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Star clusters were historically considered simple stellar populations, with all stars sharing the same age and initial chemical composition. However, the presence of chemical anomalies in globular clusters (GCs), called multiple stellar populations (MPs), has challenged star formation theories in dense environments. Literature studies show that mass, metallicity, and age are likely controlling parameters for the manifestation of MPs. Identifying the limit between clusters with/without MPs in physical parameter space is crucial to reveal the driving mechanism behind their presence. In this study, we look for MP signals in Whiting 1, which is traditionally considered a young GC. Using the Magellan telescope, we obtained low-resolution spectra within λλ = 3850–5500 Å for eight giants of Whiting 1. We measured the C and N abundances from the CN and CH spectral indices. C and N abundances have variations comparable with their measurement errors (∼ 0.1 dex), suggesting that MPs are absent from Whiting 1. Combining these findings with literature studies, we propose a limit in the metallicity vs. cluster compactness index parameter space, which relatively clearly separates star clusters with/without MPs (GCs/open clusters). This limit is physically motivated. On a larger scale, the galactic environment determines cluster compactness and metallicity, leading to metal-rich, diffuse, old clusters formed ex situ. Our proposed limit also impacts our understanding of the formation of the Sagittarius dwarf galaxy: star clusters formed after the first starburst (age≲ 8–10 Gyr). These clusters are simple stellar populations because the enriched galactic environment is no longer suitable for MP formation.

Fast radio bursts (FRBs) are flashes of unknown physical origin 1 . The majority of FRBs have been seen only once, although some are known to generate multiple flashes 2 , 3 . Many models invoke magnetically powered neutron stars (magnetars) as the source of the emission 4 , 5 . Recently, the discovery 6 of another repeater (FRB 20200120E) was announced, in the direction of the nearby galaxy M81, with four potential counterparts at other wavelengths 6 . Here we report observations that localized the FRB to a globular cluster associated with M81, where it is 2 parsecs away from the optical centre of the cluster. Globular clusters host old stellar populations, challenging FRB models that invoke young magnetars formed in a core-collapse supernova. We propose instead that FRB 20200120E originates from a highly magnetized neutron star formed either through the accretion-induced collapse of a white dwarf, or the merger of compact stars in a binary system 7 . Compact binaries are efficiently formed inside globular clusters, so a model invoking them could also be responsible for the observed bursts. The fast radio burst FRB 20200120E is shown to originate from a globular cluster in the galaxy M81, and may be a collapsed white dwarf or a merged compact binary star system.

Jellyfish are the most extreme cases of galaxies undergoing ram-pressure stripping. In order to analyse the stellar populations distribution along these galaxies, we have performed stellar population synthesis in data cubes of jellyfish from the GASP programme, using both Starlight and FADO codes.

The formation of our Milky Way can be split up qualitatively into different phases that resulted in its structurally different stellar populations: the halo and the disk components 1 – 3 . Revealing a quantitative overall picture of our Galaxy’s assembly requires a large sample of stars with very precise ages. Here we report an analysis of such a sample using subgiant stars. We find that the stellar age–metallicity distribution p ( τ , [Fe/H]) splits into two almost disjoint parts, separated at age τ  ≃ 8 Gyr. The younger part reflects a late phase of dynamically quiescent Galactic disk formation with manifest evidence for stellar radial orbit migration 4 – 6 ; the other part reflects the earlier phase, when the stellar halo 7 and the old α -process-enhanced (thick) disk 8 , 9 formed. Our results indicate that the formation of the Galaxy’s old (thick) disk started approximately 13 Gyr ago, only 0.8 Gyr after the Big Bang, and 2 Gyr earlier than the final assembly of the inner Galactic halo. Most of these stars formed around 11 Gyr ago, when the Gaia-Sausage-Enceladus satellite merged with our Galaxy 10 , 11 . Over the next 5–6 Gyr, the Galaxy experienced continuous chemical element enrichment, ultimately by a factor of 10, while the star-forming gas managed to stay well mixed. A sample of approximately 250,000 subgiant stars enables an alternative view of the Milky Way’s assembly history, especially the early formation history of the old disk and halo.

