While it is well known that mass transfer in binaries can pollute the surfaces of the accretors, it is still unclear whether this mechanism can reproduce the observed chemical inhomogeneities in globular clusters. We study the surface abundances of the accretors in low-mass binaries, as a first step towards understanding whether mass transfer in low-mass binaries is one of the potential origins of the aforementioned abundance anomalies in globular clusters. We use the Modules for Experiments in Stellar Astrophysics code to calculate binary evolutionary models with different initial donor masses between 0.9 and 1.9 $ \rm {M}_\odot$ for an initial metallicity of Z = 0.0034. The results show that in some low-mass binary systems, the accretors exhibit peculiar chemical patterns when they are still unevolved stars, e.g. C and O depletion; Na and N enhancement; constant Mg, Al and C+N+O. The abundance patterns of the accretors are significantly different from their initial abundances (or that of normal single stars), and can match the observed populations. These abundance patterns strongly depend not only on the initial parameters of binaries (donor mass, mass ratio, and orbital period), but also on the assumptions regarding mass-transfer efficiency and angular momentum loss. These results support the hypothesis that mass transfer in low-mass binaries is at least partly responsible for the unevolved anomalous stars in globular clusters. More work on binary evolutionary models and binary population synthesis is required to fully evaluate the contribution of this scenario.
Context: We present the physical and chemical properties of intermediate-mass stars models of low metallicity, evolved along the thermal pulse phase. Aims: The target of this work is to extend to low metallicities, Z=1,2 and 6 x 10^{-4}, the models previously computed for chemistries typical of Globular Clusters of an intermediate metallicity (Z=0.001), and for the most metal-rich clusters found in our Galaxy (Z=0.004); the main goal is to test the self-enrichment scenario also for metal poor Globular Clusters Methods: We calculated three grids of intermediate-mass models with metallicities Z=…
We report time-series photometry for 55 variable stars located in the central part of the globular cluster M55. The sample includes 28 newly identified objects of which 13 are eclipsing binaries. Three of these are detached systems located in the turn-off region on the cluster color-magnitude diagram. Two of them are proper motion (PM) members of M55 and are excellent candidates for a detailed follow-up study aimed at a determination of the cluster age and distance. Other detached binaries are located along the unevolved part of the cluster main sequence. Most of the variables are cluster blue…
Atomic diffusion in stars can create systematic trends of surface abundances with evolutionary stage. Globular clusters offer useful laboratories to put observational constraints on this theory as one needs to compare abundances in unevolved and evolved stars, all drawn from the same stellar population. In this paper, we show the results of an abundance study of stars in the globular cluster NGC6752 which shows weak but systematic abundances trends with evolutionary phase for Fe, Sc, Ti and Ca. The trends are best explained by a stellar structure model including atomic diffusion with efficient…
We propose that the abundance anomalies of proton-capture elements in globular clusters, such as the C-N, Na-O, Mg-Al and Na-F anti-correlations, were produced by super-massive stars with M ~ 10,000 Msun. Such stars could form in the runaway collisions of massive stars that sank to the cluster center as a result of dynamical friction, or via the direct monolithic collapse of the low-metallicity gas cloud from which the cluster formed. To explain the observed abundance anomalies, we assume that the super-massive stars had lost significant parts of their initial masses when only a small mass fra…
One crucial piece of information to study the origin of multiple stellar populations in globular clusters, is the range of initial helium abundances $Δ{Y}$ amongst the sub-populations hosted by each cluster. These estimates are commonly obtained by measuring the width in colour of the unevolved main sequence in an optical colour-magnitude-diagram. The measured colour spread is then compared with predictions from theoretical stellar isochrones with varying initial He abundances, to determine $Δ{Y}$. The availability of UV/optical magnitudes thanks to the {\sl HST UV Legacy Survey of Galactic GC…
For several decades, globular clusters have been considered the best example of simple stellar populations, hosting coeval and chemical homogeneous stars. The last decade of spectroscopic and photometric studies has revealed a more complex view of their chemical composition, with a high level of homogeneity in their iron content but star-to-star variations in some light elements. This contribution summarizes the main evidence about the chemical anomalies in the stellar content of the globular clusters, discussing also some peculiar objects with intrinsic dispersions in their iron content.
