Artur A. Hakobyan

Context . The transport of gas toward galactic centers and its connection to active galactic nucleus (AGN) activity remain key open questions in galaxy evolution. In particular, the relative roles of stellar bars and spiral arms and their possible combined influence on angular momentum redistribution and supermassive black hole fueling are not yet fully understood. Aims . We investigate the dependence of AGN activity on bar strength and spiral arm morphology in nearby spiral galaxies, with a particular emphasis on disentangling their individual and joint effects while controlling for galaxy stellar mass and color. Methods . We constructed a sample of 843 morphologically undisturbed Sa-Sd galaxies from the Mapping Nearby Galaxies at Apache Point Observatory survey in the redshift range 0.026 ≤ z ≤ 0.1. Bar strengths, stellar masses, colors, and AGN classes were adopted from the literature, while spiral arm classes (ACs), i.e., flocculent (FL), multi-armed (MA), and grand design (GD) systems, were visually determined for the entire sample using optical imaging together with bulge–disk decomposition residual maps. Results . The AGN fraction increases systematically with bar strength, from 0.10 ^+0.02 _−0.02 in unbarred galaxies to 0.34 ^+0.03 _−0.03 in strongly barred systems. After controlling for stellar mass and color, a statistically significant enhancement remains detectable primarily at intermediate stellar masses (10.5 ≲ log( M _⋆ / M _⊙ )≲11.0), whereas no significant dependence is found at higher masses. In contrast, spiral arm morphology alone does not show a robust independent connection with AGN activity once stellar mass and color are controlled, although a weak enhancement is present in intermediate-mass GD and MA systems. When bar strength and spiral AC are considered jointly, the highest AGN fraction is observed in strongly barred GD + MA galaxies (0.39 ^+0.04 _−0.04 ), while the lowest AGN fractions are found in unbarred FL systems (0.09 ^+0.03 _−0.02 ). Conclusions . Our results suggest that stellar mass and color drive the primary trend in AGN activity in nearby disk galaxies, while stellar bars and spiral arms act as secondary structural drivers. The effect of spiral arm morphology is weaker and becomes most apparent when considered jointly with bars. These findings highlight the importance of simultaneously accounting for stellar mass, color, and the combined large-scale morphological structure of galaxies when investigating the triggering of nuclear activity.

The Rossiter–McLaughlin (RM) effect produces apparent radial velocity (RV) shifts through line-profile distortions caused by a transiting planet blocking different regions of the rotating stellar surface. Because the underlying orbital RV trend can be estimated from out-of-transit observations, RM sequences provide a controlled laboratory for studying flux-induced RV variations. We compiled a sample of 1171 ESPRESSO observations of 13 targets obtained during 21 RM observing nights and trained machine learning models to reconstruct a reference RV trend from observed RVs, line-profile diagnostics, and activity indicators. Predictive performance varied substantially among stars and depended on both the strength of the RM signal and the similarity of the target star to the training sample. Applications to Sun-as-a-star observations and Proxima Centauri recovered known periodicities but did not fully remove the activity-induced variability. Larger and more diverse datasets will be required to assess the potential of this approach for mitigating activity-induced RV signals.

