Adiabatic Index Research Articles

Survey of radiative, two-temperature magnetically arrested simulations of the black hole M87* I: Turbulent electron heating

Abstract We present a set of eleven two-temperature, radiative, general relativistic magnetohydrodynamic (2TGRRMHD) simulations of the black hole M87* in the magnetically arrested (MAD) state, surveying different values of the black hole spin a*. Our 3D simulations self-consistently evolve the temperatures of separate electron and ion populations under the effects of adiabatic compression/expansion, viscous heating, Coulomb coupling, and synchrotron, bremsstrahlung, and inverse Compton radiation. We adopt a sub-grid heating prescription from gyrokinetic simulations of plasma turbulence. Our simulations have accretion rates $\dot{M}=(0.5-1.5)\times 10^{-6}\dot{M}_{\rm Edd}$ and radiative efficiencies εrad = 3 − 35 %. We compare our simulations to a fiducial set of otherwise identical single-fluid GRMHD simulations and find no significant changes in the outflow efficiency or black hole spindown parameter. Our simulations produce an effective adiabatic index for the two-temperature plasma of Γgas ≈ 1.55, larger than the Γgas = 13/9 value often adopted in single-fluid GRMHD simulations. We find moderate ion-to-electron temperature ratios in the 230 GHz emitting region of R = Ti/Te ≈ 5. While total intensity 230 GHz images from our simulations are consistent with Event Horizon Telescope (EHT) results, our images have significantly more beam-scale linear polarization (〈|m|〉 ≈ 30 %) than is observed in EHT images of M87* (〈|m|〉 < 10 %). We find a trend of the average linear polarization pitch angle ∠β2 with black hole spin consistent with what is seen in single-fluid GRMHD simulations, and we provide a simple fitting function for ∠β2(a*) motivated by the wind-up of magnetic field lines by black hole spin in the Blandford-Znajek mechanism.

The stability of anisotropic compact stars influenced by dark matter under teleparallel gravity: an extended gravitational deformation approach

In our investigation, we pioneer the development of geometrically deformed strange stars within the framework of teleparallel gravity theory through gravitational decoupling via the complete geometric deformation (CGD) technique. The significant finding is the precise solution for deformed strange star (SS) models achieved through the vanishing complexity factor scenario. Further, we introduce the concept of space-time deformation caused by dark matter (DM) content in DM haloes, leading to perturbations in the metric potentials gtt and grr components. Mathematically, this DM-induced deformation is achieved through the CGD method, where the decoupling parameter α governs the extent of DM influence. To validate our findings, we compare our model predictions with observational constraints, including GW190814 (with a mass range of 2.5-2.67M⊙) and neutron stars (NSTRs) such as EXO 1785-248 [mass=1.3-0.2+0.2M⊙], 4U 1608-52 [mass=1.74-0.14+0.14M⊙], and PSR J0952-0607 [mass=2.35-0.17+0.17M⊙]. Our investigation delves into the stability of the model by considering causality conditions, Herrera’s cracking method, the adiabatic index, and the Harrison–Zeldovich–Novikov criterion. We demonstrate that the developed model mimics a wide range of recently observed pulsars. To emphasize its compatibility, we highlight the predicted mass and radius in tabular form by varying both the parameters α and ζ1. Notably, our findings are consistent with the observation of gravitational waves from the first binary merger event. Furthermore, we compare our results with those obtained for a slow-rotating configuration. In addition to this, we discuss the moment of inertia using the Bejger–Haensel approach in this formulation.

Impact of non-metricity and matter source on the geometry of anisotropic spheres

This paper delves into the impact of extended symmetric teleparallel theory on anisotropic compact stellar structures. The explicit field equations were formulated by considering a minimum model of this extended gravity. Basically, the Darmois junction conditions are used to determine the unknown constants of metric coefficients. We explore some significant properties of the compact stars under consideration to check their viable existence in this modified framework. The Tolman–Oppenheimer–Volkoff equation assess the equilibrium state of the compact stars. Moreover, the stability analysis is defined by using methods based on sound speed (related to how disturbances propagate in the star) and adiabatic index (related to the thermodynamic behavior of the star). We find that the proposed compact stars in the f(Q,T) gravity are physically viable and stable.

