Stellar Equation Of State Research Articles

A study of white dwarf shock detonation and type Ia supernova explosion

Type Ia supernovae are not only sources of the elements of “life”, but also “standard candles” for measuring distances in the Universe. To describe the process, a hydrodynamic model of white dwarfs that takes into account the nuclear combustion of carbon is constructed. It is closed by a stellar equation of state and supplemented by a Poisson equation for the gravitational potential. The results of a mathematical simulation of a white dwarf shock detonation and subsequent explosion of a type Ia supernova are described. The computational experiments have shown that the dynamics of the shock detonation is determined by the explosion energy and the ignition point. A study of scalability and energy efficiency of the software implementation are also performed.'

Distinguishing double neutron star from neutron star-black hole binary populations with gravitational wave observations

Gravitational waves from the merger of two neutron stars cannot be easily distinguished from those produced by a comparable-mass mixed binary in which one of the companions is a black hole. Low-mass black holes are interesting because they could form in the aftermath of the coalescence of two neutron stars, from the collapse of massive stars, from matter overdensities in the primordial Universe, or as the outcome of the interaction between neutron stars and dark matter. Gravitational waves carry the imprint of the internal composition of neutron stars via the so-called tidal deformability parameter, which depends on the stellar equation of state and is equal to zero for black holes. We present a new data analysis strategy powered by Bayesian inference and machine learning to identify mixed binaries, hence low-mass black holes, using the distribution of the tidal deformability parameter inferred from gravitational-wave observations.

The hydrodynamical modeling of the Supernovae Ia explosion by means adaptive nested meshes on supercomputers

The results of the study of an explosion point of supernova type Ia (SNIa) with using of mathematical modeling on supercomputers is given in the paper. Hydrodynamical model closed by the stellar equation of state and supplemented by Poisson equation for gravitational potential is used for modeling of a white dwarf. The nuclear combustion of carbon, for which the analytical solution is constructed, is accounted in the model. A multilevel organization of computations on nested grids is used in the solution. The new highorder accuracy numerical method based on the Godunov method, the Rusanov scheme and the piecewise parabolic method on local stencil, adapted for computations on nested grids, is built. The parallel implementation is based on the idea of distributed computations, where the architecture with shared memory is used for modeling of the hydrodynamic evolution of white dwarfs, when the critical values of temperature and density are reached, a new task is launched on the architecture with distributed memory, in which the evolution of hydrodynamic turbulence leading to supersonic nuclear combustion of carbon is simulated.

Tidal deformability and I-Love-Q relations for gravastars with polytropic thin shells

The moment of inertia, the spin-induced quadrupole moment, and the tidal Love number of neutron-star and quark-star models are related through some relations which depend only mildly on the stellar equation of state. These "I-Love-Q" relations have important implications for astrophysics and gravitational-wave astronomy. An interesting problem is whether similar relations hold for other compact objects and how they approach the black-hole limit. To answer these questions, here we investigate the deformation properties of a large class of thin-shell gravastars, which are exotic compact objects that do not possess an event horizon nor a spacetime singularity. Working in a small-spin and small-tidal field expansion, we calculate the moment of inertia, the quadrupole moment, and the (quadrupolar electric) tidal Love number of gravastars with a polytropic thin shell. The I-Love-Q relations of a thin-shell gravastar are drastically different from those of an ordinary neutron star. The Love number and quadrupole moment are negative for less compact models and the I-Love-Q relations continuously approach the black-hole limit. We consider a variety of polytropic equations of state for the matter shell, and find no universality in the I-Love-Q relations. However, we cannot deny the possibility that, similarly to the neutron-star case, an approximate universality might emerge for a limited class of equations of state. Finally, we discuss how a measurement of the tidal deformability from the gravitational-wave detection of a compact-binary inspiral can be used to constrain exotic compact objects like gravastars.