We present nucleosynthesis predictions for massive (5-7 solar masses) asymptotic giant branch (AGB) stars of solar metallicity where we delay the onset of the superwind to pulsation periods of P =700-800 days. We found that delaying the superwind in solar metallicity massive AGB stars results in a larger production of s-process elements, something that would be also expected at lower metallicities. These new models and the available observations show that massive C-O core AGB stars in our Galaxy and in the Magellanic Clouds experience considerable third dredge-up (TDU). Thus, if massive AGB st…
We have computed low-mass stellar models and synthetic spectra for an initial chemical composition that includes the full C-N, O-Na, and Mg-Al abundance anticorrelations observed in second generation stars belonging to a number of massive Galactic globular clusters. This investigation extends a previous study that has addressed the effect of only the C-N and O-Na anticorrelations, seen in all globulars observed to date. We find that the impact of Mg-Al abundance variations at fixed [Fe/H] and Helium abundance is negligible on stellar models and isochrones (from the main sequence to the tip of …
We have studied the chemical composition of NGC 1806, a massive, intermediate-age globular cluster that shows a double main sequence turnoff. We analyzed a sample of high-resolution spectra (secured with FLAMES at the Very Large Telescope) for 8 giant stars, members of the cluster, finding an average iron content of [Fe/H]=--0.60 +- 0.01 dex and no evidence of intrinsic star-to-star variations in the abundances of light elements (Na, O, Mg, Al). Also, the (m_(F814W); m_(F336W)-m_(F814W)) color-magnitude diagram obtained by combining optical and near-UV Hubble Space Telescope photometry exhibit…
Lithium is created during the Big Bang nucleosynthesis and it is destroyed in stellar interiors at relatively low temperatures. However, it should be preserved in the stellar envelopes of unevolved stars and progressively diluted during mixing processes. In particular, after the first dredge-up along the RGB, lithium should be completely destroyed, but this is not what we observe today in globular clusters. This element allows to test stellar evolutionary models, as well as different types of polluters for second population stars in the multiple population scenarios. Due to the difficulty in t…
Based on spectroscopic constraints derived from NLTE line formation, we explore the likely range of stellar parameters (T_eff and log g) for the hyper-metal-poor (HMP) star HE 1327-2326. Combining the constraints from Balmer line profiles and the Ca I/II ionization equilibrium, a subgiant stage of evolution is indicated. This result is further supported by spectrophotometric observations of the Balmer jump. If a higher T_eff value was used (as favoured by some photometric calibrations), the spectroscopic analysis would indicate a turnoff-point stage of evolution. Using a stellar-structure code…
We investigated the possibility of producing helium enhanced stars in globular clusters by accreting polluted matter during the pre-main sequence phase. We followed the evolution of two different classes of pre-main sequence accreting models, one which neglects and the other that takes into account the protostellar evolution. We analysed the dependence of the final central helium abundance, of the tracks position in the HR diagram and of the surface lithium abundance evolution on the age at which the accretion of polluted material begins and on the main physical parameters that govern the prot…
Abundance anomalies observed in globular cluster stars indicate pollution with material processed by hydrogen burning. Two main sources have been suggested: asymptotic giant branch (AGB) stars and massive stars rotating near the break-up limit (spin stars). We propose massive binaries as an alternative source. We compute the evolution of a 20 Msun star in a close binary and find that it sheds about 10 Msun. The ejecta are enriched in He, N, Na, and Al and depleted in C and O, similar to the abundance patterns observed in gobular cluster stars. In contrast to the fast, radiatively driven winds …
By using the multifiber spectrograph FLAMES mounted at the ESO-VLT, we have obtained high-resolution spectra for 18 giant stars, belonging to 3 old globular clusters of the Large Magellanic Cloud (namely NGC 1786, 2210 and 2257). While stars in each cluster showed quite homogeneous iron content, within a few cents of dex (the mean values being Fe/H]= -1.75+-0.01 dex, -1.65+-0.02 dex and -1.95+-0.02 dex for NGC 1786, 2210 and 2257, respectively), we have detected significant inhomogeneities for the [Na/Fe], [Al/Fe], [O/Fe] and [Mg/Fe] abundance ratios, with evidence of [O/Fe] vs [Na/Fe] and [Mg…
