Abstract based on: arxiv.org/abs/2410.02620

In a recent work, \citet{ogawa2024celestial} proposed a model for
celestial conformal field theory (CFT) based on the $H_{3}^{+}$-Wess-Zumino-Novikov-Witten
(WZNW) model. In this paper, we extend the model advanced by \citet{ogawa2024celestial},
demonstrating how it can holographically generate tree-level MHV scattering
amplitudes for both gluons and gravitons when analytically continued
to the ultra-hyperbolic Klein space $\mathbf{R}_{2}^{2}$, thereby
offering an alternative to celestial Liouville theory. We construct
a holographic dictionary in which vertex operators and conformal primaries
in celestial CFT are derived from their worldsheet counterparts in
Euclidean $AdS_{3}$ (bosonic) string theory. Within this dictionary,
we derive the celestial stress-energy tensor, compute the two- and
three-point functions, and determine the celestial operator product
expansion (OPE). Additionally, we derive a system of partial differential
equations that characterises the celestial amplitudes of our model,
utilising the Knizhnik--Zamolodchikov (KZ) equations and worldsheet
Ward identities. In the Appendix, we provide a concise introduction
to the $H_{3}^{+}$-WZNW model, with emphasis on its connection to
Euclidean $AdS_{3}$ string theory.

This talk is structured as follows. In Section \ref{sec:Holographic-Reconstruction-of},
we begin by stating the postulates of our holographic dictionary,
and move to the derivation of its consequences, starting with the
holographic derivation of tree-level MHV amplitudes for both pure
Yang-Mills theory and Einstein's gravity. In Section \ref{sec:Correlation-Functions,-Operator},
we will demonstrate how our construction uniquely determines the two-
and three-point functions as well as the celestial OPE. Additionally,
we will examine a system of partial differential equations characterising
the celestial amplitudes, derived from our holographic dictionary
in frequency space, and, in Section \ref{sec:Discussion}, we summarise
our findings and discuss potential avenues for further research stemming
from this work.

Observations across multiple cosmological probes indicate that the Universe possesses a non-zero cosmological constant, typically associated with vacuum energy density. However, the large discrepancy between observationally inferred and theoretically predicted vacuum energy densities, together with recent analyses in which combined SnIa, H(z), BAO, LSS, BBN, and CMB datasets mildly prefer models with a time-dependent cosmological parameter [1], shows the need to re-examine the assumption that Λ represents the constant vacuum energy density.

In this study, interaction field equations derived from an action incorporating bulk influence, which reduce to the Einstein field equations in the flat-background limit [2], are utilized. A comparison of this set of equations reveals that the effective cosmological parameter is associated with the normalized bulk conformal curvature term, giving rise to an evolving effective Λ influenced by the vacuum energy structure rather than by a fixed density. This framework provides a new perspective on the interpretation of the cosmological parameter and offers a natural reframing of the cosmological constant problem, in which the small observed value of Λ can be understood as reflecting the weak curvature of the bulk while remaining consistent with current observational trends.

References

[1] Solà Peracaula, J., Gómez-Valent, A., de Cruz Pérez, J., & Moreno-Pulido, C. (2023). Running vacuum in the Universe: Phenomenological status in light of the latest observations, and its impact on the σ8 and H0 tensions. Universe, 9(6), 262. https://doi.org/10.3390/universe9060262

[2] Al-Fadhli, M. B. (2023). Gravitational, Electromagnetic and Quantum Interaction: From String to Cloud Theory. Presented at the 2nd Electronic Conference on Universe, 16 February–2 March 2023. Phys. Sci. Forum, 7(1), 55. https://doi.org/10.3390/ECU2023-14063

Antares, a long-period variable supergiant, has been known since antiquity. It is the hearth of the Scorpion constellation. It is red and rivals Mars (Ares in Greek), from which its name is derived.

The study of its light curve is mostly based on Northern hemisphere observations, when the star is low above the Summer Southern horizon.
Its nature as a long-period variable star is therefore not fully known due to the incompleteness of the long-term sample available. Catalogues and manuals of the 20^th century classified Antares as an Irregular Variable.

The photometric observations of Antares are complicated by the absence of nearby stars of comparable luminosity and by heavy airmasses in the line of sight. The AAVSO database reports for Antares only 1/18 observations of the other naked-eye supergiant Betelgeuse, before its 2020 great dimming.

The application of airmass correction to naked-eye observations with distant comparison stars is required for Antares, but it is not yet a common practice among scientists.

We examine the AAVSO V-band data and our AAVSO-SGQ series (2012-2025) of visual observations, complementing thirty years (1995-2025) of SOHO observations made with the coronograph LASCO C3, each year on 2^nd December, to have all stars within 1° of C3 field of view, with vignetting and diffraction acting as systematic effects.

