CyberSec Research

Using NIRSpec on JWST, we studied a sample of 15 intermediate-mass (1.8-4.1 Msun) young stellar objects (YSOs) previously identified with MIRI photometry in the low-metallicity NGC 346 star-forming cluster in the Small Magellanic Cloud (SMC). All objects, observed in the 1.7-5.3 micron range, show strong hydrogen recombination lines in the Paschen, Brackett, Pfund, and Humphreys series, confirming their very young ages. The spectra of 11 YSOs show prominent absorption bands from the three most important ice species (H2O, CO2, CO), marking the first detection of these ices in intermediate-mass YSOs beyond our Galaxy. In three YSOs, water ice appears to be in crystalline form. In some objects, we also detect 13CO2 and OCS ices -- never before observed beyond the Milky Way (MW) -- and methanol ice in at least one star. We compared the column densities of H2O, CO2, and CO ices with those measured in more and less massive protostars in the MW and Large Magellanic Cloud, finding that in NGC 346 ice column densities reach values nearly an order of magnitude lower than in more massive objects (~1x10^{17} cm-2 for water and ~1x10^{16} cm-2 for CO2 and CO). However, the relative proportions of the ice species abundances do not differ from those in massive MW YSOs. This suggests that metallicity may not significantly affect ice chemistry in protoplanetary discs and that, shielded by the protostellar envelope or deep in the midplane, circumstellar material is likely impervious to the radiation environment.

We develop the Kazantsev theory of small-scale dynamo generation at small Prandtl numbers near the generation threshold and restore the concordance between the theory and numerical simulations: the theory predicted a power-law decay below the threshold, while simulations demonstrate exponential decay. We show that the exponential decay is temporary and owes its existence to the flattening of the velocity correlator at large scales. This effect corresponds to the existence of a long-living virtual level in the corresponding Schrodinger type equation. We also find the critical Reynolds number and the increment of growth/decay above and under the threshold; we express them in terms of the quantitative characteristic properties of the velocity correlator, which makes it possible to compare the results with the data of different simulations.

Intensity interferometry (II) offers a powerful means to observe stellar objects with a high resolution. In this work, we demonstrate that II can also probe internal stellar kinematics by revealing a time-asymmetric Hanbury Brown and Twiss (HBT) effect, causing a measurable shift in the temporal correlation peak away from zero delay. We develop numerical models to simulate this effect for two distinct astrophysical scenarios: an emission-line circumstellar disk and an absorption-line binary system. Our simulations reveal a clear sensitivity of this temporal asymmetry to the system's inclination angle, velocity symmetry, and internal dynamics. This suggests that, with sufficiently high time resolution, II can be used to extract quantitative information about internal kinematics, offering a new observational window on stellar dynamics.

The macroscopic model for a neutron star (NS) as a perfect liquid drop at the equilibrium is extended to rotating systems with a small frequency $\omega$ within the effective-surface (ES) approach. The NS angular momentum $I$ and moment of inertia (MI) for a slow stationary azimuthal rotation around the symmetry axis is calculated by using the Kerr metric approach in the Boyer-Lindquist and Hogan forms for the perfect liquid-drop model of NSs. The gradient surface terms of the NS energy density $\mathcal{E}(\rho)$ [Equation of State] are taken into account along with the volume ones at the leading order of the leptodermic parameter $a/R \ll 1$, where $a$ is the ES crust thickness and $R$ is the mean NS radius. The macroscopic NS angular momentum $I$ at small frequencies $\omega$, up to quadratic terms, can be specified for calculations of the adiabatic MI, $\Theta=d I/d \omega$, by using Hogan's inner gravitational metric, $r\le R$. The NS MI, $\Theta=\tilde{\Theta}/(1-\mathcal{G}_{t\varphi})$, was obtained in terms of the statistically averaged MI, $\tilde{\Theta}$, and its time and azimuthal angle correlation, $\mathcal{G}_{t\varphi}$, as sumes of the volume and surface components. The MI $\Theta$ depends dramatically on its effective radius $R$ because of a strong gravitation. We found the significant shift of the Schwarzschild radius $R_{\rm S}$ to a much smaller position due to the time and azimuthal correlation term $\mathcal{G}_{t\varphi}$. The adiabaticity condition is carried out for several neutron stars in a strong gravitation case.

