The effects of parametrization of the dark energy equation of state

We investigate in detail the influence of parametrizations of the dark energy equation of state on reconstructing dark energy geometrical parameters,such as the deceleration parameter q(z) and Om diagnostic.We use a type Ia supernova sample,baryon acoustic oscillation data,cosmic microwave background information along with twelve observational Hubble data points to constrain cosmological parameters.With the joint analysis of these current datasets,we find that the parametrizations of w(z) have little influe...     

Cooling and energy loss of partially ionized hydrogen gas behind a shock wave

We studied the difference in behavior of total energy and its thermal component during the radiative cooling of partially ionized hydrogen gas. Our calculations were fulfilled for the conditions in the atmosphere of a cool star. It is shown that the attenuation of total energy loss does not interfere with the cooling rate.     

Revisiting the parametrization of equation of state of dark energy via SNIa data

In this paper, we revisit the parametrizations of the equation of state of dark energy and point out that comparing merely the  χ²  of different fittings may not be optimal for choosing the 'best' parametrization. Another figure of merit for evaluating different parametrizations based on the area of the   w ( z ) − z   band is proposed. In light of the analysis of some two-parameter parametrizations and models based on available SNIa data, the area of   w ( z ) − z   band seems to be a good figure of merit, especially in the situation that the value of  χ²_min  for different parametrizations are very close. Therefore, we argue that both the area of the   w ( z ) − z   band and  χ²_min  should be synthetically considered for choosing a better parametrization of dark energy in the future experiments.     

Hyperonic equation of state

An equation of state for cold matter at neutron star densities, >10¹⁴ gm/cm³, is evaluated. The gas is considered to be a degenerate mixture of neutrons, protons, leptons, hyperons and massive baryons. We derive the equilibrium equations including the effects of nuclear interactions among all the hadrons.     

A new parametrization of dark energy equation of state leading to double exponential potential

We show that a phenomenological form of energy density for the scalar field can provide the required transition from decelerated(q 0) to accelerated expansion(q 0) phase of the universe.We have used the latest type Ia supernova(SNIa) and Hubble parameter datasets to constrain the model parameters. The best fit values obtained from those datasets are then applied to reconstruct ωφ(z), the equation of state parameter for the scalar field. The results show that the reconstructed forms of q(z) and ω_φ(z) do not differ much from the standard ΛCDM value at the current epoch. Finally, the functional form of the relevant potential V(φ) is derived by a parametric reconstruction. The corresponding V(φ)comes out to be a double exponential potential, which has a number of cosmological implications.Additionally, we have also studied the effect of this particular scalar field dark energy sector on the evolution of matter overdensities.     

Magnetogasdynamic cylindrical shock wave in an isothermal state of a gas,II

Non-self-similar isothermal flows behind weak cylindrical shock waves are investigated. The shock is assumed to be propagating in a medium at rest with uniform density permeated by a uniform transverse magnetic field. The electrical conductivity of the gas is infinite everywhere. The total energy of the disturbance is not constant but can be made to increase at a slow rate. A comparative study has been made between the results with an without a magnetic field.     

Optimal strategies: theoretical approaches to the parametrization of the dark energy equation of state

The absence of compelling theoretical model requires the parameterizing the dark energy to probe its properties. The parametrization of the equation of state of the dark energy is a common method. We explore the theoretical optimization of the parametrization based on the Fisher information matrix. As a suitable parametrization, it should be stable at high redshift and should produce the determinant of the Fisher matrix as large as possible. For the illustration, we propose one parametrization which can satisfy both criteria. By using the proper parametrization, we can improve the constraints on the dark energy even for the same data. We also show the weakness of the so-called principal component analysis method.     

Neutron stars and the equation of state

The interior of neutron stars consists of the densest, although relatively cold, matter known in the universe. Here, baryon number densities might reach values close to ten times the nuclear saturation density. These suggest that the constituents of neutron star cores not only consist of nucleons, but also of more exotic baryons like hyperons or a phase of deconfined quarks. We discuss the consequences of such exotic particles on the gross properties and phenomenology of neutron stars. In addition, we determine the general phase structure of dense and also hot matter in the chiral parity-doublet model and confront model results with the recent constraints derived from the neutron star merger observation.     

Solar oscillations and the equation of state

Summary Accurate measurements of observed frequencies of solar oscillations are providing a wealth of data on the properties of the solar interior. The frequencies depend on solar structure, and on the properties of the plasma in the Sun. Here we consider in particular the dependence on the thermodynamic state. From an analysis of the equations of stellar structure, and the relevant aspects of the properties of the oscillations, we argue that in the convection zone one can isolate information about the equation of state which is relatively unaffected by other uncertainties in the physics of the solar interior. We review the different treatments that have been used to describe the thermodynamics of stellar plasmas. Through application of several of these to the computation of models of the solar envelope we demonstrate that the sensitivity of the observed frequencies is in fact sufficient to distinguish even quite subtle features of the physics of solar matter. This opens up the possibility of using the Sun as a laboratory for statistical mechanics, under conditions that are out of reach in a terrestrial laboratory.     

