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An Examination of the Very First Polarimetric X-Ray Observations of Radio-Quiet Active Galactic Nuclei
Considerations on Possible Directions for a Wide Band Polarimetry X-ray Mission

Journal Description

Galaxies

Galaxies

is an international, peer-reviewed, open access journal on astronomy, astrophysics, and cosmology published bimonthly online by MDPI.

Open Access— free for readers, with article processing charges (APC) paid by authors or their institutions.
High Visibility: indexed within Scopus, ESCI (Web of Science), Astrophysics Data System, INSPIRE, Inspec, and other databases.
Journal Rank: JCR - Q2 (Astronomy and Astrophysics) / CiteScore - Q2 (Astronomy and Astrophysics)
Rapid Publication: manuscripts are peer-reviewed and a first decision is provided to authors approximately 19.9 days after submission; acceptance to publication is undertaken in 4.9 days (median values for papers published in this journal in the second half of 2024).
Recognition of Reviewers: reviewers who provide timely, thorough peer-review reports receive vouchers entitling them to a discount on the APC of their next publication in any MDPI journal, in appreciation of the work done.

Impact Factor: 3.2 (2023); 5-Year Impact Factor: 2.5 (2023)

Imprint Information Journal Flyer Open Access ISSN: 2075-4434

Latest Articles

51 pages, 15203 KiB

Open AccessReview

High-Contrast Imaging: Hide and Seek with Exoplanets

by Riccardo Claudi and Dino Mesa

Galaxies 2025, 13(1), 3; https://doi.org/10.3390/galaxies13010003 - 31 Dec 2024

So far, most of the about 5700 exoplanets have been discovered mainly with radial velocity and transit methods. These techniques are sensitive to planets in close orbits, not being able to probearge star–planet separations. μ-lensing is the indirect method that allows us [...] Read more.

So far, most of the about 5700 exoplanets have been discovered mainly with radial velocity and transit methods. These techniques are sensitive to planets in close orbits, not being able to probearge star–planet separations. μ-lensing is the indirect method that allows us to probe the planetary systems at the snow-line and beyond, but it is not a repeatable observation. On the contrary, direct imaging (DI) allows for the detection and characterization ofow mass companions at wide separation (≤5–6 au). The main challenge of DI is that a typical planet–star contrast ranges from 10⁻⁶, for a young Jupiter in emittedight, to 10⁻⁹ for Earth in reflectedight. In theast two decades, aot of efforts have been dedicated to combiningarge (D ≥ 5 m) telescopes (to reduce the impact of diffraction) with coronagraphs and high-order adaptive optics (to correct phase errors induced by atmospheric turbulence), with sophisticated image post-processing, to reach such a contrast between the star and the planet in order to detect and characterize cooler and closer companions to nearby stars. Building on the first pioneering instrumentation, the second generation of high-contrast imagers, SPHERE, GPI, and SCExAO, allowed us to probe hundreds of stars (e.g., 500–600 stars using SHINE and GPIES), contributing to a better understanding of the demography and the occurrence of planetary systems. The DI offers a possible clear vision for studying the formation and physical properties of gas giant planets and brown dwarfs, and the future DI (space and ground-based) instruments with deeper detectionimits will enhance this vision. In this paper, we briefly review the methods, the instruments, the main sample of targeted stars, the remarkable results, and the perspective of this rising technique. Full article

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33 pages, 1413 KiB

Open AccessReview

Gamma-Ray Bursts: What Do We Know Today That We Did Not Know 10 Years Ago?

by Asaf Pe’er

Galaxies 2025, 13(1), 2; https://doi.org/10.3390/galaxies13010002 - 31 Dec 2024

I discuss here the progress made in the last decade on a few of the key open problems in GRB physics. These include (1) the nature of GRB progenitors, and the outliers found to the collapsar/merger scenarios; (2) jet structures, whose existence became [...] Read more.

