Abstract Blue straggler stars (BSSs) are believed to form through mass transfer in binary systems or stellar collisions. The reported presence of double BSS sequences in some globular clusters (GCs) has been interpreted as evidence that these two formation channels produce distinct sequences in color–magnitude diagrams. We reassess this claim using Hubble Space Telescope (HST) UV Globular Cluster Survey photometry of 56 Galactic GCs. We used the Hartigan dip test to test bimodality, and Akaike model comparison to test whether BSS distance distributions are better described by a mixture of two unskewed Gaussians or a skewed unimodal Gaussian model. We find no strong statistical evidence for bimodality: no cluster yields a dip test p -value below 0.15, and the Akaike model comparison favors the skewed unimodal model in 94 out of 112 cases. We reexamine NGC 7099 (M30), the prototypical case of a double BSS sequence, using three reductions of HST data. We find bimodality is detected at p = 4 × 10 −3 , versus the originally reported p ∼ 10 −5 , in the original photometry. The observed uncertainties derived from the subgiant branch widths are comparable to the suggested separation between the proposed BSS sequences, making the detection of statistically significant bimodality challenging. Our results suggest that the dip between two BSS sequences in the M30 photometry is a coincidence, and that later bifurcation claims can be explained as skew in the BSS color distribution, rather than two separate distributions.

Recent discoveries show that chemically peculiar (CP) stars also reside in globular clusters (GCs). The channels leading to chemical peculiarity are, however, still under discussion. The main goals of our investigation are to identify the CP stars in GCs and search for systematic relations among different stellar groups commonly observed in GCs and chemical peculiarity. Additionally, we search for possible correlations of the occurrence rate of CP stars with the cluster properties. We obtained BVg2 multicolour photometry for 18 southern GCs with the 1.54m Danish telescope located at La Silla observatory in Chile. The data were reduced and processed with standard tools. To avoid blending, we omitted central parts of the clusters. We determined the membership of the stars to a particular GC based on the proper motion and parallax of the stars from the Gaia DR3 catalogue. Colour-magnitude diagrams were constructed, and only stars on horizontal branch and in the region of blue stragglers were considered for further analysis. Construction of the Δ a diagram allowed us to identify CP candidate stars with high probability. To test the reliability of our approach, we used spectroscopic observations of M53. Iraf Except for four GCs, CP candidates were found in all other GCs with the average fraction of 4.2,%. We found that a significant portion of the candidates are blue stragglers. These constitute a full sample of identified CP candidates in four GCs and represent more than 60,% of the candidates in ten GCs. The a photometry was found as a powerful tool for the identification of CP stars in GCs. We discovered a possible connection between chemical peculiarity and blue straggler stars.

Abstract We present a ground-based time-series photometric study of stellar variability in four intermediate- to old-age open clusters—NGC 2192, NGC 2266, NGC 2509, and IC 1369—based on high-cadence Cousins R -band observations obtained with the 0.6 m VASISTHA telescope at the IERCOO observatory. The monitoring campaign comprises more than ∼34 hr of time-series data, providing sensitivity to short-period variability on timescales of ∼0.02–2 days. We identified between 190 and 290 probable members in each cluster using a Gaussian mixture model. Structural parameters were derived from radial density profiles fitted with King models. Fundamental parameters were further constrained using color–magnitude diagram analysis with PARSEC isochrones, yielding ages of ∼0.3–1.6 Gyr and distances of ∼2.5–3.9 kpc. From the time-series photometry, we identify four new variable stars and seven previously uncharacterized periodic variables, including δ Scuti and γ Doradus pulsators, as well as rotational variables. The detected variables exhibit periods between ∼0.12 and 0.90 days, with R -band amplitudes ranging from 0.01 to 0.20 mag. Periods were determined using Lomb–Scargle analysis of calibrated light curves. For a subset of variables, spectral energy distribution fitting was performed to derive effective temperatures (∼4300–10,000 K), radii (∼1.3–46 R ⊙ ), and luminosities (∼2–100 L ⊙ ), enabling reliable placement on the Hertzsprung–Russell diagram. We present PHOEBE light-curve modelling of the W UMa-type eclipsing binary Gaia DR3 2164531610149292288 in IC 1369, deriving its physical parameters and providing the first detailed characterization beyond its previously reported variability. These results demonstrate that combining dense-cadence ground-based observations with Gaia astrometry provides a reliable approach for identifying and characterizing variable stars in open clusters.