Theoretical Analysis of Specific Impulse at Lower Geo-Potential Altitude Range for a Paraffin-Fueled Hybrid Rocket Motor

Abstract: This paper presents a theoretical performance of paraffin-based hybrid rocket motors, focusing on the ideal specific impulse performance (Isp) at lower geo-potential atmospheric altitudes. The study considers n-docosane (C22H46) as fuel with oxidizer as nitrous oxide (NO). Simplified mathematical modelling is employed to perform the analysis. The input thermodynamic parameters are calculated for exhaust gas mixture under balanced stoichiometric chemical reaction based on fixing a standardized exhaust temperature of 1000oK in relevance with material considerations. The analysis underscores the influence of specific heat capacity and specific heat ratio on thrust coefficients. The nozzle area ratio is fixed as 2.5 and accordingly the design exit Mach number (Me) is calculated using Chebyshev iterative method by formulating a transcendental non-linear function and accordingly theoretical computations are performed based on the methodology followed. The inference reveals that the ideal specific impulse ranges from 218 to 246 seconds across lower atmospheric geo-potential altitudes. This also highlights the influence of chamber pressure on altitude as a performance assessment for propulsion system. The findings are theoretical and delivers valuable insight into application of hybrid propulsion systems for altitude operations in future.

Sustained Casing Pressure Assessment Based on Gas Leakage Rate by the State of Equation for Real Gas

ABSTRACTGas leakage is one of the most concerning issues in wells for exploring conventional or unconventional oil and gas reservoirs, carbon sequestration, and geothermal energy. Wellbore barriers, such as tubing, casing, and cement, are the primary components that prevent the undesirable flow of subsurface fluids. However, due to the complexity of the operating condition in a harsh environment, the tubing integrity is prone to failure, causing gas leakage and forming a sustained casing pressure (SCP) at the wellhead. This work proposes a prediction model considering the real gas effect when evaluating SCP. The proposed model involves gas flow, leakage, and accumulation in the wellbore. With the pressure and temperature obtained by the flow equations as boundaries, the model estimates the gas flow rate at the leakage point and SCP. Subsequently, comparing the current leakage model with the conventional method demonstrates the model's performance. Finally, the current model is applied to an ultra‐deep well to determine the leakage location by inversion. Further sensitivity studies reveal the influences of wellbore conditions on SCP, including the production rate, depth of liquid level, and annular fluid density. The study indicates that the traditional method based on ideal gas underestimates the mass flow rate by approximately 22% compared to the current model. When the adiabatic index of the conventional method is approximated as the isentropic coefficient, the mass flow rate may agree well with the current model. It is acceptable to predict the leakage flow rate by assuming that the production gas is pure methane and ignoring the influence of gas composition. The leakage position is the most influential factor for SCP. These results would help engineers predict SCP and determine the leakage location in wells.

Charged Gravastar Solutions in the Finch‐Skea Framework with f(Q)$f(\mathbb {Q})$ Gravity

AbstractIn this study, the features of a charged gravastar are investigated within the framework of gravity ( represents nonmetricity) using the Finch–Skea metric. This metric is applied to both the interior and shell regions of the charged gravastar and the field equations are derived accordingly. For the exterior regions, various black holes are considered, i.e., Reissner–Nordström, Bardeen, and Hayward regular black holes. The interior and exterior layers are matched using the Israel junction conditions, which help to determine the surface energy density and surface pressure for these black holes. Some physical properties such as proper length, entropy, energy, and the equation of state parameter are examined. The stability of the developed gravastar model is discussed through the effective potential, the causality condition and adiabatic index. It is concluded that the compact gravastar structure could be a viable alternative to black holes within this framework.

