Observations from Hubble Space Telescope (HST) and James Webb Space Telescope (JWST) continue to reveal gravitationally magnified high-redshift star candidates, resulting in a rising demand for accurate stellar bolometric corrections to facilitate comparisons between theoretical stellar evolutionary models and observational data. To address this need, we updated our YBC stellar bolometric correction database by incorporating bolometric corrections for cosmologically redshifted stars, now called the zYBC database. These bolometric corrections were derived by redshifting stellar spectra from public stellar spectral libraries, attenuated by the extinction curves for both the dust in the host galaxy and the Milky Way, followed by convolution with the transmission curves of various photometric filters. Our methodology naturally incorporates the effects due to the cosmological K-correction and the dust. In addition to the existing stellar spectral libraries in the previous version of YBC, we included stellar spectral libraries from non-local thermodynamic equilibrium models (PoWR, TLUSTY, and CMFGEN) for O, B stars, which are more appropriate for hot massive stars. In particular, the PoWR and CMFGEN models include the modelling of stellar wind. The database supports key photometric systems for high-redshift studies, such as HST/WFC3 wide-band filters, JWST/NIRCam, and Chinese Space-station Survey Telescope (CSST) equipped with its Main Survey Camera (MSC) and Multi-Channel Imager (MCI), which are pivotal facilities for the current and future observational studies of high-redshift stars. The database maintains the flexibility to incorporate additional photometric systems upon request. As examples, we present colours as functions of log Teff at various redshifts for several photometric systems, which exhibit non-monotonic behaviours and demonstrate the necessity for a dedicated modelling. In particular, we find that the relations show larger dispersions at high redshifts than in the zero redshift case. This indicates that the stellar parameters of high-redshift stars can be better determined than those of their local counterparts, given their redshifts can be reliably determined through other methods (e.g. from their host galaxies) and their photometric data are of high enough quality for physical parameter determinations via spectral-fitting methods. We also demonstrate the difference in the effects brought on by the variety in the amounts of extinctions. This updated database represents a valuable resource for high-redshift star research.

Growing evidence suggests that the stellar initial mass function (IMF) varies systematically across galaxies, deviating from the canonical Milky Way form. Such variations would modify the integrated nucleosynthetic yields and, hence, the abundance patterns used in stellar population synthesis studies. The question of how these could impact, in particular, the sodium abundance (and sodium-to-oxygen ratios) in star-forming galaxies is not fully understood. In this work, we carry out a systematic study of how high-mass IMF variations affect sodium enrichment using a one-zone galactic chemical evolution model. The model incorporates star formation histories from semi-analytic simulations and is calibrated to match the observed galaxy mass–metallicity relation. We find that varying the IMF high-mass end (and the IMF slope) could only alter the sodium abundance by less than 0.1 dex, across galaxies with stellar masses from 10 9 M ⊙ to 10 11 M ⊙ . This result is robust under different stellar models and galaxy evolution assumptions, primarily because the sodium production is similar to that of oxygen. We conclude that sodium abundance is largely insensitive to changes in the high-mass IMF and, thus, it is unlikely to negatively impact the use of sodium indices as IMF diagnostics in stellar population studies.

We present phosphorus abundance measurements for a total of 102 giant stars, including 82 stars in 24 open clusters and 20 Cepheids, based on high-resolution near-infrared spectra obtained with GIANO-B. Evolution of phosphorus abundance, despite its astrophysical and biological significance, remains poorly understood due to a scarcity of observational data. By combining precise stellar parameters from the optical, a robust line selection and measurement method, we measure phosphorus abundances using available P I lines. Our analysis confirms a declining trend in [P/Fe] with increasing [Fe/H] around solar metallicity for clusters and Cepheids, consistent with previous studies. We also report a [P/Fe]-age relation among open clusters older than 1 Gyr, indicating a time-dependent enrichment pattern. Such a pattern can be explained by the different stellar formation history of their parental gas, with more efficient stellar formation in the gas of older clusters (thus with higher phosphorus abundances). [P/Fe] shows a flat trend among Cepheids and clusters younger than 1 Gyr (along with three Cepheids inside open clusters), possibly hinting at the phosphorus contribution from the previous-generation low-mass stars. Such a trend suggests that the young clusters share a nearly common chemical history, with a mild increase in phosphorus production by low-mass stars

