Large Radius Research Articles

Line detections in photospheric radius expansion bursts from 4U 1820-303. Confirmation of previous detections and their temporal evolution

The Neutron star Interior Composition ExploreR (NICER) is the instrument of choice for the spectral analysis of type I X-ray bursts, as it provides high throughput at the X-ray CCD resolution down to 0.3 keV. Triggered by the detection of absorption and emission lines in the first four photospheric radius expansion (PRE) bursts detected by NICER, we wish to test the dependence of the absorption line energies on the inferred blackbody radius because it was reported that the absorption line energies were positively correlated with the inferred blackbody radius. This was tentatively explained by a combination of a weaker gravitational redshift and higher blueshifts in a burst with a larger blackbody radius. We thus reanalysed these four bursts and analysed another eight bursts from 4u, for which we report evidence for PRE. We first followed the spectral evolution of the burst on the shortest possible timescales (tenth of a second). We adopted two parallel continuum descriptions to characterise the photospheric expansion and line evolution. Using the accretion-enhanced model, in which the burst emission is modelled as the sum of a blackbody and a component describing the persistent emission recorded prior to the burst and multiplied by a constant ( we inferred maximum equivalent blackbody radii up to ∼ 900 km. The peak bolometric (0.1-20 keV) luminosity reached between $4-7 ergss (and even higher when absorption from a putative photoionised absorber is accounted for) in our sample of bursts. This exceeds the Eddington luminosity of a helium accretor. In individual bursts, we detected absorption lines and assessed their significance through extensive Monte Carlo simulations. To characterise the spectral lines, we used dedicated plasma codes available within spex with a phenomenological continuum. A deep search throughout the temperature-velocity parameter space was run to explore Doppler shifts and minimise the chance of becoming stuck in local minima. We detected several significant (> 99.9 % significance) absorption lines, including the 2.97 keV line that was previously reported. We do not confirm the correlation between the line energies and the inferred blackbody radius, but for some bursts with larger radii, up to four lines are reported, and the line strength is higher. From the modelling of the feature lines, a photoionised or collisionally ionised slightly redshifted (almost rest-frame) gas in emission is suggested in most cases. In particular for the burst presenting the greatest PRE, a combination of photoionisation plasma in emission and absorption is preferred, however.

Segue 2 Recently Collided with the Cetus-Palca Stream: New Opportunities to Constrain Dark Matter in an Ultra-faint Dwarf

Abstract Stellar streams in the Milky Way are promising detectors of low-mass dark matter (DM) subhalos predicted by ΛCDM. Passing subhalos induce perturbations in streams that indicate the presence of the subhalos. Understanding how known DM-dominated satellites impact streams is a crucial step toward using stream perturbations to constrain the properties of dark perturbers. Here, we cross-match a Gaia Early Data Release 3 and SEGUE member catalog of the Cetus-Palca stream (CPS) with H3 for additional radial velocity measurements and fit the orbit of the CPS using this six-dimensional (6D) data. We demonstrate for the first time that the ultra-faint dwarf Segue 2 had a recent (77 ± 5 Myr ago) close flyby (within the stream's 2σ width) with the CPS. This interaction enables constraints on Segue 2’s mass and density profile at larger radii ( O ( 1 ) kpc) than are probed by its stars ( O ( 10 ) pc). While Segue 2 is not expected to strongly affect the portion of the stream covered by our 6D data, we predict that if Segue 2’s mass within ∼ 6 kpc is 5 × 109 M ⊙, the CPS's velocity dispersion will be ∼ 40 km s−1 larger at ϕ 1 > 20° than at ϕ 1 < 0°. If no such heating is detected, Segue 2’s mass cannot exceed 109 M ⊙ within ∼ 6 kpc. The proper motion distribution of the CPS near the impact site is mildly sensitive to the shape of Segue 2’s density profile. This study presents a critical test for frameworks designed to constrain properties of dark subhalos from stream perturbations.