I discuss here the progress made in the last decade on a few of the key open problems in GRB physics. These include (1) the nature of GRB progenitors, and the outliers found to the collapsar/merger scenarios; (2) jet structures, whose existence became evident following GRB/GW170817; (3) the great progress made in understanding the GRB jet launching mechanisms, enabled by general-relativistic magnetohydrodynamic (GR-MHD) codes; (4) recent studies of magnetic reconnection as a valid energy dissipation mechanism; (5) the early afterglow, which may be highly affected by a wind bubble, as well as recent indication that in many GRBs, the Lorentz factor is only a few tens, rather than a few hundreds. I highlight some recent observational progress, including the major breakthrough in detecting TeV photons and the on-going debate about their origin, polarization measurements, as well as the pair annihilation line recently detected in GRB 221009A, and its implications for prompt emission physics. I probe into some open questions that I anticipate will be at the forefront of GRB research in the next decade. Full article

(This article belongs to the Special Issue Gamma-Ray Bursts in Multiwavelength: Theory, Observational Correlations and GRB Cosmology)

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31 pages, 1262 KiB

Open AccessArticle

Distinguishing Black Holes and Superspinars in Spinning Modified Gravity via Generalized Spin Precession Frequency

by Parthapratim Pradhan

Galaxies 2025, 13(1), 1; https://doi.org/10.3390/galaxies13010001 - 31 Dec 2024

We compute the generalized spin precession frequency (

Ω_{p}

) of a test gyroscope in a stationary spacetime, specifically for a Kerr–MOG black hole within the framework of scalar–tensor–vector gravity (STVG), also known as modified gravity (MOG). A comprehensive analysis of the [...] Read more.

We compute the generalized spin precession frequency (

Ω_{p}

) of a test gyroscope in a stationary spacetime, specifically for a Kerr–MOG black hole within the framework of scalar–tensor–vector gravity (STVG), also known as modified gravity (MOG). A comprehensive analysis of the generalized spin frequency was conducted for non-extremal Kerr–MOG black hole, extremal Kerr–MOG black hole, and naked singularity or superspinar, in comparison to non-extremal Kerr black hole, extremal Kerr black hole, and Kerr naked singularity or Kerr superspinar. The generalized spin frequency we derived can be expressed in terms of the black hole mass parameter, the angular momentum parameter, and the MOG parameter. Additionally, we distinguish between non-extremal black hole, extremal black hole, and naked singularity through the computation of the aforementioned precession frequency. Furthermore, we calculate the generalized spin frequency for various angular coordinates, ranging from the polar to the equatorial plane. Lastly, we determine three fundamental epicyclic frequencies, the Keplerian frequency, the radial epicyclic frequency, and the vertical epicyclic frequency, to differentiate these three types of objects. We also compute the periastron frequency and nodal frequency. Utilizing these frequency profiles allows for the distinction of these three compact objects. Full article

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11 pages, 1074 KiB

Open AccessReview

From the s-Process to the i-Process: A New Perspective on the Chemical Enrichment of Extrinsic Stars

by Sophie Van Eck, Riano Giribaldi, Thibault Merle, Adrian Lambotte, Drisya Karinkuzhi, Stéphane Goriely, Arthur Choplin, Nicholas Storm, Jeffrey Gerber, Lionel Siess, Maria Bergemann and Alain Jorissen

Galaxies 2024, 12(6), 89; https://doi.org/10.3390/galaxies12060089 - 23 Dec 2024

Separating stars enriched in the s- and r-processes of nucleosynthesis is usually achieved by analyzing the element ratios of s-process elements (like Ba or La) to r-process elements (like Eu). The situation becomes more complex when analyzing CEMP-rs stars, [...] Read more.

Separating stars enriched in the s- and r-processes of nucleosynthesis is usually achieved by analyzing the element ratios of s-process elements (like Ba or La) to r-process elements (like Eu). The situation becomes more complex when analyzing CEMP-rs stars, which are carbon-enriched metal-poor objects enriched in a mixture of s- and r-elements. These objects, possibly resulting from the i-process of nucleosynthesis, are notoriously difficult to classify based on elemental ratios. Recent theoretical studies have outlined, however, that the s-, i-, and r-processes produce distinct isotopic mixtures. Here, we propose to analyze a sample of stars known to be enriched in s, r, or r + s elements and to determine the odd-to-even isotopic ratio measured on atomic lines of barium, in order to validate or disprove their assignation. Full article

(This article belongs to the Special Issue Nucleosynthesis and Dust Production in AGB Stars: Open Issues and Future Perspectives)

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7 pages, 1570 KiB

Open AccessArticle

Advances in Stellar and Galactic Evolution with the Population of Planetary Nebula Progenitors from the APOGEE DR17 Survey

by Letizia Stanghellini, Verne V. Smith, Katia Cunha and Nikos Prantzos

Galaxies 2024, 12(6), 88; https://doi.org/10.3390/galaxies12060088 - 23 Dec 2024

Planetary nebulae (PNe) are the ejected gas and dust shells of evolved low- and intermediate-mass stars (LIMSs). We present an abundance comparison between PNe and their progenitors to reveal their similarities and differences since such a comparison has rarely, and not recently, been [...] Read more.