Abstract The Sagittarius dwarf spheroidal galaxy (Sgr dSph) provides us with the unique opportunity to study an ongoing Galactic cannibalistic event between our Milky Way (MW) Galaxy and a satellite dwarf galaxy. Understanding this event crucially requires memberships and high-precision metallicities. Here, we present the first major membership star catalog of the Sgr dwarf core (≈140,000 sources) and Messier 54 (M54; ≈2000 sources) with positions, proper motions, and parallaxes from the third Gaia data release (DR3), supplemented with metallicities from the Apache Point Observatory Galactic Evolution Experiment (or APOGEE). We initially isolate the Sgr dwarf core and M54 spatially from prior literature positions. Using evolutionary subsamples separated within a color–magnitude diagram, we analyze the substructures of the Sgr core and infer its positional relationship with M54 within 5D phase space. A sample of MW stars from a similar Galactic latitude were used to identify contaminants and separate member stars from the core of the Sgr dSph and M54 using a Gaussian mixture model. We present the derived proper motions, parallaxes, and metallicities for these evolutionary subsamples and demonstrate the precision of our sample using red clump (RC) standard candles. We find distance moduli for the Sgr core and M54 of ( m − M ) 0 = 16.95 8 − 0.044 + 0.044 mag and ( m − M ) 0 = 16.9 4 − 0.056 + 0.047 mag, corresponding to heliocentric distances of d = 24.63 5 − 0.49 + 0.49 kpc and d = 24.45 2 − 0.602 + 0.537 kpc, respectively. Using RC distance analysis, our results imply that there is no separation between the Sgr core and M54. Finally, we describe the metallicity distributions of the evolved stars within these two systems, finding evidence for the infall scenario.

Understanding the demographics of white dwarf--main sequence (WDMS) binaries is key to uncovering the formation of various stellar exotica and refining the details of binary stellar evolution. Despite several dedicated efforts to identify unresolved WDMS binaries, their population remains incomplete, even within a 100 pc volume-limited sample. This study aims to identify WDMS binaries hidden within the MS of the optical color-magnitude diagram (CMD), improving the completeness of WDMS binaries within a volume-limited sample of 100 pc. We used near-ultraviolet (NUV)--optical CMDs to distinguish unresolved WDMS binaries from the rest of the populations. High-precision astrometric and photometric data from DR3 and NUV data from GR6/7 were combined to construct CMDs. Using the binary spectral energy distribution (SED) fitting algorithm within the Virtual Observatory SED Analyzer (VOSA) tool, we estimated stellar parameters such as effective temperature, bolometric luminosity, and radii. The WD masses were determined using WD evolutionary models. As we used the sources that were only detected in the NUV band of GALEX, this study directly complements the majority of the previous studies. We identify 347 WDMS binary candidates within 100 pc, with 188 being newly reported. Our method predominantly identifies binaries with cooler WDs ( łe10,000 K) compared to previous studies. The WD masses range from ∼0.2 and 1.3 M_⊙, and most MS companions are of M spectral type.

Context . The recent discovery of free-floating planets (FFPs) in nearby young stellar associations suggests that these objects might be common in the Galaxy. Our search for FFPs in the young Lower Centaurus-Crux (LCC) association using the VISTA Variables in the Vía Láctea VVV and VVVX surveys revealed several candidates with distances d < 200 pc. Aims . The main goal of the paper is to identify binary FFPs among this sample. The presence of such binaries is useful to contrast two different main formation scenarios: the formation in the circumstellar disk of the parent star with subsequent ejection by dynamical interactions and the in situ formation by gravitational collapse of a protostellar cloud. Methods . We used the Gaia , VVV, VVVX, and DECaPS databases to identify pairs of low-mass objects in the LCC association sharing common proper motions. We examined the optical and near-IR color-magnitude and color-color diagrams, and visually confirmed the detections in the available optical and near-IR images. Results . We find 17 young low-mass binaries in the LCC association, with distances starting from 68 pc and projected separations ranging from 88 to 6742 au. A couple of candidates have additional faint companions that need confirmation to secure them as triple systems. Adopting an age of 17 Myr for the LCC association, we find that 14 of the components are faint enough to have planetary masses. Conclusions . Our results indicate that binaries represent ≳2% of the population of FFPs in the LCC association, and this suggests that their preferred formation mechanism is the gravitational collapse independent of a star. Also, many of the recently discovered FFPs in LCC may be unresolved giant binary planets. The wide range of colors and flux reversals observed suggests that the existence of clouds in their atmospheres is important and points to continuity with the BD populations.