Possible explanations for the formation of ear-like structures in young Type Ia supernova remnants

Several young type Ia supernova remnants (SNRs) exhibit apparent axisymmetrical deviations from spherical symmetry, manifested as two opposite ear-like protrusions in their projected morphologies. The origin of this specific feature remains debated, with several physical mechanisms proposed as possible explanations. In this work, we propose two scenarios to explain the formation of ear-like structures: (i) the feedback of cosmic-ray (CR) acceleration via an effective adiabatic index, γ_ eff, with spatial variations, and (ii) the large-scale pre-existing density gradient in the local interstellar medium (ISM). Our cylindrical hydrodynamic simulation results reveal that both scenarios can produce prominent protrusions well beyond the main shell, resembling the peculiar features observed in several young type Ia SNRs. Additionally, based on the detailed analysis of the simulation data, we attempt to elucidate the ear-formation process and suggest that the relative positions of contact discontinuity can serve as an observational diagnosis in pursuing the origin of ear-like structures. We also discuss the following points: (i) the theoretical prediction of type Ia SNRs with "ears" and the visibility of an ear-like structure in observations; (ii) the simulation-based inference concerning the emission properties of the ear-like regions and the implications of a qualitative comparison of our results with X-ray observations; and (iii) the potential combination of different scenarios. Despite various existing models, we tend to regard our proposed scenarios as potential alternatives, whereby the ear-like structures originate in pure ejecta-ISM interaction, distinct from the ejecta-circumstellar medium (CSM) or jet-CSM models.

Investigation of drafting-kissing-tumbling movement of two particles with conjugate heat transfer

The conjugate heat transfer at the particle-fluid interface and the collision between particles play a crucial role in the sedimentation process of particles. In this work, the recent volumetric lattice Boltzmann method for thermal particulate flows with conjugate heat transfer is adopted to investigate the drafting-kissing-tumbling movement in the sedimentation process of two particles in a closed channel. This volumetric lattice Boltzmann method is based on double distribution functions, with the density distribution function used for the velocity field and the internal energy distribution function used for the temperature field. It is a single-domain approach, and the nonslip velocity condition within the solid domain can be strictly ensured. The difference in thermophysical properties between the solid and fluid can be correctly handled, and the conjugate heat transfer condition can be automatically achieved without any additional treatments. Based on this particle-resolved simulation, the influences of the solid-to-fluid specific heat ratio, the Grashof number, and the particle’s initial temperature on the drafting-kissing-tumbling movement are discussed in detail. It is found that the fluid cooled by the particle and thus subjected to the downward buoyancy force can promote particle sedimentation. As the specific heat ratio increases, the particle’s temperature rises relatively slowly. In the sedimentation of two cold particles, the drafting duration and tumbling duration of the drafting-kissing-tumbling movement decrease when the heat capacity ratio increases. In contrast, the kissing duration increases as the heat capacity ratio increases. When the Grashof number increases, the heat transfer between the particle and fluid is enhanced, and the drafting duration significantly decreases while the kissing duration and tumbling duration remain almost unchanged in the sedimentation of two cold particles. The particle’s initial temperature greatly affects the occurrence moment of the drafting-kissing-tumbling movement. To be specific, the drafting-kissing-tumbling movement occurs at the earliest moment for the sedimentation of two cold particles, followed by the sedimentation of one cold and one hot particle, and the latest for the sedimentation of two hot particles. The promoting effect of the low particle’s initial temperature on the drafting-kissing-tumbling movement mainly takes place in the dragging stage and kissing stage. The particle’s initial temperature has almost no influence on the tumbling duration.

On aerobreakup in the stagnation region of high-Mach-number flow over a bluff body

In this paper, aerobreakup in the stagnation region of high-Mach-number flow over a bluff body is studied with experiments and computations. Water drops of diameter 0.51–2.30 mm were acoustically levitated at sea level along the flight path of a rectangular $100\ {\rm mm} \times 150\ {\rm mm}$ rail-gun launched projectile. This enabled the study of aerobreakup at high Mach (3.03–5.12), post-shock Mach (1.5–1.9), Weber $(5 \times 10^4\unicode{x2013}4 \times 10^5)$ and Reynolds $(6 \times 10^4\unicode{x2013}3 \times 10^5)$ numbers. High-speed backlit shadowgraphy is used to record the flow structure. Computations are made for two cases, and it was found that the drop behaviour is not dominated by viscous or surface-tension effects and can be adequately captured by treating the gas as calorically perfect with the ratio of specific heats set to 1.3 to account for thermochemical effects. To assess drop surface stability at early breakup times, results from Newton's inclination method are used to determine the flow along the drop surface and input to a linear-stability analysis; from this, it was found that viscosity and surface tension can be neglected. Moreover, the acceleration term dominates the shear term at the stagnation point, a point accentuated as a drop flattens; this relation inverts closer to the drop equator. Linear-stability analysis was insufficient for modelling late-stage aerobreakup because the predicted wavelengths were too small and the expected aerobreakup times were non-physically short. To address this discrepancy, a nonlinear instability model with constant-rate growth is used that treats the accelerated drop surface as analogous to bubbles rising through a liquid; agreement with computations is good.

