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Dionysios Gakis

Publications and source records attributed to Dionysios Gakis.

8 recordsLinked to original sources

Echoes of Star Stories: Integrating Indigenous narratives into modern stellar astrophysics

Indigenous narratives have long preserved observations of celestial phenomena, offering insights that resonate with modern astrophysical research. Deeply embedded in cultural traditions, these stories describe events such as stellar variability, supernovae, eclipses, and planetary alignments. Indigenous communities have also used the stars for navigation and calendar systems, reflecting a sophisticated understanding of celestial patterns. These narratives not only complement historical records but offer human-centric perspectives on stellar life cycles that often parallel modern models. Drawing from traditions from diverse Indigenous communities - including Aboriginal Australians, Pueblo peoples, Inuit, and Polynesians - this paper highlights the profound connections between Indigenous knowledge and stellar astrophysics. Integrating these traditions acknowledges their value, safeguards their relevance in the Space Age, and fosters mutual learning through cross-cultural collaboration. We explore how such narratives can enrich education, public engagement, and even inspire scientific hypotheses, bridging cultural heritage and modern astrophysics.

astro-ph.SR

Modeling the effectiveness of radiation shielding materials for astronaut protection on Mars

The surface of Mars is bombarded by energetic charged particles of solar and cosmic origin with little shielding compared to Earth. As space agencies are planning for crewed missions to the red planet, a major concern is the impact of ionizing radiation on astronaut health. Keeping exposure below acceptable radiation dose levels is crucial for the health of the crew. In this study, our goal is to understand the radiation environment of Mars and describe the main strategies to be adopted to protect astronauts from the harmful impacts of cosmic radiation. Specifically, we investigate the shielding properties of various materials in the Martian radiation field using the Geant4 numerical model, after validating its accuracy with in-situ instrument measurements by MSL RAD. Our results indicate that composite materials such as types of plastic, rubber or synthetic fibers, have a similar response against cosmic rays and are the best shields. Martian regolith has an intermediate behavior and therefore could be used as an additional practical option. We show that the most widely used aluminum could be helpful when combined with other low atomic number materials.

astro-ph.EP

Revisiting thermoelectric effects in the crust of neutron stars

Context. Large thermal variations have been observed in neutron stars that typically are not aligned with density gradients. Such terms may activate the Biermann battery effect, leading to thermoelectric interactions and to the generation of electromotive force. Aims. We aim to identify the possible impact of a temperature anisotropy on the crust of a neutron star can have in the evolution of its magnetic field, through the thermoelectric terms. Methods. We consider a neutron star crust with large temperature gradients, associated with long-lived hot spots, described by a localized Gaussian-type function. We simulate the interplay between the battery term and the Hall and Ohmic evolution numerically for axisymmetric systems. Results. The results indicate that for crust temperatures of $\sim$$10^9$ K the toroidal field can be amplified up to $\sim$$10^{14}-10^{15}$ G near the points of maximum temperature gradients, and it locally changes the architecture of the poloidal field lines. For internal crustal temperatures of $\sim$$10^8$ K, the temperature gradient generates fields that are lower by about two orders of magnitude. In these cases, saturation is achieved after some hundred thousand years, after which the battery and Ohmic dissipation balance each other, whereas the Hall drift contributes comparatively little to the final field strength, but it can affect its structure. Conclusions. We conclude that the thermoelectric effect can impact the overall magnetic field evolution, provided that the thermal gradient is maintained for a sufficiently long time. Neutron stars endowned with moderate-strength magnetic fields may be affected by the thermoelectric effect if the hotspots survive for timescales of a few kiloyears.

astro-ph.HE

Orbital analysis of the Pluto-Charon moon system's mutual interactions and forced frequencies

