Dorota M. Skowron

Classical Cepheids (DCEPs) are excellent standard candles expected to trace the spatial and kinematic distribution of the Galaxy’s young and dynamically cold stellar disc. Using the most precise mid-infrared DCEP distances to date combined with Gaia DR3 astrometry and line-of-sight velocities, we perform a comprehensive 6D dynamical census of the Milky Way’s DCEP population. While the vast majority exhibit the expected disc-like kinematics, we identify 18 kinematically anomalous Cepheids. These ‘rogue’ stars reside on highly inclined orbits (three at >70°), including two in retrograde motion and one with a total velocity of ∼480 km s ⁻¹ . Despite their extreme trajectories, their optical light curves are consistent with DCEP classifications. We explore whether these anomalies originate from possible classification systematics or physical processes and find only three of our sources are likely misclassified. Assuming a runaway scenario, we derive dynamical ages for the kinematic anomalies, which we find highly consistent with their Cepheid ages. Spectroscopic follow-up would be insightful as one source in particular is exceptionally metal poor ([Fe/H] —1.6 dex), which is highly atypical for a DCEP. Integrating the photometric classification with 6D kinematics will help fully characterise the Galaxy’s variable star populations.

We present an independent reassessment of the nearby long secondary period (LSP) stars analyzed by Shariat et al. (2026). By inspecting long-term All Sky Automated Survey (ASAS) light curves, together with Gaia Focused Product Release (FPR) radial velocity (RV) curves, for 221 LSP candidates (out of 224) located within 1.5 kpc from the Sun, we find that less than half of the sample (103 objects, ∼47%) exhibits convincing LSP-like behavior. The remaining part of the sample could be more naturally interpreted as that of semiregular variables, characterized by irregular or multiperiodic pulsations. This indicates that the analyzed sample is significantly contaminated by non-LSP objects and therefore is not representative of the classical period–luminosity sequence-D population. Using the gaiamock tool to predict Gaia renormalized unit weight error (RUWE) values for binary systems, we show that even the nearest LSP stars do not have to exhibit elevated RUWE values as a consequence of their binarity. We also argue that the binary-nature hypothesis for LSP stars does not lead to a discrepancy between the observed and expected distance–RUWE relation for these variables.

Modeling of complex microlensing events suffers from many difficult-to-disentangle degeneracies, especially for a source's orbital motion in a binary system. This is known as the xallarap effect. To address the degeneracies inherent in xallarap modeling, we developed a novel approach that directly samples the physical parameters of source stars (initial mass, evolutionary phase, metallicity, distance, and reddening) during Markov chain Monte Carlo fitting. In our approach the physical parameters of the source were estimated using MIST stellar evolution models. This parameterization imposes astrophysical constraints that help identify the most physically probable solutions. We tested our method on the complex microlensing event OGLE-2017-BLG-0114, which exhibits signatures that can be traced to the complexity of the source system. We successfully constrain the microlensing models, achieving improvements in the Einstein ring radius estimates by up to an order of magnitude in the case of binary source models.

Long-period variable stars (LPVs) are evolved red giant and supergiant stars whose pulsations provide unique insights into late stages of stellar evolution and serve as essential tools in modern astrophysics. Their period–luminosity and period–age relations make them valuable distance and age indicators, while their light-curve morphology, amplitudes, and multiperiodicity reveal the underlying physics of stellar interiors and mass loss. In this review, we provide an overview of the current status of LPV studies, focusing on their observational properties and applications, including: ▪ Modern classification of LPVs into Miras, semiregular variables (SRVs), and OGLE small-amplitude red giants (OSARGs), which occupy multiple period–luminosity sequences associated with different pulsation modes, chemical compositions, and evolutionary stages. ▪ Mira variables as reliable distance indicators across diverse stellar environments and their increasing role as standard candles. ▪ The increasing role of SRVs and OSARGs. ▪ Long-secondary-period variables as potential tracers of exoplanets. Together with advances in theoretical modeling, these developments establish LPVs as valuable tracers of Galactic structure, stellar populations, and the extragalactic distance scale.