Planetary nebulae (PNe) are the ejected gas and dust shells of evolved low- and intermediate-mass stars (LIMSs). We present an abundance comparison between PNe and their progenitors to reveal their similarities and differences since such a comparison has rarely, and not recently, been performed in the Milky Way. The dynamical expulsion of the outer envelope of an evolved LIMS produces the PN. We expected similarities in most

α

-element distributions across the stellar and nebular populations, given that these elements are only marginally produced and destroyed during the LIMS evolution. Differences found in the Fe and S abundances allow us to determine their depletion due to grain condensation in the post-AGB phases. Differences in N and C between PNe and their progenitors set new limits to the low- and intermediate-mass star contributions to these elements. Finally, radial metallicity gradients from evolved LIMS and PNe and Gaia-calibrated distances constrain Galactic evolution in the framework of the current chemical evolutionary models. We found the following: (1) Gas-phase iron is significantly depleted in PNe compared to their progenitor stars, with an average depletion factor of <D[Fe/H]> = 1.74 ± 0.49. (2) Sulfur is also depleted in PNe, though to a much lesser extent than iron. (3) The median enrichment levels for carbon and nitrogen relative to the median stellar population of the same metallicity are approximately [C/H] ∼ +0.3 and [N/H] ∼ +0.4, respectively. PNe with progenitors that experienced hot-bottom burning (HBB) exhibit extreme nitrogen enrichment. (4) With the data available to date, the radial metallicity gradient derived from evolved LIMSs and PNe are compatible within the uncertainties. Full article

(This article belongs to the Special Issue Nucleosynthesis and Dust Production in AGB Stars: Open Issues and Future Perspectives)

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7 pages, 2607 KiB

Open AccessArticle

Stellar Evolution and Convection in 3D Hydrodynamic Simulations of a Complete Burning Phase

by Federico Rizzuti, Raphael Hirschi, Vishnu Varma, William David Arnett, Cyril Georgy, Casey Meakin, Miroslav Mocák, Alexander StJ. Murphy and Thomas Rauscher

Galaxies 2024, 12(6), 87; https://doi.org/10.3390/galaxies12060087 - 9 Dec 2024

Our understanding of stellar evolution and nucleosynthesis is limited by the uncertainties coming from the complex multi-dimensional processes in stellar interiors, such as convection and nuclear burning. Three-dimensional stellar models can improve this knowledge by studying multi-D processes, but only for a short [...] Read more.

Our understanding of stellar evolution and nucleosynthesis is limited by the uncertainties coming from the complex multi-dimensional processes in stellar interiors, such as convection and nuclear burning. Three-dimensional stellar models can improve this knowledge by studying multi-D processes, but only for a short time range (minutes or hours). Recent advances in computing resources have enabled 3D stellar models to reproduce longer time scales and include nuclear reactions, making the simulations more accurate and allowing to study explicit nucleosynthesis. Here, we present results from 3D stellar simulations of a convective neon-burning shell from a 20 M_⊙ star, run with an explicit nuclear network from its early development to complete fuel exhaustion. We show that convection halts when fuel is exhausted, stopping its further growth after the entrainment of fresh material. These results, which highlight the differences and similarities between 1D and multi-D stellar models, have important implications for the evolution of convective regions in stars and their nucleosynthesis. Full article

(This article belongs to the Special Issue Nucleosynthesis and Dust Production in AGB Stars: Open Issues and Future Perspectives)

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18 pages, 5685 KiB

Open AccessArticle

Analysis of Habitability and Stellar Habitable Zones from Observed Exoplanets

by Jonathan H. Jiang, Philip E. Rosen, Christina X. Liu, Qianzhuang Wen and Yanbei Chen

Galaxies 2024, 12(6), 86; https://doi.org/10.3390/galaxies12060086 - 3 Dec 2024

The investigation of exoplanetary habitability is integral to advancing our knowledge of extraterrestrial life potential and detailing the environmental conditions of distant worlds. In this analysis, we explore the properties of exoplanets situated with respect to circumstellar habitable zones by implementing a sophisticated [...] Read more.