The Chinese Space Station Survey Telescope (CSST) is a flagship space mission, supported by the China Manned Space Project, designed to carry out a large-area sky survey to explore the nature of dark matter and dark energy in the Universe. The onboard multi-band imaging and slitless spectroscopic modules will enable us to obtain photometric data for billions of galaxies and stars, as well as hundreds of millions of spectroscopic measurements, advancing various scientific analyses such as galaxy clustering and weak gravitational lensing. To support the image simulations for the main survey of the CSST mission, we present a mock catalog of stars and galaxies. For stars, the mock catalog is generated using either Galaxia or TRILEGAL , both of which provide a range of stellar properties to meet the requirements of CSST image simulations. For galaxies, we built a mock light-cone up to redshift z ∼ 3.5 from the cosmological N -body simulation and populated the mock galaxy catalog from the dark matter haloes using a semi-analytical galaxy formation model. We then performed a full-sky ray-tracing simulation of weak gravitational lensing to obtain lensing shear at the position of each galaxy in the light-cone. To support both multi-band imaging and slitless spectroscopic simulations, we computed the spectral energy distribution (SED) for each galaxy based on its star formation history using a supervised deep learning model and determined the magnitudes in each band using the CSST throughputs. Finally, the properties of our mock galaxies include positions, redshifts, stellar masses, shapes, sizes, SEDs, lensing shears and magnifications. We have validated our mock catalog against observational data and theoretical models, with results showing good overall agreement. The catalog provides a flexible data set for the development of CSST image processing and can support a wide range of cosmological analyses within the CSST mission.

The ultraviolet (UV) spectral domain occupies a unique position in stellar astrophysics, serving as the bridge between the thermal continuum of photospheres and the high-energy, non-thermal processes of stellar coronae and winds. This article provides a review of stellar physics in the UV, addressing both the theoretical framework and observational applications across the Hertzsprung-Russell diagram. We explicitly structure our discussion around key scientific questions, demonstrating that accurate spectral synthesis in this regime demands Non-LTE radiative transfer codes, which in turn rely on precise atomic collision and recombination rates. We highlight how a critical scarcity of modern laboratory astrophysics data limits these models, particularly for complex ions. Moving to observational diagnostics, we review how UV spectroscopy constrains diffusion and radiatively driven winds in hot subluminous stars, and traces shock dynamics and abundance patterns in Planetary Nebulae and Supernova Remnants. In the context of star clusters, we illustrate how UV sensitivity to light-element variations (C, N, O) allows us to disentangle multiple stellar populations that appear degenerate in optical bands. We conclude that future progress depends on facilities capable of high-resolution spectroscopy, time-domain monitoring, and polarimetry to recover these diagnostic tracers and resolve the physics of stellar feedback.

Observations from HST JWST continue to reveal gravitationally magnified high-redshift star candidates, resulting in increasing demand for accurate stellar bolometric corrections to compare stellar models with observational data. We update YBC stellar bolometric correction database by incorporating bolometric corrections for cosmologically redshifted stars, called zYBC. The bolometric corrections are derived by redshifting stellar spectra from libraries, attenuated by extinction curves for both the dust in the host galaxy and the Milky Way, and followed by convolution with the transmission curves of photometric filters. Our methodology incorporates the effects due to the cosmological K-correction and the dust. Besides the spectral libraries in earlier YBC, we add NLTE-based spectral libraries for O, B stars, which are better suited for hot massive stars, particularly wind-included PoWR and CMFGEN models. The database supports key photometric systems for high-redshift studies, such as HST/WFC3, JWST/NIRCam, and CSST's MSC and MCI, and maintains the flexibility to incorporate additional photometric systems upon request. As examples, we present colors as functions of Teff at various redshifts for several photometric systems, which exhibit non-monotonic behaviors and demonstrate the necessity for a dedicated modelling. In particular, we find that the relations show larger dispersions at high redshift than the zero redshift case. This indicates that the stellar parameters of high redshift stars can be better determined than those of their local counterparts, given their redshifts reliably determined through other methods and their photometric data are of high enough quality for physical parameter determination through spectral-fitting. We also show the difference in the effect brought by the different amount of extinctions. zYBC represents a valuable resource for high-redshift star research.

