A FEROS spectroscopic study of the extreme O supergiant He 3–759

While massive star clusters have been detected in almost every galaxy with appreciable star formation, they are most prevalent in interacting and merging galaxies. As many as 95% of these clusters will ultimately be disrupted, often in the first 10 Myr, but those clusters that do survive may be the progenitors of globular clusters. Many questions exist regarding these massive clusters and the processes that lead to their formation and disruption, including the uniformity of these processes within a galaxy and between galaxies with different degrees of cluster formation (e.g., quiescent spirals, starbursts, and merging systems). To address these questions, we present a detailed spectroscopic survey of young, massive star clusters in the Antennae, one of the best examples of cluster formation in a merging galaxy. Using near-infrared imaging, we selected a sample of 117 clusters to observe with a combination of near-infrared and optical spectroscopy at the W.M. Keck Observatory. These clusters were chosen to sample the major star-forming regions within the Antennae. This is the largest spectroscopic survey of young massive star clusters in any merging galaxy. Comparing the equivalent widths of hydrogen recombination lines and CO absorption bandheads to the population synthesis models of Starburst99, we measure the age of each cluster. More than half of the clusters show the simultaneous presence of hydrogen recombination lines and CO bandheads, which is not predicted by an instantaneous burst model of cluster formation. We determine that cluster formation is better modeled by a 5 Myr duration constant rate burst of star formation, which we apply to our cluster measurements. We find the vast majority of clusters have ages between 7 and 12 Myr, with a few younger clusters. Comparing cluster ages with predictions of the temporal evolution of cluster luminosity, we find the lack of older (>12 Myr) clusters (and to a lesser extent younger ( Cluster masses are measured by comparing the extinction-corrected K-band luminosity with model luminosity predictions. We find most cluster masses are between 105 and 106 M☉ with a median cluster mass around 3.5 x 105 M☉. Substantial variation exists in masses between different regions, with the overlap region having the most massive clusters on average. These mass differences can be interpreted as size-of-sample effects and our results are consistent with a uniform cluster initial mass function throughout the Antennae. Improved spatial resolution CO (1-0) observations of the Antennae show that younger clusters coincide with areas of enhanced molecular gas concentration and, not surprisingly, also have on average higher extinctions. From two metallicity tracers, we find cluster metallicities consistent with solar values. Based on CO bandhead and SiI equivalent widths in the near-infrared spectra, we uncover strong evidence of a substantial population of M2--M4 supergiants in many of the older clusters.

The initial sizes and masses of massive star clusters provide information about the cluster formation process and also determine how cluster populations are modified and destroyed, which have implications for using clusters as tracers of galaxy assembly. Young massive cluster populations are often assumed to be unchanged since cluster formation; therefore, their distributions of masses and radii are used as the initial values. However, the first few hundred million years of cluster evolution does change both cluster mass and cluster radius, through both internal and external processes. In this paper, we use a large suite of N-body cluster simulations in an appropriate tidal field to determine the best initial mass and initial size distributions of young clusters in the nearby galaxy M83. We find that the initial masses follow a power-law distribution with a slope of −2.7 ± 0.4 , and the half-mass radii follow a lognormal distribution with a mean of 2.57 ± 0.04 pc and a dispersion of 1.59 ± 0.01 pc. The corresponding initial projected half-light radius function has a mean of 2.7 ± 0.3 pc and a dispersion of 1.7 ± 0.2 pc. The evolution of the initial mass and size distribution functions is consistent with mass-loss and expansion due to stellar evolution, independent of the external tidal field and the cluster’s initial density profile. Observed cluster sizes and masses should not be used as the initial values, even when clusters are only a few hundred million years old.