Coronal loops are plasma structures in the solar atmosphere with temperatures reaching millions of Kelvin, shaped and sustained by the magnetic field. However, their morphology and fundamental nature remain subjects of debate. By studying their cross-sectional properties and how they change along the loop and in time, we can understand their magnetic structure and heating mechanisms. In this study, we investigated the cross-sectional intensity profiles, both spatially and temporally, of two unique coronal loops, observed in the periphery of two distinct active regions by the Extreme Ultraviolet Imager (EUI/HRI$_{\rm EUV}$) on board the Solar Orbiter spacecraft. The main results of this study are 1. The lifetimes of these two loops (loop1 > 120 min \& loop2 > 50 min) are longer than the typical timescales of radiative cooling and thermal conduction. 2. Their widths determined by the FWHM of the single Gaussian fit to the cross-axis intensity profiles are greater than 6-7 pixels of EUI/HRI$_{\rm EUV}$, indicating that the loop cross-section is uniformly filled on well-resolvable scales. 3. These loops exhibited an almost constant width, both spatially and temporally (width for loop1 is 2.1 $\pm$ 0.4 Mm and for loop2 is 1.3 $\pm$ 0.2 Mm), indicating that they are stable non-expanding structures. 4. We present observational evidence that the one of the loops (loop2) is not braided, which strongly suggests that the non-expanding nature of this multi-stranded loop along its length cannot be attributed to the twist of the magnetic field lines. In conclusion, we find that these coronal loops are long, stable, multi-stranded, non-expanding structures with a uniform cross-section that persist in the corona for an unusually extended duration. This not only challenges our current understanding of the structure of the coronal magnetic field but also raises critical questions about the mechanisms

The CUbesat Solar Polarimeter (CUSP) project is a CubeSat mission planned for a launch in low-Earth orbit and aimed to measure the linear polarization of solar flares in the hard X-ray band by means of a Compton scattering polarimeter. CUSP will allow us to study the magnetic reconnection and particle acceleration in the flaring magnetic structures of our star. CUSP is a project in the framework of the Alcor Program of the Italian Space Agency aimed at developing new CubeSat missions. It is undergoing a 12-month Phase B that started in December 2024. The Compton polarimeter on board CUSP is composed of two acquisition chains based on plastic scintillators read out by Multi-Anode PhotoMultiplier Tubes for the scatterer part and GAGG crystals coupled to Avalanche PhotoDiodes for the absorbers. An event coincident between the two readout schemes will lead to a measurement of the incoming X-ray's azimuthal scattering angle, linked to the polarization of the solar flare in a statistical manner. The current status of the CUSP mission design, mission analysis, and payload scientific performance will be reported. The latter will be discussed based on preliminary laboratory results obtained in parallel with Geant4 simulations.

The isotopic ratios measured in meteoritic presolar grains are a crucial tool for tracing the nucleosynthetic origin of isotopes. In the case of silicon isotopes, two important indicators to establish the origin of presolar grains are the ratios 29Si/28Si and 30Si/28Si. To constrain theoretical predictions, the rates of key nuclear reactions influencing the abundances of 29Si and 30Si must be well known. One such reaction is 29Si(p,gamma)30P which plays a role in classical nova explosions. The aim of the present work is to determine the nonresonant cross section of the 29Si(p,gamma)30P reaction, which has not been previously measured. The activation method was employed to measure the total cross section at four proton energies between Ep = 1000 and 1430 keV. The measured cross sections were found to be significantly (a factor of 4.3+-0.6) higher than those predicted by theoretical direct capture calculations, thereby impacting the reaction rates at low astrophysical temperatures, below about 30 MK. This higher nonresonant cross section - now based on experimental data - can be used in forthcoming nucleosynthesis calculations of classical novae. As a secondary result, the 16O(p,gamma)17F cross section was also obtained and found to be in good agreement with existing literature data.