The SRG/eROSITA All-Sky Survey: Large-scale view of the Centaurus cluster

The Centaurus cluster is one of the brightest and closest clusters. Previous comprehensive studies were done only in its brightest part (r<30'), where the centers of the main substructures (Cen 30 and Cen 45) are located, and only a small fraction of the outskirts has been studied. Through this work, we aim to characterize the intracluster medium (ICM) morphology and properties of the Centaurus cluster out to the radius within which the density is 200 times the critical density of the Universe at the redshift of the cluster, R_ We utilized the combined five SRG/eROSITA All-Sky Survey data (eRASS:5) to perform X-ray imaging and spectral analyses in various directions out to large radii. We employed some image manipulation methods to enhance small- and large-scale features. Surface brightness profiles out to 2R_200 were constructed to quantify the features. We acquired the gas temperature, metallicity, and normalization per area profiles out to R_200. We compared our results with previous Centaurus studies, cluster outskirts measurements, and simulations. Comprehensive sky background analysis was done across the field of view in particular to assess the variation of the eROSITA Bubble emission that partially contaminates the field. The processed X-ray images show the known sloshing-induced structures in the core, such as the cool plume, cold fronts, and ram pressure-stripped gas. The spectra in the core (r≤11 kpc ) are better described with a two-temperature (2T) model than an isothermal model. With this 2T analysis, we measured a lower temperature from the cooler component (∼!1.0 keV ) and a higher metallicity (∼!1.6Z_⊙), signifying an iron bias. In the intermediate radial range, the temperature peaks at ∼!3.6 keV and we observed prominent surface brightness and normalization per area excesses in the eastern sector (Cen 45 location). Temperature enhancements near the location of Cen 45 imply that the gas is shock-heated due to the interaction with Cen 30. We reveal that the eastern excess emission extends even further out, reaching R_500. The peak excess of normalization is located at ∼!23' from the center ($8'$ behind the center of Cen 45) with a $45%$ and $7.7σ$ above the full azimuthal value. This might be the tail or ram pressure-stripped gas from Cen 45. There is a temperature decrease of a factor of about two to three from the peak to the outermost bin at -R_ . We find good agreement between the outer temperatures (r>R_2500) with the temperature profile from simulations and the temperature fit from cluster outskirts measurements. We detect significant surface brightness emission to the sky background level out to R_200 with a $3.5σ$, followed by $2.9σ$ at 1.1R_200. The metallicity at -R_ is low but within the ranges of other outskirts studies. We present the first whole azimuth beyond ∼!30' measurement of the ICM morphology and properties of the Centaurus cluster, and increasing the probed volume by a factor of almost 30. While the cluster core is rich in features as a result of active galactic nucleus feedback and sloshing, the cluster outskirts temperature of Centaurus follows the temperature profile of clusters in simulations as well as the temperature fit from other cluster outskirts measurements.

Численное исследование естественной конвекции в кольцевом канале с подвижным внутренним цилиндром

This paper presents a numerical study of the natural convection of a viscous incompressible fluid that completely fills a horizontal annular channel. The temperature difference at the outer boundary produces convection, which can cause the rotation of the inner cylinder around its axis. The cylinder is assumed to rotate without friction. The Boussinesq approximation is used as a mathematical model. The problem is solved numerically by the algorithm SIMPLER. The equation of motion of the inner cylinder is derived from the law of conservation of angular momentum. The choice of a numerical integration method for the equation of motion of the inner cylinder does not affect the result. The influence of the dimensionless parameters on the flow structure and the angular velocity of the inner cylinder was studied. Dimensionless parameters vary within the ranges: Grashof number 103...2·105, Prandtl number 0.5...15, the dimensionless thermal conductivity of the cylinder 0...∞, and the cylinder inner radius 0.05...0.9. Numerical simulations have shown that the flow structure affects the angular velocity of the inner cylinder, which reaches its maximum in the case of a single-cell flow and at a minimum internal radius. As the Grashof number increases, the maximum possible angular velocity of the inner cylinder increases monotonically and is realized at large inner radius values. The Prandtl number has an opposite effect on the cylinder rotation velocity. If it decreases, then the angular velocity becomes extremely high at smaller inner radius values. An increase in the dimensionless thermal conductivity leads to the growth of the angular velocity, but it reaches a maximum at a smaller inner radius. The influence of the considered parameters on the angular velocity is determined by the change in the structure of the flow in the annular channel. The research results may be useful in the development of technical devices with an annular channel and a rotating inner cylinder.