Here we discuss the X-ray emission properties from the hot thermalized plasma that results from the collisions of individual stellar winds and supernovae ejecta within rich and compact star clusters. We propose a simple analytical way of estimating the X-ray emission generated by super star clusters and derive an expression that indicates how this X-ray emission depends on the main cluster parameters. Our model predicts that the X-ray luminosity from the star cluster region is highly dependent on the star cluster wind terminal speed, a quantity related to the temperature of the thermalized ejecta. We have also compared the X-ray luminosity from the super stellar cluster (SSC) plasma with the luminosity of the interstellar bubbles generated from the mechanical interaction of the high-velocity star cluster winds with the interstellar medium (ISM). We found that the hard (2.0-8.0 keV) X-ray emission is usually dominated by the hotter SSC plasma, whereas the soft (0.3-2.0 keV) component is dominated by the bubble plasma. This implies that compact and massive star clusters should be detected as pointlike, hard X-ray sources embedded into extended regions of soft diffuse X-ray emission. We also compared our results with predictions from the population synthesis models that take into consideration binary systems and found that in the case of young, massive, and compact super star clusters the X-ray emission from the thermalized star cluster plasma may be comparable to or even larger than that expected from the high-mass X-ray binary (HMXB) population.

The numerous and massive young star clusters in blue compact galaxies (BCGs) are used to investigate the properties of their hosts. We test whether BCGs follow claimed relations between cluster populations and their hosts, such as the the fraction of the total luminosity contributed by the clusters as function of the mean star formation rate density; the $V$ band luminosity of the brightest youngest cluster as related to the mean host star formation rate; and the cluster formation efficiency (i.e., the fraction of star formation happening in star clusters) versus the density of the SFR. We find that BCGs follow the trends, supporting a scenario where cluster formation and environmental properties of the host are correlated. They occupy, in all the diagrams, the regions of higher SFRs, as expected by the extreme nature of the starbursts operating in these systems. We find that the star clusters contribute almost to the 20 % of the UV luminosity of the hosts. We suggest that the BCG starburst environment has most likely favoured the compression and collapse of the giant molecular clouds, enhancing the local star formation efficiency, so that massive clusters have been formed. The estimated cluster formation efficiency supports this scenario. BCGs have a cluster formation efficiency comparable to luminous IR galaxies and spiral starburst nuclei (the averaged value is about 35 %) which is much higher than the 8 - 10 % reported for quiescent spirals and dwarf star-forming galaxies.

Dwarf galaxies can have very high globular cluster specific frequencies, and the GCs are in general significantly more metal-poor than the bulk of the field stars. In some dwarfs, such as Fornax, WLM, and IKN, the fraction of metal-poor stars that belong to GCs can be as high as 20%–25%, an order of magnitude higher than the 1%–2% typical of GCs in halos of larger galaxies. Given that chemical abundance anomalies appear to be present also in GCs in dwarf galaxies, this implies severe difficulties for self-enrichment scenarios that require GCs to have lost a large fraction of their initial masses. More generally, the number of metal-poor field stars in these galaxies is today less than what would originally have been present in the form of low-mass clusters if the initial cluster mass function was a power-law extending down to low masses. This may imply that the initial GC mass function in these dwarf galaxies was significantly more top-heavy than typically observed in present-day star forming environments.

Two independent sets of arguments lead us to conclude that the progenitors of superintense bursts (with an energy yield larger than that for ordinary supernovae by one or two orders of magnitude) are born in massive dense star clusters, but generally flare up only after they have left the cluster; these are the same objects that are the progenitors of gamma-ray bursts (GRBs). Each of the giant stellar arcs which are grouped into multiple systems of stellar complexes in the LMC and NGC 6946 could only be produced by a single powerful energy release near its center. The progenitors of these systems of arc-shaped stellar complexes must have had a common source nearby, and it could only be a massive star cluster. Such clusters are actually known near both systems. On the other hand, calculations of the dynamical evolution of star clusters show that close binary systems of compact objects are formed in the dense central parts of the clusters and are then ejected from them during triple encounters. Mergers of the components of such systems are believed to be responsible for GRBs. Since their progenitors are ejected from the cluster before merging, the arc-shaped stellar complexes produced by GRBs are observed near (but not around) the parent clusters. If a considerable fraction of the GRB progenitors are formed as a result star encounters in massive star clusters, and if the GRBs themselves trigger star formation near the parent clusters, then observations of GRBs in star-forming regions are consistent with their origin during mergers of pairs of compact objects.