This dissertation focuses on the reconstruction of Equations of State (EoSs) describing the interior of compact stars, using modern machine learning and deep learning methods. The pipeline is based on data from mass-radius (M-R) curves, obtained by numerically solving the Tolman-Oppenheimer-Volkoff equations for a wide range of admissible EoSs. The manuscript is divided into a Theoretical Part (Chs. 1-4) and a Computational Part (Chs. 5-7). The theoretical chapters analyze the properties of neutron and quark stars, the physical constraints of viable EoS models, and introduce regression algorithms (Decision Tree, Random Forest, Gradient Boosting, XGBoost) and neural networks with normalization and dropout techniques. The computational part presents the generation of artificial EoSs for hadronic and quark stars (MIT bag, CFL), the numerical solution of the TOV equations, data preparation, and hyperparameter tuning. Results include training and evaluation of models using MSE/MSLE metrics, learning curves for neural networks, and reconstruction of 21 hadronic and 20 quark star EoSs. Source code and tools for reproducibility and future research are provided. The work aims to establish a reusable and scalable framework, strengthening the connection between theoretical astrophysics and computational science.

Massive stars are often born in triples, where gravitational dynamics and stellar interactions play a crucial role in shaping their evolution. One such pathway includes the merger of the inner binary, transforming the system to a binary with a distinct formation history. Therefore, the interpretation of observed binary properties and their inferred formation history may require the consideration of a potential triple origin. We aim to investigate the population of stellar mergers in massive hierarchical triples. Specifically, we assess how frequently mergers occur, and characterise the properties of the post-merger binaries and their subsequent evolution. We combine the triple population synthesis code TRES, which self-consistently models stellar evolution, binary interaction, and gravitational dynamics, with the binary population synthesis code SeBa to simulate 10^5 dynamically stable, massive triples from the zero-age main sequence through merger and post-merger evolution. We explore the effects of a range of physical models for the initial stellar properties, mass transfer, and merger. We find that stellar mergers are a common outcome, occurring in 20-32% of massive triples. Most mergers happen relatively early in the evolution of the system and involve two main-sequence (MS) stars, producing rejuvenated merger remnants that can appear significantly younger than their tertiary companions. Consequently, we predict that 2-10% of all wide MS+MS binaries (P>100 days) have a measurable age discrepancy, and serve as a promising way to identify merged stars. The post-merger systems preferentially evolve into wide, eccentric binaries, with ~80% avoiding further interaction. However, a notable fraction (16-22%) undergoes a second mass-transfer phase, which may result in the formation of high-mass X-ray binaries or mergers of compact objects that spiral in via gravitational-wave emission.

Current models of binary systems often depend on simplified approach of the radiation field, which are unlikely to accurately capture the complexities of asymmetric environments. We investigate the dynamical and chemical implications of a 3D asymmetric radiation field that accounts for the optical properties of sub-structures present in a protoplanetary disk, as well as the inclusion of a secondary radiation source in binary systems. We conducted a series of 3D-SPH hydrodynamical simulations using PHANTOM, coupled with the 3D Monte Carlo radiative transfer code MCFOST, to compute disc temperatures on-the-fly. We explored different binary-disk orientations (0$^o$ and 30$^o$) for an eccentric binary, along with a constant dust-to-gas ratio and dust as a mixture prescription. We also simulated an outburst event as an example of a drastic increase in luminosity. Heating from the secondary star inflates the outer disk, increasing the aspect ratio facing the companion by about 25% in inclined cases compared to 10% in coplanar ones. Dust settling in the mid-plane enhances extinction along the disk plane, making the coplanar case cooler than the inclined one on the side of the disk facing the companion. Besides, heating causes a shift in the snow line for species with freeze-out temperatures below 50 K, depending on the disk-binary inclination and binary phase. During outbursts, the aspect ratio doubles on the star-facing side and increases by 50% on the opposite side in inclined cases. The snow line shift would impact all the species considered in the outburst case. Disk heating in binaries depends on stellar properties, orbital phase, and disk local and global characteristics. This results in temperature asymmetries, especially during secondary star outbursts, leading to variations in aspect ratio and snow lines that can affect chemistry and planet formation.

We report the discovery of two binary systems, each consisting of a slightly bloated G-type main-sequence star and an unseen companion, identified through photometric data from TESS and radial velocity variation from Gaia. High-resolution spectroscopy confirms orbital periods of 1.37 and 2.67 days with circular orbits. The visible components have masses of $\sim 0.9\,M_\odot$, while the minimum masses of the unseen companions are $1.073^{+0.058}_{-0.060} M_\odot$ and $0.919^{+0.049}_{-0.051} M_\odot$, respectively. Assuming tidal synchronization, we estimate the companion masses to be $1.12^{+0.10}_{-0.08} M_\odot$ and $1.02^{+0.15}_{-0.10} M_\odot$. The absence of detectable spectral features from the companions rules out main-sequence stars of these masses, suggesting that the unseen companions are likely O/Ne or C/O massive white dwarfs. The short orbital periods imply that these systems are post-common envelope binaries. Their subsequent evolution is uncertain, with possible outcomes including cataclysmic variables, Type Ia supernovae, or accretion-induced collapse, depending on the nature of future mass transfer.