Using spectra obtained with the VLT/FORS2 and Gemini-S/GMOS instruments we have investigated carbon, nitrogen and sodium abundances in a sample of red giants in the Small Magellanic Cloud cluster Kron 3. The metallicity and luminosity of this cluster are comparable to those of Galactic globular clusters although with a notably younger age of $\sim$ 6.5 Gyr. Specifically we have investigated the strengths of the CH ($λ$ 4300 A) and CN ($λ$ 3800, $λ$ 4215) molecular bands finding a bimodality of CN band-strengths and a CH/CN anti-correlation. Application of spectrum synthesis techniques reveals …
We used three sets of high-resolution spectra acquired with the multifiber facility FLAMES at the Very Large Telescope of the European Southern Observatory to investigate the chemical and kinematical properties of a sample of 42 horizontal branch (HB) stars, 18 Blue Straggler Stars (BSSs) and 86 main sequence turn-off and sub-giant branch stars in the nearby globular cluster NGC 6397. We measured rotational velocities and Fe, O and Mg abundances. All the unevolved stars in our sample turn out to have low rotational velocites (v sin i< 10\kms), while HB stars and BSSs show a broad distribution,…
Blue horizontal-branch (BHB) stars with $T_{\rm eff}$ approximately larger than 11500 K show several observational anomalies. In globular clusters, they exhibit low rotational velocities, abundance anomalies (as compared to cluster abundances), photometric jumps and gaps and spectroscopic gravities lower than predicted by canonical models. It is commonly believed that the low rotational velocities of these stars permit atomic diffusion to be efficient in their atmosphere thereby causing the observed anomalies. Recent detections of vertical stratification of iron (and some other chemical elemen…
This paper presents the chemical abundance analysis of 217 stars in the metal-poor globular cluster NGC 6752, distributed from the turn-off to the lower red giant branch. Aluminium and Lithium abundances were derived through spectral synthesis applied to spectra collected with FLAMES, both in GIRAFFE and UVES modes. The work aims to gain insight into the nature of the polluter(s) responsible for the abundance variations and C-N, Na-O, Al-Mg anti-correlations associated with the multiple population phenomenon. We found a plateau at A(Li)=2.33$\pm$0.06 dex in unevolved stars, with the average Li…
Large abundance anomalies have been previously detected in Horizontal Branch B-type stars. We present the first high resolution study of isotopic anomalies and chemical abundances in six Horizontal Branch B-type stars in globular clusters NGC6397 and NGC6752 and compare them to those observed in HgMn stars. We obtained high-resolution (up to R~115000) UVES spectra of a representative sample of six B-type stars (T183, T191, T193, B652, B2151, B2206). It is the first time an abundance analysis is performed for all elements for which spectral lines were detected in UVES spectra of Horizontal Bran…
Globular clusters are of paramount importance for testing theories of stellar evolution and early galaxy formation. Strong evidence for multiple populations of stars in globular clusters derives from observed abundance anomalies. A puzzling example is the recently detected Mg-K anticorrelation in NGC 2419. We perform Monte Carlo nuclear reaction network calculations to constrain the temperature-density conditions that gave rise to the elemental abundances observed in this elusive cluster. We find a correlation between stellar temperature and density values that provide a satisfactory match bet…
We present photometry for all bright red giant branch (RGB), horizontal branch (HB), and asymptotic giant branch (AGB) stars within 10' of the center of M13. We find support for the idea that the population of HB stars redder than the primary group are noticeably evolved, which resolves a disagreement between distance moduli derived from the tip of the RGB and from stars near the instability strip. The sharp cut at the red end of the HB provides strong evidence that stars from the dominant HB group must still be undergoing blue loops, implying that diffusion is being inhibited. We argue that M…
