POSYDON Version 2: Population Synthesis with Detailed Binary-evolution Simulations across a Cosmological Range of Metallicities

Context. Grids of stellar models, computed with the same physical ingredients, allow one to study the impact of a given physics on a broad range of initial conditions and they are a key ingredient for modeling the evolution of galaxies. Aims. We present here a grid of single star models for masses between 0.8 and 120 M⊙, with and without rotation for a mass fraction of heavy element Z = 0.006, representative of the Large Magellanic Cloud (LMC). Methods. We used the GENeva stellar Evolution Code. The evolution was computed until the end of the central carbon-burning phase, the early asymptotic giant branch phase, or the core helium-flash for massive, intermediate, and low mass stars, respectively. Results. The outputs of the present stellar models are well framed by the outputs of the two grids obtained by our group for metallicities above and below the one considered here. The models of the present work provide a good fit to the nitrogen surface enrichments observed during the main sequence for stars in the LMC with initial masses around 15 M⊙. They also reproduce the slope of the luminosity function of red supergiants of the LMC well, which is a feature that is sensitive to the time-averaged mass loss rate over the red supergiant phase. The most massive black hole that can be formed from the present models at Z = 0.006 is around 55 M⊙. No model in the range of mass considered will enter into the pair-instability supernova regime, while the minimal mass to enter the region of pair pulsation instability is around 60 M⊙ for the rotating models and 85 M⊙ for the nonrotating ones. Conclusions. The present models are of particular interest for comparisons with observations in the LMC and also in the outer regions of the Milky Way. We provide public access to numerical tables that can be used for computing interpolated tracks and for population synthesis studies.

Using the Wide Field Planetary Camera 2 on the Hubble Space Telescope, we have obtained a deep color-magnitude diagram in V- and I-band equivalents for more than 2000 stars in a patch of the outer disk of the Large Magellanic Cloud LMC). Aperture photometry is feasible from these data with good signal-to-noise ratio for stars with V ≤ 25, which allows us for the first time to construct a color magnitude diagram for LMC disk stars on the lower main sequence, extending beyond the oldest main sequence turnoff point. We analyze the structure of the main-sequence band and overall morphology of the color-magnitude diagram to obtain a star formation history for the region. A comparison between the distribution of stars across the main-sequence band for M_v ≤ 4 and a stellar population model constrains historical star formation rates within the past 3 Gyr. The stellar populations in this region sample the outer LMC disk for stars with ages of 1 Gyr or older that have had time to spatially mix. The structure of the main-sequence band requires that star formation occurred at a roughly constant rate during most of the past ≈ 3 Gyr. However, the distribution of subgiant stars indicate that a pronounced peak in the star formation rate likely occurred about 2 Gyr ago, prior to which the star formation rate had not been enhanced for several Gyr. Studies over timescales of more than 3 Gyr require a separation of the effects of star formation history and the chemical evolution on the LMC color-magnitude diagrams, which is difficult to achieve without additional constraints. If lower main-sequence stars in the LMC have moderate metallicities, then the age for most LMC disk stars is less than about 8 Gyr.

We present the first detailed kinematic analysis of the proper motions (PMs) of stars in the Magellanic Bridge, from both the Gaia Data Release 2 catalog and from Hubble Space Telescope (HST) Advanced Camera for Surveys data. For the Gaia data, we identify and select two populations of stars in the Bridge region, young main-sequence (MS) and red giant stars. The spatial locations of the stars are compared against the known H i gas structure, finding a correlation between the MS stars and the H i gas. In the HST fields our signal comes mainly from an older MS and turnoff population, and the PM baselines range between ∼4 and 13 yr. The PMs of these different populations are found to be consistent with each other, as well as across the two telescopes. When the absolute motion of the Small Magellanic Cloud is subtracted out, the residual Bridge motions display a general pattern of pointing away from the Small Magellanic Cloud toward the Large Magellanic Cloud. We compare in detail the kinematics of the stellar samples against numerical simulations of the interactions between the Small and Large Magellanic Clouds, and find general agreement between the kinematics of the observed populations and a simulation in which the Clouds have undergone a recent direct collision.