ON THE PIECEWISE PARABOLIC METHOD FOR COMPRESSIBLE FLOW WITH STELLAR EQUATIONS OF STATE

The piecewise parabolic method and related schemes are widely used to model stellar flows. Several different methods for extending the validity of these methods to a general equation of state (EOS) have been proposed over time, but direct comparisons among one-another and exact solutions with stellar EOSs are not widely available. We introduce some simple test problems with exact solutions run with a popular stellar EOS and test how two existing codes with different approaches to incorporating general gases perform. The source code for generating the exact solutions is made available.

Constraints on kilohertz quasi-periodic oscillation models and stellar equations of state from SAX J1808.4–3658, Cyg X-2 and 4U 1820–30

We test the relativistic precession model (RPM) and the magnetohydrodynamics Alfvén wave oscillation model (AWOM) for the kilohertz (kHz) quasi-periodic oscillations (QPOs) from sources with measured neutron star (NS) masses and twin kHz QPO frequencies. For the RPM, the derived NS masses of Cyg X-2, SAX J1808.4–3658 and 4U 1820–30 are 1.96 ± 0.10, 2.83 ± 0.04 and 1.85 ± 0.02 M⊙, respectively. These are, respectively, ∼30, 100 and 40 per cent higher than the measured results 1.5 ± 0.3, <1.4 and |$1.29^{+0.19}_{-0.07}$| M⊙. For the AWOM, where the free parameter of the model is the density of the star, we infer the NS radii to be around 10–20 km for the above three sources. Based on this, we can infer the matter compositions inside the NSs with the help of the equations of state. In particular, for SAX J1808.4–3658, the AWOM shows a lower mass density for its NS than those of the other known kHz QPO sources, with a radius range of 17–20 km, which excludes the strange quark matter inside its star.

Model-independent comparisons of pulsar timings to scalar–tensor gravity

Observations of pulsar timing provide strong constraints on scalar–tensor theories of gravity, but these constraints are traditionally quoted as limits on the microscopic parameters (like the Brans–Dicke coupling, for example) that govern the strength of scalar–matter couplings at the particle level in particular models. For binary pulsars whose all five post-Keplerian parameters have been measured, we present fits to timing data directly in terms of the phenomenological couplings (masses, scalar charges, moment of inertia sensitivities and so on) of the stars involved, rather than to the more microscopic parameters of a specific model. For instance, for the double pulsar PSR J0737-3039A/B, we find at the 68% confidence level that the masses are bounded by 1.28 < mA/m⊙ < 1.34 and 1.19 < mB/m⊙ < 1.25, while the scalar charge-to-mass ratios satisfy |aA| < 0.21, |aB| < 0.21 and |aB − aA| < 0.002, independent of the details of the scalar–tensor model involved, and of assumptions about the stellar equations of state. Whenever it is possible to do so, we urge observers to express their results in this more model-independent way, which potentially can then be used by theorists to constrain a great variety of specific models by computing the fit quantities as functions of the microscopic parameters in any particular model. For the Brans–Dicke and quasi-Brans–Dicke models, the constraints coming from the double pulsar obtained in this manner are consistent with, and slightly weaker than, those quoted in the literature.

The estimations of neutron star mass and radius by the kHz QPOs

AbstractThe kHz quasi‐periodic oscillations (QPOs) have been detected by the RXTE satellite in about thirty neutron stars (NSs) in low mass X‐ray binaries (LMXBs), which are usually interpreted to be related to the Keplerian motions in the orbit close to NS surface where the accreted matter is sucked onto the star. Based on the MHD Alfvén wave oscillation model and the relativistic precession model for the neutron star (NS) kHz QPOs, estimations of mass M and radius R of some NSs are given, which can give clues to evaluate the models. Furthermore, comparisons with theoretical M ‐R relations by stellar equations of state (EOSs) are presented (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The applications of the MHD Alfvén wave oscillation model for kHz quasi‐periodic oscillations