Stellar Metallicities from DECam u-band Photometry: A Study of Milky Way Ultra-faint Dwarf Galaxies

Abstract We conducted an in-depth analysis of candidate member stars located in the peripheries of three ultra-faint dwarf (UFD) galaxy satellites of the Milky Way (MW): Boötes I (Boo1), Boötes II (Boo2), and Segue I (Seg1). Studying these peripheral stars has previously been difficult due to contamination from the MW foreground. We used u-band photometry from the Dark Energy Camera (DECam) to derive metallicities to efficiently select UFD candidate member stars. This approach was validated on Boo1, where we identified both previously known and new candidate member stars beyond five half-light radii. We then applied a similar procedure to Boo2 and Seg1. Our findings hinted at evidence for tidal features in Boo1 and Seg1, with Boo1 having an elongation consistent with its proper motion and Seg1 showing some distant candidate stars, a few of which are along its elongation and proper motion. We find two Boo2 stars at large distances consistent with being candidate member stars. Using a foreground contamination rate derived from the Besançon Galaxy model, we ascribed purity estimates to each candidate member star. We recommend further spectroscopic studies on the newly identified high-purity members. Our technique offers promise for future endeavors to detect candidate member stars at large radii in other systems, leveraging metallicity-sensitive filters with the Legacy Survey of Space and Time and the new, narrowband Ca HK filter on DECam.

Planar tetracoordinate beryllium in σ-aromatic Li4Be and Na4Be clusters: A missing member in first-octal row planar tetracoordinate family.

While planar tetracoordinate (pt) centers have been extensively explored from carbon to other octal-row elements or their heavier analogs, their counterparts involving alkali (A) and alkaline-earth metals (Ae) remain elusive due to the large atomic radius and absence of p orbitals. In this work, we found six hitherto unknown anionic ptA (A4A-) and neutral ptAe (A4Ae) centers through an extensive exploration of potential energy surfaces. The D4h-symmetry ptBe structures in Li4Be and Na4Be emerge as the lowest-energy configurations, and all the other ptA/ptAe structures are higher in energy or saddle points. The global-minimum ptBe structure can be described as Be- with a 2s12px12py1 electronic configuration, forming three σ electron sharing interactions with quartet Li4+/Na4+ motifs. The delocalized σ orbitals contribute to σ aromaticity, thereby enhancing the overall stability of these intriguing title ptBe species. Furthermore, these ptBe systems can be encapsulated within the [n]cycloparaphenylene nanoloop (n = 7, 8) thermochemically spontaneously, without any disturbance in planarity in the ptBe moiety, where the systems get stabilized by a predominant electrostatic interaction between Li4/Na4 and the nanoloop.

Molecular Engineering to Construct MoS2 with Expanded Interlayer Spacing and Enriched 1T Phase for "Rocking-Chair" Aqueous Calcium-Ion Pouch Cells.

The moderate working voltage and high capacity of transition metal dichalcogenides (TMDs) make them promising anode materials for aqueous calcium-ion batteries (ACIBs). However, the large radius and two charges of Ca2+ cause TMDs to exhibit poor performance in ACIBs. Therefore, effective regulation strategies are crucial for enabling the application of TMDs in ACIBs. Herein, MoS2 with expanded interlayer spacing and an enriched 1T phase (ES-1T-MoS2) is constructed by molecular engineering and reported as an anode material for ACIBs. Molecular engineering increases the capacity of MoS2 from 29.4 to 91.2 mAh g-1 and improves its rate performance from 20 to 76.1 mAh g-1 at 2.0 A g-1. ES-1T-MoS2 also shows a -20 to 50 °C wide temperature working capability. Furthermore, the capacity improvement reasons and the calcium storage mechanism of ES-1T-MoS2 are revealed through density functional theory calculations and in situ/ex situ characterizations. Finally, a "rocking-chair" aqueous calcium-ion pouch cell with a Prussian blue analogue cathode and ES-1T-MoS2 anode is assembled. The pouch cell exhibits a life of 150 cycles with over 90.8% capacity retention at 0 and 25 °C. This work demonstrates that molecular engineering is an effective strategy to improve the calcium storage performance of TMDs and promotes the advancement of ACIBs.