Stars are the most fundamental components of the visible universe. They provide reference points in the sky, distance markers for cosmologists and tracers of dark matter; serve as the basis for understanding the composition and origin of our solar system; and are companions used to find planets, brown dwarfs, and black holes. Our ability to understand and to use stars effectively for these and other applications depends on knowledge of stellar life cycles. Our view of stellar evolution has changed and progressed rapidly in the past 5 to 10 years, due in part to technological advances, including bigger and better charge-coupled devices, improved ground-based telescopes such as Keck I and II, and continued spectacular results from orbiting satellites such as the Hubble Space Telescope, the Rontgen X-ray Satellite, and the Solar Heliospheric Observatory. Given this progress, the recent discoveries of extrasolar planets (see News story on p. [1336][1]) and brown dwarfs, and the possibility that we may soon identify a black hole and a source for gamma ray bursts using the characteristics of star-like objects near these phenomena, it seems fitting to highlight some advances in stellar research in the nine Articles in this special issue. A star is born by the gravitational collapse of a cloud of dust and gas. If the condensing spherical mass cannot grow large enough, then it slowly gets colder and fades, becoming a brown dwarf. If it grows enough to sustain hydrogen fusion, a main sequence star, like our sun, is formed. Eventually hydrogen is exhausted, the star expands to a red giant as it feeds off other nuclear reactions, and finally the red giant collapses into a small, dense white dwarf, when these reactions cease. In some cases, the collapsing star throws off excess mass as a gaseous envelope, called a planetary nebula. More massive stars tend to die in rapid energetic outbursts called supernovae explosions that produce white dwarfs; or if the remnant core is too massive, collapse continues until most energetic particles are converted to neutrons that stabilize the core into a neutron star. The most massive stars, with the shortest lifetimes and the most energetic supernovae explosions, cannot collapse into white dwarfs or neutron stars; instead, they collapse into an even smaller, denser core from which not even light can escape: a black hole. The first five Articles discuss the early part of the stellar cycle. Kulkarni (p. [1350][2]) focuses on the direct detection of a brown dwarf and discusses how brown dwarfs may be a link between planets and stars. O'Dell and Beckwith (p. [1355][3]) describe the dynamics of stellar birth, focusing on star-forming regions in the Orion Nebula, which show that young stars do not just collapse into spheres of gas but produce jets, winds, and possible planet-forming disks of dust. Arnett and Bazan (p. [1359][4]) discuss the latest progress in understanding nucleosynthetic processes, including the identification of presolar grains in meteorites and the use of high-energy density laser experiments to simulate thermonuclear reactions. Neuhauser (p. [1363][5]) focuses on the x-ray emissions from protostars, which can be seen through molecular clouds that surround protostars. Many protostars are binaries and interact with the circumstellar disk, possibly allowing planet formation within the disks. Burgarella (p. [1370][6]) reviews globular clusters, which are regions of colliding galaxies that produce bursts of star-forming regions encased in dense envelopes of gas. Globular clusters help us understand galaxy formation and the evolution and age of the universe. The death of stars is perhaps most obvious in supernovae explosions. Supernova 1987A, because it was discovered early in its explosive history and was relatively close to Earth, has provided a wealth of information about the chemical and physical evolution of supernovae. Chevalier (p. [1374][7]) reviews the progress made in understanding Type II supernovae, particularly 1987A and 1993 J. Nomoto, Iwamoto, and Kishimoto (p. [1378][8]) discuss Type Ia supernovae, which are believed to represent the thermonuclear explosions of accreting white dwarfs and are used to estimate the age of the universe and distance to its edge. On the quieter side of stellar death, Weinberger and Kerber (p. [1382][9]) describe the diverse morphologies of and applications for planetary nebulae. Spectroscopic observations of planetary nebulae in the halos of galaxies confirm that we cannot account for most of the dark matter in the universe. Finally, Kaaret and Ford (p. [1386][10]) review x-ray binaries, which consist of a neutron star or black hole and a compact star. Understanding how material flows from the star to the black hole will help to identify the event horizon of a black hole. [1]: /lookup/doi/10.1126/science.276.5317.1336 [2]: /lookup/doi/10.1126/science.276.5317.1350 [3]: /lookup/doi/10.1126/science.276.5317.1355 [4]: /lookup/doi/10.1126/science.276.5317.1359 [5]: /lookup/doi/10.1126/science.276.5317.1363 [6]: /lookup/doi/10.1126/science.276.5317.1370 [7]: /lookup/doi/10.1126/science.276.5317.1374 [8]: /lookup/doi/10.1126/science.276.5317.1378 [9]: /lookup/doi/10.1126/science.276.5317.1382 [10]: /lookup/doi/10.1126/science.276.5317.1386

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.