Context. The radiation field consisting of hydrogen recombination lines and continuum emission might significantly affect the hydrogen-level populations in ultra- and hypercompact (U/HC) H II regions. The escape probability approximation was used to estimate the effect of the radiation field in previous models for calculating hydrogen-level populations. The reliability of this approximation has not been systematically studied, however. Aims. We investigate the appropriate ranges of previous models with the escape probability approximation and without the effects of the radiation field. We create a new model for simulating the integrated characteristics and the spatially resolved diagnostics of the hydrogen recombination lines throughout H II regions. Methods. We developed a new nl model with a full radiative transfer treatment of the radiation field causd by hydrogen recombination lines and continuum emission to calculate the hydrogen-level populations and hydrogen recombination lines. We then compared the level populations and the corresponding hydrogen recombination line intensities simulated by the new model and previous models. Results. We studied the applicability and the valid parameter ranges of previous models. Radiation fields exhibit negligible effects on the level populations in classical and UC H II regions. With the modified escape probability, the model with the escape probability approximation is suitable for most HC H II regions. The improved new model performs better in the HC H II region with an extremely high emission measure. To address the high computational costs inherent in numerical models, we trained a precise machine-learning model to enable a rapid estimation of hydrogen-level populations and the associated hydrogen recombination lines.

The eccentric short-period O-star binary BD+60 497 is an interesting laboratory to study tidal interactions in massive binary systems, notably via the detection and characterisation of apsidal motion. The rate of apsidal motion in such systems can help us constrain their age and gain insight into the degree of mass concentration in the interior of massive stars. Spectroscopic data collected over two decades are used to reconstruct the individual spectra of the stars and to establish their epoch-dependent radial velocities. An orbital solution, explicitly accounting for apsidal motion is adjusted to the data. Space-borne photometric time series are analysed with Fourier methods and with binary models. We derive a rate of apsidal motion of $6.15^{+1.05}_{-1.65}$ degree/yr which suggests an age of $4.13^{+0.42}_{-1.37}$ Myr. The disentangled spectra unveil a curious change in the spectral properties of the secondary star between the epochs 2002-2003 and 2018-2022 with the secondary spectrum appearing of earlier spectral type over recent years. Photometric data show variability at the 6 mmag level on the period of the binary system which is hard to explain in terms of proximity effects. Whilst the rate of apsidal motion agrees well with theoretical expectations, the changes in the reconstructed secondary spectrum hint at a highly non-uniform surface temperature distribution for this star. Different effects are discussed that could contribute to the photometric variations. The currently most-likely explanation is a mix of proximity effects and tidally excited oscillations

We run numerical simulations to study high-power wind accretion in a massive binary system during a high mass loss event. The system consists of an evolved primary star with a zero age main sequence mass of $ M_{1} = \rm 100~M_{\odot}$ and a hot secondary star with a mass ranging from $ M_{2} = \rm 30-80~M_{\odot}$, orbiting in a circular orbits with periods between 455 and 1155 days. We initiate a weak eruption event with mass loss at a rate of $10^{-3}~\rm {M_{\odot}}\rm~yr^{-1}$ for 1.5 years. During this event, a fraction of the mass lost by the primary is accreted onto the secondary, with the accretion rate being dependent on the orbital and stellar parameters. From the set of simulations, we derive an analytical relation describing the dependence of the mass accretion rate on the orbital period and stellar mass ratio. We also identify the transitional orbital period for which Roche lobe overflow begins to dominate over wind accretion. We find that accretion leads to a reduction in the effective temperature of the secondary star. However, the mass average accretion rate we obtain in the simulations is low enough for the secondary to remain in thermal equilibrium and avoid radial expansion.