Context. The orbits of the four small moons in the Pluto-Charon system, Styx, Nix, Kerberos and Hydra, are circumbinary, as the former form a binary dwarf planet. Consequently, the orbit of each one of them is characterized by a number of frequencies, arising by the central binary and the mutual gravitational interactions. Aims. In this work, we identify the most prominent of these forced frequencies using Fast Fourier Transformations. Methods. Two methods are implemented, a semi-analytic and a numerical one, and comparisons are being made. Results. The results indicate that as a first approximation, moon orbits may well be modelled as the superposition of a series of inevitable oscillations, induced by Pluto and Charon, deviating from circular ones, even if the eccentricity is set to zero. Moreover, the mutual gravitational effects are significant in their long term evolution, especially for the lighter moons Styx and Kerberos, activating modes that dominate the low-frequency region of the power spectrum. This becomes evident through the comparison of simulations where only one moon is included along with the binary dwarf planet and simulations of the entire six-body system. These modes become noticeable over long integration times and may affect the orbits of the lighter moons of the system.

astro-ph.EP

Crewed Missions to Mars: Modeling the Impact of Astrophysical Charged Particles on Astronauts and Their Health

The impact of exposure to astrophysical ionizing radiation on astronaut health is one of the main concerns in planning crewed missions to Mars. Astronauts will be exposed to energetic charged particles from Galactic and Solar origin for a prolonged period with little protection from a thin spacecraft shield in transit and from the rarefied Martian atmosphere when on the surface. Adverse impacts on astronaut health include, for example, Acute Radiation Syndrome, damage to the nervous system, and increased cancer risk. We rely on medical studies to assess the impact of enhanced radiation dose levels on various physiological systems and the overall health of astronauts. Using a combination of radiation measurements and numerical modeling with the GEANT4 package, we calculate the distribution of radiation dose in various human body organs for various expected scenarios simulated with a model human phantom. We suggest mitigation strategies, such as improved ways of shielding and dietary supplements, and make recommendations for the safety of astronauts in future crewed missions to Mars.

physics.space-ph

Orbits of the TOI-1338 and TIC-172900988 systems

Recent observations by TESS revealed the existence of circumbinary planets in the systems of TOI-1338 and TIC-172900988. The purpose of this work is to model the planetary orbits in these two systems and study them under the perspective of previous theoretical models. Each planet's distance from the barycenter through time is simulated using n-body integrations and is compared with outcomes from a semi-analytic, a geometric and a Keplerian-based approach. Furthermore, we infer the most prominent frequencies of both planets' orbits induced by the central binaries. We confirm that both systems appear to be stable. Lastly, we examine the implications of an additional candidate planet in TOI-1338 system finding that an extra, 48 M$_\oplus$ planet that has been hinted from observations could be located at 0.8 AU without generating any radical changes to the orbits of the other members of the system.

astro-ph.EP

Radiation protection and shielding materials for crewed missions on the surface of Mars

A potential crewed mission to Mars would require us to solve a number of problems, including how to protect astronauts against the devastating effects of energetic charged particles from Solar and Galactic sources. The radiation environment on Mars is of particular interest, since maintaining optimal absorbed doses by astronauts is crucial to their survival. Here, we give an overview of the conditions on Mars, as determined by theoretical models and in-situ measurements, and present the main proposed strategies to mitigate radiation exposure while on Mars. Specifically, we focus on the passive shielding technique. Several widely used materials, along with some innovative ones and combinations of those, are studied for their behavior against Solar Energetic Particle Events and Galactic Cosmic Rays in the Martian environment. For that purpose, we implement the GEANT4 package, a Monte-Carlo numerical model developed by CERN, which is specifically applied to simulate interactions of radiation with matter. A description of our model will be given, followed by outputs of the numerical model. We conclude that hydrogen-rich materials act as better attenuators, as expected, but other materials can be helpful against cosmic rays too.

astro-ph.EP

Orbit determination of the moons of the Pluto-Charon system

Four small moons (Styx, Nix, Kerberos and Hydra) are at present known to orbit around the barycenter of Pluto and Charon, which are themselves considered a binary dwarf-planet due to their relatively high mass ratio. The central, non-axisymmetric potential induces moon orbits inconvenient to be described by Keplerian osculating elements. Here, we report that observed orbital variations, may not be the result of orbital eccentricities or observational uncertainties, but may be due to forced oscillations caused by the central binary. We show, using numerical integration and analytical considerations, that the differences reported on their orbital elements, may well arise from this intrinsic behavior rather than limitations on our instruments.

astro-ph.EP