Astrophysics and Space Science最新文献

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.

The irregular satellites within the Jupiter system hold high scientific value due to their potential to contain clues about the early evolution of the solar system. This paper proposes an optimization algorithm for multiple irregular satellites flyby trajectories, which includes the powered gravity assist from Galilean moons. The algorithm is based on beam search and uses virtual trajectories to determine potential flyby targets, solving for trajectories that satisfy constraints on velocity increment, mission duration, and perijove radius. By changing the initial orbital period and optimizing the number of branches with Galilean moon gravity assist during branching, and comparing with the case without Galilean moon gravity assist, the effects of these factors on the number of irregular satellite flybys are summarized. The simulation results show that the algorithm can effectively solve for orbits that flyby multiple irregular satellites for initial orbits of different periods. The three trajectories obtained can serve as references for future missions.

Time-optimal orbital transfers with soft terminal conditions are studied in this work. First, a two-layer thrust continuation method is devised. The unfavorable thrust continuation path is handled by switching between different solution curves. Second, the proposed method is applied to solving time-optimal transfers under two- or three-body dynamics with Cartesian coordinates to verify its effectiveness. The near conservation of the product between the time of flight and the thrust level is observed for general orbital transfers. A linear variation of this quantity with eccentricity is also illustrated when the difference in eccentricity between the initial and terminal orbits is large enough.

Inspired by the recent literature, we study the Einstein–de Sitter cosmological model coupled with a generalization of the relation between the redshift and time dilation of the kind (delta t_{0} = delta t_{e} (1+z)^{n}). We find that this model fits the experimental data regarding 1048 supernovae, in a way which is competitive with the standard (Lambda mathrm{CDM}) model and without the need of introducing a non-zero cosmological constant. Since the existence of dark energy, as a main ingredient of the composition of the cosmos, is still under debate, we propose our formalism as an example of an alternative description of the cosmological scenario.

There will be an unprecedented increase in the number of galaxies observed as a result of the current and upcoming surveys. Consequently, data-driven approaches have become the main tools for deciphering and evaluating this massive volume of data. Computer vision combined with deep learning has proven most effective for recognizing galaxy morphology but most of the conventional deep learning models are large in terms of parameters due to which computational cost, risk of overfitting increases. In this paper, we proposed a lightweight convolutional neural network (CNN) model using separable convolution which helps to reduce trainable parameters of the model. Further, Efficient Channel Attention (ECA) mechanism is used to focus on important features. ECA focuses on features channel wise without dimensionality reduction which reduces the computational overhead. Performance of proposed model named as “DIAT-DSCNN-ECA-Net” is evaluated on two datasets such as Galaxy Zoo 2, Galaxy Zoo DECaLS, each having seven different types of galaxies, achieved an accuracy of 90.81% and 94.17% respectively at the cost of 1.8 Mega-Byte model size, 0.13 million parameters, 1.04 Floating Point Operations (FLOPs). The outcomes of the experiments demonstrate that the proposed approach can outperform the existing CNN models.

The YangBaJing Hybrid Array (YBJ-HA) is located at the Yangbajing international cosmic ray observatory, Tibet, China. It consists of 115 scintillation detectors (SDs) and 16 underground water Cherenkov muon detectors (MDA). Its main physical goal is to observe the (gamma )-ray sources in the 100 TeV energy region, so the array must have very good pointing accuracy. To achieve this performance, the time response of the scintillator detectors must have high consistency. This paper introduces a characteristic plane method (offline calibration method) to calibrate the time of each scintillator detector and uses the moon shadow analysis results to test the time calibration accuracy.