Properties of interacting quark star in light of Rastall gravity

Abstract This study explores the properties of quark stars (QS) formulated with an interacting quark matter equation of state (EoS) within the framework of Rastall gravity, a modified theory of gravity. We derive the mass-radius relationships and calculate the maximum gravitational masses and their corresponding radii, comparing these results under both Rastall gravity and general relativity. Our analysis incorporates recent observational data, including the GW190425 event, to constrain the model parameters ($\bar{\lambda}, \eta, B_{\rm eff}$). We also assess the stability of these quark stars by evaluating their static stability, adiabatic index, and sound velocity profiles, \textcolor{blue}{thus confirming their viability within the modified gravitational framework}.

Stability of compact stars in a uniform density background cloud

We are discussing a scenario where a compact star (neutron star, NS) is embedded in a thin, uniform density background cloud (a remnant cloud after a supernova or a cloud generated from the late stages of a star e.g., a planetary nebula or asymptotic red giant phases) and its effect on the stability of the compact star. Due to the thin background cloud, the spacetime geometry is minimally deformed allowing us to employ the technique of minimal geometric decoupling (MGD). Assuming a uniform background cloud density simplifies the problem, and through the MGD method, one can take Θtt=Θ>0 0$$\\end{document}]]>, where Θ is the density of the cloud. The background cloud interacts with the compact star through a coupling strength α. By varying α, one can tune the cloud density to analyze the stability of the embedded compact star. We found that for α<3×10-5, all the thermodynamic quantities are well-behaved, indicating a stable configuration. Once the coupling parameter exceeds 3×10-5, the adiabatic index drops below Γmax′, triggering a gravitational collapse. Beyond this limit of α, the pressure and speed of sound also become non-physical. At the end, we have used the M-R curve generated from the solution to determine the radii of a few compact stars, namely PSR J1614-2230, PSR J0952-0607, GW190814, and GW200210. Furthermore, we have discussed the possibility of the secondary component of GW200210 i.e. the less massive compact object with an upper mass of 3.3M⊙, which may be a stellar black hole with a Schwarzschild radius RBH=9.746 km. However, if the mass is 2.83M⊙ as observed, then its predicted minimum radius is 10.74 km, corresponding to α=0. This radius is far beyond RBH=8.357 km and therefore is most probably a massive NS in the mass gap.

Charged Stellar Structures with Adler-Finch-Skea Geometry in Ricci-inverse Gravity

Abstract We obtain a class of charged, anisotropic, and spherically symmetric solution characterized by the function $f(\mathcal{R},\mathcal{A})=\mathcal{R}+\alpha \mathcal{A}$, where $\mathcal{R}$ is the Ricci scalar, $\mathcal{A}$ is anticurvature scalar, and $\alpha$ is the coupling constant. The model was developed by utilizing the Karmarkar condition to produce the radial metric coefficient and adopting the time metric coefficient as proposed by Adler. We formulated the model by assuming a specific type of charge distribution within the stellar interior. We elaborated boundary conditions to ensure the continuity of the spacetime, considering the exterior spacetime as Bardeen's solution. In addition, we analyzed several physical entities, including the energy density, pressure components, anisotropy in pressure, energy conditions, equation of state, surface redshift, compactness factor, adiabatic index, sound speed, and Tolman-Oppenheimer-Volkoff equilibrium condition. The aforementioned conditions were satisfied, indicating that the solutions derived in this study are within the physically acceptable range.