II型超新星瞬发爆炸模型的激波传播

本文专门讨论II型超新星瞬发爆炸模型中的激波形成点、激波波阵面的位置、激波速度和激波能量等一系列的问题 .研究发现在外星核区除存在首级激波外 ,还存在次级激波 ;激波波阵面的准确位置应在熵变化的最大点和外星核区中人为粘性压的最大点之间 ;并给出激波能量的不同定义量之间的大小关系     

Emulating the OPAL equation of state

The equation of state for the structure of the Sun and stars has to be precise to allow comparisons with observations, i.e., helioseismic inversions of thermodynamic quantities. Among the two of the most popular formalisms are (1) the OPAL equation of state developed at Livermore and (2) the Mihalas-Hummer-Däppen (MHD) equation of state. While OPAL has a solid theoretical foundation, and matches the observational data better, the MHD formalism is more intuitive, easy to realize, and has the possibility of adjustable parameters. Furthermore, it an open-source product in contrast to the proprietary OPAL. Recently a version of MHD has been obtained by including the so-called “Plank-Larkin partition function” and by adding scattering-state terms. The resulting formalism matches OPAL rather well. Here, we report on the next logical step, the implementation of this MHD upgrade into the simple and popular CEFF equation of state. Such an implementation will make it a flexible and convenient tool, allowing an approximative on-line implementation of OPAL in solar and stellar models.     

SN 2005cs in M51 – I. The first month of evolution of a subluminous SN II plateau

Early-time optical observations of supernova (SN) 2005cs in the Whirlpool Galaxy (M51) are reported. Photometric data suggest that SN 2005cs is a moderately underluminous Type II plateau SN (SN IIP). The SN was unusually blue at early epochs (   U − B ≈−0.9  about three days after explosion) which indicates very high continuum temperatures. The spectra show relatively narrow P Cygni features, suggesting ejecta velocities lower than observed in more typical SNe IIP. The earliest spectra show weak absorption features in the blue wing of the He  i 5876-Å absorption component and, less clearly, of Hβ and Hα. Based on spectral modelling, two different interpretations can be proposed: these features may either be due to high-velocity H and He  i components, or (more likely) be produced by different ions (N  ii , Si  ii ). Analogies with the low-luminosity, ⁵⁶Ni-poor, low-velocity SNe IIP are also discussed. While a more extended spectral coverage is necessary in order to determine accurately the properties of the progenitor star, published estimates of the progenitor mass seem not to be consistent with stellar evolution models.     

The state equation of superdense degenerate plasma

On the basis of general requirements, the parameters of the hadronic plasma and the state equation of degenerate superdense matter in the whole range of pressures are improved and corrected.     

The equation of state for neutron stars

We present the results of an improved analysis of the equation of state of matter from metallic to nuclear density which takes into account: (a) in the region of low densities (?<10⁴ g cm^?3), the electron correlation energy in the lattice; (b) in the region of medium density (10⁴11 g cm^?3) a more refined discussion of the electron energy; (c) in the region of large densities (4.3×10¹¹ g cm^?3相似文献   

Neutron star models based on an improved equation of state

Using an equation of state for cold degenerate matter which takes nuclear forces and nuclear clustering into account, neutron star models are constructed. Stable models were obtained in the mass range above 0.065M and density range 10^14.08 to 10^15.4 gm/cm³. All of these models were found to be bound. The outer crystalline layer of the star was found to have a thickness of 200 m or more depending on the mass of the model.     

A note on the energy loss from pulsars

We respond to comments made in a recent letter by Karimet al. (1984), and show that the examples of interstellar absorption at2800 Å that they have presented so far can all be attributed to overexposure of the IUE detectors.     

Variable equation of state for Bianchi type-VI<Subscript>0</Subscript> dark energy models

We present dark energy models in an anisotropic Bianchi type-VI₀ (B-VI₀) space-time with a variable equation of state (EoS). The EoS for dark energy ω is found to be time dependent and its existing range for derived models is in good agreement with the recent observations of SNe Ia data (Knop et al. in Astrophys. J. 598:102 2003), SNe Ia data with CMBR anisotropy and galaxy clustering statistics (Tegmark et al. in Astrophys. J. 606:702, 2004b) and latest a combination of cosmological datasets coming from CMB anisotropies, luminosity distances of high redshift type Ia supernovae and galaxy clustering (Hinshaw et al. in Astrophys. J. Suppl. 180:225, 2009; Komatsu et al. in Astrophys. J. Suppl. 180:330, 2009). The cosmological constant Λ is found to be a positive decreasing function of time and it approaches a small positive value at late time (i.e. the present epoch) which is corroborated by results from recent supernovae Ia observations. The physical and geometric aspects of the models are also discussed in detail.