Context. The direct empirical determination of the local value of the Hubble constant ( H ₀ ) has markedly advanced thanks to improved instrumentation, measurement techniques, and distance estimators. However, combining determinations from different estimators is nontrivial due to their correlated calibrations and different analysis methodologies. Aims. Using covariance weighting and leveraging community expertise, we have constructed a rigorous and transparent “Distance Network” to find a consensus value and uncertainty for the locally measured Hubble constant. Methods. Experts across all relevant distance measurement domains were invited to critically review the available datasets spanning parallaxes, detached eclipsing binaries, masers, Cepheids, the tip of the red giant branch, Miras, carbon-rich asymptotic giant branch stars, Type Ia (SNe Ia) and Type II supernovae, surface brightness fluctuations, the fundamental plane, and Tully–Fisher relations. Before any calculations, the group voted for first-rank indicators to define a “baseline” Distance Network. Other indicators were included to assess the robustness and sensitivity of the results. We provide open-source software and data products to support full transparency and future extensions of this effort. Results. Our key findings are as follows: (1) The local H ₀ is robustly determined, with first-rank indicators internally consistent within their uncertainties. (2) A covariance-weighted combination yields a relative uncertainty of 1.1% (baseline) or 0.9% (all estimators). (3) The contribution from SNe Ia is consistent across compilations of optical or NIR magnitudes. (4) Removing either Cepheids or the tip of the red giant branch has a minimal effect on the central value of H ₀ . (5) Replacing SNe Ia with galaxy-based indicators changes H ₀ by less than 0.1 km s ⁻¹ Mpc ⁻¹ while doubling its uncertainty. (6) The baseline result is H ₀ = 73.50 ± 0.81 km s ⁻¹ Mpc ⁻¹ , 7.1 σ from the early Universe plus ΛCDM result 67.24 ± 0.35 km s ⁻¹ Mpc ⁻¹ and 5.0 σ from BBN+BAO within a flat ΛCDM DESI DR2 (68.51 ± 0.58 km s ⁻¹ Mpc ⁻¹ ). Conclusions. A networked approach, such as the one presented here, is invaluable for enabling further progress in Hubble constant measurements, as it provides the much needed advances in accuracy and precision without overreliance on any single method, sample, or group.

The Wesenheit function is widely used to reduce the effects of interstellar reddening in distance measurements. Its construction, however, relies on the assumption of a universal extinction curve and on fixed values of the total-to-selective extinction ratio, R _V . Recent studies have shown that R _V varies significantly across the Milky Way and between different galaxies, raising concerns about systematic biases in Wesenheit magnitudes and period–Wesenheit relations. In this work, we discuss the impact of nonuniversal extinction on Wesenheit indices by combining the R _V -dependent extinction curve with a grid of stellar atmosphere models. We compute the integrated extinction in optical and near-infrared passbands, derive R _V -dependent R coefficients for multiple Wesenheit indices, and examine how changes in R _V propagate into Wesenheit magnitudes and Cepheid distances in our Galaxy. We find that the R coefficients in the Wesenheit functions vary strongly with R _V . For classical Cepheids in the Milky Way disk, variations of R _V within the typical observed range (2.6–3.6) can lead to substantial differences in the Wesenheit function, reaching ± 0.7 mag from the mean for the Gaia-based Wesenheit index W _G and resulting in distance errors of almost 40%. Near-infrared Wesenheit indices are much less sensitive to R _V changes. Our results clearly show that accounting for variable R _V is essential when applying period–Wesenheit relations, particularly in the optical regime, or that near- or mid-infrared-based distances should be used. While we present this effect for classical Cepheids, it applies to all pulsating stars for which period–Wesenheit relations are used to infer distances.

A population of free-floating planets is known from gravitational microlensing surveys. None have a directly measured mass, owing to a degeneracy with the distance, but the population statistics indicate that many are less massive than Jupiter. We report a microlensing event—KMT-2024-BLG-0792/OGLE-2024-BLG-0516, which was observed from both ground- and space-based telescopes—that breaks the mass-distance degeneracy. The event was caused by an object with 0.219 − 0.046 + 0.075 Jupiter masses that is either gravitationally unbound or on a very wide orbit. Through comparison with the statistical properties of other observed microlensing events and predictions from simulations, we infer that this object likely formed in a protoplanetary disk (like a planet), not in isolation (like a brown dwarf). Dynamical processes then ejected it from its birthplace, producing a free-floating object.

