Astrophysics最新文献

In this paper we have examined the Bianchi type-V model within the framework of linear model which is a special case of broader f (R, T) gravity framework proposed by Harko et al. Here we have considered the f (R, T) = R+ 2λT where λ is an arbitrary constant, R is the Ricci scalar, T is the trace of stress energy momentum tensor. To find the solution of field equations, we have assumed the condition that cosmic jerk parameter j is directly proportional to negative of deceleration parameter q, namely j ∝ – q. Different cases of Hubble parameter H, spacial volume V, deceleration parameter q, energy density ρ, pressure of matter p and cosmological constant Λ are discussed. Also, we have discussed the physical and geometrical properties of the model. Our results match with the observations.

The study presents estimates of the atmospheric mass-loss rate for the planet WASP-195 b. The F-type star WASP-195 hosts a planet whose mass is only 0.104 that of Jupiter, yet their radii are comparable. The planet thus has a low mean density (ρ = 0.16± 0.06 g/cm³). To evaluate the atmospheric mass outflow, two approaches were applied: (1) an approximate relation corresponding to the energy-limited escape model, and (2) a method based on interpolation of results obtained from a grid of hydrodynamic upper-atmosphere models. The stellar XUV photon fluxes were estimated using an analytical relation linking F_XUV and the chromospheric activity index ({{mathrm{log}R}^{mathrm{^{prime}}}}_{HK}). The obtained estimates of the atmospheric mass-loss rate Ṁ were compared as a function of ({{mathrm{log}R}^{mathrm{^{prime}}}}_{HK}) variations for both the energy-limited and hydrodynamic escape models. The calculations show that the atmospheric mass loss of the exoplanet is significant even in the case of low chromospheric activity of the host star. The maximum value of Ṁ may reach 7.3·10¹¹ g/s (according to the hydrodynamic model under conditions of high stellar activity).

Observational data on quasiperiodic variations of the period derivative of pulsars are considered. Characteristic radii of the superfluid regions of pulsars are obtained under the assumption that these variations represent collective elastic oscillations of the superfluid vortex lattice (Tkachenko modes). The derived values of the radii were compared with those obtained from observations of the X-ray satellites NICER and XMM-Newton. These values were also compared with those inferred from the equation of state. The simple model of Tkachenko waves requires further refinement. This method can be used for approximate estimation of pulsar radii in certain cases.

We investigate the magnetic field and other physical properties of the star HD 149438 ( τ Sco). The analysis shows that this object exhibits all the key characteristics typical of other known magnetic O-type stars. Furthermore, all magnetic O-type stars display properties closely resembling those of SrCrEu + Si + He-w + He-r peculiar stars, suggesting a common origin and evolutionary pathway for these objects. The results also indicate that the hypotheses proposed in the literature (that magnetic stars originate from close binary systems) are not supported by observational evidence.

We have analyzed 20 low-frequencies (LF) type-II radio bursts associated with solar coronal mass ejections (CMEs), coronal holes (CHs), and solar flares observed during the solar cycle 23 and solar cycle 24, which consist of the period of year 1997 to year 2015. A total number of 505 types-II radio bursts were observed during the above period out of which only 20 types II radio bursts have frequencies ≤ 1 MHz. The time duration of 20 LF type II bursts ranges from 5 min to 2020 min. On investigation of 17 type-II bursts associated CMEs, solar flares, and coronal holes we also found that 12 types-II burst related CMEs observed when there were CHs and solar flares within 10° and 5 type-II burst-associated CMEs found when there were CHs and solar flares within 30°, respectively. In this paper we have done statistical analysis of low frequency type II radio bursts and related solar phenomena. The LF type-II radio bursts start after the peak time of associated solar flares. In this paper each LF type-II radio burst and other related solar phenomena are seen and analyzed to understand the origin of LF type-II radio bursts from the Sun in the latest scenario of solar heliophysics.

In the context of f (R, T) theory, we address the viscous ghost dark energy model for a homogeneous and isotropic flat universe. To talk about the universe′s evolution, we derive the Hubble, deceleration, and effective equation of state parameters. We analyze the computed cosmological parameters in light of the model parameters′ best fit values and limit the model parameters using the most recent cosmological observations. The deceleration parameter shows a smooth phase transition of the universe from decelerated to accelerated expansion. The universe remains in accelerating phase in future also which shows that dark energy will dominate the universe in future as well. We observe that effective equation of state in our model never cross the phantom divide line and approaches to ΛCDM model in the future. We observe current values of the Hubble parameter ({H}_{0}={66.567}_{-1.048}^{+1.151}) and ({H}_{0}={69.582}_{-0.813}^{+0.905}) for SNIa+OHD+SLS and SNIa+OHD+SH0ES, respectively. The estimated current ages of the universe are 15. 13 Gyr for SNIa+OHD+SLS and 15. 61 Gyr for the SNIa+OHD+SH0ES. We observe that the present model is able to resolve H0 tension and age problem of the standard ΛCDM model. Further, we apply higher order geometric analysis namely statefinder jerk, snap, and lerk parameters to discriminate our model with existing DE models. The trajectories in r - s and r - q planes pass through ΛCDM model during the evolution but the overall evolution of the trajectories differs from any known dark energy model.