Antares variability measured with SOHO encompasses 0.4 magnitudes and 30 years. V-band data start in 1983 and have large gaps in time and range over 0.9 magnitudes. SGQ series spans over 0.6 magnitudes, with the same personal equation. A Markovian process may affect the visual data, showing an overall brightening in the last decade, partially explainable with a small variability of Spica, used as reference star.

The detection of periods and their modulations is possible by integrating SOHO with other independent measures, and we discuss some preliminary analyses.

After the celebrated active region (AR) 3664, whose flares produced aurorae down to the Mediterranean area on 10-11 May 2024, AR3697 had a rhythmic behavior, which allowed us to predict flares’ timings with accuracy down to the hour in June 2024. Similar behavior was registered in AR4274 and AR4299 in November-December 2025, and again it produced the largest flare of 2025, on 11 November 2025, with geo-effectiveness again reaching locations significantly far from the magnetic poles.
Moreover, these active regions maintained their characteristics (timing and flaring directions) after two whole solar rotations.

An elevated degree of sympatheticity with nearby AR has been also verified: in nearby regions, they triggered other flares or they were excitated to flare, with a connecting time of up to one hour.
Sympathetic flares are usually considered when they are less than 20 minutes apart, but the predictability of such events extended this timescale.

The flaring events of 8 December 2025 with the AR4299 and the nearby giant complex AR4294-96-98 are analyzed in detail, with particular attention given to the sequence M1.1 at 13:05UT in AR4294 and C4.82 in AR4299 at 14:06UT: the propagation of the perturbing wave was visible through the excitation of spotless plages between the two classified ones. These observations were made with the H-alpha telescope.

The cooling phases of the flare X1.95 from AR4299 and M5.05, X1.2 and X5.16, sources of Solar Particle Events (SPEs) from AR 4274 (the same region one rotation before), and the one M9.75 of AR3697 (SPE and Ground Level Enhancement) are also analyzed in X-rays from GOES Satellite data.

High-velocity (25-30000 km/s) lines (HVFs) of Ca II were first discovered in the spectrum of SN1994D in 1999, and since then, they have been proven to be ubiquitous. Despite this, their exact origin remains unclear to this day. They appear to form in a higher-velocity layer above the photosphere (10-15000 km/s) and show varying strengths and velocity evolutions from object to object.

We used early-phase optical and near-infrared spectra of 56 Type Ia supernovae, observed over the span of 15 years by the Hobby–Eberly Telescope (McDonald Observatory, Texas) to determine the velocity, strength, and evolution of these features.

After using SYNOW to model the high-velocity features of these spectra, we compare them to Gaussian fits of the lines. We look at how velocity determination from the Gaussian fitting of spectral lines compares to velocities gained from the modelling of spectra. We confirm a connection between the light-curve width and the strength (and velocity) of the HVFs, with slow decliners showing stronger (and faster) high-velocity lines. We also demonstrate how high-velocity (HV) and normal-velocity (NV) Wang subtypes have differently behaving high-velocity features, which leads to interesting possibilities in the discussion of the progenitor systems of Type Ia supernovae.

The apsidal motion in close eccentric binaries is a means to unveil the internal structure of stars. I make use of it to test the internal mixing processes in stars with the stellar evolution code GENEC.

The apsidal motion is the slow precession of an eccentric orbit with time. Its rate depends on the tidal interactions occurring between the stars through k2, a measure of the star’s inner density profile. The apsidal motion rate is commonly derived from the eclipses’ times of minima, made possible thanks to high precision TESS/Kepler observations. I propose an innovative approach: derive the apsidal motion rate from radial velocities obtained over a long timescale combined with light curves to get high-accuracy consistent physical and orbital parameters for the binaries. I highlight recent results concerning the two most massive binaries studied this way.

Confronted to observations of massive stars, standard non-rotating single star models usually predict stars with too low a density contrast; the well-known k2-discrepancy. I built bespoke GENEC stellar evolution models including tidally-enhanced/suppressed rotational mixing for the twin massive binary HD 152248. The models reveal the instabilities allowing to reproduce the stellar density profiles: advecto-diffusive models better reproduce k2 than magnetic models. A large overshooting is necessary to converge towards the observed k2, yet alone is not sufficient. While a change in metallicity or mass-loss rate has no significant impact on k2, a larger initial helium abundance allows to better reproduce the k2. Yet, a super-solar helium abundance is not observationally supported. These analyses highlight the need for a process in the stars that slows down the increase of their radius with time. It paves the way for the next generation of models.

