Astrophysics and Space Science最新文献

In this brief review, we describe the key role played by Big Bang Nucleosynthesis (BBN) in today’s ‘Precision Cosmology’, focusing in particular on the precise determination of the primordial abundance of deuterium. We describe the development of the ideas and methods of BBN research from their beginnings more than 75 years ago to the latest developments, and conclude with a forward look to likely advances expected towards the end of the current decade.

We report the tidal disruption of very low–mass stars (VLMS) by a (100,M_{odot }) black hole (BH) using five hydrodynamical models (M1–M5) with stellar masses of 0.08, 0.16, 0.24, 0.32, 0.40 (M_{odot }) and radii of 0.132, 0.230, 0.319, 0.401, 0.480 (R_{odot }), respectively. Although all stars begin from the same initial position, their dynamical evolution diverges rapidly as they approach pericenter. The lowest–mass star (M1) undergoes the strongest early deformation due to its weak self–gravity, while higher–mass stars remain more structurally coherent. Surface–density maps show that massive stars retain dense cores after disruption, whereas lower–mass stars become more fully stretched and stripped. All models exhibit a sharp reduction in bound mass near pericenter, marking the moment when tidal forces exceed stellar self–gravity. M1 retains the least post–pericenter mass, while M4 and M5 preserve the largest fraction of their initial mass. Mass–loss histories indicate that M1 experiences the greatest fractional loss, whereas M5 loses the largest absolute amount of material ((sim 0.027) (M_{odot })). Mass loss becomes efficient once the star crosses its tidal radius, after which all curves plateau as the remnant recedes. Debris morphology depends strongly on stellar mass and the compactness. Also, the bound debris to BH increases from 0.0409 (M_{odot }) for the least massive star to 0.2064 (M_{odot }) for the most massive one, reflecting the deeper gravitational potential and broader energy spread of more compact stars during tidal disruption. Overall, the results show that stellar compactness governs tidal deformation, mass loss, and debris structure in VLMS tidal disruption events (TDEs).

The MONDian theory of AQUAL (AQUAdratic Lagrangian) and the theory of GRAS (GRavitational Anti-Screening) are alternatives to the theory of dark matter. When these theories are applied to galaxy dynamics they are in excellent agreement with observations including the galactic RAR (Radial Acceleration Relationship). However, when applied to galaxy clusters they do not explain the bulk of the missing mass. This manuscript develops a modified version of the GRAS/AQUAL field equation that can be extended to galaxy clusters. It involves just a single free parameter. The new field equation is then applied to a sample of galaxy clusters and checked against modeled galaxies and solar system constraints. Further to this, the modified field equation leads to an understanding of the difference between the galactic RAR and the RAR recently found for clusters.

Magnetohydrodynamic (MHD) instabilities in accretion disks play a crucial role in mediating energy transfer, and their physical properties have therefore been extensively investigated. In this study, we analytically examine particle acceleration driven by such instabilities in the accretion disk around a Kerr black hole, focusing on the regime where particles experience strong frame-dragging effects. Using the linear growth rate of the magnetorotational instability (MRI) obtained from general relativistic MHD analysis, we explore the dependence of the turbulent acceleration timescale on the MRI growth rate and the turbulent Alfvén Mach number. Because the MRI growth rate depends on the black hole spin, the pressure anisotropy with respect to the magnetic field, and the plasma beta, the acceleration timescale becomes shorter as these parameters increase. By incorporating the energy loss timescale, we estimate the maximum Lorentz factor of the particles where the acceleration and loss timescales are balanced. Furthermore, we solve the steady-state Fokker-Planck equation including both energy diffusion and radiative loss terms to obtain the particle energy distribution. The resulting steady-state spectra and mean particle energy clearly demonstrate that the particle acceleration efficiency increases with the MRI growth rate throughout the accretion disk, over a broad range of spin and plasma beta values. Our results suggest that higher spin and pressure anisotropy enhance particle energies, which could be relevant for understanding high-energy astrophysical phenomena in active galactic nuclei.

