New Astronomy Reviews最新文献

Wormholes have captured the interest of scientists and readers of science fiction as a fascinating possibility for traveling through huge cosmic distances or presumably enabling time travel. This review aims to present a thorough overview of wormholes in the context of modified gravity, highlighting the theoretical foundations, and significant developments in this rapidly developing area. In this article, we first give an overview of the concept of a wormhole in the different frameworks of modified gravities. Then this is followed by the wormhole framework, embedding diagrams, the existence of the wormholes, and thorough analysis in various modified gravities like $f\left(R\right)$, $f\left(G\right)$, $f\left(T\right)$, $f(R,L_m)$, $f\left(R,T\right)$, $f\left(Q\right)$, and $f\left(Q,T\right)$. For distinct purposes, various models have been created. Many authors have used a variety of techniques to get a solution. Hereafter, the emphasis will be on the stability analysis of wormholes through different approaches namely Energy Conditions (ECs), Tolman–Oppenheimer–Volkoff (TOV), Volume Integral Quantifier(VIQ), Active Gravitational Mass(AGM), and Total Gravitational Energy (TGE). The possible observable features of such wormholes are briefly discussed. All the solutions are organized by the suggested framework to demonstrate broad concepts of wormhole solutions.

Stellar multiplicity is among the oldest and richest problems in astrophysics. Binary stars are a cornerstone of stellar mass and radius measurements that underpin modern stellar evolutionary models. Binaries are the progenitors of many of the most interesting and exotic astrophysical phenomena, ranging from type Ia supernovae to gamma ray bursts, hypervelocity stars, and most detectable stellar black holes. They are also ubiquitous, accounting for about half of all stars in the Universe. In the era of gravitational waves, wide-field surveys, and open-source stellar models, binaries are coming back stronger than a nineties trend. Much of the progress in the last decade has been enabled by the Gaia mission, which provides high-precision astrometry for more than a billion stars in the Milky Way. The Gaia data probe a wider range of binary separations and mass ratios than most previous surveys, enabling both an improved binary population census and discovery of rare objects. I summarize recent results in the study of binary stars brought about by Gaia, focusing in particular on developments related to wide ($a\gtrsim 100$  au) binaries, evidence of binarity from astrometric noise and proper motion anomaly, astrometric and radial velocity orbits from Gaia DR3, and binaries containing non-accreting compact objects. Limitations of the Gaia data, the importance of ground-based follow-up, and anticipated improvements with Gaia DR4 are also discussed.

The recent hint of correlated $\gamma$-ray and neutrino emission from the blazar TXS 0506+056 has renewed interest in blazars as the source of high-energy neutrinos, in which the possible neutrino emission involved hadronic acceleration in the jet of blazars. In this review, we summarize the recent advances in the multi-wavelength and neutrino observations of blazars. We focus on the discussion of the current understanding of blazar emission processes in the leptonic and hadronic model. The future multi-messenger observations combining electromagnetic and neutrino measurements will help us to constrain blazar emission models and understand the origins of the high-energy $\gamma$-rays and neutrinos.

Modern radio telescopes will generate, on a daily basis, data sets on the scale of exabytes for systems like the Square Kilometre Array (SKA). Massive data sets are a source of unknown and rare astrophysical phenomena that lead to discoveries. Nonetheless, this is only plausible with the exploitation of machine learning to complement human-aided and traditional statistical techniques. Recently, there has been a surge in scientific publications focusing on the use of machine/deep learning in radio astronomy, addressing challenges such as source extraction, morphological classification, and anomaly detection. This study provides a comprehensive and concise overview of the use of machine learning techniques for the morphological classification of radio galaxies. It summarizes the recent literature on this topic, highlighting the main challenges, achievements, state-of-the-art methods, and the future research directions in the field. The application of machine learning in radio astronomy has led to a new paradigm shift and a revolution in the automation of complex data processes. However, the optimal exploitation of machine/deep learning in radio astronomy, calls for continued collaborative efforts in the creation of high-resolution annotated data sets. This is especially true in the case of modern telescopes like MeerKAT and the LOw-Frequency ARray (LOFAR). Additionally, it is important to consider the potential benefits of utilizing multi-channel data cubes and algorithms that can leverage massive datasets without relying solely on annotated datasets for radio galaxy classification.