This White Paper presents the scientific rationale and instrument concept for HRMOS (High-Resolution Multi-Object Spectrograph), a next-generation instrument proposed for the ESO Very Large Telescope within the VLT 2030 roadmap. Current and planned facilities offer either multi-object spectroscopy or ultra-high spectral resolution, but not both. HRMOS fills this gap by combining very high spectral resolution, multi-object capability, and radial-velocity stability, enabling transformative studies in Galactic and extragalactic astrophysics. The baseline design provides a resolving power of R = 80000, radial-velocity precision of 10 m s-1 (goal: 5 m s-1), simultaneous observations of 50-60 targets, and broad optical coverage down to 385 nm. These capabilities enable precise measurements of elemental abundances, isotopic ratios, line profiles, and radial velocities for large stellar samples, including crowded fields, star clusters, the Galactic bulge, and nearby dwarf galaxies. HRMOS will address key questions on the age of the oldest stellar populations through nucleocosmochronology, the formation and survival of planetary systems, the assembly history of the Milky Way and satellites, the origin of the heaviest elements, stellar evolution, and the chemical and dynamical properties of the interstellar and circumgalactic medium. It will bridge large spectroscopic surveys and the next generation of extremely large telescopes, with strong synergies with 4MOST, Gaia, TESS, PLATO, the proposed Haydn mission, and future ELT instruments. Building on VLT/FLAMES heritage, HRMOS represents a strategic investment for European astronomy in the 2030s.

Synthetic-template subtraction is widely used to measure chromospheric activity in large spectroscopic surveys. However, many solar-like FGK stars show systematically negative Ca II infrared triplet (IRT) residual indices, implying that the observed line cores are deeper than those predicted by parameter-matched templates. We investigate this effect using solar-like stars from LAMOST DR9, MaStar, and XSL DR3, measuring activity indices (R+) for both the Ca II H K and IRT lines in a uniform framework. We find that observational effects, including atmospheric-parameter offsets, treatment of the instrumental line-spread function, and propagated measurement uncertainties, contribute to scatter but do not explain the systematic negative bias in R+_IRT. The results instead suggest that the negative bias most likely arises because photospheric templates underestimate the depth of the IRT cores, likely owing to missing chromospheric structure and, to a lesser extent, NLTE effects. An empirical increase in the adopted microturbulent velocity deepens the synthetic IRT cores and partially mitigates the negative offset. In addition, R+ values derived from different synthesis configurations show systematic offsets but generally preserve strong linear correlations, indicating that they can be cross-calibrated. These results clarify the origin of negative Ca II IRT residual indices and help interpret template-dependent systematics in chromospheric activity measurements based on synthetic-template subtraction.

Open clusters are the primary birthplaces of stars in the Milky Way disk, yet their neutron star progeny are rarely found within them, presumably due to supernova-induced kicks that eject them at birth. Here we report the arcsec-level localization of the pulsar PSR J1921+3745 to the tidal tail of NGC 6791, one of the oldest and most massive open clusters. Our N-body simulation shows that more than 95

