Astrophysics and Space Science最新文献

We conduct a systematic study on the effects of rapid rotation on predicted Be star observables. We use the three-dimensional Monte Carlo radiative transfer code, hdust, to model a comprehensive range of Be star subtypes at varying rotation rates. Using these models, we predict (V) magnitude and photometric color, H(alpha ) line profiles, and polarization at UV wavelengths as well as in the (V)-band for Be stars from B0 to B8. For each spectral subtype, we investigate the effects of disk density on the produced observables. We find that reddening and brightening effects of gravity darkening may cause rapidly-rotating stars to appear more evolved than they truly are. Rotational effects on the H(alpha ) line profile shape may reduce line intensity for Be stars viewed at low inclinations and increase line intensity for those viewed at high inclinations. Additionally, rapid rotation can significantly impact the measured equivalent width of the line produced by a star with a moderate to high density disk, especially at high inclinations. When the star-disk system is viewed near edge-on, gravity darkening can result in stronger H(alpha ) emission than would otherwise be expected for a disk of a given density. We also find that the competing effects of rapid rotation and H i opacity cause the slope of the polarized continuum (the polarization color) to be very sensitive to changes in the stellar rotation rate. This quantity offers a strong diagnostic for the rotation rate of Be stars.

Optical interferometry is an observational technique that provides the highest spatial resolutions available in the optical. By interfering light from separate telescopes, and measuring the properties of the resulting interference pattern, it is possible to retrieve information about the night sky at spatial resolutions equal to the separation of the telescopes, overcoming the diffraction limit of a single telescope. In long baseline amplitude optical interferometry, the beams of light from the telescopes are transported to a central location and physically interfered. The interference is achieved via an instrument known as a beam combiner. In this review, I discuss the functionality of a beam combiner. I begin with a mathematical explanation of how interference fringes are produced and what information these interference fringes contain. This is followed by a discussion of how interference fringes are generated and measured in practise for the most common beam combination schemes, for both pupil plane and image plane combination and how these schemes can be realised in bulk optics or integrated optics. I also provide a detailed summary of the various design considerations that can affect the functionality of a beam combiner. Finally, I discuss current and future work in long baseline amplitude optical interferometry.

Accurate prediction of solar activity and geomagnetic disturbances is essential for reducing the risks posed by space weather to technological systems. This study presents a hybrid deep learning model that integrates Long Short-Term Memory (LSTM) and Gated Recurrent Unit (GRU) networks for simultaneous forecasting of sunspot numbers and geomagnetic indices (Ap and Kp). Using comprehensive datasets spanning solar cycles 20 to 24 (1964–2016), the model employs advanced preprocessing techniques including Savitzky-Golay filtering and normalization and a 60-day sliding window to capture complex temporal dependencies. Model performance, evaluated via 8-fold cross-validation, demonstrates high predictive accuracy, achieving R² values of 0.9943 for sunspot numbers, 0.970 for Kp, and 0.923 for Ap, with low RMSE values. Heatmaps highlighted low RMSE across most time segments, confirming model robustness. Our results confirm a strong correlation between high Ap-index values and increased sunspot activity. Extreme event analysis demonstrates reliable detection of high-intensity geomagnetic storms, with substantial improvements in probability of detection and false alarm rates relative to NOAA/SWPC benchmarks. Comparative assessments show that the hybrid LSTM-GRU model outperforms standalone deep learning and conventional approaches, offering both aggregate skill and operationally relevant performance, even in the presence of severe class imbalance. The hybrid LSTM–GRU model demonstrates clear advantages over standalone LSTM and GRU architectures, with correlation analysis confirming strong links between sunspot activity and the Ap-index, underscoring the model’s ability to capture solar-terrestrial interactions. The proposed LSTM-GRU model demonstrates significant potential for real time space weather forecasting and offers a scalable framework for extended solar-terrestrial predictive analysis.

