Astronomy

15 pages, 2712 KiB

Open AccessReview

by Donald V. Reames

Astronomy 2025, 4(1), 5; https://doi.org/10.3390/astronomy4010005 - 18 Mar 2025

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High-energy particles may be accelerated widely in stellar coronae; probably by the same processes we find in the Sun. Here, we have learned of two physical mechanisms that dominate the acceleration of solar energetic particles (SEPs). The highest energies and intensities are produced in “gradual” events where shock waves are driven from the Sun by fast and wide coronal mass ejections (CMEs). Smaller, but more numerous “impulsive” events with unusual particle compositions are produced during magnetic reconnection in solar jets and flares. Jets provide open magnetic field lines where SEPs can escape. Closed magnetic loops contain this energy to produce bright, hot flares; perhaps even contributing to heating the low corona in profuse nanoflares. Streaming protons amplify Alfvén waves upstream of the shocks. These waves scatter and trap SEPs and, in large events, modify the element abundances and flatten the low-energy spectra upstream. Shocks also re-accelerate the residual ions from earlier impulsive events, when available, that characteristically dominate the energetic heavy-ion abundances. The large CME-driven shock waves develop an extremely wide longitudinal span, filling much of the inner heliosphere with energetic particles. Full article

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10 pages, 428 KiB

Open AccessArticle

Deciphering the Electron Spectral Hardening in AMS-02

by Carmelo Evoli

Astronomy 2025, 4(1), 4; https://doi.org/10.3390/astronomy4010004 - 28 Feb 2025

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Abstract

We analyze the electron cosmic-ray spectrum from AMS-02, focusing on the spectral hardening around 42 GeV. Our findings confirm that this feature is intrinsic to the primary electron component rather than a byproduct of contamination from primary positron sources. Even under conservative assumptions, [...] Read more.

7 σ

, strongly indicating a genuine spectral break. Accordingly, we introduce a new, more realistic parametric fit, which we recommend for the next round of AMS-02 analyses. Once the sources of systematic uncertainties are better constrained, this refined approach can either reinforce or refute our conclusions, providing a clearer understanding of the observed electron spectrum. If confirmed, we propose that this hardening most likely arises from interstellar transport or acceleration effects. Full article

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14 pages, 4574 KiB

Open AccessArticle

Line Ratio in the C-like Ion Spectrum O III: Testing Atomic Theory Predictions Through the Observation of Galaxies

by Julius Richard Dreisbach, Dominik J. Bomans and Elmar Träbert

Astronomy 2025, 4(1), 3; https://doi.org/10.3390/astronomy4010003 - 10 Feb 2025

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Abstract

The progress made in atomic structure computations has indicated that certain line ratios of forbidden transitions may be slightly different from earlier assumptions. In order to check this theory, we evaluate previous observations of dwarf galaxies by the UVES spectrograph at the VLT telescope on ESO Paranal for the line ratios of branched decays in C-like oxygen ions [O III] that are insensitive to the local environment. Our findings show that the observed line ratio for [O III] (

r = 3.005 \pm 0.237

) aligns with recent theoretical predictions based on more sophisticated models, while it deviates from older computations. Additionally, the analysis of line profiles suggests that, in some cases, the spectral resolution was insufficient to fully resolve dynamic substructures within the galaxies. Our results emphasize the importance of improved data quality and consistency for future studies, especially for future searches of finestructure constant variations at higher redshifts using this method. Full article

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20 pages, 593 KiB

Open AccessArticle

Tracing the Evolution of the Emission Properties of Carbon-Rich AGB, Post-AGB, and PN Sources

by Silvia Tosi and Ester Marini

Astronomy 2025, 4(1), 2; https://doi.org/10.3390/astronomy4010002 - 20 Jan 2025

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Abstract

Understanding the transition from the Asymptotic Giant Branch (AGB) to the Planetary Nebula (PN) phase is crucial for advancing our knowledge of galaxy evolution and the chemical enrichment of the universe. In this manuscript, we analyze 137 carbon-rich, evolved low- and intermediate-mass stars (LIMSs) from both the Magellanic Clouds (MCs) and the Milky Way (MW). We focus on AGB, post-AGB, and PN sources, tracing the evolution of their emission through spectral energy distribution (SED) modeling. Consistent with previous studies, we observe that more evolved LIMSs exhibit cooler dust temperatures and lower optical depths. Amorphous carbon (amC) is the dominant dust species in all the evolutionary stages examined in this work, while silicon carbide (SiC) accounts for 5–30% of the total dust content. Additionally, we analyze color–color diagrams (CCDs) in the infrared using data from IRAC, WISE, and 2MASS, uncovering significant evolutionary trends in LIMS emission. AGB stars evolve from bluer to redder colors as they produce increasing amounts of dust. Post-AGB and PN sources are clearly differentiated from AGB stars, reflecting shifts in both effective stellar and dust temperatures as the stars transition through these evolutionary phases. Full article

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11 pages, 347 KiB

Open AccessArticle

An Investigation of the Entropy Associated with a Collapsing Molecular Cloud

by Gemechu Muleta Kumssa

Astronomy 2025, 4(1), 1; https://doi.org/10.3390/astronomy4010001 - 17 Jan 2025

Viewed by 683

Abstract

The investigation of entropy variation during the star formation process within collapsing molecular clouds represents a significant field of inquiry in astrophysics. As the cloud contracts, the presence of gaseous components contributes to an increase in entropy; however, the degree of this entropy change is contingent upon the spatial constraints imposed on the gases. In this research endeavor, I perform a comprehensive analysis of entropy dynamics on a microcosmic level within the contracting cloud, adhering to the tenets of the second law of thermodynamics. The initial focus centers on a turbulent cloud consisting of N particles, each with a mass of M, which succumbs to gravitational forces. It becomes evident that for the collapse to transpire, the gravitational energy must surpass the opposing pressure forces, resulting in the swift movement of particles throughout the cloud and ultimately facilitating a shift in entropy. Full article

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Astronomy, Volume 4, Issue 1 (March 2025) – 5 articles

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