Most structural and evolutionary properties of galaxies strongly rely on the stellar initial mass function (IMF), namely the distribution of the stellar mass formed in each episode of star formation 1 – 4 . The IMF shapes the stellar population in all stellar systems, and so has become one of the most fundamental concepts of modern astronomy. Both constant and variable IMFs across different environments have been claimed despite a large number of theoretical 5 – 7 and observational efforts 8 – 15 . However, the measurement of the IMF in Galactic stellar populations has been limited by the relatively small number of photometrically observed stars, leading to high uncertainties 12 – 16 . Here we report a star-counting result based on approximately 93,000 spectroscopically observed M-dwarf stars, an order of magnitude more than previous studies, in the 100–300 parsec solar neighbourhood. We find unambiguous evidence of a variable IMF that depends on both metallicity and stellar age. Specifically, the stellar population formed at early times contains fewer low-mass stars compared with the canonical IMF, independent of stellar metallicities. In more recent times, however, the proportion of low-mass stars increases with stellar metallicity. The variable abundance of low-mass stars in our Milky Way establishes a powerful benchmark for models of star formation and can heavily affect results in Galactic chemical-enrichment modelling, mass estimation of galaxies and planet-formation efficiency. A direct star-counting method of about 93,000 M-dwarf stars in the solar neighbourhood indicates a variable stellar initial mass function that depends on both metallicity and stellar age.

The Chinese Space Station Telescope (CSST) is a cutting-edge two-meter astronomical space telescope currently under construction. Its primary Survey Camera (SC) is designed to conduct large-area imaging sky surveys using a sophisticated seven-band photometric system. The resulting data will provide unprecedented data for studying the structure and stellar populations of the Milky Way. To support the CSST development and scientific projects related to its survey data, we generate the first comprehensive Milky Way stellar mock catalogue for the CSST SC photometric system using the TRILEGAL stellar population synthesis tool. The catalogue includes approximately 12.6 billion stars, covering a wide range of stellar parameters, photometry, astrometry, and kinematics, with magnitude reaching down to g =27.5 mag in the AB magnitude system. The catalogue represents our benchmark understanding of the stellar populations in the Milky Way, enabling a direct comparison with the future CSST survey data. Particularly, it sheds light on faint stars hidden from current sky surveys. Our crowding limit analysis based on this catalogue provides compelling evidence for the extension of the CSST Optical Survey (OS) to cover low Galactic latitude regions. The strategic extension of the CSST-OS coverage, combined with this comprehensive mock catalogue, will enable transformative science with the CSST.

Stars strongly impact their environment, and shape structures on all scales throughout the universe, in a process known as “feedback.” Due to the complexity of both stellar evolution and the physics of larger astrophysical structures, there remain many unanswered questions about how feedback operates and what we can learn about stars by studying their imprint on the wider universe. In this white paper, we summarize discussions from the Lorentz Center meeting “Bringing Stellar Evolution and Feedback Together” in 2022 April and identify key areas where further dialog can bring about radical changes in how we view the relationship between stars and the universe they live in.