We derive [K/Fe] abundance ratios for 119 stars in the globular cluster NGC 2808, all of them having O, Na, Mg and Al abundances homogeneously measured in previous works. We detect an intrinsic star-to-star spread in the Potassium abundance. Moreover [K/Fe] abundance ratios display statistically significant correlations with [Na/Fe] and [Al/Fe], and anti-correlations with [O/Fe] and [Mg/Fe]. All the four Mg deficient stars ([Mg/Fe]<0.0) discovered so far in NGC 2808 are enriched in K by ~0.3 dex with respect to those with normal [Mg/Fe]. NGC 2808 is the second globular cluster, after NGC 2419,…
Context. Abundance anomalies have been observed in field sdB stars and in nearly all Horizontal Branch (HB) stars of globular clusters with Teff > 11 000K whatever be the cluster metallicity. Aims. The aim is to determine the abundance variations to be expected in sdB stars and in HB stars of metallicities Z \geq 0.0001 and what observed abundances teach us about hydrodynamical processes competing with atomic diffusion. Methods. Complete stellar evolution models, including the effects of atomic diffusion and radiative acceleration, have been computed from the zero age main-sequence for metalli…
Disentangling distinct stellar populations along the red-giant branches (RGBs) of Globular Clusters (GCs) is possible by using the pseudo two-color diagram dubbed chromosome map (ChM). One of the most intriguing findings is that the so-called first-generation (1G) stars, characterized by the same chemical composition of their natal cloud, exhibit extended sequences in the ChM. Unresolved binaries and internal variations in helium or metallicity have been suggested to explain this phenomenon. Here, we derive high-precision Hubble Space Telescope photometry of the GCs NGC6362 and NGC6838 and bui…
Abundance anomalies in globular clusters provide strong evidence for multiple stellar populations within each cluster. These populations are usually interpreted as distinct generations, with the currently observed second-generation stars having formed in part from the ejecta of massive, first-generation "polluter" stars, giving rise to the anomalous abundance patterns. The precise nature of the polluters and their enrichment mechanism are still unclear. Even so, the chemical abundances measured in second-generation stars within the globular cluster NGC 2419 provide insight into this puzzling p…
We observe two metal-poor main sequence stars that are members of the recently-discovered Sylgr stellar stream. We present radial velocities, stellar parameters, and abundances for 13 elements derived from high-resolution optical spectra collected using the Magellan Inamori Kyocera Echelle spectrograph. The two stars have identical compositions (within 0.13 dex or 1.2 sigma) among all elements detected. Both stars are very metal poor ([Fe/H] = -2.92 +/- 0.06). Neither star is highly enhanced in C ([C/Fe] < +1.0). Both stars are enhanced in the alpha elements Mg, Si, and Ca ([alpha/Fe] = +0.32 …
In the globular cluster NGC2808, a quasi-standard initial lithium abundance is derived for a red giant belonging to the `extreme' population, characterized by a large helium overabundance, and by abundances of proton capture elements typical of nuclear processing in gas at very high temperatures, where the initial lithium has been fully destroyed. The observations of lithium in such extreme cluster stars are important to test different models for the formation of multiple populations in old Globular Clusters. In the asymptotic giant branch (AGB) scenario, fresh lithium is synthetized during th…
Abundance anomalies observed in globular cluster stars indicate pollution with material processed by hydrogen burning. Two main sources have been suggested: asymptotic giant branch stars and massive stars rotating near the break-up limit. We discuss the potential of massive binaries as an interesting alternative source of processed material. We discuss observational evidence for mass shedding from interacting binaries. In contrast to the fast, radiatively driven winds of massive stars, this material is typically ejected with low velocity. We expect that it remains inside the potential well of …
The aim of Galactic archaeology is to recover the history of our Galaxy through the information encoded in stars. An unprobed assumption of this field is that the chemical composition of a star is an immutable marker of the gas from which it formed. It is vital to test this assumption on open clusters, group of stars formed from the same gas. Previous investigations have shown that unevolved stars in clusters are chemically homogeneous within the typical uncertainties of these analysis, i.e. 15$\%$ of the elemental abundances. Our strictly differential analysis on five members of the Pleiades …