This is the fourth paper in the series of analyses of times of minimum and period variations in W UMa eclipsing binaries, and the implications for mass exchange in these systems. In this paper, the systems discussed in Papers 1–3 for which analyses are relatively complete are sorted into three sections, arranged in order of degree of confidence that mass exchange occurrence is established and that the present rate of exchange has been determined. The higher-confidence systems are: 44 Boo, RZ Cam, VW LMi, AU Ser; the intermediate-confidence systems are: TY Boo, V1191 Cyg, AP Leo, ER Ori; and the lower-confidence systems are: AB And, XY Boo, AC Boo, AH Cnc, VW Cep, EZ Hya, XY Leo, W UMa, and GR Vir. A fourth section describes promising cases that require additional data and/or analyses. For the higher-confidence systems, selected analyses of the times of minimum and of the most comprehensive light curve (LC) and radial velocity (RV) suites of data have yielded mean mass changes to the more massive component of dM₁/dt = +2.37 (13) × 10⁻⁷ M_ʘ/y for 44 Boo, +4.00 (14) × 10⁻⁸ M_ʘ /y for RZ Com, +2.53 (7) × 10⁻⁷ M_ʘ /y for VW LMi, and -1.98 (11) × 10⁻⁷ M_ʘ /y for AU Ser.

Proto-planetary discs, the birth environment of planets, are an example of a structure commonly found in astrophysics, accretion discs. Identifying the mechanism responsible for accretion is a long-standing problem, dating back several decades. The common picture is that accretion is a consequence of turbulence, with several instabilities proposed for its origin. While traditionally this field used to be a purely theoretical endeavour, the landscape is now changing thanks mainly to new observational facilities such as the ALMA radio interferometer. Thanks to large improvements in spatial and spectral resolution and sensitivity (which have enabled the study of disc substructure, kinematics and surveys of large disc populations), multiple techniques have been devised to observationally measure the amount of turbulence in discs. This review summarises these techniques, ranging from attempts at direct detection of turbulence from line broadening, to more indirect approaches that rely on properties of the dust or consider the evolution of global disc properties (such as masses, radii and accretion rates) for large samples, and what their findings are. Multiple lines of evidence suggest that discs are in fact not as turbulent as thought one decade ago. On the other hand, direct detection of turbulence in some discs and the finite radial extent of dust substructures and in some cases the finite vertical extent strongly indicate that turbulence must be present at some level in proto-planetary discs. It is still an open question whether this amount of turbulence is enough to power accretion or if this is instead driven by other mechanisms, such as MHD winds.

The Shower Database (SD) of the Meteor Data Center (MDC) had been operating on the basis of stream-naming rules which were too complex and insufficiently precise for 15 years. With a gradual increase in the number of discovered meteor showers, the procedure for submitting new showers to the database and naming them led to situations that were inconsistent with the fundamental role of the SD — the disambiguation of stream names in the scientific literature. Our aim was to simplify the meteor shower nomenclature rules. We proposed a much simpler set of meteor shower nomenclature rules, based on a two-stage approach, similar to those used in the case of asteroids. The first stage applies to a new shower just after its discovery. The second stage concerns a repeatedly observed shower, the existence of which no longer raises any doubts. Our proposed new procedure was approved by a vote of the commission F1 of the IAU in July 2022.

The question “are we alone in the universe?” has been asked through the ages and is beginning to be addressed by deploying spacecraft and advanced observatories capable of detecting biological signatures. Apart from the certainty that life exists on the Earth, there is no clear evidence at the time of writing for extra-terrestrial life (also termed exo-life). Although the sheer number of potentially habitable extrasolar planets in our galaxy alone makes a compelling case for widespread exo-life if taken in isolation, the constraints on the emergence of life imposed by chemistry and biology provide a counterbalance to this optimistic view. In the absence of any clear sign of exo-life and therefore our ignorance about whether it exists or not, the only way forward is to apply scientific knowledge in a rational way to discriminate between different scenarios until such a time that real evidence is forthcoming, if at all. This article reviews the main features of current astrobiological research to speculate on the likelihood of each critical transition in the development of living entities, emphasising the involvement of chemistry and informational macromolecules. It concludes that carbon-based compounds may be widespread on and in exoplanets, but the organisation of these prebiotic molecules into cellular structures with anything like the complexity of the primitive organisms on Earth could be very rare or non-existent. However, if such organisms do arise, the path to multicellularity and the functional organisation required for human capabilities may not be so daunting. Some of the key genetic features required for this development may already be present in primitive cells ready to be activated or repurposed.

The study of ultraluminous X-ray sources (ULXs) has changed dramatically over the last decade. In this review we first describe the most important observations of ULXs in various wavebands, and across multiple scales in space and time. We discuss recent progress and current unanswered questions. We consider the range of current theories of ULX properties in the light of this observational progress. Applying these models to neutron-star ULXs offers particularly stringent tests, as this is the unique case where the mass of the accretor is effectively fixed.

The overwhelming majority of theories on dark matter either introduce exotic, never discovered experimentally subatomic particles or change the physical laws. In this brief review I discuss three theories that do not do this, so that they are preferable from the viewpoint of the Occam razor principle.