MR Imaging Reveals Dynamic Aggregation of Multivalent Glycoconjugates in Aqueous Solution.

Glycoconjugates forming from the conjugation of carbohydrates to other biomolecules, such as proteins, lipids, or other carbohydrates, are essential components of mammalian cells and are involved in numerous biological processes. Due to the capability of sugars to form multiple hydrogen bonds, many synthetic glycoconjugates are desirable biocompatible platforms for imaging, diagnostics, drugs, and supramolecular self-assemblies. Herein, we present a multimeric galactose functionalized paramagnetic gadolinium (Gd(III)) chelate that displays spontaneous dynamic aggregation in aqueous conditions. The dynamic aggregation of the Gd(III) complex was shown by the concentration-dependent magnetic resonance (MR) relaxation measurements, nuclear magnetic resonance dispersion (NMRD) analysis, and dynamic light scattering (DLS). Notably, these data showed a nonlinear relationship between magnetic resonance relaxation rate and concentrations (0.03-1.35 mM), and a large DLS hydrodynamic radius was observed in the high-concentration solutions. MR phantom images were acquired to visualize real-time dynamic aggregation behaviors in aqueous solutions. The in situ visualization of the dynamic self-assembling process of multivalent glycoconjugates has rarely been reported.

Long-Range Corrections for Molecular Simulations with Three-Body Interactions.

Due to their computational intensity, long-range corrections of three-body interactions are particularly desirable, while there is no consensus of how to devise a cutoff scheme. A cutoff correction scheme for three-body interactions in molecular simulations is proposed that does not rest on complex integrals and can be implemented straightforwardly. For a limited number of configurations, the three-body interactions are evaluated for a desired and a very large cutoff radius to determine the required corrections.

Long Time Evolution of Concentrated Vortex Rings with Large Radius

Long Time Evolution of Concentrated Vortex Rings with Large Radius

Physicochemical properties of CF3OCF = CF2 in high-voltage switchgear: part 1. Decomposition characteristics and thermodynamic parameters

Abstract Recently, perfluoromethyl vinyl ether (CF3OCF = CF2) has been proposed to have the potential to replace SF6, but its arc-extinguishing ability is still unknown. To provide the necessary basic parameters for further study of arc extinguishing characteristics, this series of papers focuses on the physical and chemical properties of CF3OCF = CF2 arc plasma. The research content of this paper is decomposition characteristics and thermodynamic parameters. The geometric configuration, rotational inertia, and vibration frequency characteristics of CF3OCF = CF2 and its decomposition products were obtained at the B3LYP/6–311G(d,p) level. The energy of each particle was calculated at the CCSD(T)/cc-pVTZ level. The most likely decomposition path was CF3OCF = CF2→CF3+CF2CFO→CF2+CFO. The equilibrium compositions of arc plasma were calculated by the mass action law model. The main components of pure CF3OCF = CF2 after arc were C3F8, C3F6, C2F6, CF4, and CO. When CO2 was mixed in CF3OCF = CF2, with the increase of CO2 content, the main components after arc gradually became CF2O, CF4, CO2, and CO. The thermodynamic parameters such as density, specific enthalpy, and specific heat at constant pressure were obtained by thermodynamic relationship. Through ρC p and ρh, it can be inferred that the radial heat transfer capacity of CF3OCF = CF2 is not as good as SF6, but the energy dissipation capacity of axial heat convection is stronger than SF6, and the arc presents the characteristics of large radius and long length.