A novel dark energy model with an equation of state (EoS) given by: ω(z) = ω₀ + ω₁ z/(1 + ln(1 + z)) is proposed, which effectively captures the transition from deceleration to acceleration in the cosmic expansion. This model is motivated by recent observational measurements, all of which indicate a significant shift in the dynamics of the universe at intermediate redshifts (z ≈ 0.5). By utilizing a logarithmic damping term, the model allows for a smooth, redshift-dependent transition between matter, dark energy, and radiation domination, and avoids the abrupt transitions often observed in simpler models. In particular, the parameters ω₀ and ω₁ govern the early and late-time evolution of the equation of state. ω₀ characterizes the late-time asymptotic behaviour, while ω₁ controls the rate of transition at intermediate redshifts. The model is further examined from multiple theoretical standpoints: we establish its correspondence with viscous and logotropic dark fluids, reconstruct its scalar field theoretical counterpart (quintessence-type), and derive the effective equation of state ω_eff (z), which confirms a smooth transition to an accelerating phase near z ~ 0.5. Furthermore, we perform a thermodynamic analysis to verify the validity of the generalized second law of thermodynamics (GSL) and ensure the model′s stability. The speed of sound, which is essential for stability analysis, is computed and analysed to ensure it remains positive and stable across all redshifts, ensuring that the model adheres to physical constraints. We analysed the present model with updated observational Hubble parameter data from cosmic chronometers, performing a full chi-square minimization and model selection analysis using the Akaike Information Criterion (AIC). The resulting best-fit parameters indicate a close match with data, yielding a ({chi }_{min}^{2}) comparable to ΛCDM and CPL, and a ∆AIC that confirms the present model as a competitive phenomenological alternative within the current cosmological framework.

In this paper, we have considered Friedmann-Robertson-Walker space-time in the context of f(G) theory of gravitation. The viscosity in presence of Chaplygin gas is taken as source of matter. Field equations are solved by using linearly varying deceleration parameter. Statefinder diagnostic pair {r, s} is also investigated. It is found that the model is stable.

This study investigates the periodic spatial distribution of various groups of pulsars. The analysis of observational data for more than 3,800 pulsars revealed that the characteristic distance of normal radio pulsars from the Galactic plane increases with their characteristic age. Millisecond pulsars of all types (including pulsars located in Galactic globular clusters, millisecond gamma- and X-ray pulsars, and radio millisecond pulsars of the Galactic field) are very old objects, with average characteristic ages of several billion years. They are distributed in a wide layer around the Galactic plane, with a characteristic spread σ of about 500 parsecs. The GRS population with relatively large periods differs significantly from normal radio pulsars. While the maximum of the period distribution for normal radio pulsars corresponds to log(P₀) = – 0.3, for GRS pulsars it is shifted toward smaller values. These GRS pulsars are located in a thin layer near the Galactic plane, with a characteristic scale height σ of about 160 parsecs, and their average age is about one hundred thousand years.

Based on a series of studies recently carried out at the Crimean Astrophysical Observatory, we have analyzed the chemical composition of 20 G- and K-type giants with magnetic fields, comparing them with the chemical composition of 7 non-magnetic giants. Significant differences have been found in the lithium abundance and in the [N/C] ratio between magnetic and non-magnetic giants; both of these parameters are known to be sensitive indicators of stellar evolution. Lithium is shown to be absent in almost all non-magnetic giants, in full agreement with theoretical predictions concerning the evolution of giants that have undergone deep convective mixing during the first dredge-up. In contrast, most magnetic giants retain detectable lithium in their atmospheres. Furthermore, the [N/C] ratio, examined as a function of stellar age and mass, displays systematic differences between magnetic and non-magnetic giants. Possible explanations for the observed differences in the chemical composition of these two types of giants are discussed. It is also shown that the total C+N+O abundance, which apparently remains constant throughout the evolution, exhibits a clear dependence on [Fe/H] not only for the giants analyzed here, but also across a wider range of metallicity from –2.5 to +0.3.