Ultra-faint dwarf galaxies (UFDs) are among the oldest and most metal-poor stellar systems in the Universe. Their metallicity distribution encodes the fossil record of the earliest star formation, feedback, and chemical enrichment, providing crucial tests of models of the first stars, galaxy assembly, and dark matter halos. However, due to their faint luminosities and the limited number of bright giants, spectroscopic studies of UFDs typically probe only small stellar samples. Here, we present an analysis of multi-epoch Hubble Space Telescope and James Webb Space Telescope observations of the UFD Boötes I. Using a deep color–magnitude diagram in the F606W and F322W2 bands, extending from the subgiant branch to the M dwarfs, and stellar proper motions to identify likely members, we obtained an unprecedentedly clean census of the system. The exquisite quality of the diagram, combined with the sensitivity of M-dwarf colors to metallicity, allowed us to constrain the metallicity distribution in a large stellar sample. As a first step, we then exploited the metallicity sensitivity of M-dwarf colors to derive the metallicity distribution function. We find that most of the stars ∼85% have [Fe/H] < −2, and that roughly ∼17% have [Fe/H] < − 3. Then, we derived the binary fraction in Boötes I. This is crucial since binaries can bias kinematic mass estimates, affect stellar population analyzes, and shape the photometric signatures used to infer metallicity. We find that 20 ± 2% of stellar systems in Boötes I are binaries with mass ratios larger than 0.4, corresponding to a total binary fraction of ∼30%. This value is comparable to the binary fractions observed in globular clusters of similar stellar mass, suggesting that the presence of dark matter does not significantly affect the binary properties of Boötes I.

A significant fraction of stars in both the Galactic field and stellar clusters belong to binary systems. Understanding their properties is therefore fundamental for a comprehensive picture of stellar structure, stellar evolution, and cluster dynamics. Despite extensive work on binaries in clusters, key questions remain open, particularly concerning photometric binaries among low-mass stars. While the binary fraction among field stars shows a strong dependence on stellar mass, studies of star clusters have so far suggested an approximately constant fraction across the limited mass range explored. Moreover, the mass function (MF) of very low-mass stars remains poorly constrained in clusters older than a few hundred million years. In this work, we used deep Hubble Space Telescope imaging of the intermediate-age open cluster NGC 2158 to investigate its binary population and derive the luminosity and MFs down to ∼0.14 M ⊙ . This dataset enables the first detailed study of binaries in this cluster. We obtained a global binary fraction of 38%, which is consistent with that observed in other open clusters, and detected a clear mass dependence: the fraction decreases from ∼52% at 1.0 M ⊙ to ∼11% at 0.2 M ⊙ . This trend mirrors that seen in the Galactic field, which suggests that binaries in NGC 2158 and field populations share similar properties. The MF of NGC 2158 is best described by three regimes: high-mass stars ( α = −2.49 ± 0.19), low-mass stars ( α = −1.11 ± 0.09), and very low-mass stars ( α = −0.08 ± 0.07). The slope change near 1.0 M ⊙ agrees with recent open cluster surveys, although we find a deficit of stars at the lowest masses ( M ≲ 0.3 M ⊙ ). Finally, we identify a discontinuity in the main sequence around M ∼ 0.3 M ⊙ . We explore the possibility that this feature traces the 3 He-driven instability predicted by stellar models, analogous to the ‘Jao Gap’ observed in the colour–magnitude diagram of nearby field stars.