The bar pattern speed of the Large Magellanic Cloud (LMC) has been measured using data, suggesting the presence of a slow pattern that is perhaps not rotating at all. Numerical simulations of interacting LMC-SMC systems were able to reproduce a bar stoppage. Here, we report on the first measurement of the bar pattern speed of the LMC as a function of the evolutionary phase of its stellar populations. Astrometric and photometric data of ∼11 million LMC stars from DR3 were used to build five evolutionary phases, from less to more evolved stars. The Dehnen method, a new procedure to derive bar pattern speeds from kinematics of particles in N-body simulations, was applied to the LMC stellar populations. We observe a modulation of the bar pattern speed with the evolutionary phase, meaning that different LMC stellar populations exhibit different pattern speeds, ranging from -0.9 to 6.6 ̨mskpc. Moreover, less evolved stars have a lower pattern speed, while the bar of more evolved phases tends to rotate faster. The LMC bar is thus extremely slow, ruling out the presence of bar corotation within the disc, in agreement with a previous claim, but this time observed with various stellar populations. It is the first time that a pattern speed is measured separately for different stellar populations in any galaxy. The LMC pattern speed cannot be simply resumed to a singular value, but instead is an overlay of different patterns depending on the evolutionary phase of the stars. Future releases will be crucial to investigate more deeply the relations of the pattern speed with important astrophysical parameters of stars, such as their age and metallicity, which will be helpful to constrain the chemo-dynamical evolution of the LMC bar.

In this paper, we discuss the main properties of the family of star clusters in the Large Magellanic Cloud (LMC). To this aim. we first review the stellar models for low and intermediate mass stars in occurrence of convective overshoot, and then describe an algorithm suitable for a theoretical approach to cluster photometry (colour magnitude (C-M) diagrams. luminosity functions (LF). integrated magnitudes and colours, and total masses). The Johnson BV CCD photometry of the stellar content of NGC 1866 is presented and used to test the theoretical models for intermediate mass stars with convective overshoot. The conclusion of this analysis is that these models fit the C-M diagram and LF of NGC 1866 much better than any other type of model in literature. In addition to this, the Johnson BV CCD photometry of NGC 3 831, another populous cluster of LMC somewhat older than NGC 3 866, is presented and analyzed by means of the same stellar models. In particular, we discuss the effect of unresolved binary stars on alleviating the discrepancy between the age determined from the main sequence stars and the age determined from the red giant stars. Following an idea advanced by Eggen and Iben (1988, 1989) to explain the existence of blue stragglers in young disk aggregates, we explore the possibility that delayed star formation has taken place in NGC 1831. The final part of the review deals with the integrated photometry of star clusters. Theoretical calibrations between integrated colours and age as a function of the chemical composition are briefly discussed. The aim is to derive the age distribution function of LMC cluster and interpret the physical cause of their bimodal distribution in (B-V). The main result is that bimodality simply originates from the kind of age distribution that LMC clusters happen to possess, whereas all other explanations put forward in literature play a marginal role. Furthermore, it is argued that the age distribution function show two peaks of major activity of cluster formation, whose ages are centered at about 1.5×10 and 2×10 yr ago. We also present new estimates of the total mass of many clusters derived from their integrated BV magnitudes. Finally a few words are addressed to the issue of the history of star formation in the field stars of LMC.KeywordsStar FormationMain SequenceStar ClusterLuminosity FunctionStellar ModelThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Context. The quest to detect dormant stellar-mass black holes (BHs) in massive binaries (i.e. OB+BH systems) is challenging; only a few candidates have been claimed to date, all of which must still be confirmed. Aims. To search for these rare objects, we study 32 Galactic O-type stars that were reported as single-lined spectroscopic binaries (SB1s) in the literature. In our sample we include Cyg X-1, which is known to host an accreting stellar-mass BH, and HD 74194, a supergiant fast X-ray transient, in order to validate our methodology. The final goal is to characterise the nature of the unseen companions to determine if they are main-sequence (MS) stars, stripped helium stars, triples, or compact objects such as neutron stars (NSs) or stellar-mass BHs. Methods. After measuring radial velocities and deriving orbital solutions for all the systems in our sample, we performed spectral disentangling to extract putative signatures of faint secondary companions from the composite spectra. We derived stellar parameters for the visible stars and estimated the mass ranges of the secondary stars using the binary mass function. Variability observed in the photometric TESS light curves was also searched for indications of the presence of putative companions, degenerate or not. Results. In 17 of the 32 systems reported as SB1s, we extract secondary signatures, down to mass ratios of ∼0.15. For the 17 newly detected double-lined spectroscopic binaries (SB2s), we derive physical properties of the individual components and discuss why they have not been detected as such before. Among the remaining systems, we identify nine systems with possible NS or low-mass MS companions. For Cyg X-1 and HD 130298, we are not able to extract any signatures for the companions, and the minimum masses of their companions are estimated to be about 7 M⊙. Our simulations show that secondaries with such a mass should be detectable from our dataset, no matter their nature: MS stars, stripped helium stars or even triples. While this is expected for Cyg X-1, confirming our methodology, our simulations also strongly suggest that HD 130298 could be another candidate to host a stellar-mass BH. Conclusions. The quest to detect dormant stellar-mass BHs in massive binaries is far from over, and many more systems need to be scrutinised. Our analysis allows us to detect good candidates, but confirming the BH nature of their companions will require further dedicated monitorings, sophisticated analysis techniques, and multi-wavelength observations.