Periodic orbits (POs) play a central role in the circular restricted three-body problem (CRTBP). This paper introduces a method to search for POs by identifying single- and multiple-revolution fixed points in chosen Poincare maps that describe the CRTBP dynamics, with a theoretical capability to detect all fixed points across arbitrary revolution counts exhaustively.First, high-order transfer maps (HOTMs), represented as polynomials, are constructed within the differential algebra (DA) framework for both planar and spatial CRTBP to map states between successive Poincare section crossings, with the Jacobi constant used to reduce the number of independent variables. Next, an automatic domain splitting (ADS) strategy is employed to generate subdomains, preserving HOTM accuracy, with an integrated feasibility estimation to reduce ADS's computation burden.Then, a two-stage HOTM-based polynomial optimization framework is introduced, first identifying combinable subdomain sequences and then refining the fixed point solutions. Finally, the method is applied to the Earth-Moon CRTBP, identifying POs up to nine revolutions in the planar case and four in the spatial case. Known families such as distant retrograde orbits (DROs) and Lyapunov orbits are recovered, along with a previously undocumented family that exhibits a hybrid character between DROs and Lyapunov orbits.

Stellar rotation is a fundamental parameter governing a star's magnetic activity and evolution. The Transiting Exoplanet Survey Satellite (TESS) provides high-precision photometric data ideal for measuring rotation periods via brightness modulations from starspots. This paper presents a detailed analysis of the star TIC 445493624 using 2-minute cadence data from TESS Sector 58. We process the light curve using a custom pipeline to perform outlier removal, binning, and Savitzky-Golay detrending to isolate the stellar variability. A Lomb-Scargle periodogram of the cleaned data reveals a single, dominant periodic signal at 3.638 days with a power of 0.43, corresponding to a negligible false-alarm probability. The phase-folded light curve at this period is highly coherent and exhibits a stable, non-sinusoidal morphology indicative of large-scale magnetic features or spot groups.

Magnetic reconnection is a fundamental and omnipresent energy conversion process in plasma physics. Novel observations of fields and particles from Parker Solar Probe (PSP) have shown the absence of reconnection in a large number of current sheets in the near-Sun solar wind. Using near-Sun observations from PSP Encounters 4 to 11 (Jan 2020 to March 2022), we investigate whether reconnection onset might be suppressed by velocity shear. We compare estimates of the tearing mode growth rate in the presence of shear flow for time periods identified as containing reconnecting current sheets versus non-reconnecting times, finding systematically larger growth rates for reconnection periods. Upon examination of the parameters associated with reconnection onset, we find that 85% of the reconnection events are embedded in slow, non-Alfvenic wind streams. We compare with fast, slow non-Alfvenic, and slow Alfvenic streams, finding that the growth rate is suppressed in highly Alfvenic fast and slow wind and reconnection is not seen in these wind types, as would be expected from our theoretical expressions. These wind streams have strong Alfvenic} flow shear, consistent with the idea of reconnection suppression by such flows. This could help explain the frequent absence of reconnection events in the highly Alfvenic, near-Sun solar wind observed by PSP. Finally, we find a steepening of both the trace and magnitude magnetic field spectra within reconnection periods in comparison to ambient wind. We tie this to the dynamics of relatively balanced turbulence within these reconnection periods and the potential generation of compressible fluctuations.

Asteroseismology, the study of stellar oscillations, and stellar modeling both offer profound insights into the fundamental properties and evolution of stars. With pySYD, a new open-source Python package, we were able to constrain the asteroseismic global parameters, $\nu_{max}$ and $\Delta\nu$, for 82 solar-like oscillating subgiant and lower red giant stars, filling in the region between the Kepler dwarfs and giants. Using asteroseismic scaling relations, we were able to compute seismic masses, radii, and surface gravities for our entire sample with average errors of 0.21 $M_{\bigodot}$, 0.27 $R_{\bigodot}$, and 0.06 dex respectively. Using 4 stellar modeling grids we determine and compare stellar ages for our sample. We find that our age distribution from stellar modeling is consistent with other local star samples. We find small consistent offsets from model predictions across our regime, but offsets were worse at higher gravities (log(g) $\geq$ 3.5 dex), suggesting the need for better calibration. Finally, we discuss our sample in the context of galactic archaeology and show how ages like these could be used to identify and study binary system evolution and galactic evolution in the future. All in all, we show that asteroseismology can be successfully performed with TESS data and can continue to make an impact on our understanding of stellar physics and galactic archaeology.