Stars are extremely important astronomical objects that constitute the pillars on which the Universe is built, and as such, their study has gained increasing interest over the years. White dwarf stars are not the exception. Indeed, these stars constitute the final evolutionary stage for more than 95% of all stars. The Galactic population of white dwarfs conveys a wealth of information about several fundamental issues and are of vital importance to study the structure, evolution and chemical enrichment of our Galaxy and its components—including the star formation history of the Milky Way. Several important studies have emphasized the advantage of using white dwarfs as reliable clocks to date a variety of stellar populations in the solar neighborhood and in the nearest stellar clusters, including the thin and thick disks, the Galactic spheroid and the system of globular and open clusters. In addition, white dwarfs are tracers of the evolution of planetary systems along several phases of stellar evolution. Not less relevant than these applications, the study of matter at high densities has benefited from our detailed knowledge about evolutionary and observational properties of white dwarfs. In this sense, white dwarfs are used as laboratories for astro-particle physics, being their interest focused on physics beyond the standard model, that is, neutrino physics, axion physics and also radiation from “extra dimensions”, and even crystallization. The last decade has witnessed a great progress in the study of white dwarfs. In particular, a wealth of information of these stars from different surveys has allowed us to make meaningful comparison of evolutionary models with observations. While some information like surface chemical composition, temperature and gravity of isolated white dwarfs can be inferred from spectroscopy, and the total mass and radius can be derived as well when they are in binaries, the internal structure of these compact stars can be unveiled only by means of asteroseismology, an approach based on the comparison between the observed pulsation periods of variable stars and the periods predicted by appropriate theoretical models. The asteroseismological techniques allow us to infer details of the internal chemical stratification, the total mass, and even the stellar rotation profile. In this review, we first revise the evolutionary channels currently accepted that lead to the formation of white-dwarf stars, and then, we give a detailed account of the different sub-types of pulsating white dwarfs known so far, emphasizing the recent observational and theoretical advancements in the study of these fascinating variable stars.

We present a scenario for the formation of super star clusters (with masses larger than 10 5 M ⊙ ) in which multiple generations of star formation will occur. We stress that the gas left over (∼50%) from first generation (1G) star formation should be retained in such massive clusters (thanks to their deep potential wells, with escape speeds larger than 10 km/s) and be available for a second or even third generation of stars, with the basic HeCNONaMgAl chemical anomalies observed in globular clusters, the latter assumed to be the descendents of these super star clusters. One new feature of this model is the role of C + cooling of the dense warm trapped neutral or ionized gas which defines a characteristic temperature of ∼100 K, leading to a second generation (2G) of stars with a top-heavy IMF (M > 5 M ⊙ ). The ashes of the 2G very massive stars (VMS, M > 100 M ⊙ ) sampled in this IMF quickly pollute and dilute the left-over pristine gas with their slow winds (that cannot escape the cluster), while the majority of massive stars develop fast winds (that actually can escape from the cluster). Meanwhile, much of the remaining dense T = 100 K gas contracts gravitationally in the massive cluster and may reach densities of the order of 10 9 cm −3 , in which case the Jeans mass drops to about 0.2 M ⊙ and leads to a substantial low-mass pre-MS 3G population (most likely on a very short timescale). In this way, we may solve both the mass budget and the excess Helium problem in proto-globular clusters, while also explaining the Na-O and Mg-Al anti-correlations resulting from hot H-burning of very massive stars at 45MK and 75MK, respectively.

We review the current knowledge about nuclear star clusters (NSCs), the spectacularly dense and massive assemblies of stars found at the centers of most galaxies. Recent observational and theoretical works suggest that many NSC properties, including their masses, densities, and stellar populations, vary with the properties of their host galaxies. Understanding the formation, growth, and ultimate fate of NSCs, therefore, is crucial for a complete picture of galaxy evolution. Throughout the review, we attempt to combine and distill the available evidence into a coherent picture of NSC evolution. Combined, this evidence points to a clear transition mass in galaxies of ∼109M⊙ where the characteristics of nuclear star clusters change. We argue that at lower masses, NSCs are formed primarily from globular clusters that inspiral into the center of the galaxy, while at higher masses, star formation within the nucleus forms the bulk of the NSC. We also discuss the co-existence of NSCs and central black holes, and how their growth may be linked. The extreme densities of NSCs and their interaction with massive black holes lead to a wide range of unique phenomena including tidal disruption and gravitational-wave events. Finally, we review the evidence that many NSCs end up in the halos of massive galaxies stripped of the stars that surrounded them, thus providing valuable tracers of the galaxies’ accretion histories.