Anisotropic quark stars in gravity* *Takol Tangphati was Supported by Walailak University under the New Researcher Development scheme (WU67268). A. Pradhan expresses gratitude to the IUCCA in Pune, India, for offering facilities under associateship programs. In addition, İzzet Sakallı thanks TÜBİTAK, ANKOS, and SCOAP3 for their contributions. Takol Tangphati and İzzet Sakallı also appreciate COST Actions CA21106 and CA22113 for

We investigated the impact of gravity on the internal structure of compact stars, expecting this theory to manifest prominently in the high-density cores of such stars. We considered the algebraic function , where α represents the matter-geometry coupling constant. We specifically chose the matter Lagrangian density to explore compact stars with anisotropic pressure. To this end, we employed the MIT bag model as an equation of state. Subsequently, we numerically solved the hydrostatic equilibrium equations to obtain mass-radius relations for quark stars (QSs), examining static stability criteria, adiabatic index, and speed of sound. Finally, we used recent astrophysical data to constrain the coupling parameter α, which may lead to either larger or smaller masses for QSs, compared to their counterparts in general relativity.

Computational Study on Flow Characteristics of Shocked Light Backward-Triangular Bubbles in Polyatomic Gas

This study computationally examined the Richtmyer–Meshkov instability (RMI) evolution in a helium backward-triangular bubble immersed in monatomic argon, diatomic nitrogen, and polyatomic methane under planar shock wave interactions. Using high-fidelity numerical simulations based on the compressible Navier–Fourier equations based on the Boltzmann–Curtiss kinetic framework and simulated via a modal discontinuous Galerkin scheme, we analyze the complex interplay of shock-bubble dynamics. Key findings reveal distinct thermal non-equilibrium effects, vorticity generation, enstrophy evolution, kinetic energy dissipation, and interface deformation across gases. Methane, with its molecular complexity and higher viscosity, exhibits the highest levels of vorticity production, enstrophy, and kinetic energy, leading to pronounced Kelvin–Helmholtz instabilities and enhanced mixing. Conversely, argon, due to its simpler atomic structure, shows weaker deformation and mixing. Thermal non-equilibrium effects, quantified by the Rayleigh–Onsager dissipation function, are most significant in methane, indicating delayed energy relaxation and intense turbulence. This study highlights the pivotal role of molecular properties, specific heat ratio, and bulk viscosity in shaping RMI dynamics in polyatomic gases, offering insights on uses such as high-speed aerodynamics, inertial confinement fusion, and supersonic mixing.

Slowly rotating relativistic stars in [formula omitted] theory of gravity

In this paper, we investigate slowly rotating compact stars within the framework of f(R) gravity in a self-consistent way. We derive the equations describing the structure of compact stars in f(R) gravity and solve both the exterior and interior problems simultaneously. The study includes an equation of state. We considerably analyze the moment of inertia and its dependence on the stellar mass and the f(R) gravity. Future studies of the moment of inertia of compact stars may help determine the difference between f(R) gravity and general relativity, providing information on the high field domain of gravity. For our current analysis, we consider five different compact stars, namely, PSR1903+327, PSR1937+21, PSRJ1614−2230, CenX−3, and VelaX−1. To determine the viability of our model, we conduct a number of physical experiments, such as the analysis of energy density and pressure components, equilibrium and energy conditions, stability analysis, and the adiabatic index. The relationship between stellar objects’ mass, energy density, radius, and moment of inertia is observed. Our findings indicate that obtained results are physically adequate and consistent with the model we considered.

Effect of Phenolic Resin Pyrolysis on Thermal Properties of SiFRP Composites under High Heating Rates

Abstract During the ablation process of resin-based composite materials, residual carbon generated from pyrolysis of phenolic resin change the thermal performance of the material. Phenolic resin pyrolysis process depends on temperature and heating rate. Based on the mass loss curves of phenolic resin under high heating rates (5K/s, 50K/s, 100K/s), the study analyzes the variations in thermal performance parameters of SiFRP composite materials with temperature assuming phenolic resin and silica fibres are in parallel in heat transferring. As temperature rising, the pyrolysis conversation ratio of phenolic resin increases, thermal conductivity and specific heat capacity of the composites increase accordingly. The greater the heating rates, the smaller the change of thermal conductivity and specific heat capacity of composites at the same temperature. The complete pyrolysis temperature of phenolic resin is positively correlated with the temperature rise rate. When the temperature rise rate is very large (such as 100K/s), the influence of pyrolysis on the thermal properties of composites can be ignored at medium and low temperature.