Lewis Basicity Enhanced SrFeO3-δ-Based Perovskite Fuel Electrode for Surface and Bulk Processes Co-Accelerated CO2 Reduction in Solid Oxide Electrolysis Cells.

Electroreduction of CO2 to CO by solid oxide electrolysis cells (SOEC) is an effective means to realize carbon neutralization. However, the sluggish kinetics at SOEC fuel electrode impedes its further practical application. Herein, the doping strategy of cesium ion (Cs1+) is employed to develop a series of perovskite-type fuel electrode materials, i.e., CsxSr1-xFe0.9Nb0.1O3-δ (x = 0.05, 0.1, 0.15). Combining the results of experiments and theoretical calculations, it is found that the introduction of Cs1+ into A-site of SrFeO3-δ-based perovskite accelerates the reaction kinetics of CO2 adsorption and dissociation due to increased lattice oxygen basicity caused by the low electronegativity of Cs1+. In addition, in comparison to Sr2+, the larger ionic radius and lower valence of Cs1+ is beneficial to decrease formation energy of oxygen vacancy and migration barrier of oxygen ion in perovskite bulk. Because of those merits brought by Cs1+ doping, the La0.8Sr0.2Ga0.8Mg0.2O3-δ electrolyte-supported electrolysis cells with Cs0.1Sr0.9Fe0.9Nb0.1O3-δ fuel electrode presents satisfied current density of 2205mA cm-2 at 1.6V and 850°C. The stable long-term operation of electrolysis cells is also demonstrated at applied current density of 600mA cm-2 and 800 °C for 100 h.

A dynamics-based density profile for dark haloes – III. Parameter space

Abstract In the previous paper of this series, we proposed a new function to fit halo density profiles out to large radii. This truncated Einasto profile models the inner, orbiting matter as ρorb∝exp [ − 2/α (r/rs)α − 1/β (r/rt)β] and the outer, infalling term as a power-law overdensity. In this paper, we analyse the resulting parameter space of scale radius rs, truncation radius rt, steepening α, truncation sharpness β, infalling normalisation δ1, and infalling slope s. We show that these parameters are non-degenerate in averaged profiles, and that fits to the total profiles generally recover the underlying properties of the orbiting and infalling terms. We study the connection between profile parameters and halo properties such as mass (or peak height) and accretion rate. We find that the commonly cited dependence of α on peak height is an artefact of fitting Einasto profiles to the actual, truncated profiles. In our fits, α is independent of mass but dependent on accretion rate. When fitting individual halo profiles, the parameters exhibit significant scatter but otherwise follow the same trends. We confirm that the entire profiles are sensitive to the accretion history of haloes, and that the two radial scales rs and rt particularly respond to the formation time and recent accretion rate. As a result, rt is a more accurate measure of the accretion rate than the commonly used radius where the density slope is steepest.

KIC 7914906: An eclipsing heartbeat star with tidally excited oscillations and gamma Doradus/delta Scuti hybrid pulsations

Abstract We present the eclipsing heartbeat star KIC 9704906 with tidally excited oscillations (TEOs) and gamma Doradus/delta Scuti hybrid pulsations. The derived parameters show that it has an orbital period of $P = 8.7529108(1)$ d, a high eccentricity of $e = 0.467(3)$, and a high inclination of $i = 78.^{\!\!\!\circ }81(6)$. The mass ratio $q = 0.981(5)$ and relative radii (radius divided by the semi-major axis) $r_1 = 0.0639(2)$ and $r_2 = 0.0715(4)$ indicate that the secondary component has a smaller mass and a larger radius, and may have evolved off the main sequence. The eight derived TEO candidates, $n = 3$, 4, 5, 6, 7, 12, 40, and 44 harmonics, are consistent with or close to the dominant spherical harmonic $l=2$, $m=0$, or $\pm 2$, assuming that the spin and orbital axes are aligned, and the pulsations are adiabatic and standing waves. We also identify eight independent frequency candidates, but one of them, ${f_{7}}$, is more like a modulation of a quasi-periodic signal and the orbits. According to the g-mode frequencies, we find that the rotation period of one component is 11.52(29) d. Although the masses and radii cannot be further constrained due to the lack of sufficient high-precision spectra, fascinating phenomena in the Fourier spectra are evident and valuable in this system.

