Astrophysics and Space Science最新文献

In the current study, we have considered three different parameterizations of deceleration parameter to describe the cosmological dynamics of the accelerating universe in (f(Q)) gravity. The power law symmetric teleparallel gravity with a specific form (f(Q)= Q + n Q^{m}) is assumed for the modelling purpose. Here, (m) and (n) are constants and (Q) is the non-metricity term that describes the gravitational interaction in space time. We constructed the field equations depending on the power law (f(Q)) gravity and parameters are extracted using experimental observations. Latest observational datasets of BAO, (H(z)) and Pantheon are utilized to predict the best fit values of parameters and current value of Hubble constant. The Markov Chain Monte Carlo (MCMC) algorithm has been used to decide the best plausible values of parameters. We numerically represent the physical and geometrical features of the models and thoroughly explore their development. We analyzed our models using the jerk and Om diagnosis that depict the derived cosmic models are different from the (Lambda )CDM model expressing late time accelerated expansion of cosmos with phantom type of the universe. We also discussed the viability of models by the analysis of energy conditions.

We present an alternative model of unusual type-IIP SN 2018gj. Despite the short plateau and early gamma-ray escape seeming to favor low-mass ejecta, our hydrodynamic model requires a large ejected mass (≈23 (M_{odot })). The high ejecta velocity, we find from hydrogen lines in early spectra, is among the crucial constraints on the hydrodynamic model. We recover the wind density that rules out a notable contribution of the circumstellar interaction to the bolometric luminosity. The early radioactive gamma-ray escape is found to be due to the high velocity of ⁵⁶Ni, whereas the asymmetry of the H(alpha ) emission is attributed to the asymmetry of the ⁵⁶Ni ejecta. The available sample of type-IIP supernovae studied hydrodynamically in a uniform way indicates that the asymmetry of the ⁵⁶Ni ejecta is probably their intrinsic property. Hydrogen lines in the early spectra of SN 2018gi and SN 2020jfo are found to imply a clumpy structure of the outer ejecta. With two already known similar cases of SN 2008in and SN 2012A we speculate that the clumpiness of the outer ejecta is inherent to type-IIP supernovae related to the red supergiant explosion.

The research on solar flare predicting holds significant practical and scientific value for safeguarding human activities. Current solar flare prediction models have not fully considered important factors such as time step length, nor have they conducted a comparative analysis of the physical features in multiple models or explored the consistency in the importance of features. In this work, based on SHARP data from SDO, we build 9 machine learning-based solar flare prediction models for binary “Yes” or “No” class prediction within the next 24 hours, and study the impact of different time steps and other factors on the forecasting performance. The main results are as follows. (1) The predictive performance of eight deep learning models shows an increasing trend as the time step length increases, and the models perform the best at the length of 40. (2) In predicting solar flares of ≥C class and ≥M class, the True Skill Statistic(TSS) of deep learning models consistently outperforms that of baseline model. For the same model, the TSS for predicting ≥M class flares generally exceeds that for predicting ≥C class flares. (3) The Brier Skill Score (BSS) of deep learning models significantly surpasses that of baseline model in predicting ≥C class flares. However, the BSS scores of the nine models are comparable for predicting ≥M class flares. For the same model, the BSS for predicting ≥C class flares is generally higher than that for predicting ≥M class flares. (4) Through feature importance analysis of multiple models, the common features that consistently rank at the top and bottom are identified.

This study reveals a new feature of many solar jets: a group height, which links their acceleration and velocity.

The acceleration and velocity ((a), (V)) for jets such as spicules, often displayed as scattergraphs, show a strong correlation. This can be represented empirically by the equation, (V = pa + q), where (p) and (q ) are two arbitrary non-zero constants.