The investigation of exoplanetary habitability is integral to advancing our knowledge of extraterrestrial life potential and detailing the environmental conditions of distant worlds. In this analysis, we explore the properties of exoplanets situated with respect to circumstellar habitable zones by implementing a sophisticated filtering methodology on data from the NASA Exoplanet Archive. This research encompasses a thorough examination of 5595 confirmed exoplanets listed in the Archive as of 10 March 2024, systematically evaluated according to their calculated average surface temperatures and stellar classifications of their host stars, taking into account the biases implicit in the methodologies used for their discovery. Machine learning, in the form of a Random Forest classifier and an XGBoost classifier, is applied in the classification with high accuracies. The feature importance analysis indicates that our approach captures the most important parameters for habitability classification. Our findings elucidate distinctive patterns in exoplanetary attributes, which are significantly shaped by the spectral classifications and mass of the host stars. The insights garnered from our study both inform refinement of existing models for managing burgeoning exoplanetary datasets, and lay foundational groundwork for more in-depth explorations of the dynamic relationships between exoplanets and their stellar environments. Full article

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6 pages, 256 KiB

Open AccessCommunication

Insight on AGB Mass-Loss and Dust Production from PNe

by Silvia Tosi

Galaxies 2024, 12(6), 85; https://doi.org/10.3390/galaxies12060085 - 2 Dec 2024

The asymptotic giant branch (AGB) phase, experienced by low- and intermediate-mass stars (LIMSs), plays a crucial role in galaxies due to its significant dust production. Planetary nebulae (PNe) offer a novel perspective, providing valuable insights into the dust production mechanisms and the evolutionary [...] Read more.

The asymptotic giant branch (AGB) phase, experienced by low- and intermediate-mass stars (LIMSs), plays a crucial role in galaxies due to its significant dust production. Planetary nebulae (PNe) offer a novel perspective, providing valuable insights into the dust production mechanisms and the evolutionary history of LIMSs. We selected a sample of nine PNe from the Large Magellanic Cloud (LMC), likely originating from single stars. By modeling their spectral energy distributions (SEDs) with photoionization techniques, we successfully reproduced the observed photometric data, spectra, and chemical abundances. This approach enabled us to constrain key characteristics of the central stars (CSs), dust, and gaseous nebulae, which were then compared with predictions from stellar evolution models. By integrating observational data across ultraviolet (UV) to infrared (IR) wavelengths, we achieved a comprehensive understanding of the structure of the PNe in our sample. The results of the SED analysis are consistent with evolutionary models and previous studies that focus on individual components of the PN, such as dust or the gaseous nebula. Our analysis enabled us to determine the metallicity, the progenitor mass of the CSs, and the amount of dust and gas surrounding the CSs, linking these properties to the previous AGB phase. The PN phase provides critical insights into the physical processes active during earlier evolutionary stages. Additionally, we found that higher progenitor masses are associated with greater amounts of dust in the surrounding nebulae but lower amounts of gaseous material compared to sources with lower progenitor masses. Full article

(This article belongs to the Special Issue Nucleosynthesis and Dust Production in AGB Stars: Open Issues and Future Perspectives)

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9 pages, 7780 KiB

Open AccessArticle

On the Ionization Tolerance of C₂₀ Fullerene in Ground and Excited Electronic States in Planetary Nebulae

by SeyedAbdolreza Sadjadi and Quentin Andrew Parker

Galaxies 2024, 12(6), 84; https://doi.org/10.3390/galaxies12060084 - 30 Nov 2024

As the smallest member of the fullerene family,

C_{20}

is yet to be discovered in planetary nebulae. In this work, we present a quantum chemical study via density functional theory (DFT) and partially by the MP2 on the ionization tolerance of this [...] Read more.