Context . The Milky Way’s nuclear star cluster (NSC) offers a unique laboratory for studying the formation and evolution of dense stellar systems around a supermassive black hole. Previous work suggests that most stars in the NSC are old; however, the detailed age and metallicity distributions remain uncertain. Aims . We aim to constrain the star formation history (SFH) and metallicity of a poorly explored region of the NSC located about 3 pc from Sagittarius A*. Methods . We analysed NACO/VLT observations in the intermediate-band filter centred at 2.24 μm, complemented by H -band imaging, and constructed colour–magnitude diagrams and completeness-corrected K -band luminosity functions. We clearly identified the red clump and red giant branch bumps. The SFH was derived by fitting cumulative luminosity functions with theoretical models from MIST, PARSEC, and BaSTI, which spans a wide range of ages and metallicities, and by employing Monte Carlo sampling to estimate uncertainties. We also constrained the metallicity of the stellar population, further refined by spectroscopic data from the literature. Results . The NSC stellar population is predominantly old and metal-rich, with 75.6 ± 9.5% of the stellar mass formed ≳10 Gyr ago and a median metallicity of [M/H] ~ +0.35. Significant contributions come from an intermediate-age population around 2–3 Gyr (20.8 ± 8.7%), while minor components appear at ~400 Myr (0.9 ± 0.8%) and 20 Myr (3.6 ± 1.4%), the latter representing a small but non-negligible young population. Systematic uncertainties related to stellar evolution models, binning, photometric range, unresolved binaries, and filter selection are considered. Conclusions . Our findings indicate that the NSC formed predominantly in an early episode, with a substantial contribution from a star formation episode 2–3 Gyr ago and minor younger components. The metallicity estimates are consistent with spectroscopic measurements, and the results agree with the stellar population properties of the inner NSC and the inner nuclear stellar disc, providing useful constraints on the transition between these two structures.

Abstract Using Hubble Space Telescope (HST) imaging, we searched for candidate magnetically active binary (AB) counterparts to low-luminosity Chandra X-ray sources in the globular cluster 47 Tucanae (NGC 104). We consider the catalog of 300 Chandra sources within the half-light radius of 47 Tuc compiled by C. O. Heinke et al. (2005). The 10-band HST imaging used to identify these sources spans the range from 275–658 nm. We generated a cross-matched catalog of objects detected by photometric reductions. We used this catalog to search for counterparts within Chandra source error circles using color–magnitude diagrams and a color–color plot. We found 88 likely ABs that lie primarily in the binary sequence region above the main sequence, 10 possible ABs within this region, 12 sub-subgiants, three subgiants, six blue stragglers, and two yellow stragglers. Of these 121 proposed counterparts, 67 are newly identified. One of the ABs appears to be a pre-cataclysmic variable. Our analysis of the spatial distribution of these proposed counterparts indicates that they are significantly more centrally concentrated than the main-sequence turnoff stars. The inferred characteristic mass for these objects is 1.3 ± 0.1 M ⊙ . A comparison of the numbers of X-ray detected ABs above L X = 10 30 erg s −1 in four globular clusters indicates that their relative numbers can be approximately predicted by the mass and binary fractions of each cluster, suggesting that ABs are a primordial population.

Abstract OB stars generally form in open clusters within the Milky Way’s thin disk, so when they are found at high Galactic latitudes, it is thought that they were ejected from their birth clusters during the past few tens of millions of years. Using Gaia Data Release 3 data, we traced the kinematic trajectories of 39 high-latitude B-type stars and 447 Galactic open clusters with high-quality astrometry to search for moments of past intersection. In cases where we found matching trajectories, we also considered the clusters’ Hertzsprung–Russell diagrams to confirm parent–orphan pairs have matching ages. Further analysis of the clusters’ core environments allowed us to determine a probable ejection mechanism. Through these paternity tests, we have identified possible origins for five of these orphaned B-type stars. Here we present the likely travel times, ejection velocities, and a discussion of the runaway mechanism for each case. We also identify one star whose trajectory did not bring it near the disk during the time period of our analysis, and we discuss its possible origins as well.