Forbush decreases (FDs) are short-term reductions in the time-intensity flux of cosmic rays (CRs). Their spectacular and unpredictable intensity variations present their detection, timing, magnitude estimation, and cataloging as one of the most difficult tasks in space weather research. Due to the paucity of accurate event lists, new methods of event detection and FD catalogs continue to appear in the literature. But validation of either the old or new lists remains an open field. This work intends to remind the astrophysicist and space weather community that dramatic modifications of the age-long manual/semi-manual FD event cataloging are long overdue. In this present era of extremely fast and sophisticated computer algorithms, a complementary automated list of all the manually created FD catalogs, especially the IZMIRAN (Pushkin Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation, Russian Academy of Sciences, http://spaceweather.izmiran.ru/eng/dbs.html) (which is the currently available and widely used lists) should have been developed, the old lists expanded and new catalogs created. A Fourier decomposition technique, which guarantees the sinusoidal fidelity of the input and output signals, is presented. This highlights the possibility of a complete algorithm-based FD cataloging with the aim of stimulating other automated methodological approaches. The impossibility of creating accurate FD lists without first disentangling the contribution from diurnal CR anisotropy and the 11-year solar oscillation was qualitatively analyzed. Using a set of CR data from Moscow station, we automatically created four different FD catalogs − FD1 from the raw data without adjusting for the contribution from the 11-year solar cycle oscillation or CR anisotropy, FD2 after adjusting for the 11-year solar cycle effects, FD3 after adjusting for CR anisotropy and FD4 after adjusting for both the 11-year and CR anisotropy. This allows us to practically demonstrate the possible discrepancies among different FD catalogs and the attendant bias implications on FD-based space weather investigation. Given the detailed analyses performed, FD4 is the most accurate FD list. This serves as an evidence that accurate FD catalog is realizable. We also establish that several of the events in FD1, FD2 and FD3 catalogs may be intensity reductions/spurious events arising from other CR phenomena like anisotropies.

Stellar rotation has long been recognized as important to the evolution of stars, by virtue of the chemical mixing it can induce and how it interacts with binary mass transfer. Binary interaction and rapid rotation are both common in massive stars and involve processes of angular momentum distribution and transport. An important question is how this angular momentum transport leads to the creation of two important classes of rapidly rotating massive stars, Be stars defined by disklike emission lines, and Bn stars defined by rotationally broadened absorption lines. A related question is what limits this rotation places on how conservative the mass transfer can be. Central to addressing these issues is knowledge of how close to rotational break-up stars can get before they produce a disk. Here we calculate diagnostics of this rotational criticality using the continuum polarization arising from a combination of rotational stellar distortion (i.e., oblateness) and redistribution of stellar flux (i.e., gravity darkening), and compare polarizations produced in the von Zeipel approximation with the approach of Espinosa Lara & Rieutord (ELR). Both produce similar photospheric polarizations that rise significantly in the far ultraviolet (FUV) for B stars, with a stronger signal in the von Zeipel case. For early main-sequence and subgiant stars, it reaches a maximum of (sim 1)% at 140 nm for stars rotating at 98% of critical, when seen edge-on. Rotational rates above 80% critical result in polarizations of several tenths of a percent, at high inclination. Even at a low inclination of (i=40^{circ }), models at 98% critical show polarization in excess of 0.1% down to 200 nm. These predicted stable signal strengths indicate that determinations of near-critical rotations in B stars could be achieved with future spectropolarimetric instrumentation that can reach deep into the FUV, such as CASSTOR, the Polstar mission concept, or the POLLUX detector design.

We study the effects of pair creation on the radiation emerging from black holes under the assumption that the magnetic fields are vortex driven. In particular, for a sufficiently broad range of supermassive black holes, we investigated the energies at which photons undergo decay under the influence of a strong magnetic field, producing electron-positron pairs. Depending on particular physical parameters, it has been shown that in certain scenarios high or very high energy emission generated by black holes will be strongly suppressed, thus, will be unable to escape a zone where radiation is generated. In particular, photons with energies exceeding (sim 1text{ GeV}) will never leave the magnetosphere if they are generated at the scale 10(R_{g}) and the threshold is of the order of (1text{ TeV}), if the emission is produced at (sim 100; R_{g}). Analysing the process versus the black hole mass, assuming the region (100; R_{g}), it has been shown that for the considered lowest mass, the photons with energies (250text{ GeV}) will never leave the black hole and for the considered highest mass the corresponding value is (sim 250text{ TeV}).