As the smallest member of the fullerene family,

C_{20}

is yet to be discovered in planetary nebulae. In this work, we present a quantum chemical study via density functional theory (DFT) and partially by the MP2 on the ionization tolerance of this molecule in the space environment. Considering that the ionization and excitation phenomena play key roles in demonstrating the lifetime of a molecule, we examined both ground and excited electronic-state potential energy surfaces (PES) of

C_{20}

and its cations

{C_{20}}^{q +}

. Our theoretical results indicate that the

C_{20}

cage tolerates a positive charge as high as

13 +

by characterizing local minimum geometries on both the abovementioned electronic states. The results are backed by characterizing both

{C_{20}}^{12 +}

and

{C_{20}}^{13 +}

as local minimum geometries at the MP2 level of computations. We also explored, theoretically and systematically, scenarios in which the electronic structure of neutral

C_{20}

is excited to very high spin multiplicity (beyond triplet state), and local minimum molecular geometries with cage structures are well characterized. We anticipate that such structural resistance to excitation and ionization delivers a prolonged lifetime necessary for the spectroscopic detection of this interesting molecule and its cations in space and potentially in planetary nebulae (PN). Full article

(This article belongs to the Special Issue Origins and Models of Planetary Nebulae)

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32 pages, 3308 KiB

Open AccessReview

Primer on Formation and Evolution of Hydrogen-Deficient Central Stars of Planetary Nebulæ and Related Objects

by Marcelo M. Miller Bertolami

Galaxies 2024, 12(6), 83; https://doi.org/10.3390/galaxies12060083 - 29 Nov 2024

We present a brief review of the formation and evolution of hydrogen-deficient central stars of planetary nebulae. We include a detailed description of the main observable features of both the central stars and their surrounding nebulae and review their main classifications. We also [...] Read more.

We present a brief review of the formation and evolution of hydrogen-deficient central stars of planetary nebulae. We include a detailed description of the main observable features of both the central stars and their surrounding nebulae and review their main classifications. We also provide a brief description of the possible progenitor systems of hydrogen-deficient central stars as well as of transients, which are closely connected to the formation of these stars. In particular, we offer a detailed theoretical explanation of the main evolutionary scenarios, both single and binary, which is devised to explain these stars and nebulae. Particular emphasis is made in the description of the so-called born again scenario, their quantitative predictions, and uncertainties. Finally, we discuss the pros and cons of both binary and single evolution channels, draw some conclusions, and discuss open questions in the field. Full article

(This article belongs to the Special Issue Origins and Models of Planetary Nebulae)

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8 pages, 888 KiB

Open AccessArticle

Dusty Common Envelope Evolution

by Lionel Siess, Luis C. Bermúdez-Bustamante, Orsola De Marco, Daniel J. Price, Miguel González-Bolívar, Chunliang Mu, Mike Y. M. Lau, Ryosuke Hirai and Taïssa Danilovich

Galaxies 2024, 12(6), 82; https://doi.org/10.3390/galaxies12060082 - 29 Nov 2024

We present the first hydrodynamical simulations of common envelope evolution that include the formation of dust and the effect of radiation pressure on dust grains. We performed smoothed particle hydrodynamics simulations of the CE evolution for two systems made of a 1.7 [...] Read more.

We present the first hydrodynamical simulations of common envelope evolution that include the formation of dust and the effect of radiation pressure on dust grains. We performed smoothed particle hydrodynamics simulations of the CE evolution for two systems made of a 1.7

M_{⊙}

and 3.7

M_{⊙}

AGB star primary with a 0.6

M_{⊙}

binary companion. The results of our calculations indicate that dust formation has a negligible impact on the gas dynamics essentially because dust forms in the already unbound material. The expansion and cooling of the envelope yield very early and highly efficient production of dust. In our formalism, which does not consider dust destruction, almost 100% of the available carbon that is not locked in CO condensates in dust grains. This massive dust production, thus, strongly depends on the envelope mass and composition, in particular, its C/O ratio, and has a considerable impact on the observational aspect of the object, resulting in a photospheric radius that is approximatively one order of magnitude larger than that of a non-dusty system. Full article

(This article belongs to the Special Issue Nucleosynthesis and Dust Production in AGB Stars: Open Issues and Future Perspectives)

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14 pages, 565 KiB

Open AccessArticle

A Parameter Study of 1D Atmospheric Models of Pulsating AGB Stars

by Henry A. Prager, Lee Anne M. Willson, Joyce A. Guzik, Michelle J. Creech-Eakman and Qian Wang

Galaxies 2024, 12(6), 81; https://doi.org/10.3390/galaxies12060081 - 29 Nov 2024

Using the atmospheric pulsation code written by George Bowen, we have performed a parameter study examining the effects of modifying various parameters of models of oxygen-rich AGB atmospheres pulsating in the fundamental and first-overtone modes. For each pulsation mode, we have examined the [...] Read more.