Abstract The presence (and nature) of variations in the stellar initial mass function (IMF) at substantially subsolar masses and metallicities ( m < 0.5 M ⊙ and [M/H] ≲ −1, respectively) remains poorly constrained. Predictions from simulations vary widely, while observationally, resolved star studies of ultrafaint dwarf (UFD) galaxies suffer from small sample sizes and background galaxy contamination due to low projected stellar densities. As an alternative metal-poor target, we measure the IMF from resolved stars toward a carefully selected field in the Small Magellanic Cloud, leveraging a plethora of independent constraints on the target field stellar population including distributions of distance, age, and metallicity. We resolve >15,000 stars down to 0.16 M ⊙ within a single pointing of NIRCam on board JWST, using an observing strategy that minimizes contamination from point-source-like background galaxies. We explore three different functional forms of the IMF, forward modeling observed color–magnitude diagrams and luminosity functions. We find a best-fit single power law IMF slope of α = −1.61 − 0.03 + 0.03 , consistent with UFDs probed down to similar limiting masses. Fitting a broken power-law IMF, we find low- and high-mass slopes of α 1 = −1.44 − 0.04 + 0.04 and α 2 = −2.17 − 0.11 + 0.11 , respectively, consistent with solar neighborhood values. Assuming a lognormal IMF, we find a characteristic mass and lognormal width of m c = 0.1 2 − 0.03 + 0.03 M ⊙ and σ = 0.61 − 0.06 + 0.07 M ⊙ , respectively, allowing for characteristic masses lower than local values as seen in some simulations as well as low-metallicity Galactic clusters. Lastly, we quantify the impact of assumptions required in our analysis and discuss potential future improvements.

Abstract Observations show that multiple stellar populations (MPs) are ubiquitous in globular clusters. The Hubble Space Telescope (HST) has been a pivotal tool for previous photometric studies of MPs. The Chinese Space Station Survey Telescope (CSST) is a 2 m telescope scheduled for launch. One of its imaging instruments, the Survey Camera (SC), combines ultraviolet sensitivity comparable to that of HST with a significantly larger field of view, making it well-suited for conducting large-scale photometric surveys of MPs within extensive stellar stream structures. In this work, we perform mock observations of the stellar stream Palomar 5 to assess the feasibility of detecting MPs with the CSST/SC. The results indicate that the CSST/SC cannot resolve MPs in stellar streams at distances comparable to Palomar 5 (≳20 kpc) with one or 10 150 s exposures. This fundamental limitation arises from the absence of the precise proper motions required to disentangle stream members. We estimate that successful resolution would require the target stream to be ≲8 kpc under a 150 s exposure. Furthermore, using theoretical color–magnitude diagrams, we find that the CSST/SC g band provides an optimal balance between contamination rate and completeness rate for member identification in the cluster’s core. However, this approach fails in the stream due to severe field star contamination. Therefore, future CSST observations of Palomar 5 and its tidal tails will employ multiple epochs across several bands to obtain the deep photometry and proper motion data for a definitive MP analysis.

ABSTRACT We perform a non-local thermodynamic equilibrium spectroscopic analysis of eight OB-type stars in the solar neighbourhood, using high-resolution spectra obtained with the 0.8-m telescope at Ankara University Kreiken Observatory together with archival European Southern Observatory/Ultraviolet and Visual Echelle Spectrograph data. Stellar parameters were determined from hydrogen Balmer lines, silicon ionization equilibria, and helium line profiles, while chemical abundances were derived for He, C, N, O, Ne, Mg, Al, Si, S, and Fe. HD 37032 is newly identified as a He-rich peculiar star ([He/H] $\approx +0.33$ dex), with asymmetric H $\alpha$ and [O i] $\lambda$5577 features revealing the presence of circumstellar material. HD 46660, HD 46883, and HD 7694 likewise exhibit mild helium enrichment relative to the solar abundance. HD 29309 is found to be a double-lined spectroscopic binary hosting a Barium-star secondary. A Hertzsprung–Russell diagram analysis indicates that most of the stars are located close to the main sequence, whereas HD 46883 is classified as a blue supergiant with no strong signatures of CNO mixing. These findings provide new constraints of the chemical diversity and evolutionary pathways of early-type stars in the local Galactic environment.