We investigate the viability of a model in which the chemical anomalies among Globular Cluster stars are due to accretion of gas onto the protostellar discs of low mass stars. This model has been suggested as a way to reduce the large initial cluster masses required by other models for the formation of multiple stellar generations. We numerically follow the evolution of the accreting stars, and we show that the structure of the seed star does not remain fully convective for the whole duration of the accretion phase. Stellar populations showing discrete abundances of helium in the core, that se…
AGB stars have long been held responsible for the important star-to-star variations in light elements observed in Galactic globular clusters. We analyse the main impacts of a first generation of rotating intermediate-mass stars on the chemical properties of second-generation globular cluster stars. The rotating models were computed without magnetic fields and without the effects of internal gravity waves. They account for the transports by meridional currents and turbulence. We computed the evolution of both standard and rotating stellar models with initial masses between 2.5 and 8 Msun within…
NGC5824 is a massive Galactic globular cluster suspected to have an intrinsic spread in its iron content, according to the strength of the calcium triplet lines. We present chemical abundances of 117 cluster giant stars using high-resolution spectra acquired with the multi-object spectrograph FLAMES. The metallicity distribution of 87 red giant branch stars is peaked at [Fe/H]=-2.11+-0.01 dex, while that derived from 30 asymptotic giant branch stars is peaked at [Fe/H]=-2.20+-0.01 dex. Both the distributions are compatible with a null spread, pointing out that this cluster did not retain the e…
Several scenarios have been proposed to explain the presence of multiple stellar populations in globular clusters. Many of them invoke multiple generations of stars to explain the observed chemical abundance anomalies, but it has also been suggested that self-enrichment could occur via accretion of ejecta from massive stars onto the circumstellar disc of low-mass pre-main sequence stars. These scenarios imply different initial conditions for the kinematics of the various stellar populations. Given some net angular momentum initially, models for which a second generation forms from gas that col…
Anomalous surface abundances are observed in a fraction of the low-mass stars of Galactic globular clusters, that may originate from hot-hydrogen-burning products ejected by a previous generation of massive stars. We present and investigate a scenario in which the second generation of polluted low-mass stars can form in shells around cool supergiant stars within a young globular cluster. Simulations of low-Z massive stars (M$_{\rm i}\sim$ 150$-$600 M$_{\odot}$) show that both core-hydrogen-burning cool supergiants and hot ionizing stellar sources are expected to be present simulaneously in you…
A small fraction of the halo field is made up of stars that share the light element (Z<=13) anomalies characteristic of second generation globular cluster (GC) stars. The ejected stars shed light on the formation of the Galactic halo by tracing the dynamical history of the clusters, which are believed to have once been more massive. Some of these ejected stars are expected to show strong Al enhancement at the expense of shortage of Mg, but until now no such star has been found. We search for outliers in the Mg and Al abundances of the few hundreds of halo field stars observed in the first eigh…
Oxygen abundances of 67 dwarf stars in the metallicity range -1.6<[Fe/H]<-0.4 are derived from a non-LTE analysis of the 777 nm O I triplet lines. These stars have precise atmospheric parameters measured by Nissen and Schuster, who find that they separate into three groups based on their kinematics and alpha-element (Mg, Si, Ca, Ti) abundances: thick-disk, high-alpha halo, and low-alpha halo. We find the oxygen abundance trends of thick-disk and high-alpha halo stars very similar. The low-alpha stars show a larger star-to-star scatter in [O/Fe] at a given [Fe/H] and have systematically lower o…
