Astrophysics and Space Science最新文献

Observations by the James Webb Space Telescope (JWST) have led to a series of groundbreaking discoveries that challenge our current understanding of early galaxy formation. A large number of galaxies have been surprisingly identified during the epoch of cosmic dawn, the redshift of (zsim 11-14), 13.4 to 13.5 billion years ago, far exceeding theoretical predictions. Additionally, many faint AGNs hosting supermassive black holes have been discovered at (z>4). What was happening in the early universe? This article provides an overview of these latest findings.

To understand better the polarized radiative transfer near the surface of rotating massive stars that remain nearly spherically symmetric, we use plane-parallel stellar atmosphere models to explore the unique opportunity presented by the Öhman effect. This effect refers to the predicted variation in linear polarization across a rotationally broadened absorption line, due to the interaction of that line with the spatially varying continuum polarization across the face of a strongly scattering photosphere, such as found in hot stars. Even if the rotation is weak enough for the star to remain spherically symmetric, the Öhman effect persists because differential absorption induced by the rotational Doppler shift of the line breaks the symmetry that would otherwise cancel the continuum polarization in the absence of that line. Neglecting rotational distortion effects, the net polarization across the line vanishes, yet resolved line profiles display a telltale triple-peak polarization pattern, with one strong polarization peak at line center and two smaller ones in the line wings at a position angle that is rotated 90 degrees from the line center. The far ultraviolet (FUV) is emphasized because both the polarization amplitude and the specific luminosity are greatest there for photospheres with effective temperatures between about 15,000 and 20,000 K. Additionally, larger polarizations result for lower-gravity atmospheres. There is a high density of spectral lines in the FUV, leading to a rich “second stellar spectrum” in linear polarization (analogous to the “second solar spectrum”) that is made observable with stellar rotation. Some hot stars exhibit extreme rotation, which suppresses the polarimetric amplitude for the forest of weaker FUV lines, but a few strong lines such as the Siiv 140 nm doublet still give observable polarizations at high rotation speeds even before rotational distortion effects of the atmosphere are considered. Thus polarizations at the level of 0.1% to 1% are achievable across individual lines for a wide variety of B-type stars. We highlight the prospects for accessing the unique information encoded in the Öhman effect with future moderate-resolution spaceborne spectropolarimetric missions in the FUV.

The HEliospheric pioNeer for sOlar and interplanetary threats defeNce (HENON) mission is a CubeSat Space Weather mission, designed to operate in a Sun-Earth Distant Retrograde Orbit (DRO) at more than 10 million km from the Earth. HENON will embark payloads tailored for Space Weather (SWE) observations, i.e., a high-resolution energetic particle radiation monitor, a Faraday cup, and a magnetometer enabling it to provide quasi-real-time monitoring of the interplanetary conditions in deep space. HENON has many important goals, such as demonstrating CubeSat capabilities in deep space, including long-duration electric propulsion with periodic telemetry and command, and robust attitude control for deep-space operations. It will pave the way for a future fleet of spacecraft on DROs, providing continuous near real-time measurements for SWE forecasting. This paper focuses on the mission analysis performed for phase A/B, with the main goal of defining a baseline transfer trajectory to a heliocentric DRO in co-orbital motion with the Earth. The proposed transfer leverages a rideshare opportunity on a mission escaping Earth’s gravity field, most likely one headed toward the Sun–Earth L₂ region, and relies exclusively on on-board electric propulsion to reach deep space, making it a pioneering demonstration of this approach and the technology. Under appropriate assumptions on the electric propulsion system performances, s/c mass and propellant budget, it will be shown that the HENON target DRO can be reached in about 1 year, taking into account also periodic interruptions of thrusting to allow for Telemetry, Tracking and Command.

Gravitational Waves (GWs) provide a powerful means for cosmological distance estimation, circumventing the systematic uncertainties associated with traditional electromagnetic (EM) indicators. This work presents a model for estimating distances to binary black hole (BBH) mergers using only GW data, independent of EM counterparts or galaxy catalogs. By utilizing the intrinsic properties of the GW signal, specifically the strain amplitude and merger frequency, our model offers a computationally efficient preliminary distance estimation approach that could complements existing Bayesian parameter estimation pipelines. In this work, we examine a simplified analytical expression for the GW luminosity distance derived from General Relativity (GR), based on the leading-order quadrupole approximation. Without incorporating post-Newtonian (PN) or numerical relativity (NR) corrections, or modeling spin, eccentricity, or inclination, we test how closely this expression can reproduce distances reported by full Bayesian inference pipelines. We apply our model to 87 events from the LIGO-Virgo-Kagra (LVK) Gravitational Wave Transient Catalogues (GWTC), computing distances for these sources. Our results demonstrate consistent agreement with GWTC-reported distances, further supported by graphical comparisons that highlight the model’s performance across multiple events.

We present a theoretical analysis of the ³He((alpha ), (gamma ))⁷Be radiative capture reaction, using pionless effective field theory (EFT) at the leading order. What sets our approach apart is the unique combination of direct capture mechanisms and resonant processes that involve the (7/2^{-}) excited state of ⁷Be at 429 keV. By rigorously examining electromagnetic multipole transitions, we’ve managed to achieve a theoretical uncertainty of just 4.1% for the astrophysical S-factor. Our calculated value of (S(0) = 0.511 pm 0.021text{ keV}cdot )b aligns impressively with the recommended experimental value of (0.529 pm 0.018text{ keV}cdot )b. At the temperatures found in the solar core ((T_{9} = 0.015)), our reaction rate of ((9.2 pm 0.4) times 10^{3}text{ cm}^{3}text{ mol}^{-1}text{ s}^{-1}) helps to clear up some long-standing discrepancies in stellar models. Interestingly, our multipole decomposition shows a surprising persistence of M1 contributions (35.2% at resonance) that goes beyond what typical single-particle models would predict, underscoring the significance of two-body currents. The theoretical uncertainties we encountered are mainly due to EFT truncation errors (2.8%) and variations in low-energy constants (2.1%). These findings have direct implications for solar neutrino flux predictions and calculations of primordial lithium abundance.