Stars with outflows impinging on ambient gas experience accelerations due to the gravitational feedback from the interaction morphology between the outflow and the ambient gas. Such "negative dynamical friction" (NDF), in contrast to the conventional "dynamical friction" (DF), is studied for its impact on the dynamics of open clusters (OCs) immersed in a uniform ambient gas. We modify the N-body integration code REBOUND with both NDF and DF implemented according to the outflow conditions of each star in a consistently constructed OC. The evolution of stars is also involved in determining the gas-star interactions throughout their stellar lives. Compared to DF-only and gas-free models with identical initial conditions, the NDF-affected cluster is puffier and evaporates faster, as indicated by various diagnostics, including lower velocity dispersions and larger half-mass and half-light radii. Neutron stars with fast winds are expelled from the cluster due to their intensive NDF effect, even without the "kicks" by asymmetric supernovae. Exploration of parameter space confirms that the NDF effect is generally enhanced with higher ambient gas densities, in qualitative agreement with the expression of acceleration. Outflow-ambient interactions should be considered for the proper interpretation of the stellar dynamics evolution in clusters.

Individual stars located near the caustics of galaxy clusters can undergo extreme magnification when crossing micro-caustics, rendering them observable even at cosmological distances. Though most massive stars are likely reside in binary systems rather than as single star, the influence of binary star system on magnification events is severely under-explored. In this work, we simulate the light curves produced by detached binary stars crossing micro-caustics, aiming to characterize their unique observational signatures.Using high-resolution magnification maps generated by the GPU-PMO-CAUSTIC algorithm and PARSEC stellar models with red-shifted magnitude, we examined the impact of binary star parameters and crossing geometries on microlensing magnification patterns. Our simulations reveal that binary stars produce diverse light curve features, including overlapping peaks, plateau-like structures, and time-variable color-magnitude differences. These features, particularly the distinct temporal variations in spectral energy distributions, offer diagnostic tools for distinguishing binary systems from single stars.We further demonstrate the potential of multi-band photometry using the Chinese Space Station Telescope's Multi-Channel Imager (CSST-MCI) to capture these variations.Our findings provide theoretical support for identifying binary systems in future caustic-crossing events, enabling more accurate characterization of high-redshift stellar populations.

We present an analysis of the young stellar moving group ASCC 127 using Gaia DR3 data, significantly expanding its membership to 3971 stars—double the number identified in previous studies. Using kinematic and distance criteria, ASCC 127 is divided into five subgroups (Groups 1−5) with ages spanning from 15 to 32 Myr. Groups 1−5 are spatially linked to the Cepheus Flare star-forming region, revealing potential evidence of four sequential star formation episodes at approximately 32 Myr, 20 Myr, 15 Myr, and 7 Myr. Through dust and gas mapping, we identify a spatial cavity extending several tens of parsecs, which may have resulted from feedback processes such as supernovae associated with earlier generations of stars in the region. This structure, along with the larger Loop III feature, indicates that feedback from massive stars likely influenced the interstellar medium (ISM). By integrating young stellar populations with ISM studies, we provide a detailed picture of the feedback-driven star-formation history in the Cepheus Flare region.

Open clusters offer unique opportunities to study stellar dynamics and evolution under the influence of their internal gravity, the Milky Way's gravitational field, and the interactions with encounters. Using the Gaia DR3 data for a catalog of open clusters within 500 parsecs that exhibit tidal features reported by the literature, we applied a novel method based on 3D principal component analysis to select a ``golden sample'' of nearby open clusters with minimal line-of-sight distortions. This approach ensures a systematic comparison of 3D and 2D structural parameters for tidally perturbed clusters. The selected golden sample includes Blanco 1, Melotte 20, Melotte 22, NGC 2632, NGC 7092, NGC 1662, Roslund 6, and Melotte 111. We analyzed these clusters by fitting both 2D and 3D King Profiles to their stellar density distributions. Our results reveal systematic discrepancies, with most of the golden sample clusters exhibiting larger 3D tidal radii compared to their 2D counterparts, demonstrating that the 2D projection effects bias the measured cluster size. Furthermore, the 3D density profiles show stronger deviations from the King profiles at the tidal radii (Δ ρ_ 3D > Δ ρ_ 2D ), highlighting enhanced sensitivity to tidal disturbances. Additionally, we investigated the spatial distribution of cluster members relative to their bulk motion in the Galactic plane. We find that some clusters exhibit tidal features oriented perpendicular to their direction of motion, which can be attributed to the fact that the current surveys only detect the curved inner regions of the tidal features. In conclusion, this work offers a golden sample of nearby open clusters that are most reliable for 3D structure analysis and underscores the necessity of 3D analysis when characterizing OC morphological asymmetries, determining cluster size, and identifying tidal features.