We have used previously published observations of the CO emission from the Antennae (NGC 4038/4039) to study the detailed properties of the supergiant molecular complexes with the goal of understanding the formation of young massive star clusters. Over a mass range from 5 × 106 to 9 × 108 M☉ the molecular complexes follow a power-law mass function with a slope of -1.4 ± 0.1, which is very similar to the slope seen at lower masses in molecular clouds and cloud cores in the Galaxy. Compared with the spiral galaxy M51, which has a similar surface density and total mass of molecular gas, the Antennae contain clouds that are an order of magnitude more massive. Many of the youngest star clusters lie in the gas-rich overlap region, where extinctions as high as AV ~ 100 mag imply that the clusters must lie in front of the gas. Young clusters found in other regions of the galaxies can be as far as 2 kpc from the nearest massive cloud, which suggests that either young clusters can form occasionally in clouds less massive than 5 × 106 M☉ or that these young clusters have already destroyed their parent molecular clouds. Combining data on the young clusters, thermal and nonthermal radio sources, and the molecular gas suggests that young massive clusters could have formed at a constant rate in the Antennae over the last 160 Myr and that sufficient gas exists to sustain this cluster formation rate well into the future. However, this conclusion requires that a very high fraction of the massive clusters that form initially in the Antennae do not survive as long as 100 Myr. Furthermore, if most young massive clusters do survive for long periods, the Antennae must be experiencing a relatively short burst of cluster formation to prevent the final merger remnant from exceeding the observed specific frequency of star clusters in elliptical galaxies by a wide margin. Finally, we compare our data with two models for massive star cluster formation and conclude that the model in which young massive star clusters form from dense cores within the observed supergiant molecular complexes is most consistent with our current understanding of this merging system.

We present an SPH parameter study of the dynamical effect of photoionization from O--type stars on star--forming clouds of a range of masses and sizes during the time window before supernovae explode. Our model clouds all have the same degree of turbulent support initially, the ratio of turbulent kinetic energy to gravitational potential energy being set to $E_{\rm kin}/|E_{\rm pot}|$=0.7. We allow the clouds to form stars and study the dynamical effects of the ionizing radiation from the massive stars or clusters born within them. We find that dense filamentary structures and accretion flows limit the quantities of gas that can be ionized, particularly in the higher density clusters. More importantly, the higher escape velocities in our more massive (10$^{6}$M$_{\odot}$) clouds prevent the HII regions from sweeping up and expelling significant quantities of gas, so that the most massive clouds are largely dynamically unaffected by ionizing feedback. However, feedback has a profound effect on the lower--density 10$^{4}$ and 10$^{5}$M$_{\odot}$ clouds in our study, creating vast evacuated bubbles and expelling tens of percent of the neutral gas in the 3Myr timescale before the first supernovae are expected to detonate, resulting in clouds highly porous to both photons and supernova ejecta.

We numerically investigate whether and how gaseous ejecta from AGB stars can be converted into new stars within originally massive star clusters (MSCs) in order to understand the origin of multiple stellar populations in globular clusters (GCs). We adopt a scenario in which (i) MSCs with masses of M_s can be formed from high-mass, high-density giant molecular clouds (GMCs) in their host galactic building blocks embedded in dark matter halos at high redshifts and (ii) their evolution therefore can be significantly influenced by M_s, their initial locations, and physical properties of their hosts. Our 3D hydrodynamical simulations show that gaseous ejecta from AGB stars can be retained within MSCs and consequently converted into new stars very efficiently in the central regions of MSCs, only if M_s exceed a threshold mass (M_th) of ~10^6 M_sun. The new stars can correspond to the ``second generation (SG)'' of stars with higher Na and lower O abundances observed in GCs. Star formation efficiencies during the formation of SG stars within MSCs with M_s > M_th can be rather high (0.3-0.9) so that very compact new clusters within original MSCs can be formed. M_s should be as large as 10^6-10^7 M_sun to explain the observed large fraction of SG stars in the present ordinary Galactic GCs, because new stars can consist of only 1-4% among all stars for the standard IMF. Nuclear MSCs are found to retain much more effectively the AGB ejecta and convert more efficiently the gas into new stars owing to much deeper gravitational potential of their hosts. We suggest that both M_s and their locations within their hosts can determine whether abundance spread can be seen only in light elements or even in heavy ones.