Mapping the integrated 21 cm emission line from dark matter-tracing neutral hydrogen gas is the primary science goal for MeerKLASS (MeerKAT’s Large Area Synoptic Survey). Prior to the arrival of MeerKAT, this intensity mapping technique had only been tested on a couple of pre-existing single-dish radio telescopes with a handful of observational hours with which to make early pioneering detections. The 64-dish MeerKAT array, precursor to the SKA Observatory (SKAO), can scan the sky in auto-correlation (or single-dish) mode and perform intensity mapping across large sky areas, presenting the exciting potential for a wide area (({gtrsim },10{,}000,{mathrm{deg}}^{2})) spectroscopic survey across redshift (0.4,{<},z,{<},1.45). Validating the single-dish mode of observation for a multi-dish array and developing the analysis pipeline with which to make unbiased measurements has presented major challenges to this endeavour. In this work, we overview the advances in the field that have facilitated a robust analysis framework for single-dish intensity mapping, and review some results that showcase its success using early MeerKLASS surveys. We demonstrate our control of foreground cleaning, signal loss and map regridding to deliver detections of cosmological clustering within the intensity maps through cross-correlation power spectrum measurements with overlapping galaxy surveys. Finally, we discuss the prospects for future MeerKLASS observations and forecast its potential, making our code publicly available: https://github.com/meerklass/MeerFish.

The sample copula of order (m) provides an approximation to the copula that characterizes the dependence structure of a set of random variables. In this work, we first derive the sample copula of order (m) for a random vector (X = (X_{1},ldots ,X_{d})), with (d geq 2), by extending previously established results for the bivariate case. Based on the definition of a parametric copula with piecewise constant density, we show that the maximum likelihood estimation of the density parameters coincides with the elements employed in the definition of the sample copula of order (m), under the condition (2 le m le n), where (m) is an integer divisor of (n), and (n) denotes the given sample size. In the second part, we present an application of the sample copula of order (m) as a complementary alternative for estimating the cosmological parameters (H_{0}) and (Omega _{m0}), the current values of the Hubble constant and the matter density, respectively. This is carried out using a sample of observations of the redshift (z), the Hubble parameter (H), and its measurement error. To this end, several probability distributions, in addition to the Gaussian distribution, are proposed to model the observed error in the variable (H). Moreover, the applicability of this methodology is highlighted in the context of limited sample sizes.

This study investigates the (beta )-decay properties of (sd)-shell nuclei using the proton-neutron quasiparticle random phase approximation (pn-QRPA) model. The computed Gamow-Teller (GT) strength distributions show decent agreement with the measured data. The calculated (beta )-decay half-lives show good agreement with the previous shell model calculations. The computed log (textit{ft}) values align well with the available experimental data, validating the consistency of the theoretical approach. A key advancement of this work is the calculation of stellar weak interaction rates performed without assuming the Brink-Axel hypothesis for the estimation of GT distributions from parent excited states. The sum of (beta ^{-}) and positron capture ((beta ^{-}) + PC) rates were compared with earlier predictions from the shell model. The percentage contribution of (beta ^{-}) and PC is also investigated under stellar conditions. At low density and high temperature ((rho =10^{7}text{ g}/text{cm}^{3}), T = 30 GK) the pn-QRPA calculation compare well with the shell model and differs at most by a factor 10. Our findings may provide crucial and refined nuclear inputs for astrophysical simulations of (r)- and (s)-process nucleosynthesis.

The Gaia Alerts system is providing alerts on a variety of objects displaying a significant photometric change detected by the Gaia satellite from one passage to the next one over the same region of the sky. Among the ∼23,000 alerts published until the end of 2022, around 13% concern AGN or quasar candidates. At the same time, a different method to detect variations specifically in galactic nuclei was tested on Gaia data during a one year period only, leading to another set of candidates. We have embarked on a spectroscopic ground-based follow-up of some of those, to establish their true nature, and reveal the various phenomena leading to a change in magnitude of those AGN. The present paper deals with radio-quiet objects, while the radio-loud ones will be described in a companion paper. We confirm, on one hand, the AGN/quasar nature of 64 new candidates alerted by Gaia, and, on the other hand, obtained second-epoch spectra of over 200 known AGN also alerted for large photometric variations. The observed phenomena show a large variety described in this paper: from Flares to Tidal Disruption Events (TDEs) and a large number of Changing Look Quasars (CLQs, 56 secure ones, plus 23 probable ones), not forgetting some rarer events like SNe, microlensing events or Extreme Coronal Line Emitters. This sample shows that variability is an excellent tool to detect new quasars, especially radio-quiet ones that otherwise would be undetected, and that a significant fraction of variable AGN/quasars, around 10%, presents the CLQ phenomenon. Some of the new CLQs are followed-up to monitor further changes and measure time scales.

We present calculations concerning the surface gravitational redshift in neutron star at finite temperature using general relativity. The method is presented explicitly in detail. Numerical applications are shown and discussed. Confrontation with experiment and other calculations is performed with some success. In addition to use more complex cold or hot equation of state for the nuclear matter than the free neutron one to describe massive neutron stars, this work shows that we can also take into account the temperature.