data assimilation. This study explores the bias characteristics of those temperature channels near 4.3 µm with and without a fast NLTE model implemented in the Community Radiative Transfer Model (CRTM). It is shown that the biases of those temperature channels can reach up to 12 K and are dramatically reduced to below 4 K during daytime by the fast NLTE model with small differences from the nighttime biases. However, the biases after applying the NLTE correction remain large for CrIS upper atmospheric temperature channels during both daytime and nighttime. A further investigation suggests that the remaining biases in those temperature channels mainly originate from the cold biases in the stratospheric temperature profiles of National Center for Environmental Prediction Global Forecast System (GFS) forecasts as input to CRTM. The cold biases reach the maximum of about 8 K near the tropics at about 1 hPa, decreasing toward higher latitudes and lower altitudes.

The Numerical Simulation of Liquid-Vapor Stratified Flow in Horizontal Metal-Foam Tubes.

In this paper, a boiling stratified flow model in a metal-foam tube is proposed. First, based on Branuer non-equilibrium gas-liquid interface model, a force balance on the gas-liquid interface in metal-foam is calculated. The shape of the interface of upper gas phase and lower liquid phase in metal foam tube is obtained. As for the lower liquid phase, the energy conservation equations of liquid and metal foam are formulated, which account for porosity and fiber diameter of foam on the basis of non-local thermal equilibrium model (NTEM), respectively. Therefore, a profile of temperature difference between liquid and metal foam can be obtained. For the upper gas phase, an empirical correlation developed by other researchers is utilized to obtain temperature difference between gas phase and solid wall. In addition, the variation of the Reynolds number with increasing mass quality along the flow direction is acquired. Ultimately, an average circumference heat transfer coefficient is calculated. The results of circumference heat transfer coefficient agree well with available experimental data, showing the prediction of the proposed stratified flow model is feasible. The reason resulting in discrepancies between the prediction and experiment data is also illustrated.

NON-LOCAL THERMAL EQUILIBRIUM MODEL ATMOSPHERES FOR THE HOTTEST WHITE DWARFS: SPECTRAL ANALYSIS OF THE COMPACT COMPONENT IN NOVA V4743 Sgr

Half a year after its outburst in 2002 September, nova V4743 Sgr evolved into the brightest supersoft X-ray source in the sky with a flux maximum around 30 Å. We calculated grids of synthetic energy distributions based on non-local thermal equilibrium model atmospheres for the analysis of the hottest white dwarfs (WDs) and present the result of fits to Chandra and XMM-Newton grating X-ray spectra of V4743 Sgr of outstanding quality, exhibiting prominent resonance lines of C v, C vi, N vi, N vii, and O vii in absorption. The nova reached its highest effective temperature (Teff = 740 ± 70 kK) around 2003 April and remained at that temperature at least until 2003 September. We conclude that the WD is massive, ≈1.1–1.2 M☉. The nuclear-burning phase lasted for 2–2.5 years after the outburst, probably the average duration for a classical nova. The photosphere of V4743 Sgr was strongly carbon deficient (≈0.01 times solar) and enriched in nitrogen and oxygen (>5 times solar). Especially the very low C/N ratio indicates that the material at the WD's surface underwent thermonuclear burning. Thus, this nova retained some of the accreted material and did not eject all of it in outburst. From 2003 March to September, the nitrogen abundance is strongly decreasing; new material is probably already being accreted at this stage.

Simulation study on the reaction process based single stage metal hydride thermal compressor

The reasonable use of waste heat has been one of the promising ways in increasing the utilization ratio of energy, while a metal hydride-based single stage thermal compressor shows great potential in recycling power from low grade heat. As the core part of a metal hydride-based hydrogen compressor, the reactor integrating reaction and heat transfer deserves in-depth investigation. In order to analyze the dynamic working process of a reactor, a numerical simulation based on a non-local thermal equilibrium model was conducted at 50 × 20 grid. Under the given conditions, the dynamic characteristics of both absorption and desorption were examined, and it was found that the reaction fraction X reaches 0.9 after 450 s during the absorption, while the reverse process of desorption takes about 300 s. The largest bed temperature differences during absorption and desorption are 11 K and 25 K, respectively. A pronounced asymmetry between absorption and desorption was observed, which is not desirable in the working of MHTC. However, by changing the transport conditions properly, it is possible to improve the situation, which was shown in the simulation results.

Identifying heat and mass transfer characteristics of metal hydride reactor during adsorption—Parameter analysis and numerical study

The performance of a metal hydride reactor is highly dependent on its transport process. It is important to know how the transport process affects the performance, therefore two key parameters—heat transfer controlled reaction rate and mass transfer controlled reaction rate were introduced to explain this kind of effect. Moreover, a brief discussion about how to use the new parameters was given. In order to analyze the reactor performance and heat/mass transfer characteristics, a non-local thermal equilibrium model describing the actual adsorption process was formulated, and numerical simulations were carried out. Then the two parameters were applied for the same purpose. The result of the parameter analysis coincided well with that of the numerical simulation, which approved the validity of the two parameters in identifying heat and mass transfer characteristics of the metal hydride reactor.

CYG X-1 = V1357 CYG investigation based on high-resolution optical spectroscopy of 2002–2004

We report the results of Cyg X-1 spectral monitoring for 2002–2004. A comparison of observed and non-local thermal equilibrium model profiles for H I, He I and Mg II is given, taking into account the tidal distortion of the Cyg X-1 optical component and its illumination by X-ray emission from the secondary component. We set limits on the main characteristics of the optical component, T eff=30 400±500K and log g=3.31±0.07, and on the overabundances of He and Mg, [He]/[H]=0.43±0.06 dex and [Mg]/[H]=0.75±0.15 dex. The Doppler images were reconstructed by an improved Doppler tomography method on the basis of nine He II (λ=4686 A) profiles from 2003 (the ‘soft’ X-ray state) and six profiles from 2004 (the ‘hard’ X-ray state). This allowed us to set limits on the black-hole-to-supergiant mass ratio (1/4)≤slant M X/M O≤slant (1/3).

NUMERICAL SIMULATION OF FORCED CONVECTION HEAT TRANSFER IN POROUS PLATE CHANNELS USING THERMAL EQUILIBRIUM AND NONTHERMAL EQUILIBRIUM MODELS

A numerical study of fluid flow and convection heat transfer in a plate channel filled with metallic or nonmetallic particles using both the local thermal equilibrium model and a nonlocal thermal equilibrium model is presented in this paper. The numerical simulation results are compared with our experimental data, and a new modified thermal dispersion conductivity model is presented. The effects of the assumption of local thermal equilibrium versus nonlocal thermal equilibrium and the thermal dispersion effect on the convection heat transfer are investigated. The nonlocal thermal equilibrium model is appropriate for either nonmetallic or metallic porous media. The velocity distribution and the temperature fields are presented.