The Milky Way is a dynamic and evolving system shaped by numerous merger events throughout its history. These mergers bring stars with kinematic and dynamic properties differing from the main stellar population. However, it remains uncertain whether any of the Galactic supernova remnants can be attributed to such a merger origin. In this work, we compare the progenitor of Kepler’s supernova to its nearby stars, “alien” stars, and in situ Milky Way stellar populations. We uncover the abnormal kinematics and dynamics of Kepler’s supernova and propose that its progenitor may have an extragalactic origin. We call the Type Ia supernovae (SNe Ia) produced by stars accreted into the Milky Way through merger events “alien SNe Ia” since they are cosmic immigrants. We estimate the rate of alien SNe Ia exploded recently using two methods: through galactic chemical evolution, and through a method without considering exact star formation history, introduced for the first time in this paper. We consider the past accretion of a few major satellite galaxies—Kraken, Gaia–Enceladus–Sausage, the Helmi streams, Sequoia, Sagittarius, Wukong/LMS-1, and Cetus—assuming these were dry mergers. The first method yields 1.5 × 10 −5 –5.0 × 10 −5 yr −1 , while the second method yields a comparable 3.1 − 1.1 + 1.8 × 1 0 − 5 yr − 1 as the rate estimates for recent alien SNe Ia. These estimates represent lower bounds because we assumed no postmerger star formation.

Recent literature reports a color deviation between observed Gaia color-magnitude diagrams (CMDs) and theoretical model isochrone predictions, particularly in the very low-mass regime. To assess its impact on cluster age determination via isochrone fitting, we quantified the color deviations for three benchmark clusters, Hyades, Pleiades, and Praesepe, both for the Gaia color (BP-RP) and (G-RP). In general, the (G-RP) color deviations are smaller than the (BP-RP) ones. Empirical color correction functions based on these benchmarks are derived for the currently available MIST and PARSEC 1.2S isochrone models. Applying the correction functions to 31 additional open clusters and 3 moving groups results in a significantly improved alignment between the isochrones and observed CMDs. With our empirical corrections, isochrones provide age estimates consistent with literature values obtained through the spectral Lithium Depletion Boundary method, validating the effectiveness of our approach. The corresponding metallicities with PARSEC 1.2S also show a good agreement with the spectroscopic results. The empirical color correction function we present in this work offers a tool for a consistent age determination within the full mass range of stellar clusters using the isochrone fitting method.

Open clusters (OCs) are the primary contributors to the native stellar population of the Milky Way disk. Neutron stars (NSs) -- remnants of massive stars -- in the disk should also originate from OCs, along with the resulting pulsars. However, no NS associated with any OC has been validated, presumably due to high-velocity kicks, exceeding the OC's escape velocity, imparted by supernova explosions. We have achieved arcsec-level localization (~1/1000 of the OC's tidal radius) to place the newly found pulsar PSR J1921+3745 in the tidal tail as well as near the tidal radius edge of NGC 6791, one of the oldest and most massive Galactic OCs. Our N-body simulation shows that, for an OC like NGC 6791, a substantial fraction of NSs with low-velocity kicks, peaking around 3 km/s in our calculation, remain in the cluster's tidal tails. The radio PSR J1921+3745 in the cluster tidal tail thus represents a snapshot of an escaping NS from an OC to the Galactic disk field. Further identification and characterization of pulsars associated with OCs are crucial to probe the origin of Galactic disk neutron stars and their co-evolution with the clusters.