Using the atmospheric pulsation code written by George Bowen, we have performed a parameter study examining the effects of modifying various parameters of models of oxygen-rich AGB atmospheres pulsating in the fundamental and first-overtone modes. For each pulsation mode, we have examined the effects of adjusting the dust condensation temperature, dust condensation temperature range, pulsation amplitude, dust opacity, and metallicity. Our model grids are generated with the constraint that their luminosities are chosen to span the range of observed mass loss rates at a chosen mass. The dust condensation temperature, pulsation amplitude, and dust opacity have strong effects on the ultimate location and shape of the final model grids in the mass luminosity plane. The mass loss rate evolution of the fundamental and first-overtone mode models show a significant difference in behavior. While the fundamental mode models exhibit the typically assumed power–law relation with mass and luminosity, the first-overtone mode models show significant non-power law behavior at observed mass loss rates. Effectively, models in the first-overtone mode require somewhat higher luminosities to reach the same mass loss rates seen in fundamental mode models of the same mass, consistent with observed AGB stars. Full article

(This article belongs to the Special Issue Nucleosynthesis and Dust Production in AGB Stars: Open Issues and Future Perspectives)

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19 pages, 1396 KiB

Open AccessReview

Fifty Years After the Discovery of the First Stellar-Mass Black Hole: A Review of Cyg X-1

by Jiachen Jiang

Galaxies 2024, 12(6), 80; https://doi.org/10.3390/galaxies12060080 - 21 Nov 2024

Around 50 years ago, the famous bet between Stephen Hawking and Kip Thorne on whether Cyg X-1 hosts a stellar-mass black hole became a well-known story in the history of black hole science. Today, Cyg X-1 is widely recognised as hosting a stellar-mass [...] Read more.

Around 50 years ago, the famous bet between Stephen Hawking and Kip Thorne on whether Cyg X-1 hosts a stellar-mass black hole became a well-known story in the history of black hole science. Today, Cyg X-1 is widely recognised as hosting a stellar-mass black hole with a mass of approximately 20 solar masses. With the advancement of X-ray telescopes, Cyg X-1 has become a prime laboratory for studies in stellar evolution, accretion physics, and high-energy plasma physics. In this review, we explore the latest results from X-ray observations of Cyg X-1, focusing on its implications for black hole spin, its role in stellar evolution, the geometry of the innermost accretion regions, and the plasma physics insights derived from its X-ray emissions. This review primarily focuses on Cyg X-1; however, the underlying physics applies to other black hole X-ray binaries and, to some extent, to AGNs. Full article

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10 pages, 3558 KiB

Open AccessArticle

The New Deep-Underground Direct Measurement of ²²Ne(α, γ)²⁶Mg with EASγ: A Feasibility Study

by Daniela Mercogliano, Andreas Best and David Rapagnani

Galaxies 2024, 12(6), 79; https://doi.org/10.3390/galaxies12060079 - 20 Nov 2024

²²Ne(α, γ)²⁶Mg is pivotal in the understanding of several open astrophysical questions, as the nucleosynthesis beyond Fe through the s-process, but its stellar reaction rate is still subject to large uncertainties. These mainly arise from its extremely low rate in [...] Read more.

²²Ne(α, γ)²⁶Mg is pivotal in the understanding of several open astrophysical questions, as the nucleosynthesis beyond Fe through the s-process, but its stellar reaction rate is still subject to large uncertainties. These mainly arise from its extremely low rate in the Gamow energy region, whose measurement is hampered by the unavoidable presence of the cosmic ray background noise. A possibility to overcome this issue is to perform the measurement in a quasi background-free environment, such as that offered by the underground Bellotti Ion Beam Facility at LNGS. This is the key idea of EASγ experiment. In this study, the signal from the de-excitation of the compound nucleus ²⁶Mg has been simulated and its detection has been investigated both on surface and deep-underground laboratories. The simulation results show the enhancement in sensitivity achieved by performing the measurement deep underground and with an additional shielding, yielding to unprecedented sensitivity. Full article

(This article belongs to the Special Issue Nucleosynthesis and Dust Production in AGB Stars: Open Issues and Future Perspectives)