Context . Elemental abundances of FGK stars can be derived routinely from high-resolution optical spectra, but this remains considerably more difficult for cooler stars. Even estimates of the chemical composition of planet-host stars are central for studies of planet formation and interior composition, however. Machine-learning methods offer a practical route for inferring otherwise inaccessible abundances from more widely available stellar data. Aims . We investigate how much information on stellar metallicity and selected elemental abundances is encoded in kinematic and orbital properties, and whether these empirical relations can be used to estimate abundances and abundance ratios relevant for the characterisation of exoplanet hosts. Methods . We used a large APOGEE DR17 sample of red giant stars as the main training set and an independent HARPS sample of nearby FGK dwarfs for the external validation. From astrometry and radial velocities, we derived Galactic velocities, orbital parameters, and stellar population-membership probabilities. We benchmarked several machine-learning regressors, optimised the strongest models, and analysed the feature importance using gain-based metrics, permutation importance, single-feature models, and SHapley Additive exPlanations (SHAP) values. We also explored the prediction for C and O from Mg, Si, and [Fe/H] and derived simple empirical relations between selected abundance ratios (Fe/Si, Mg/Si, C/O, and Fe/O) and metallicity. Results . Kinematic information alone recovers only a limited fraction of the variance in stellar metallicity, with a clear performance ceiling at Root Mean Square Error (RMSE) ∼0.20 dex. The most informative predictor is the maximum vertical orbital excursion, Z _max , followed by radial orbital parameters. When [Fe/H] is combined with kinematic information, the abundances of C, O, Mg, and Si are predicted significantly more accurately than with the baseline approximation [X / H] = [Fe / H]. In contrast, when C and O are predicted from Mg, Si, and [Fe/H], most of the predictive power is already contained in the elemental abundances themselves, with Mg being the dominant contributor, and the addition of kinematic information provides little improvement. The trained models reproduced the main abundance trends associated with Galactic chemical evolution. We found that the slopes of the relations between Fe/Si, Mg/Si, C/O, and Fe/O and metallicity differ slightly between the HARPS and APOGEE samples, with fractional differences generally below 17%. Conclusions . Stellar kinematics and orbital parameters contain only limited information on stellar metallicity, and the predictive performance is primarily constrained by the intrinsic information content of the input features and not by the choice of the model architecture. Nevertheless, when combined with metallicity, these parameters enable accurate predictions of elemental abundances, including those that are difficult to determine directly. This provides a practical framework for extending chemical characterisation to stars for which detailed abundance determinations are challenging.

Scientific citations are widely used to assess impact, yet they often reflect more than just quality or relevance. In astronomy, we analyzed 255,000 peer-reviewed articles published between 2000 and 2025 to examine how journals are cited in relation to their output and authorship patterns. We found that multidisciplinary journals attract far more citations—up to nine times their share of publications—while specialized journals receive fewer. Additionally, authors are more likely to cite articles within the journal in which they publish. These trends have weakened but persist, influenced by topical focus, author habits, and strategic referencing. Our findings reveal structural biases in how visibility is conferred through citations. Recognizing these patterns is essential for interpreting metrics fairly and fostering more equitable referencing practices across the scientific publishing landscape.

Context. The composition of rocky planets is strongly driven by the primordial materials in the protoplanetary disk, which can be inferred from the abundances of the host star. Understanding this compositional link is crucial for characterizing exoplanets. Aims. We aim to investigate the relationship between the compositions of low-mass planets and their host stars. Methods. We determined the primordial compositions of host stars using high-precision present-day stellar abundances and stellar evolutionary models. These primordial abundances were then input into a stoichiometric model to estimate the composition of planet-building blocks. Additionally, we employed a three-component planetary interior model (core, mantle, and water in different phases) to estimate planetary compositions based only on their radius and mass. Results. We find that although stellar abundances vary over time, relevant abundance ratios such as Fe/Mg remain relatively constant during the main sequence evolution for low temperature stars. A strong correlation is found between the iron-to-silicate mass fraction of protoplanetary disks and planets, while no significant correlation was observed for water mass fractions. The Fe/Mg ratio varies significantly between planets and their stars, indicating substantial disk-driven compositional diversity, and this ratio also correlates with planetary radius. Conclusions. While stellar abundances, as a proxy of the composition of protoplanetary disk, provide a baseline for planetary composition, significant deviations arise due to complex disk processes, challenging the assumption of a direct, one-to-one elemental relationship between stars and their planets.