BaPb2(CO3)2F2: Enhancing Birefringence by a Multiunit Synergy Effect.

Carbonates are ideal birefringent materials because their π-conjugated [CO3] groups are beneficial for large anisotropy. How to maximize the birefringence and maintain a wide transmittance simultaneously in carbonates is a great challenge. Adopting a multiple-functional group synergy strategy by combining alkaline-earth metal Ba2+ with large ion radius, PbO6F distorted hexagonal pyramid, and paralleled π-conjugated CO3 triangles together, we synthesized a new metal carbonate crystal, BaPb2(CO3)2F2. BaPb2(CO3)2F2 features a typical two-dimensional layered structure, which consists of a PbCO3F monolayer parallel to the ab plane and Ba2+ cations filled between the monolayers. BaPb2(CO3)2F2 exhibits a large experimental birefringence of approximately 0.245@546 nm and a short ultraviolet absorption edge down to 264 nm (Eg = 4.70 eV), indicating that it is a promising ultraviolet birefringent material. The reported highest experimental birefringence index among carbonate crystals is surpassed by BaPb2(CO3)2F2. The theoretical calculation shows that the contribution percentages for total birefringence are 57 and 43% for the CO3 group and PbO3F group, respectively. Our work offers a new route for designing new birefringent materials with excellent comprehensive performance.

Vibrational Spectroscopy of Perovskite Ferroelectrics

Ferroelectric and antiferroelectric materials are technologically important by the richness of applications such as piezoelectric, pyroelectric, electro-optic, elasto-optic, and nonlinear optic effects. Especially, oxides with a perovskite structure are very important. Its chemical formula is ABO3, where A is a cation with a larger ionic radius, and B is a cation with a smaller ionic radius. Various elements are available in A- and B-sites. For example, the large piezoelectricity of well-known Pb(ZrxTi1−x)O3 (PZT) solid solutions was found in a morphotropic phase boundary (MPB). The very high dielectric constant, colossal piezoelectric effect, and large electro-optic effect are induced by ferroelectric phase transitions. Such excellent functionalities are closely related to lattice dynamical instability. The vibrational spectroscopy, i.e., Raman scattering, Brillouin scattering, far-infrared, and terahertz time-domain spectroscopy, is a powerful tool for lattice dynamical anomalies. This paper intended a brief review of vibrational spectroscopy on ferroelectric phase transitions of advanced perovskite oxides.

Multiple Redox‐Active Centers in An Azatriangulenetrione‐Based Covalent Organic Framework for High‐Capacity, High‐Rate and Ultra‐Stable Sodium‐Ion Batteries

AbstractSodium‐ion batteries (SIBs) suffer from sluggish kinetics, large volume change, and limited specific capacity due to the large radius of Na+. These issues can be solved through using covalent organic frameworks (COFs) as electrodes. Herein, an azatriangulenetrione‐containing COF (denoted as CityU‐33) was designed and synthesized as an electrode material for SIBs. Due to its inherent abundance of multiple redox‐active sites and fast intercalation kinetics, CityU‐33 delivered a high discharge capacity of 410.4 mAh g−1 at 0.1 A g−1 and showed remarkable long‐term cycling stability, where a discharge capacity of 288 mAh g−1 at 0.2 A g−1 with 97 % retention over 2000 cycles was achieved, making it the top COF electrode material for SIBs.

Multiple Redox-Active Centers in An Azatriangulenetrione-Based Covalent Organic Framework for High-Capacity, High-Rate and Ultra-Stable Sodium-Ion Batteries.