This research investigates cosmic dynamics within the context of (f(mathcal{R},mathcal{L}_{m})) gravity, concentrating on a binary mixing of perfect fluid and dark energy in a Plane Symmetric space-time. By incorporating the non-linear form of (f(mathcal{R},mathcal{L}_{m})) as (f(mathcal{R},mathcal{L}_{m})=frac{mathcal{R}}{2}+mathcal{L}_{m}^{alpha }), it investigates late-time cosmic acceleration and the transition from matter-dominated to dark energy-dominated epochs. The analysis includes the quintessence and Chaplygin gas models, demonstrating their role in the dynamics of energy density, effective pressure, and anisotropy. The model is validated through parameterization using observational data, such as Hubble parameter datasets, which result in an excellent level of agreement with empirical findings. Advanced diagnostics, like the jerk, statefinder, and (Om) diagnostics, show that (f(mathcal{R},mathcal{L}_{m})) gravity is different from previous cosmological models. This lets us explain the expansion of the universe in terms of geometry. This study provides a strong basis for future research on modified gravity, anisotropic cosmological models, and the role of dark energy in the evolution of the universe.

In this paper, I discuss the macroscopic properties of the ultracompact object XTE J1814-338, whose inferred mass and radius read (M = 1.21 pm ) 0.05 (M_{odot }) and R = 7.0 ± 0.4 km as a dark matter-admixed strange star. By using the neutralino as WIMP dark matter with a fixed Fermi momentum, I calculated the moment of inertia, the gravitational redshift, the dimensionless tidal parameter, and the total amount of dark matter for a 1.2(M_{odot }) star. At the end, I study the role of the neutralino’s mass.

The properties of kinetic Alfvén waves (KAWs) are investigated in the negative positive ion electron (NPIE) plasma of Titan’s ionosphere. The concentrations of positive and negative ions in dayside and nightside regions of this ionosphere are different, whereas the magnitude of the ambient magnetic field also varies. The plasma data indicate the possibility for the existence of very low frequency and long wavelength kinetic Alfvén waves (KAWs) in this environment. The frequencies and wavelengths of these waves are estimated considering only the two kinds of positive (HCNH^{+}) and negative (CN^{-}) ions, which have dominant concentrations. Extremely small magnetic field (mid B_{0} mid simeq 0.0002) (G) can sustain KAWs with extremely small frequencies of the order of (simeq 0.004text{ rad}/text{s}) and very long wavelengths of the order of a thousand kilometres along field lines while the Titan is about a million kilometres away from Saturn’s surface. It is pointed out that the weakly nonlinear KAWs can also give rise to electromagnetic solitary waves similar to the Earth’s upper ionosphere. The formation of solitary structures by the nonlinear KAWs is also investigated using the appropriate normalization of spatial coordinates in parallel and perpendicular directions with respect to the ambient magnetic field. The almost stationary electromagnetic pulses may appear in Titan’s ionosphere moving at very small speeds.

This paper investigates the application of knot theory to the classification of orbit families in the Circular Restricted Three-Body Problem (CR3BP). Motivated by the infinite variety of possible orbits—many of which remain unnamed and uncataloged—this paper applies polynomial knot invariants, primarily the Alexander polynomial, to establish a relation between knot structures and orbital trajectories. An algorithm is developed to extract knot types from three-dimensional trajectories enabling the identification and differentiation of complex orbit families. Knot theory topics explored and correlated to CR3BP trajectories include the torus knot and unknot. The findings provide a novel topological framework for understanding CR3BP dynamics, offering both theoretical understanding and practical modeling in astrodynamics for multi-body gravitational systems.

We explore the possibility of retrieving cosmological information along with its inherent uncertainty from 21-cm tomographic data at intermediate redshift. The first step in our approach consists of training an encoder, composed of several three dimensional convolutional layers, to cast the neutral hydrogen 3D data into a lower dimension latent space. Once pre-trained, the featurizer is able to generate 3D grid representations which, in turn, will be mapped onto cosmology ((Omega _{mathrm{m}}), (sigma _{8})) via likelihood-free inference. For the latter, which is framed as a density estimation problem, we consider a Bayesian approximation method which exploits the capacity of Masked Autoregressive Flow to estimate the posterior. It is found that the representations learned by the deep encoder are separable in latent space. Results show that the neural density estimator, trained on the latent codes, is able to constrain cosmology with a precision of (R^{2} ge 0.91) on all parameters and that most of the ground truth of the instances in the test set fall within (1sigma ) uncertainty. It is established that the posterior uncertainty from the density estimator is reasonably calibrated. We also investigate the robustness of the feature extractor by using it to compress out-of-distribution dataset, that is either from a different simulation or from the same simulation but at different redshift. We find that, while trained on the latent codes corresponding to different types of out-of-distribution dataset, the probabilistic model is still reasonably capable of constraining cosmology, with (R^{2} ge 0.80) in general. This highlights both the predictive power of the density estimator considered in this work and the meaningfulness of the latent codes retrieved by the encoder. We believe that the approach prescribed in this proof of concept will be of great use when analyzing 21-cm data from various surveys in the near future.