Magnetic fields can drastically change predictions of evolutionary models of massive stars via mass-loss quenching, magnetic braking, and efficient angular momentum transport, which we aim to quantify in this work. We use the mesa software instrument to compute an extensive main-sequence grid of stellar structure and evolution models, as well as isochrones, accounting for the effects attributed to a surface fossil magnetic field. The grid is densely populated in initial mass (3–60 M⊙), surface equatorial magnetic field strength (0–50 kG), and metallicity (representative of the Solar neighbourhood and the Magellanic Clouds). We use two magnetic braking and two chemical mixing schemes and compare the model predictions for slowly rotating, nitrogen-enriched (‘Group 2’) stars with observations in the Large Magellanic Cloud. We quantify a range of initial field strengths that allow for producing Group 2 stars and find that typical values (up to a few kG) lead to solutions. Between the subgrids, we find notable departures in surface abundances and evolutionary paths. In our magnetic models, chemical mixing is always less efficient compared to non-magnetic models due to the rapid spin-down. We identify that quasi-chemically homogeneous main sequence evolution by efficient mixing could be prevented by fossil magnetic fields. We recommend comparing this grid of evolutionary models with spectropolarimetric and spectroscopic observations with the goals of (i) revisiting the derived stellar parameters of known magnetic stars, and (ii) observationally constraining the uncertain magnetic braking and chemical mixing schemes.

Most globular clusters in the local group show multiple stellar populations, a pattern defined by variations of several light elements, with some also displaying dispersions in heavy elements. Since helium is the most immediate product of hydrogen burning, almost all models suggest that second-generation stars should show different levels of helium enrichment than first-generation stars. Therefore, investigating helium enrichment in stellar populations of globular clusters can constrain different theoretical models. Using the deep photometry carried out by the Hubble Space Telescope, we have analyzed the morphologies of the main sequences of four Large Magellanic Cloud globular clusters, Hodge 11, NGC 1841, NGC 2210, and NGC 2257. We aim to constrain the helium distributions among their main sequence stars. We found a clear broadening of main sequences in all four clusters. After excluding the effects of photometric errors, differential reddening, unresolved binaries, and metallicity dispersions, this broadening would suggest a substantial helium enrichment in their star populations, ranging from delta Y =0.08 to delta Y ≥0.12, depending on the presumed helium distributions. Helium-enriched stars are comparable in number to normal stars within these clusters, and may even dominate, comprising approximately ∼40% to over 80% of the total stellar population. However, given the great distance of the Large Magellanic Cloud, it is difficult to rule out the significant impact of crowding, so our results may only represent an upper limit.

Galactic star formation scaling relations show increased scatter from kpc to sub-kpc scales. Investigating this scatter may hold important clues to how the star formation process evolves in time and space. Here, we combine different molecular gas tracers, different star formation indicators probing distinct populations of massive stars, and knowledge of the evolutionary state of each star-forming region to derive the star formation properties of ∼150 star-forming complexes over the face of the Large Magellanic Cloud (LMC). We find that the rate of massive star formation ramps up when stellar clusters emerge and boost the formation of subsequent generations of massive stars. In addition, we reveal that the star formation efficiency of individual giant molecular clouds (GMCs) declines with increasing cloud gas mass ( ). This trend persists in Galactic star-forming regions and implies higher molecular gas depletion times for larger GMCs. We compare the star formation efficiency per freefall time ( ) with predictions from various widely used analytical star formation models. While these models can produce large dispersions in similar to those in observations, the origin of the model-predicted scatter is inconsistent with observations. Moreover, all models fail to reproduce the observed decline of with increasing in the LMC and the Milky Way. We conclude that analytical star formation models idealizing global turbulence levels and cloud densities and assuming a stationary star formation rate (SFR) are inconsistent with observations from modern data sets tracing massive star formation on individual cloud scales. Instead, we reiterate the importance of local stellar feedback in shaping the properties of GMCs and setting their massive SFR.