ABSTRACT The stellar initial mass function (IMF) is among the most fundamental distributions in astrophysics, defined as the mass spectrum of stars produced in a single star-formation event. Even in the solar neighbourhood, where measurements can be conducted via star counting, disentangling the IMF from observational effects remains challenging. In this work, we introduce a new parametrization of the stellar IMF in the 100-pc solar neighbourhood, leveraging the high-precision astrometric and photometric data from Gaia DR3: we model the colour–magnitude diagram of the field star population while accounting for observational uncertainties, Malmquist bias, Lutz–Kelker bias, variations in the mass–luminosity relation arising from metallicity differences and the effects of unresolved binaries. In particular, we synthesize the binary population with a process imitating the dynamical evolution observed in star clusters to enforce that all components are drawn from the same IMF, while simultaneously recovering the observed present-day mass-ratio distribution. We determine an averaged stellar IMF over $0.25\lt m\lt 1.0~{\rm M}_{\odot }$ that aligns with canonical IMFs but achieves significantly tighter constraints: $\alpha _1=0.75^{+0.06}_{-0.04}$, $\alpha _2=2.07^{+0.04}_{-0.03}$, and a break point at $m_{\mathrm{break}}=0.40^{+0.01}_{-0.01}$ $\mathrm{{\rm M}_{\odot }}$. Our inference also yields an averaged binary fraction over $0.25\lt m\lt 1.0~{\rm M}_{\odot }$ of approximately 26 per cent, and constrains the Gaia Data Release 3 angular resolution to $1.11^{+0.11}_{-0.08}$ arcsec. We also provide the $\xi$-parameter for our IMF, which is $0.5070_{-0.0096}^{+0.0068}$, to facilitate direct comparison with other IMF determinations.

Abstract The ∼200,000 stars observed by the Kepler mission have provided unprecedented constraints across astrophysics. With the advent of modern spectroscopic and photometric surveys, new limits in stellar characterizations are within reach. In this work, we report a compilation of atmospheric parameters ( T eff , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo>(</mml:mo> <mml:mi>g</mml:mi> <mml:mo>)</mml:mo> </mml:math> , and [M/H]) for the Kepler stars by crossmatching with several spectroscopic and spectro-photometric surveys. We use these to calculate bolometric corrections, which combined with color–magnitude diagram information from Gaia yield self-consistent luminosities on a survey-by-survey basis. These properties will aid in future explorations of Kepler data toward new astrophysical insights. We make our catalog publicly available online in Zenodo (doi: 10.5281/zenodo.18620911 ).

Abstract The supernova remnant (SNR) S147 contains the pulsar PSR J0538+2817 and a likely unbound binary companion, HD 37424. It is the only good Galactic candidate for a binary unbound by a core-collapse supernova. Using Gaia DR3 parallaxes and photometry, we select the stars local to SNR S147 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>ϖ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>S</mml:mi> <mml:mn>147</mml:mn> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>0.7</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.05</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.04</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em"/> <mml:mspace width="0.1em"/> <mml:mtext>mas</mml:mtext> <mml:mspace width="0.1em"/> </mml:math> ) in a cylinder with a projected radius of 100 pc and a parallax range of 0.614 &lt; ϖ &lt; 0.787 mas (a length of ≃360 pc). We individually model the most luminous of these stars. The two most luminous single stars are the unbound binary companion, HD 37424, and HD 37367, with estimated masses of (13.51 ± 0.05) M ⊙ and (14.30 ± 0.09) M ⊙ , respectively. The two most luminous binary systems are the spectroscopic binary HD 37366 and the eclipsing binary ET Tau, which have primary masses of (20.9 ± 0.12) and (16.7 ± 0.09) M ⊙ , respectively. We model the Gaia color–magnitude diagram of this local stellar population using both single stars and a model consisting of noninteracting binaries using solar metallicity PARSEC v2.0 isochrones. For both models, the estimated age distributions of the 439 M G &lt; 0 mag stars favor a high-mass progenitor of 21.5–41.1 M ⊙ for the supernova.