We present Hubble Space Telescope (HST) photometric results for NGC 6402, a highly reddened very luminous Galactic globular cluster (GC). Recent spectroscopic observations of its red giant stars have shown a quite peculiar behavior in the chemistry of its multiple populations. These results have prompted UV and optical HST observations aimed at obtaining the cluster's "Chromosome map" (ChM), an efficient tool to classify GCs and characterize their multiple populations. We find that the discontinuity in the abundance distributions of O, Mg, Al and Na inferred from spectroscopy is more nuanced i…
A number of stellar sources have been advocated as the origin of the enriched material required to explain the abundance anomalies seen in ancient globular clusters (GCs). Most studies to date have compared the yields from potential sources (asymptotic giant branch stars (AGBs), fast rotating massive stars (FRMS), high mass interacting binaries (IBs), and very massive stars (VMS)) with observations of specific elements that are observed to vary from star-to-star in GCs, focussing on extreme GCs such as NGC 2808, which display large He variations. However, a consistency check between the result…
An abundance analysis is presented for IRAS 18095+2704 (V887 Her), a post-AGB star and proto-planetary nebula. The analysis is based on high-resolution optical spectra from the McDonald Observatory and the Special Astrophysical Observatory. Standard analysis using a classical Kurucz model atmosphere and the line analysis program MOOG provides the atmospheric parameters: Teff = 6500 K, log g = +0.5, and a microturbulent velocity Vt = 4.7 km/s and [Fe/H] = -0.9. Extraction of these parameters is based on excitation of FeI lines, ionization equilibrium between neutral and ions of Mg, Ca, Ti, Cr, …
NGC 2419 is a peculiar Galactic globular cluster in terms of size/luminosity, and chemical abundance anomalies. Here, we present Strömgren $uvby$ photometry of the cluster. Using the gravity- and metallicity-sensitive $c_1$ and $m_1$ indices, we identify a sample of likely cluster members extending well beyond the formal tidal radius with an estimated contamination by non-members of only 1%. We derive photometric [Fe/H] of red giants, and depending on which literature metallicity relation we use, find reasonable to excellent agreement with spectroscopic [Fe/H]. We demonstrate explicitly that t…
We present a model for the concurrent formation of globular clusters (GCs) and supermassive stars (SMSs, $>10^3\,{\rm M}_\odot$) to address the origin of the HeCNONaMgAl abundance anomalies in GCs. GCs form in converging gas flows and accumulate low-angular momentum gas, which accretes onto protostars. This leads to an adiabatic contraction of the cluster and an increase of the stellar collision rate. A SMS can form via runaway collisions if the cluster reaches sufficiently high density before two-body relaxation halts the contraction. This condition is met if the number of stars $\gtrsim10^6$…
Globular cluster stars show chemical abundance patterns typical of hot-CNO processing. Lithium is easily destroyed by proton capture in stellar environments, so its abundance may be crucial to discriminate among different models proposed to account for multiple populations. In order to reproduce the observed O-Na anticorrelation and other patterns typical of multiple populations, the formation of second generation stars must occur from the nuclearly processed stellar ejecta, responsible of the chemical anomalies, diluted with pristine gas having the composition of first generation stars. The l…
Blue straggler stars (BSSs) are brighter and bluer than the main-sequence (MS) turnoff and more massive than MS stars.Two scenarios for their formation have been proposed: collision-induced stellar mergers (COL-BSSs),or mass-transfer in binary systems (MT-BSSs).Depleted surface abundances of C and O are expected for MT-BSSs,whereas no chemical anomalies are predicted for COL-BSSs. Both MT and COL-BSSs should rotate fast,but braking mechanisms may intervene with efficiencies and time-scales unknown,thus preventing a clear prediction of the expected rotational velocities. In this context,an exte…
With the aim to constrain multiple populations in the metal-poor globular cluster NGC6397, we analyse and discuss the chemical compositions of a large number of elements in 21 red giant branch stars. High-resolution spectra were obtained with the FLAMES/UVES spectrograph on VLT. We have determined non-LTE abundances of Na and LTE abundances for the remaining 21 elements, including O, Mg, Al, alpha, iron-peak, and neutron-capture elements, many of which have not previously been analysed for this cluster. We have also considered the influence of possible He enrichment in the analysis of stellar …