Recently, a novel class of modified gravity theories has been proposed, wherein Einstein’s General Relativity (GR) is extended by incorporating a quadratic energy–momentum term of the form (T_{mu nu }T^{mu nu } ), coupled via a constant parameter (alpha ). The corresponding field equations deviate from the Einstein equations only in the presence of matter. Analytical studies indicate that, without interaction, the energy-momentum squared term remains subdominant, mainly enabling non-singular Big Bang scenarios. In this work, we investigate this framework in a homogeneous and isotropic cosmological background. We show that, in its minimal form, the theory does not naturally explain late-time cosmic acceleration. Although a cosmological constant can remedy this, it introduces an effective dark energy component with positive pressure during the matter era, distorting large-scale structure formation. To overcome this, we derive an analytical dark energy form by redefining its equation of state and imposing boundary conditions consistent with early- and late-time cosmology. The resulting phenomenological model alleviates the coincidence and fine-tuning problems and ensures classical stability. Observational constraints confirm good agreement with current data, though a statefinder analysis shows that, while the model mimics (Lambda )CDM today, it deviates in the far future as the acceleration rate increases.

The nature of dark matter (DM) and its effects on Milky Way galactic evolution are outstanding problems in astrophysics, space science, and cosmology. Quantitative models for the distribution of DM in the solar system are needed to clarify the role of DM in the Milky Way and to enable and guide direct and indirect DM detection searches. The dynamics, energetics, and composition of the solar corona are important elements in the Sun-heliosphere connection. By adopting a kinetic collisionless approach, we present the first quantitative model for the spatial distribution of DM in the solar corona. Our results show that the predicted DM mass density can be a fraction of plasma mass densities predicted by models of the solar corona. We find that the DM density in the lower corona is mass-dependent with the largest DM density occurring for a DM mass on the order of several proton masses.

We present an investigation of the classical nova V615 Vul that erupted on July 29, 2024. The results obtained are based on our multicolor observations at AI SAS, the M. R. Štefánik Observatory in Hlohovec and MAO NASU, as well as photometry from the AAVSO and ASAS-SN database. We also present spectral observations obtained with the 1.3-meter telescope at Skalnaté Pleso Observatory, when the very broad asymmetric H_α emission line dominated during the nova’s decline. From the constructed light curves, we determined the brightness decay rates at 2 and 3 magnitudes, obtaining (t_{2,V} = 5^{d}).2, (t_{3,V} = 8^{d}).9 and (t_{3,B} = 11^{d}).6, which corresponds to very fast novae. The light curve shows a variation with a period of 6.5 days from day 12 to about day 90 after the outburst maximum. We also detected and analyzed short-term variations in (V), (R_{c}) and (I_{c}) starting around day 35. We suppose that the found oscillation of 0^d.2238 ± 0.005 is the orbital period of V615 Vul. We calculated color indices and estimated the color temperature of the system. The tracks in the (V-R_{c}) and (R_{c}-I_{c}) diagrams exhibit loop-like variations caused by the broad H_α emission line, while the tracks in (U-B)/(B-V) diagram reflect changes in color temperature during the outburst - behavior typical of many cataclysmic variables and novae. We also estimated the main parameters of the system, such as the absolute magnitude, distance, extinction, and mass of the white dwarf.

Angular momentum transport is a fundamental process shaping the structure, evolution, and lifespans of stars and disks across a wide range of astrophysical systems. Be stars offer a valuable environment for studying viscous transport of angular momentum in massive stars, thanks to their rapid rotation, observable decretion disks, and likely absence of strong magnetic fields. This study analyzes angular momentum loss in 40 Be binary simulations spanning a range of orbital separations and companion masses, using a smoothed-particle hydrodynamics (SPH) code. A novel framework is introduced to define the outer disk edge based on the behaviour of the azimuthal velocity, streamlining the analysis of angular momentum transport within the system. Applying this framework reveals that systems with smaller truncation radii tend to reaccrete a larger fraction of their angular momentum during dissipation, thereby inhibiting the stars ability to regulate its surface rotation. Modification of this rate may alter the star’s mass-injection duty cycle or long-term evolutionary track. Finally, a subset of the simulations were post-processed using the Monte Carlo radiative transfer code HDUST, generating synthetic observables including H(alpha ) line profiles, V-band polarization, and UV polarization. Suggestions for observational verification of the dynamical results are demonstrated using the simulated data.