We present a comparative study of the molecular gas in two galaxies from the Legacy ExtraGalactic UV Survey (LEGUS) sample: barred spiral NGC 1313 and flocculent spiral NGC 7793. These two galaxies have similar masses, metallicities, and star formation rates, but NGC 1313 is forming significantly more massive star clusters than NGC 7793, especially young massive clusters (<10 Myr, >104 M ⊙). Using Atacama Large Millimeter/submillimeter Array (ALMA) CO(2–1) observations of the two galaxies with the same sensitivity and resolution (13 pc), we directly compare the molecular gas in these two similar galaxies to determine the physical conditions responsible for their large disparity in cluster formation. By fitting size–line width relations for the clouds in each galaxy, we find that NGC 1313 has a higher intercept than NGC 7793, implying that its clouds have higher kinetic energies at a given size scale. NGC 1313 also has more clouds near virial equilibrium than NGC 7793, which may be connected to its higher rate of massive cluster formation. However, these virially bound clouds do not show a stronger correlation with young clusters than with the general cloud population. We find surprisingly small differences between the distributions of molecular cloud populations in the two galaxies, though the largest of those differences is that NGC 1313 has higher surface densities and lower freefall times.

Massive star clusters (SCs) have been proposed as additional contributors to Galactic Cosmic rays (CRs), to overcome the limitations of supernova remnants (SNR) to reach the highest energy end of the Galactic CR spectrum. Thanks to fast mass losses through collective stellar winds, the environment around SCs is potentially suitable for particle acceleration up to PeV energies. A handful of star clusters has been detected in gamma-rays confirming the idea that particle acceleration is taking place in these environments. Here we present a new anal- ysis of Fermi-LAT data collected towards a few massive young star clusters and estimate the contribution of these types of sources to the bulk of CRs. We then briefly discuss the observational prospects for ASTRI and CTAO.

Extended main-sequence turnoffs (eMSTOs) are a common feature in color–magnitude diagrams (CMDs) of young and intermediate-age star clusters in the Magellanic Clouds. The nature of eMSTOs is still debated. The most popular scenarios are extended star formation and ranges of stellar rotation rates. Here, we study implications of a kink feature in the main sequence (MS) of young star clusters in the Large Magellanic Cloud (LMC). This kink shows up very clearly in new Hubble Space Telescope observations of the 700 Myr old cluster NGC 1831 and is located below the region in the CMD where multiple or wide MSs, which are known to occur in young clusters and thought to be due to varying rotation rates, merge together into a single MS. The kink occurs at an initial stellar mass of 1.45 ± 0.02 M ⊙; we posit that it represents a lower limit to the mass below which the effects of rotation on the energy output of stars are rendered negligible at the metallicity of these clusters. Evaluating the positions of stars with this initial mass in CMDs of massive LMC star clusters with ages of ∼1.7 Gyr that feature wide eMSTOs, we find that such stars are located in a region where the eMSTO is already significantly wider than the MS below it. This strongly suggests that stellar rotation cannot fully explain the wide extent of eMSTOs in massive intermediate-age clusters in the Magellanic Clouds. A distribution of stellar ages still seems necessary to explain the eMSTO phenomenon.