Bolometric light-curve modeling reveals extremely high ejecta masses in SLSNe-I

I present the bolometric light-curve modeling of 98 hydrogen-poor superluminous supernovae (SLSNe- I) using three power input scenarios of the Minim code: the magnetar model, and the constant-density and the steady-wind versions of circumstellar interaction scenario (CSM models). Quasi-bolometric fluxes were constructed from ZTF g- and r-band photometry, while ejecta velocities were estimated from spectroscopic measurements. The modeling indicates that 14 events favor the magnetar scenario, the light curves of 39 objects are better described by circumstellar interaction, and 45 events show consistent light curves with either mechanism. Magnetar fits yield spin periods and magnetic field strengths in agreement with previous studies, but imply substantially larger ejecta masses. The mean ejecta mass for magnetar-powered models is 34.25 M ☉ (in the range between 1.53 and 198.1 M ☉), while circumstellar interaction models produce even higher values, with mean masses of 116.82 M ☉ for the constant-density case and 105.99 M ☉ for the steady-wind case. These large ejecta masses arise in part from the assumption of an electron-scattering opacity of κ = 0.2 and from higher inferred ejecta velocities. Overall, the results suggest that SLSNe-I, regardless of whether they are powered by a central engine or circumstellar interaction, originate from the explosions of extremely massive progenitor stars.

For many years, detailed observational and statistical study of chemically peculiar stars has shown that the overabundance of heavy radioactive chemical elements in their atmospheres is not well understood. We try to explain the overabundance of a few of them by the explosion of possible supernovae in binary or multiple-stellar systems, i.e., to search for a chemically peculiar star and pulsar pair by traceback motion study in the galaxy. We analyzed trajectories of 529 chemically peculiar stars in our sample and 28 pulsars from the ATNF catalogue, with ages that do not exceed 100 Myr, and obtained several possible candidate pairs (CP star plus pulsar), which may have been at the same place at the same time. Moreover, our traceback study of them with the young stellar clusters (Hunt and Reffert, 2023) led to the identification of several possibly parental stellar groups. However, more detailed studies are needed to estimate the ages of chemically peculiar stars and detect their locations of formation, as well as using statistical methods to assess whether the possible supernova ejection is the result of coincidence or not. Obviously, modeling of the best chemically peculiar star–pulsar pairs will also allow us to explain the processes occurring in these stars.

We present a preliminary analysis of magnetic activity cycles in a sample of solar-type stars for which both primary and possible secondary magnetic cycles were identified from long-term chromospheric activity indicators. Magnetic cycle periods were estimated using time series of the S-index and Bisector Inverse Slope (BIS) for stars selected from the catalogue of Gaia Radial Velocity (RV) standard stars. Significant periodicities were determined using the Generalized Least Squares (GLS) periodogram.
Eight stars in the sample show possible evidence of secondary magnetic cycles. For these stars exhibiting dual-cycle signatures, we computed the period and amplitude ratios to investigate the potential coexistence of distinct dynamo modes. The period ratios span a broad range from 0.3 to 2.2, suggesting both shorter secondary cycles—consistent with the 1.6-year solar quasi-biennial oscillations—and longer secondary cycles relative to the primary one, which may indicate possible cycle dominance switching or beating dynamo behavior. The amplitude ratios range from 0.1 to 1.4, implying that, in some stars, the secondary signal is weak and possibly noise-dominated, while in others it is comparable in strength to the primary cycle.
Although preliminary, these results support the idea that multi-periodic magnetic activity is not uncommon among solar-type stars and may offer valuable constraints for dynamo theory.

The Saha ionization equation is conventionally introduced in the context of astrophysics as a means of calculating ionization balance in stellar atmospheres. However, its origins are thoroughly rooted in classical chemical equilibrium and plasma chemistry. The present study reinterprets the Saha equation as a high-temperature version of normal chemical equilibrium, representing the reversible process of the neutral atom's transformation into an ion and a free electron as a thermodynamically controlled chemical reaction. With the inclusion of temperature, pressure, ionization potential, electron density, and partition functions, the structure of the Saha formulation resembles that of the conventional equilibrium constants and Gibbs free energy relations of chemical thermodynamics.

This chemically based view shows that stellar ionization is not just a physical process but a natural extension of chemical principles operating under extreme conditions. The discussion emphasizes the importance of plasma chemistry in determining the relative populations of atomic and ionic species, consequently dictating opacity, spectral line strengths, and radiative transfer in stellar photospheres and chromospheres. The Saha equation, when viewed from a chemical perspective, offers a unifying framework starting from laboratory thermodynamic behavior through to the astrophysical plasma environment. In this manner, the concept of an astrochemical pathway is reinforced by establishing the continuity between microscopic chemical reactions and macroscopic stellar properties throughout successive stages of stellar evolution.