Context. Long secondary periods (LSPs) are observed in about one-third of pulsating red giants, yet this phenomenon remains unexplained. Four key observational constraints anchor the discussion: (i) a ∼30% occurrence rate in semi-regular variable AGB stars (SRVs) with a much lower rate (or complete absence thereof) in regularly pulsating Mira-type AGB stars (Miras); (ii) ∼50% of LSP stars show a secondary mid-infrared (MIR) minimum; (iii) Keplerian fits to radial-velocity (RV) curves favour the argument of periastron ω > 180°; and (iv) the RV-light curve phase lag clusters around − π /2. Aims. We test whether a close-in, eccentric low-mass companion that only spends part of its orbit within the giant’s dust-formation (wind-launching) zone can match all four empirical facts. Methods. Guided by observed RV amplitudes and periods of ∼500–1500 days, we adopted a companion mass of M ₂ ∈ [0.08, 0.25] M _⊙ , orbital separation of a ∈ [1.5, 3] au, and eccentricty of e ≤ 0.6. Next, we took the dust condensation radius of R _cond ∼ 2.5 − 3 au for SRVs (larger for Miras when scaling with luminosity). We computed the time-in-dust fraction f _dust (time with r ≥ R _cond ) and applied line-of-sight criteria: an LSP requires an orbital inclination of i ≥ i _LSP and f _dust ≥ f _min , while a secondary MIR minimum interpreted as secondary eclipse further requires i ≥ i _ecl > i _LSP and a superior conjunction. We tested the first three empirical facts analytically, then modelled the RV-light phase offset with 3D hydrodynamical simulations. Results. Our proposed scenario explains the observed excess of ω > 180°. For SRV-like parameters, we obtained an LSP detectability of ∼31.6 ± 0.1%, while Mira-type conditions yield ∼3.0 ± 0.1%; for both scenarios, the conditional secondary MIR eclipse fraction is ∼44%. Our hydrodynamical models place the optical-depth peak just downstream of the companion near apastron, then shift it to ∼90 − 225° phase offsets later in the orbit. This result is consistent with the RV-light offsets. Conclusions. A time-in-dust geometric selection for low-mass companions in close eccentric orbits is sufficient to explain the four key empirical facts constraining the LSP mechanism.

Some previous studies have suggested that massive and intermediate-mass primordial black holes (PBHs) could comprise a substantial fraction of dark matter in the Universe. Such black holes, if they existed in the Milky Way halo, would give rise to long-duration microlensing events that may potentially last for years. However, earlier searches were not sufficiently sensitive to detect such events. Here, we present the results of searches for long-timescale gravitational microlensing events toward the Small Magellanic Cloud (SMC) using nearly 20 yr of photometric observations collected by the Optical Gravitational Lensing Experiment (OGLE) from 2001–2020. We found six events, three of which are new discoveries. We use a sample of five events to measure the microlensing optical depth toward the SMC τ = (0.32 ± 0.18) × 10⁻⁷ and the event rate Γ = (1.18 ± 0.57) × 10⁻⁷ yr⁻¹ star⁻¹. The properties of the detected events are consistent with lenses originating from known stellar populations within the SMC or in the Milky Way disk. No events with timescales longer than 1 yr were detected, which provides competitive limits on the fraction of massive compact objects, including PBHs, in the Milky Way dark matter halo. Together with the earlier OGLE studies of microlensing events in the direction of the Large Magellanic Cloud, these observations rule out PBHs and other compact objects with masses ranging from 10⁻⁸–10³ M _⊙ as dominant components of dark matter.

Blue large-amplitude pulsators (BLAPs) are a recently discovered class of short-period pulsating variable stars. In this work, we present new information on these stars based on photometric and spectroscopic data obtained for known and new objects detected by the Optical Gravitational Lensing Experiment (OGLE) survey. BLAPs are evolved objects with pulsation periods in the range of 3–75 minutes, stretching between subdwarf B-type stars and upper main-sequence stars in the Hertzsprung–Russell diagram. In general, BLAPs are single-mode stars pulsating in the fundamental radial mode. Their phase-folded light curves are typically sawtooth-shaped, but many longer-period objects exhibit an additional bump. The long-term OGLE observations show that the period change rates of BLAPs are usually of the order of 10⁻⁷ yr⁻¹ and in a quarter of the sample are negative. The spectroscopic data indicate that BLAPs form a homogeneous group in the period, surface gravity, and effective temperature spaces. However, we observe a split into two groups in terms of helium-to-hydrogen content. The atmospheres of He-enriched BLAPs are more abundant in metals (about 5 times) than the atmosphere of the Sun. We discover that BLAPs obey a period–gravity relationship and we use the distance to OGLE-BLAP-009 to derive a period–luminosity relation. Most of the stars observed in the OGLE Galactic bulge fields seem to reside in the bulge, while the remaining objects likely are in the foreground Galactic disk.