This study reanalyses the ((a), (V)) data for nine different groups of jets, in order to test an alternative proposal that a simpler relationship directly links ((a), (V)) to the mean height for the group of jets, without needing the empirical constants (p ) and (q). A standard mathematical test – plotting log((a)) against log((V)), tests whether (V sim a^{n}) and if so, gives the value of n. When this is done for a wide range of jets the index (n) is consistently found to be close to 0.5

The nine groups of jets include spicules, macrospicules and dynamic fibrils. The result, (V sim a)^0.5, or equivalently (V^{2} = ka), with only one constant, provides as close a match to the data as the equation (V = pa + q), which requires two unknown constants. It is found that the constant (k), is a known quantity: just twice the mean height, (overline{s}), of the group of jets being analysed. This then gives the equation (V^{2} =2 a overline{s}), for the jets in the group. This more succinct relationship links the acceleration and maximum velocity of every jet in the group to a well-defined quantity – the mean height of the group of spicules, without needing extra constants

This paper presented an analysis of geomagnetic disturbance observed on the ground during geomagnetic storms with different intensities in 2015 using the meridian chain data at geomagnetic mid and low latitudes. Ground observation records superimpose varying types of space-current system and noise interference. Geomagnetic disturbance with variation of discontinuity and irregularities are difficult to identify and distinguish. We proposed a variational mode decomposition (VMD) algorithm for reconstructing geomagnetic horizontal ((H)) disturbance signals. We decomposed the geomagnetic signals into geomagnetic disturbance signals, diurnal variation signals, and noise disturbance signals using the VMD algorithm. Intrinsic mode functions (IMFs) were selected to form the reconstructed signal, which represented a geomagnetic disturbance during a geomagnetic storm. We investigated the decreased amplitude of (H) component obtained from the reconstructed signals during main phase of geomagnetic storms with different geomagnetic storms intensities and seasons at mid and low latitudes. The maximum values of gradient variation of (H ) component disturbance with geomagnetic latitude cosine are near magnetic latitude 30°N during geomagnetic storms with different intensities and seasons. Ionopheric structural changes in the low-to-mid latitude transition zone maybe the primary cause. The result provides a reference for the complex coupling relationship between the ionosphere and magnetosphere during geomagnetic storms.

Detection of weak signals remains challenging in astrophysics. This is particularly applicable in the investigation of Forbush events. There is thus, a paucity of catalogs of small-amplitude Forbush decreases (FDs). Detail investigations of the space-weather implications of small FDs are, thus, lacking in the literature. Recently, large catalogs of weak FDs, for the first time, have been published. This work employs the newly created lists of small-amplitude FDs to investigate the statistical link between small FDs and solar-geomagnetic variables. The solar-geomagnetic variables were obtained from the OMNI database. A simple coincident R software code was employed in matching the related solar-geomagnetic variables with the weak Forbush events. The FD dates were taken as the input signal. Scatter plots of FDs against interplanetary magnetic field (IMF), solar wind speed (SWS), planetary K-index (Kp) and planetary A-index (Ap) reveal a negative relationship, while that of FDs against disturbance storm time index (Dst) shows a positive relationship. Statistical significance of these relations were tested. The small-amplitude FDs and solar-geomagnetic variables at Potchefstroom (PTFM) station register statistically significant relations. Non-statistically significant correlation between the small-amplitude FDs and solar-geomagnetic variables were obtained at South Pole (SOPO) station, with the exception of FD-SWS that reveals statistically significant correlation. The differences in the correlation results obtained at the two stations (PTFM and SOPO) could be attributed to the differences in the characteristics of the NM stations. These results suggest that geomagnetic storm indices play important role in the evolution of FDs.