Physical viability of [formula omitted] gravity corrected charged anisotropic solutions

This article explores the impact of charge on the properties of relativistic compact star candidates exhibiting anisotropic distribution in f(Q) gravity, where Q represents a nonmetricity. A specific model of this gravitational theory is considered to simplify the system and establish the precise field equations that regulate the interaction between matter and geometry. Analysis of the static spherical symmetric structures is conducted by considering the non-singular feasible solutions. The Darmois constraints are used to determine the unknown constants in the metric coefficients. We explore different physical characteristics in the interior of charged compact stars to check their viability. Sound velocity and adiabatic index techniques are employed for stability state analysis. Proposed charged stars are determined to be both viable and stable in this theoretical framework.

General isotropic charged fluid spheres within the matter coupling gravity formalism

In this paper, we proposed a new mathematical model of charged isotropic compact relativistic object in the f(R,T) gravity. In the f(R,T) framework, the gravitational action involves the trace of the energy–momentum tensor T and the Ricci scalar R. We choose a specific f(R,T) framework such that f(R,T)=R+2χT, in which χ represents the coupling parameter, which is responsible for measuring the deviation from the standard Einstein’s general theory of relativity (GR). A short overview of the modified f(R,T) gravity theory is presented and the field equations are formulated in this novel modified gravity. It is shown that for a specific limit of the coupling parameter, the standard GR can be restored from the considered f(R,T) model. We modeled, mathematically, a specific charged compact star XTEJ1739−217 (M=1.51M⨀, R=10.9 km), within the f(R,T) extended gravity theory framework by taking benefit from the well studied Heintzmann IIa ansatz [H. Heintzmann, Z. Physik 228, 489–493 (1969)]. To examine the reliability and physical plausibility of Our model, different physical characteristics such as the energy density and pressure, electric field, energy conditions, stability analysis via Herrara cracking technique and the adiabatic index, equilibrium conditions under different forces, mass–radius relationship, compactness and surface red-shift are studied carefully, which are essential for confirming the model’s physical feasibility. The mathematically established results are more accurately represented by graphical illustrations for the various chosen values of the coupling parameter χ. In this study, we also compared our findings with the standard GR results and the observational facts, and we inferred that for nonzero values of the coupling parameter, χ, our results in f(R,T) framework are closely related to the observational facts in comparison with the standard GR. We conclude that our presented model is well consistent with all the requirements and viably modeled the compact star XTEJ1739−217.

Properties of white dwarf stars within rainbow gravity

We investigate the properties of anisotropic white dwarf stars within the rainbow gravity adopting for matter content the Chandrasekhar model based on an ideal Fermi gas at zero temperature. We study in detail the effects of the anisotropic factor on stellar mass and radius, the speed of sound, and the relativistic adiabatic index in both radial and tangential directions. We find that causality is never violated, whereas the stability criterion based on the relativistic adiabatic index is not met when the objects are characterized by a positive anisotropic factor close to the Chandrasekhar limit. We present this significant observation here for the first time, to the best of our knowledge.

Impact of a Near-Surface Plasma Region on the Bow Shock Wave and Aerodynamic Characteristics of a High-Speed Model in Xenon

The main objective of this study is to demonstrate the active influence on the location of the bow shock wave, as well as on the parameters of an aerodynamic body, of a gas discharge organized near the frontal surface, between the body and the bow shock wave. The research is carried out using both experimental and numerical methods at the freestream Mach number M = 6.8. The working gas is xenon. It is shown that the location of the steady bow shock wave, along with the current and power of the discharge, is associated with the change in the adiabatic index of the plasma created by the discharge, which, in turn, is determined by plasma parameters such as the degrees of nonequilibrium and the degree of ionization. It is shown that the adiabatic index with the power supplied to the impact zone in the range of 30–120 kW can both increase and decrease in the range of 1.25–1.288. A study of the discharge-created plasma zone is conducted, and the correspondence between the gas discharge current and power and the average parameters in the plasma zone created by the discharge are presented. A good agreement between the numerical and experimental data is shown. The results obtained can be useful in the development of control systems for high-speed flows based not only on the effects of heating but also on the impact of plasma parameters.