We explore the stellar structure of the Large Magellanic Cloud (LMC) disk using data from the Survey of the MAgellanic Stellar History and the Dark Energy Survey. We detect a ring-like stellar overdensity in the red clump star count map at a radius of ∼6° (∼5.2 kpc at the LMC distance) that is continuous over ∼270° in position angle and is only limited by the current data coverage. The overdensity shows an amplitude up to 2.5 times higher than that of the underlying smooth disk. This structure might be related to the multiple arms found by de Vaucouleurs. We find that the overdensity shows spatial correlation with intermediate-age star clusters, but not with young (<1 Gyr) main-sequence stars, indicating the stellar populations associated with the overdensity are intermediate in age or older. Our findings on the LMC overdensity can be explained by either of two distinct formation mechanisms of a ring-like overdensity: (1) the overdensity formed out of an asymmetric one-armed spiral wrapping around the LMC main body, which is induced by repeated encounters with the Small Magellanic Cloud (SMC) over the last Gyr, or (2) the overdensity formed very recently as a tidal response to a direct collision with the SMC. Although the measured properties of the overdensity alone cannot distinguish between the two candidate scenarios, the consistency with both scenarios suggests that the ring-like overdensity is likely a product of tidal interaction with the SMC, but not with the Milky Way halo.

We present a spectroscopic analysis of main sequence (MS) stars in the young globular cluster NGC 1818 (age ∼40 Myr) in the Large Magellanic Cloud. Our photometric survey of Magellanic Clouds clusters has revealed that NGC 1818, similar to other young objects with ages ≲600 Myr, displays not only an extended MS turnoff (eMSTO), as observed in intermediate-age clusters (age ∼1–2 Gyr), but also a split MS. The most straightforward interpretation of the double MS is the presence of two stellar populations: a sequence of slowly rotating stars lying on the blue-MS (bMS) and a sequence of fast rotators, with rotation close to the breaking speed, defining a red-MS (rMS). We report the first direct spectroscopic measurements of projected rotational velocities for the double MS, eMSTO, and Be stars of a young cluster. The analysis of line profiles includes non-local thermodynamic equilibrium effects, required for correctly deriving values. Our results suggest that: (i) the mean rotation for bMS and rMS stars is =71 ± 10 km s−1 (σ = 37 km s−1) and = 202 ± 23 km s−1 (σ = 91 km s−1), respectively; (ii) eMSTO stars have different , which are generally lower than those inferred for rMS stars, and (iii) as expected, Be stars display the highest values. This analysis supports the idea that distinct rotational velocities play an important role in the appearance of multiple stellar populations in the color–magnitude diagrams of young clusters, and poses new constraints on the current scenarios.

Most galaxies, including the Milky Way, harbor a central supermassive black hole (SMBH) weighing millions to billions of solar masses. Surrounding these SMBHs are dense regions of stars and stellar remnants, such as neutron stars (NSs) and black holes (BHs). NSs and possibly BHs receive large natal kicks at birth on the order of hundreds of kilometers per second. The natal kicks that occur in the vicinity of an SMBH may redistribute the orbital configuration of the compact objects and alter their underlying density distribution. We model the effects of natal kicks on a Galactic center (GC) population of massive stars and stellar binaries with different initial density distributions. Using observational constraints from stellar orbits near the GC, we place an upper limit on the steepness of the initial stellar profile and find it to be core-like. In addition, we predict that 30%–70% of compact objects become unbound from the SMBH due to their kicks and will migrate throughout the Galaxy. Different BH kick prescriptions lead to distinct spatial and kinematic distributions. We suggest that the Nancy Grace Roman Space Telescope may be able to distinguish between these distributions and thus be able to differentiate between natal kick mechanisms.

Using the Binary Population and Spectral Synthesis code BPASS, we have calculated the rates, timescales and mass distributions for binary black hole mergers as a function of metallicity. We consider these in the context of the recently reported 1st LIGO event detection. We find that the event has a very low probability of arising from a stellar population with initial metallicity mass fraction above Z=0.010 (Z>0.5Zsun). Binary black hole merger events with the reported masses are most likely in populations below 0.008 (Z<0.4Zsun). Events of this kind can occur at all stellar population ages from ~3 Myr up to the age of the universe, but constitute only 0.1 to 0.4 per cent of binary BH mergers between metallicities of Z=0.001 to 0.008. However at metallicity Z=0.0001, 26 per cent of binary BH mergers would be expected to have the reported masses. At this metallicity the progenitor merger times can be close to ~10Gyr and rotationally-mixed stars evolving through quasi-homogeneous evolution, due to mass transfer in a binary, dominate the rate. The masses inferred for the black holes in the binary progenitor of GW\,150914 are amongst the most massive expected at anything but the lowest metallicities in our models. We discuss the implications of our analysis for the electromagnetic follow-up of future LIGO event detections.