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16 pages, 3064 KiB

Open AccessArticle

TOrsion-Bar Antenna: A Ground-Based Detector for Low-Frequency Gravity Gradient Measurement

by Satoru Takano, Tomofumi Shimoda, Yuka Oshima, Ching Pin Ooi, Perry William Fox Forsyth, Mengdi Cao, Kentaro Komori, Yuta Michimura, Ryosuke Sugimoto, Nobuki Kame, Shingo Watada, Takaaki Yokozawa, Shinji Miyoki, Tatsuki Washimi and Masaki Ando

Galaxies 2024, 12(6), 78; https://doi.org/10.3390/galaxies12060078 - 20 Nov 2024

The Torsion-Bar Antenna (TOBA) is a torsion pendulum-based gravitational detector developed to observe gravitational waves in frequencies between 1 mHz and 10 Hz. The low resonant frequency of the torsion pendulum enables observation in this frequency band on the ground. The final target [...] Read more.

The Torsion-Bar Antenna (TOBA) is a torsion pendulum-based gravitational detector developed to observe gravitational waves in frequencies between 1 mHz and 10 Hz. The low resonant frequency of the torsion pendulum enables observation in this frequency band on the ground. The final target of TOBA is to observe gravitational waves with a 10 m detector and expand the observation band of gravitational waves. In this paper, an overview of TOBA, including the previous prototype experiments and the current ongoing development, is presented. Full article

(This article belongs to the Special Issue Challenges for Third-Generation Gravitational Wave Detectors and Beyond)

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19 pages, 3178 KiB

Open AccessReview

Exploring the Components of Cosmogenic UHECR, Neutrino, and Diffuse Gamma Ray from High-Energy Astrophysical Objects

by Fangsheng Min, Hong Lu and Yiqing Guo

Galaxies 2024, 12(6), 77; https://doi.org/10.3390/galaxies12060077 - 18 Nov 2024

The development of multimessenger astrophysics allows us to probe various background particles from the distant early universe. Up to now, much effort has been made researching the emission and radiation of diverse steady or transient astrophysical sources. We review the potential accelerating, escaping, [...] Read more.

The development of multimessenger astrophysics allows us to probe various background particles from the distant early universe. Up to now, much effort has been made researching the emission and radiation of diverse steady or transient astrophysical sources. We review the potential accelerating, escaping, propagating, and radiation process of high-energy particles under specific circumstances for regular astrophysical sources and briefly discuss the underlying contribution from their emissions to the intensity of ultrahigh-energy cosmic ray, TeV-PeV cosmic neutrino, and the diffuse gamma-ray background, aiming to find a possible common origin. Full article

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5 pages, 600 KiB

Open AccessCommunication

Stellar Ages of TESS Stars, Adopting Spectroscopic Data from Gaia GSP-Spec

by Elisa Denis, Patrick de Laverny, Andrea Miglio, Alejandra Recio-Blanco, Pedro Alonso Palicio, Josefina Montalban and Carlos Abia

Galaxies 2024, 12(6), 76; https://doi.org/10.3390/galaxies12060076 - 14 Nov 2024

The Gaia DR3 GSP-spec/TESS (GST) catalog combines asteroseismic data from NASA’s TESS mission with spectroscopic data from ESA’s Gaia mission, and contains about 116,000 Red Clump and Red Giant Branch stars, surpassing previous datasets in size and precision. The Bayesian [...] Read more.

The Gaia DR3 GSP-spec/TESS (GST) catalog combines asteroseismic data from NASA’s TESS mission with spectroscopic data from ESA’s Gaia mission, and contains about 116,000 Red Clump and Red Giant Branch stars, surpassing previous datasets in size and precision. The Bayesian tool PARAM is used to estimate stellar ages using MESA models for, currently, 30,297 stars. This GST catalog, which includes kinematics and chemical information, is adopted for studying the Milky Way’s structure and evolution, in particular its thin and thick disk components. Full article

(This article belongs to the Special Issue Nucleosynthesis and Dust Production in AGB Stars: Open Issues and Future Perspectives)

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13 pages, 1877 KiB

Open AccessArticle

Classification of Variable Star Light Curves with Convolutional Neural Network

by Almat Akhmetali, Timur Namazbayev, Gulnur Subebekova, Marat Zaidyn, Aigerim Akniyazova, Yeskendyr Ashimov and Nurzhan Ussipov

Galaxies 2024, 12(6), 75; https://doi.org/10.3390/galaxies12060075 - 12 Nov 2024

The classification of variable stars is essential for understanding stellar evolution and dynamics. With the growing volume of light curve data from extensive surveys, there is a need for automated and accurate classification methods. Traditional methods often rely on manual feature extraction and [...] Read more.

