arXiv subjects

A. Kasikov

Publications and source records attributed to A. Kasikov.

5 recordsLinked to original sources

arXiv2026

A decade of radial-velocity monitoring of rho Leo: moment analysis and periodic variability

We investigate the origin of long-term spectroscopic and photometric variability in the blue supergiant rho Leo, with particular emphasis on distinguishing between intrinsic pulsations and variability induced by a possible companion. Our analysis is based on an 11.5-year spectroscopic time series obtained at Tartu Observatory, complemented by high-cadence, high-resolution spectroscopy from the Hertzsprung SONG telescope and space-based photometry from TESS. We studied line-profile variability using normalised moments of the HeI 6678, HeI 5875, and SiIII 4552 lines. Periodic signals were identified using the generalised Lomb-Scargle periodogram with iterative pre-whitening, and their temporal stability was examined with the weighted wavelet Z-transform. We detect a persistent periodic signal at P = 16.46 d in the first and third moments, present throughout the full observing interval, with a radial-velocity amplitude of 3.9 km/s. This signal is also present in the SONG data and is visible in multiple spectral lines, indicating a global origin. Photometric observations reveal a dominant variability timescale near 33 d, approximately twice the spectroscopic period. The stable 16.46-day period, present throughout the entire observing interval, most likely results from non-radial pulsations of the supergiant. A binary origin of the signal is not excluded, but distinguishing between these scenarios is complicated by the supergiant's complex variability pattern.

astro-ph.SR↗

arXiv2026

Painting a Family Portrait of the Yellow Super- and Hypergiants in the Milky Way I. Constraining the Distances and Luminosities

Context. Distances to evolved massive stars in the Milky Way are not well constrained by Gaia parallaxes due to their brightness and variability. This makes it difficult to determine their fundamental stellar parameters, such as radius or luminosity, and infer their evolutionary states. Aims. We aim to improve the distance estimates of Yellow Hypergiants (YHGs) and Yellow Supergiants (YSGs) by identifying possible cluster and association memberships. Using these distances, we derived updated luminosities and revised their positions in the Hertzsprung-Russell diagram. Methods. We compiled from the literature a sample of 35 luminous yellow massive stars (YHGs and the most luminous YSGs). We used Gaia DR3 astrometry to identify possible membership in clusters and OB associations. We derived distances by combining the parallaxes of nearby co-moving stars. We independently validated these distances by comparing the stellar radial velocities to the Galactic H I kinematic map. We combined angular diameters and effective temperature values from the literature with the new distances to estimate luminosities. Results. We improved the distance estimates for 28 of the 35 stars through association with co-moving stellar groups. For an additional six stars, we provided distance estimates based on the H I kinematic map. For one star, the distance remains unclear. Most YSGs are members of young stellar populations, while the environments of the YHGs are more diverse, and for some of them, their origin populations remain unclear. We derived updated luminosities for a subset of 20 stars. Most YHGs have luminosities above log L/L = 5.4, while YSGs occupy a wider range of luminosities and the most luminous YSGs have luminosities similar to YHGs.

astro-ph.SR↗

arXiv2026

AGN in Gaia Alerts: from flares to Changing Look Quasars

The Gaia Alerts system is providing alerts on a variety of objects displaying a significant photometric change detected by the Gaia satellite from one passage to the next one over the same region of the sky. Among the over 22000 alerts published until the end of 2022, around 13 percent concern AGN or quasar candidates. We have embarked on a spectroscopic ground-based follow-up of some of those (including some selected by a different method specifically in galactic nuclei), to establish their true nature, and reveal the various phenomena leading to a change in magnitude of those AGN. The present paper deals with radio-quiet objects, while the radio-loud ones will be described in a companion paper. We confirm, on one hand, the AGN/quasar nature of 64 new candidates alerted by Gaia, and, on the other hand, obtained second-epoch spectra of over 200 known AGN also alerted for large photometric variations. The observed phenomena show a large variety: from Flares to Tidal Disruption Events (TDEs) and a large number of Changing Look Quasars (CLQs, 56 secure ones, plus 23 probable ones), not forgetting some rarer events like SNe, microlensing events or Extreme Coronal Line Emitters. This sample shows that variability is an excellent tool to detect new quasars, especially radio-quiet ones that otherwise would be undetected, and that a significant fraction of variable AGN/quasars, around 10 percent, presents the CLQ phenomenon. Some of the new CLQs are followed-up to monitor further changes and measure time scales.

astro-ph.HE↗

arXiv2025

Atmospheric dynamics of the hypergiant RW Cep during the Great Dimming

The hypergiant RW Cep is one of the largest stars in our galaxy. The evolution and mass loss of such stars has profound effects on their surrounding regions and the galaxy as a whole. Between 2020 and 2024, RW Cep experienced a historic mass-loss event known as the Great Dimming. This study provides a spectroscopic analysis of RW Cep during the Great Dimming. We examine its atmospheric dynamics and place it in the context of the star's variability behaviour since the early 2000s. We conducted high-cadence spectroscopic observations of RW Cep during the dimming event using the Tartu Observatory 1.5-meter telescope and the Nordic Optical Telescope. We analysed the atmospheric dynamics by measuring the radial velocities and line depths of Fe I and other spectral lines. The radial velocities of the Fe I lines reveal a vertical velocity gradient of 10-20 km/s in the atmosphere, correlating with the strength of the spectral lines. Stronger lines, formed in higher atmospheric layers, have higher radial velocities. We measured the systemic velocity at -50.3 km/s. During the dimming, radial velocities were affected by additional emission from the ejected gas, which was blue-shifted relative to the absorption lines. Post-dimming, we observed large-scale atmospheric motions with amplitude ~25 km/s. Strong resonance lines of Ba II, K I, Na I and Ca I showed stable central emission components at -56 km/s, likely of circumstellar origin.

astro-ph.SR↗

arXiv2024

Yellow hypergiant V509 Cas -- stable in the 'yellow void'

The yellow hypergiant star V509 Cas is currently undergoing an extreme phase of evolution. Having experienced eruptive mass-loss outbursts in the 20th century, the star's effective temperature reached record high values in the early 2000s. However, since then, the star's behaviour has displayed an unprecedented level of stability. In spite of that, the star could be traversing through the 'yellow void' instability region. To describe the current evolutionary state of V509 Cas, we analysed its variability using photometric and spectroscopic data collected over recent years. By comparing our findings with historical records, we aim to determine whether the star's surface shows signs of stabilisation. Additionally, we investigate the variability of emission components in the wings of certain spectral lines to highlight the contribution of the circumstellar gaseous disc to this phenomenon. Our spectroscopic monitoring observations were carried out at Tartu Observatory over the course of seven years, supplemented by echelle spectra obtained at the Nordic Optical Telescope, as well as publicly available photometric data from Gaia, AAVSO, and AAVSO's Bright Star Monitor programme. We estimated the variability of effective temperature and radial velocity from the spectral time series and correlated it with the brightness variability of V509 Cas. The results indicate that the star's average brightness level has remained stable throughout the observed period, with an amplitude of variability ~0.1 mag. While the amplitude of short-term temperature fluctuations has decreased compared to the early 2000s, the variability of the radial velocity remains similar to historical values from the early 20th century. Moreover, we show how the variable radial velocity affects the emission components in some absorption lines (e.g. Sc II) and how that follows the hypothesis of a disc surrounding the star.

astro-ph.SR↗