Dynamics of nonlinear ion-acoustic waves (IAWs) are studied for Venus’ lower atmosphere at an altitude of (200-1000) km. Two-soliton, nonlinear solitary and periodic waves in a three-component plasma consisting of (H^{+}) and (O^{+}) ions with kappa distributed electrons are studied. Using the reductive perturbation technique (RPT), the Korteweg-de Vries (KdV) equation is derived and a Planar dynamical system is formed for the KdV equation using a travelling wave transformation. A phase portrait is drawn to analyze nonlinear wave behaviors by adjusting the parameters (kappa ) (spectral index), (gamma ) (unperturbed number density ratio), and (V) (travelling wave speed). Increasing values of (kappa ) amplify amplitudes for solitary and periodic waves, narrow down the width of the solitary wave, and broaden the width of the periodic wave. Increasing value of (gamma ) boosts amplitude of the solitary wave with unchanged width, while amplitude of the nonlinear periodic wave decreases and width widens. Increasing value of (V) enhances amplitudes and reduces widths for both solitary and periodic waves. Two-soliton solutions for the KdV equation are studied using the Hirota direct method. Increasing value of (gamma ) reduces amplitude of the soliton without affecting the width and increasing value of (kappa ) reduces width of the soliton. Phase shift for two-soliton is also shown and found that for different values of (kappa ), the phase shift increases on increasing value of (gamma ). The findings of our result aid in understanding the dynamics of nonlinear waves and two-soliton solutions in Venus’ lower ionosphere.

White dwarfs are the dense, burnt-out remnants of the vast majority of stars, condemned to cool over billions of years as they steadily radiate away their residual thermal energy. To first order, their atmosphere is expected to be made purely of hydrogen due to the efficient gravitational settling of heavier elements. However, observations reveal a much more complex situation, as the surface of a white dwarf (1) can be dominated by helium rather than hydrogen, (2) can be polluted by trace chemical species, and (3) can undergo significant composition changes with time. This indicates that various mechanisms of element transport effectively compete against gravitational settling in the stellar envelope. This phenomenon is known as the spectral evolution of white dwarfs and has important implications for Galactic, stellar, and planetary astrophysics. This invited review provides a comprehensive picture of our current understanding of white dwarf spectral evolution. We first describe the latest observational constraints on the variations in atmospheric composition along the cooling sequence, covering both the dominant and trace constituents. We then summarise the predictions of state-of-the-art models of element transport in white dwarfs and assess their ability to explain the observed spectral evolution. Finally, we highlight remaining open questions and suggest avenues for future work.

In this work, we investigate the existence of Hilbert or gravitational repulsion of a test charged particle near the Kerr–Newman anti-de Sitter black hole. We found that the dynamical motion of the test charged particle is significantly affected by the black hole charge and spin, probably due to the electrostatic interaction and the curvature behaviour of spacetime. We also obtain the various conditions under which a freely falling test particle towards the black hole experiences the Hilbert repulsion or attraction as viewed by a distant observer.

We investigate cosmological constraints on the Variable Chaplygin gas model parameters with latest observational data of the Fast Radio Bursts and compare the results with previous constraints obtained using SNe Ia (Pantheon+SHOES), Gamma Ray Bursts, Baryon Acoustic Oscillations and Hubble parameter observational data. The Variable Chaplygin gas model is shown to be compatible with these datasets. We have obtained tighter constraints on model parameters (B_{s}) and (n), using the FRB data set. By using the Markov chain Monte Carlo (MCMC) method we obtain, (B_{s})=(0.18pm 0.10), (n=1.10pm 1.15) and (H_{0})= (70.46pm 0.66) with the SNe Ia data set, (B_{s})= (0.09pm 0.06), (n= 0.44pm 0.89 ) and (H_{0}=70.57pm 0.64 ) with the FRB data set, (B_{s})=(0.16pm 0.11), (n=1.06pm 1.25) and (H_{0})= (70.37pm 0.65) with the BAO data set, (B_{s})=(0.05pm 0.000), (n=1.46pm 0.23) and (H_{0})= (70.21pm 0.57) with the H(z) data set and (B_{s})=(0.20pm 0.11), (n=1.25pm 1.17) and (H_{0})= (70.37pm 0.64) with the GRB data set. A good agreement for (H_{0}) is observed from these data sets.