We use N-body simulations to model the 12 Gyr evolution of a suite of star clusters with identical initial stellar mass functions over a range of initial cluster masses, sizes, and orbits. Our models reproduce the distribution of present-day global stellar mass functions that is observed in the Milky Way globular cluster population. We find that the slope of a star cluster's stellar mass function is strongly correlated with the fraction of mass that the cluster has lost, independent of the cluster's initial mass, and nearly independent of its orbit and initial size. Thus, the mass function - initial mass relation can be used to determine a Galactic cluster's initial total stellar mass, if the initial stellar mass function is known. We apply the mass function - initial mass relation presented here to determine the initial stellar masses of 33 Galactic globular clusters, assuming an universal Kroupa initial mass function. Our study suggests that globular clusters had initial masses that were on average a factor of 4.5 times larger than their present day mass, with three clusters showing evidence for being 10 times more massive at birth.

Three new obscured Milky Way clusters were detected as surface density peaks in the 2MASS point source catalog during our on-going search for hidden globular clusters and massive Arches-like star clusters. One more cluster was discovered serendipitously during a visual inspection of the candidates. The first deep J, H ,a ndKs imaging of the cluster (IBP 2002) CC 01 is presented. We estimated a cluster age of ∼1-3 Myr, distance modulus of (m − M)0 = 12.56 ± 0.08 mag (D = 3.5 kpc), and extinction of AV ∼ 7.7 mag. We also derived the initial mass function slope for the cluster: Γ= −2.23 ± 0.16 (ΓSalpeter = −2.35). The integration over the initial mass function yielded a total cluster mass Mtotal ≤ 1800 ± 200 M� . CC 01 appears to be a regular, not very massive star cluster, whose formation has probably been induced by the shock front from the nearby H region Sh 2-228.

Context.The fate of stars largely depends on the amount of mass lost during the end stages of evolution. For single stars with an initial mass between ∼8–30M⊙, most mass is lost during the red supergiant (RSG) phase, when stellar winds deplete the H-rich envelope. However, the RSG mass-loss rate (Ṁ) is poorly understood theoretically, and so stellar evolution models rely on empirically derived mass-loss rate prescriptions. However, it has been shown that these empirical relations differ largely, with differences up to 2 orders of magnitude.Aims.We aim to derive a new mass-loss rate prescription for RSGs that is not afflicted with some uncertainties inherent in preceding studies.Methods.We have observed CO rotational line emission towards a sample of RSGs in the open cluster RSGC1 that all are of a similar initial mass. The ALMA CO(2–1) line detections allowed us to retrieve the gas mass-loss rates (ṀCO). In contrast to mass-loss rates derived from the analysis of dust spectral features (ṀSED), the data allowed us a direct determination of the wind velocity and no uncertain dust-to-gas correction factor was needed.Results.Five RSGs in RSGC1 have been detected in CO(2–1). The retrievedṀCOvalues are systematically lower thanṀSED. Although only five RSGs in RSGC1 have been detected, the data allow us to propose a new mass-loss rate relation for M-type red supergiants with effective temperatures between ∼3200 and 3800 K that is dependent on the luminosity and initial mass, and that is valid during the phase where nuclear burning determines the evolution along the RSG branch. The new mass-loss rate relation is based on the newṀCOvalues for the RSGs in RSGC1 and on priorṀSEDvalues for RSGs in four clusters, including RSGC1. The newṀ-prescription yields a good prediction for the mass-loss rate of some well-known Galactic RSGs that are observed in multiple CO rotational lines, includingαOri,μCep and VX Sgr. Moreover, there are indications that a stronger, potentially eruptive, mass-loss process is occurring during some fraction of the RSG lifetime, suggesting that RSGs might experience a phase change in mass loss leading to the wind mass-loss rate dominating the RSG evolution at that stage.Conclusions.Implementing a lower mass-loss rate in evolution codes for massive stars has important consequences as to the nature of their end-state. A reduction of the RSG mass-loss rate implies that quiescent RSG mass loss is not enough to strip a single star’s hydrogen-rich envelope. Upon core collapse such single stars would explode as RSGs. Mass-loss rates of order ∼6 times higher would be needed to strip the H-rich envelope and produce a Wolf-Rayet star while evolving back to the blue side of the Hertzsprung–Russell diagram. Future observations of a larger sample of RSGs in open clusters should allow a more stringent determination of theṀCO–luminosity relation and a sharper diagnostic as to when the phase change in mass loss is occurring.