Context. The determination of planetary densities from the masses derived with the radial velocity (RV) and transit-timing variation (TTV) methods reveals discrepancies. Specifically, planets detected through RV exhibit higher densities than those detected through TTV, even though their radii are similar. Understanding the origins of these discrepancies is crucial and timely, especially with upcoming ground- and space-based missions dedicated to exoplanet research.Aims. We explored the possibility that the discrepant mass/densities in the TTV and RV populations might be linked to the properties of the environments in which these planets are formed.Methods. For the largest currently available sample of FGK-type stars hosting low-mass TTV and RV planets, we determined the host star abundances. Then, by employing a simple stoichiometric model, we used these abundances to estimate the iron-to-silicate mass fraction (f_iron) and the water-mass fraction (w_f) of the protoplanetary disks. We also calculated the kinematic properties of the host stars.Results. We observed an indication that the hosts of TTV planets have slightly higherf_ironand lowerw_fvalues than their RV counterparts. This suggests that TTV planets (without considering their atmospheres) are denser than RV planets on average, which implies that larger atmospheres on TTV planets are required to account for their overall lower densities. However, we also note differences in the properties of the planets, such as their orbital periods, and variations in the quality of the spectroscopic data, which may have an impact on these results.Conclusions. Exploring the TTV-RV mass and/or density discrepancy through a chemical analysis of the host star holds promise for future research, particularly with larger sample sizes and higher-quality data. Meanwhile, the provided detailed host star abundances can be employed to study the composition of the planets within the current sample, thereby contributing to a better understanding of the aforementioned discrepancy.

Among the diverse progenitor channels leading to Type Ia Supernovae (SNe Ia), there are explosions originating from white dwarfs with sub-Chandrasekhar masses. These white dwarfs undergo detonation and explosion triggered by primary detonation in the helium shell, which has been accreted from a companion star. The double-detonation model predicts a correlation between the age of the progenitor system and the near peak brightness: the younger the exploding progenitors, the brighter the SNe. In this paper, we present our recent achievements on the study of SNe Ia properties in different locations within host galactic discs and the estimation of their progenitor population ages. Observationally, we confirm the validity of the anticipated correlation between the SN photometry and the age of their progenitors.

Compositions of rocky exoplanets: The interior compositions of small rocky exoplanets cannot be observed directly but are expected to relate to the composition of the host star. Adibekyan et al . analyzed a sample of rocky exoplanets, inferring the planets’ iron fractions by combining their masses and radii with an interior structure model. The iron fractions of the host stars were calculated from stellar elemental abundances. The two iron fractions, that of the planets and that of the stars, correlate with each other, but the slope is steeper than 1, indicating that planet formation processes modify the compositions of rocky planets. —KTS

Context. Revealing the mechanisms shaping the architecture of planetary systems is crucial for our understanding of their formation and evolution. In this context, it has been recently proposed that stellar clustering might be the key in shaping the orbital architecture of exoplanets. Aims. The main goal of this work is to explore the factors that shape the orbits of planets. Methods. We performed different statistical tests to compare the properties of planets and their host stars associated with different stellar environments. Results. We used a homogeneous sample of relatively young FGK dwarf stars with radial velocity detected planets and tested the hypothesis that their association to phase space (position-velocity) over-densities (“cluster” stars) and under-densities (“field” stars) impacts the orbital periods of planets. When controlling for the host star properties on a sample of 52 planets orbiting around cluster stars and 15 planets orbiting around field stars, we found no significant difference in the period distribution of planets orbiting these two populations of stars. By considering an extended sample of 73 planets orbiting around cluster stars and 25 planets orbiting field stars, a significant difference in the planetary period distributions emerged. However, the hosts associated with stellar under-densities appeared to be significantly older than their cluster counterparts. This does not allow us to conclude as to whether the planetary architecture is related to age, environment, or both. We further studied a sample of planets orbiting cluster stars to study the mechanism responsible for the shaping of orbits of planets in similar environments. We could not identify a parameter that can unambiguously be responsible for the orbital architecture of massive planets, perhaps, indicating the complexity of the issue. Conclusions. An increased number of planets in clusters and in over-density environments will help to build large and unbiased samples which will then allow to better understand the dominant processes shaping the orbits of planets.