Recent studies have increasingly identified extended main sequence turn-off (eMSTO) phenomena in Galactic open clusters, yet the number of such clusters with sufficient spectroscopic information for member stars remains limited. Unlike most studies that rely on fitting isochrones based on color-magnitude diagram (CMD) morphology to account for varying rotational velocities, our approach leverages LAMOST spectral data to compute actual rotational velocity distributions for confirmed cluster members, along with parameters such as metallicity, differential extinction, and rotational inclination, to utilize PARSEC isochrones for fitting the cluster CMDs. We systematically surveyed all known Galactic open clusters and selected 12 clusters where rotational velocity distributions could be reliably calculated for detailed fitting. Our results successfully reproduced the eMSTO phenomenon observed in these clusters. For the majority of clusters, considering only differential extinction and variations in rotational velocity adequately explains the position and morphology of the MSTO. For some intermediate-age clusters, incorporating rotational inclination additionally accounts for the broadening of the MSTO. This study underscores the importance of spectroscopic data in understanding eMSTO phenomena and provides a probable explanation for interpreting the combined effects of differential extinction, rotation, and inclination on the CMDs of Galactic open clusters.

We present a homogeneous catalog of 1232 open clusters with precisely determined ages, metallicities, distances, extinctions, and stellar mass function (MF) slopes, derived from Gaia DR3 data. The parameters are inferred using the Mixture Model for Open clusters (MiMO), a novel Bayesian framework for modeling clusters in the color–magnitude diagram. By explicitly accounting for field-star contamination as a model component, MiMO removes the conventional need for stringent membership preselection, allowing for a more complete inclusion of member stars, and thereby enhancing both precision and robustness. Our results broadly agree with existing catalogs but offer improved precision. For each cluster, we provide the best-fit age, metallicity, distance, extinction, and MF slope, along with their full likelihood chains and photometric membership probabilities for individual stars. We further identify an “MF Prime” subsample of 163 clusters with high-quality data, for which the MF estimates are considered most reliable. The catalog and an open-source implementation of MiMO are made publicly available to the community.

We present a homogeneous catalog of 1,232 open clusters with precisely determined ages, metallicities, distances, extinctions, and stellar mass function (MF) slopes, derived from Gaia DR3 data. The parameters are inferred using the Mixture Model for Open clusters (MiMO), a novel Bayesian framework for modeling clusters in the color-magnitude diagram. By explicitly accounting for field-star contamination as a model component, MiMO removes the conventional need for stringent membership preselection, allowing for a more complete inclusion of member stars and thereby enhancing both precision and robustness. Our results broadly agree with existing catalogs but offer improved precision. For each cluster, we provide the best-fit age, metallicity, distance, extinction, and MF slope, along with their full likelihood chains and photometric membership probabilities for individual stars. We further identify an “MF Prime” subsample of 163 clusters with high-quality data, for which the MF estimates are considered most reliable. The catalog and an open-source implementation of MiMO are made publicly available to the community.

In this study, we develop a membership identification method and apply it for 30 open clusters (OCs) within 200 pc of the Sun using astrometric data of Gaia DR3. By accounting for projection effects that distort apparent stellar motions, our approach converts astrometric data into accurate five-dimensional positions and velocities. This approach enables better identification of members in nearby OCs. We then compare our refined membership lists with previous catalogs, revealing more members in most OCs, but also the identification of elongated structures in Melotte 25 (Hyades), NGC 2632 (Praesepe), Melotte 111 (Coma Berenices), Platais 3, Melotte 22 (Pleiades), NGC 2451A, Platais 9, IC 2391, Platais 8, UPK 640, and HSC 2986, which we studied in detail. An analysis of the ages of their members reveals that the members within and outside of the tidal radius are distinctly coeval, further validating our methodology. This study suggests that for OCs in the solar neighborhood, correcting for the projection effect is very important for the identification of OC members.