Aims . We have compiled a list of blue lurker (BL) candidates in open clusters using the available rotation catalogs. Blue lurkers are rejuvenated main-sequence stars hidden among normal main-sequence stars on color-magnitude diagrams of star clusters. In comparison to BLs, blue straggler stars, which formed via similar mass transfers or mergers, occupy a distinct space in color-magnitude diagrams. Methods . Blue lurkers can be identified by their unusually faster rotation compared to similar mass stars, which is a signature of recent accretion, or by the presence of a companion (e.g., an extremely low mass white dwarf), which can only be formed by mass donation. We searched for fast-rotating stars on the main sequence of open clusters using Kepler, TESS, and spectroscopic rotation indicators, such as rotation periods and v sin i measurements. Results . We identified 97 new BL candidates across 35 open clusters, almost tripling the previously known sample of 36. Based on the estimated completeness of ≈3%, thousands of BLs are likely hidden within the cluster population. Detailed spectroscopic and time series analyses will be essential to confirming their mass-transfer histories.

In a dissipative system such as star or a galaxy, the emitted photons are decoupled from matter particles and may therefore be considered as part of a closed system to which the second law of thermodynamics applies. In the present work, I defined a global entropy using a statistical approach that accounts for the contributions of both matter particles and photons. The statistical contribution of radiation is described as a photon gas in the definition of this global entropy. The increase in global entropy can foster structure formation –rather than disorder– because structures such as stars and galaxies are efficient in dissipating energy in the form of photons, and thus in producing entropy. I show that stars generate a nearly equal amount of specific entropy and, therefore, a comparable number of photons per unit mass over their lifetime on the main sequence of the Hertzsprung–Russell (HR) diagram. This suggests that the main sequence of the HR diagram constitutes a locus of convergence toward a universal specific entropy production by stars. I then examined the implications of this approach for the star-formation main sequence in galaxies and found a similar result. The emergence of organized structures in cosmic history reflects the second law, as organized matter is efficient in generating entropy through the slicing of energy into lower frequency photons. This is also reflected in the dominant contribution of low-frequency photons to the extragalactic background light. Finally, in this paper I briefly discuss how this perspective may provide insight into the possibility of the existence of life elsewhere in the Universe.

Context. Massive stars emit copious amounts of radiation, profoundly affecting their environment in galaxies and contributing to the reionization of the Universe. However, their evolution and thus their ionizing feedback are still not fully understood. One of the largest gaps in current stellar evolution calculations is the lack of a model for the mass ejections that occur when the stars reach the Eddington limit, such as during a Luminous Blue Variable (LBV) phase. Aims. Here, we aim to remedy this situation by providing a physically motivated and empirically calibrated method applicable in any 1D stellar evolution code to approximate the effect of such mass loss on stellar evolution. Methods. We employed the 1D stellar evolution code MESA, in which we implement a new mass-loss prescription that becomes active when stellar models inflate too much when reaching the Eddington limit. We used lines of constant inflation factors in the Hertzsprung-Russell diagram (HRD) for a simple empirical calibration of the threshold value. We calculated synthetic massive-star stellar populations using grids of single-star models with this mass loss prescription compared them with the observed populations in the Large and Small Magellanic Clouds. Further, with already computed grids of binary evolution models, we investigated the impact of binarity on our predictions. Results. Our single-star models reproduce key features of the observed stellar populations, namely, (i) the absence of stars located beyond the Humphreys-Davidson limit; (ii) an upper limit of red supergiant (RSG) luminosities; (iii) the faintest observed single Wolf-Rayet (WR) stars; (iv) the absolute number of O-stars, WRs, and RSGs; (v) WO stars in low metallicity environments; and (vi) the positions of LBV stars in the HRD. We show that binarity still plays an important role in explaining the observed WR stars. However, a large fraction of the binary population can also be explained via self-stripping. At the same time, our binary population explains the 70% binary fraction of O-stars and the 40% binary fraction of WR stars. However, our synthetic population also has caveats, such as an overproduction of bright H-free WN stars. Conclusions. Our results show that the effect of the Eddington-limit induced mass ejections on the structure and evolution of massive stars can remove the tension between predicted and observed massive star populations. A more fundamental treatment of these effects, particularly for hydrogen-poor stars, is needed to fully comprehend massive star evolution.