AbstractIn this paper, we improve the previous work on the MHD Alfvén wave oscillation model for the neutron star (NS) kHz quasi‐periodic oscillations (QPOs), and compare the model with the updated twin kHz QPO data. For the 17 NS X‐ray sources with the simultaneously detected twin kHz QPO frequencies, the stellar mass M and radius R constraints are given by means of the derived parameter A in the model, which is associated with the averaged mass density of the star as 〈ρ 〉 = 3M /(4πR3) ≃ 2.4 × 1014 (A /0.7)2 g/cm3, and we also compare the M ‐R constraints with the stellar equations of state. Moreover, we also discuss the theoretical maximum kHz QPO frequency and maximum twin peak separation, and some expectations on SAX J1808.4–3658 are mentioned, such as its highest kHz QPO frequency ∼ 870 Hz, which is about 1.4–1.5 times less than those of the other known kHz QPO sources. The estimated magnetic fields for both Z sources (about Eddington accretion rate $ {\dot M}_{\rm Edd} $) and Atoll sources (∼ 1% $ {\dot M}_{\rm Edd} $) are approximately ∼109 G and ∼108 G, respectively. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Unidimensional SPH simulations of reactive shock tubes in an astrophysical perspective

Smoothed Particle Hydrodynamics (SPH) is a Lagrangian method widely used for the modelling of a large variety of astrophysical fluid flows in more than one dimension. Simulations of thermonuclear explosions in stars require, besides the hydrodynamic equations, a realistic equation of state, an energy source term, and a set of nuclear kinetic equations to follow the composition changes of the gas during the explosion. The implementation of a realistic stellar equation of state, and the coupling of hydrodynamics and nuclear burning are investigated in the framework of the simple shock tube geometry. We present and discuss the results of a series of SPH simulations of a detonation in the presence of (1) a single exothermic nuclear reaction, and (2) a restricted network of nuclear reactions. Our results are compared to those of identical simulations performed by other authors using a different hydrodynamic method.

Ha, ha, ha, ha, staying alive, staying alive: A radio pulsar with an 8.5-s period challenges emission models

AbstractWe report the discovery of the longest known radio pulsar period. PSR J2144–3933, previously thought to have a period of 2.84 s, actually has a period of 8.51 s. Under the usual assumptions about the stellar equation of state, this pulsar has an average surface dipolar magnetic field strength of ~ 2.0 × 1012G. According to popular theories of the emission mechanism this pulsar should not be emitting radio waves because its long period and magnetic field strength make pair creation impossible for all reasonable magnetic field configurations. Either assumptions about the equation of state are incorrect, or the emission theories must be revised.

Neutrino-cooled Accretion: Rotation and Stellar Equation of State

Neutrino-cooled Accretion: Rotation and Stellar Equation of State

Soft X-ray pulses from neutron star glitches

view Abstract Citations (49) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Soft X-Ray Pulses from Neutron Star Glitches van Riper, Kenneth A. ; Epstein, Richard I. ; Miller, Guy S. Abstract Glitches release energy in the interiors of neutron stars. In some models of glitches, such as those involving the sudden transfer of angular momentum to the stellar crust from a more rapidly rotating neutron superfluid, the energy liberated is large enough (about 10 exp 38 to 10 exp 43 ergs) to noticeably raise the temperature of the stellar surface. These temperature enhancements last from hours to months, depending on the stellar equation of state and the depth at which the heating occurs. Publication: The Astrophysical Journal Pub Date: November 1991 DOI: 10.1086/186193 Bibcode: 1991ApJ...381L..47V Keywords: Neutron Stars; Stellar Interiors; X Ray Astronomy; Equations Of State; Rosat Mission; Stellar Rotation; Superfluidity; X Ray Telescopes; Astrophysics; DENSE MATTER; STARS: EVOLUTION; STARS: INTERIORS; STARS: NEUTRON; STARS: X-RAYS full text sources ADS |