The classification of variable stars is essential for understanding stellar evolution and dynamics. With the growing volume of light curve data from extensive surveys, there is a need for automated and accurate classification methods. Traditional methods often rely on manual feature extraction and selection, which can be time-consuming and less adaptable to large datasets. In this work, we present an approach using a convolutional neural network (CNN) to classify variable stars using only raw light curve data and their known periods, without the need for manual feature extraction or hand-selected data preprocessing. Our method utilizes phase-folding to organize the light curves and directly learns the variability patterns crucial for classification. Trained and tested on the Optical Gravitational Lensing Experiment (OGLE) dataset, our model demonstrates an average accuracy of 88% and an F1 score of 0.89 across five well-known classes of variable stars. We also compared our classification model with the Random Forest (RF) classifier and showed that our model gives better results across all of the classification metrics. By leveraging CNN, our approach does not need manual feature extraction and can handle diverse light curve shapes and sampling cadences. This automated, data-driven method offers a powerful tool for classifying variable stars, enabling efficient processing of large datasets from current and future sky surveys. Full article

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19 pages, 637 KiB

Open AccessReview

An Objective Classification Scheme for Solar-System Bodies Based on Surface Gravity

by Dimitris M. Christodoulou, Silas G. T. Laycock and Demosthenes Kazanas

Galaxies 2024, 12(6), 74; https://doi.org/10.3390/galaxies12060074 - 6 Nov 2024

We introduce succinct and objective definitions of the various classes of objects in the solar system. Unlike the formal definitions adopted by the International Astronomical Union in 2006, group separation is obtained from measured physical properties of the objects. Thus, this classification scheme [...] Read more.

We introduce succinct and objective definitions of the various classes of objects in the solar system. Unlike the formal definitions adopted by the International Astronomical Union in 2006, group separation is obtained from measured physical properties of the objects. Thus, this classification scheme does not rely on orbital/environmental factors that are subject to debate—the physical parameters are intrinsic properties of the objects themselves. Surface gravity g is the property that single-handedly differentiates (a) planets from all other objects (and it leaves no room for questioning the demotion of Pluto), and (b) the six largest (

g > 1

m

s^{- 2}

) of the large satellites from dwarf planets. Large satellites are separated from small satellites by their sizes and masses/densities, which may serve as higher-order qualifiers for class membership. Size considerations are also sufficient for the classification of (i) main-belt asteroids (except possibly Ceres) as small solar-system bodies similar in physical properties to the small satellites; and (ii) a group of large Kuiper-belt objects as dwarf planets similar in physical properties to the large (but not the largest) satellites in our solar system. The selection criteria are simple and clear and reinforce the argument that body shape and environmental factors need not be considered in stipulating class membership of solar as well as extrasolar bodies. Full article

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8 pages, 1261 KiB

Open AccessCommunication

Spectroscopy of a Sample of RV Tauri Stars Without IR Excess

by Kārlis Puķītis and Karina Korenika

Galaxies 2024, 12(6), 73; https://doi.org/10.3390/galaxies12060073 - 6 Nov 2024

We observed high-resolution optical spectra of 11 RV Tauri stars without IR excess, with the primary goal of searching for chemical depletion patterns. Using equivalent widths of absorption lines, we calculated the photospheric parameters and chemical element abundances for five stars in the [...] Read more.

We observed high-resolution optical spectra of 11 RV Tauri stars without IR excess, with the primary goal of searching for chemical depletion patterns. Using equivalent widths of absorption lines, we calculated the photospheric parameters and chemical element abundances for five stars in the sample: HD 172810, V399 Cyg, AA Ari, V457 Cyg, and V894 Per. Only the abundance pattern of V457 Cyg suggests depletion. In the spectrum of this star, TiO lines are also observed in the emission, in addition to metal emissions. V457 Cyg is likely a binary system that was once surrounded by a circumbinary disc. In the spectrum of V894 Per, we find a set of spectral lines that appear to belong to another star, corroborating that it is an eclipsing variable rather than an RV Tauri star. The high overabundance of sodium may result from mass transfer within the binary system. Full article

(This article belongs to the Special Issue Nucleosynthesis and Dust Production in AGB Stars: Open Issues and Future Perspectives)

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