Sodium-ion batteries (SIBs) suffer from sluggish kinetics, large volume change, and limited specific capacity due to the large radius of Na+. These issues can be solved through using covalent organic frameworks (COFs) as electrodes. Herein, an azatriangulenetrione-containing COF (denoted as CityU-33) was designed and synthesized as an electrode material for SIBs. Due to its inherent abundance of multiple redox-active sites and fast intercalation kinetics, CityU-33 delivered a high discharge capacity of 410.4 mAh g-1 at 0.1 A g-1 and showed remarkable long-term cycling stability, where a discharge capacity of 288 mAh g-1 at 0.2 A g-1 with 97 % retention over 2000 cycles was achieved, making it the top COF electrode material for SIBs.

Assessing the robustness of the Galactic rotation curve inferred from the Jeans equations using Gaia DR3 and cosmological simulations

Context. Several authors have recently applied Jeans modelling to Gaia-based datasets to infer the circular velocity curve for the Milky Way. These works have consistently found evidence for a continuous decline in the rotation curve beyond ~15 kpc, which may indicate the existence of a light dark matter (DM) halo. Aims. Using a large sample of Gaia DR3 data, we aim to derive the rotation curve of the Milky Way using the Jeans equations, and to quantify the role of systematic effects, both in the data and those inherent to the Jeans methodology under the assumptions of axisym-metry and time independence. Methods. We used data from the Gaia DR3 radial velocity spectrometer sample, supplemented with distances inferred through Bayesian frameworks, to determine the radial variation of the second moments of the velocity distribution for stars close to the Galactic plane. We used these profiles to determine the rotation curve using the Jeans equations under the assumption of axisym-metry and explored how they vary with azimuth and position above and below the plane of the Galactic disc. We applied the same methodology to an N-body simulation of a Milky Way-like galaxy impacted by a satellite akin the Sagittarius dwarf, and to the Auriga suite of cosmological simulations. Results. The circular velocity curve we infer for the Milky Way is consistent with previous findings out to ~15 kpc, where our statistics are robust. Due to the larger number of stars in our sample, we are able to reveal evidence of disequilibrium and deviations from axisymmetry closer in. For example, we find that the second moment of VR flattens out at R ≳ 12.5 kpc, and that the second moment of Vϕ is different above and below the plane for R ≳ 11 kpc. Our exploration of the simulations indicates that these features are typical of galaxies that have been perturbed by external satellites. From the simulations, we also estimate that the difference between the true circular velocity curve and that inferred from Jeans equations can be as high as 15%, but that it is likely of the order of 10% for the Milky Way. This is higher than the systematic uncertainties associated with the observations or those linked to most modelling assumptions when using the Jeans equations. However, if the density of the tracer population were truncated at large radii instead of being exponential as often assumed, this could lead to the erroneous conclusion of a steeply declining rotation curve. Conclusions. We find that steady-state axisymmetric Jeans modelling becomes less robust at large radii, indicating that particular caution must be exercised when interpreting the rotation curve inferred in those regions. A more careful and sophisticated approach may be necessary for precision measurements of the DM content of our Galaxy.

Ab-initio atomistic insights into lead-free perovskites for Photovoltaics Technology

The primary objective of this manuscript is to enhance knowledge and comprehension of hexagonal perovskites KNiX3 (X=I, Br). The world is fascinated by halide perovskites because of their advancements in opto-electronic applications, particularly in the photovoltaic field. KNiI3 and KNiBr3 halide perovskites correspond to the two anion I and Br. Here we aim to investigate how opto-electronic applications are affected by the substitution of cations and anion. Within the framework of density functional theory (DFT), the structural, electronic, and optical properties have been computed using the PBE GGA approximation. Br-based perovskites have a shorter bond length than I-based compounds because iodine ions have a larger ionic radius than bromine ions. Thus, the lattice constant increases from I-based potassium halide perovskites to Br-based potassium halide perovskites. The values of the lattice constant match the results of experimental computations. The Density of States (DOS) curves and band structure are used to examine the contribution of the bands. From Br to I, there is a reduction in the band gap, indicating an improvement in photovoltaic applications. Various energy ranges are used to calculate the optical spectra, dielectric function, absorption coefficient, optical reflectivity, and refractive index. KNiX3 (X=I, Br) compound reveals low reflection value which is typically an excellent choice for photovoltaic and opto-electronic applications.