We present [C/Fe] and [N/Fe] abundance ratios and CH(λ4300) and S(λ3883) index measurements for 94 red giant branch (RGB) stars in the Sculptor dwarf spheroidal galaxy from VLT/VIMOS MOS observations at a resolving power R= 1150 at 4020 Å. This is the first time that [N/Fe] abundances are derived for a large number of stars in a dwarf spheroidal. We found a trend for the [C/Fe] abundance to decrease with increasing luminosity on the RGB across the whole metallicity range, a phenomenon observed in both field and globular cluster giants, which can be interpreted in the framework of evolutionary …
MgAl burning chain in M 54: the globular cluster-like properties of a nuclear star cluster
Osservatorio Astrofisico di Arcetri（阿切尔奇天文台）；Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna（博洛尼亚大学物理与天文学系）；INAF, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna（意大利国家天体物理研究所博洛尼亚空间科学与天体物理观测站）；INAF, Osservatorio Astronomico di Roma（意大利国家天体物理研究所罗马天文台）
We present [K/Fe] abundance ratios for a sample of 450 stars in Omega Centauri, using high resolution spectra acquired with the multi-object spectrograph FLAMES@VLT. Abundances for Fe, Na and Mg were also derived. We detected intrinsic K variations in the analysed stars. Moreover, [K/Fe] shows a significant correlation with [Na/Fe] and anti-correlation with [Mg/Fe]. The presence of a clear-cut Mg-K anti-correlation makes Omega Centauri the third stellar system, after NGC 2419 and NGC 2808, hosting a sub-population of stars with [Mg/Fe]<0.0 dex, K-enriched in the case of Omega Centauri by ~0.3 …
Non-LTE analysis (LTE is local thermodynamic equilibrium) of the oxygen abundances for 51 Galactic A-, F- and G-type supergiants and bright giants is performed. In contrast with carbon and nitrogen, oxygen does not show any significant systematic anomalies in their abundances log E(O). There is no marked difference from the initial oxygen abundance within errors of the log E(O) determination across the Teff interval from 4500 to 8500 K and the log g interval from 1.2 to 2.9 dex. This result agrees well with theoretical predictions for stellar models with rotation. With our new data for oxygen …
Stellar populations contain the most important information about star clus- ter formation and evolution. Until several decades ago, star clusters were believed to be ideal laboratories for studies of simple stellar populations (SSPs). However, discoveries of multiple stellar populations in Galactic globular clusters have expanded our view on stellar populations in star clusters. They have simultaneously generated a number of controversies, particularly as to whether young star clusters may have the same origin as old globular clusters. In addition, extensive studies have revealed that the SSP …
We continue exploring our understanding of the collective properties of X-ray binaries in the stellar fields (i.e., outside globular clusters) of normal galaxies, introduced in Paper I of this series, where we considered high-mass X-ray binaries (HMXBs). In this paper (Paper II of the series) and the companion paper (Paper III of the series), we consider low-mass X-ray binaries (LMXBs), whose evolutionary scenario is very different from that of HMXBs. In this paper, we consider the evolution of primordial binaries upto the stage where the neutron star just formed in the supernova explosion of …
The origin of the chemical anomalies in star clusters is still an open question, although much effort has been employed both from a theoretical and observational point of view. The exploration of whether such multiple stellar populations are found based on certain properties of clusters has represented a compelling line of investigation so far. Here I report an overview of the results obtained from our latest surveys aimed at characterising the phenomenon of chemical variations in star clusters that are much younger with respect to the ancient globular clusters. The fundamental question we are…
Photospheric abundances of C, N, O, and Na were determined by applying the synthetic spectrum-fitting technique to 34 snap-shot high-dispersion spectra of 22 RR Lyr stars covering a metallicity range of -1.8 <[Fe/H] < 0.0, with an aim of investigating the mixing mechanism in the interior of low-mass giant stars by examining the abundance anomalies of these elements possibly affected by the evolution-induced dredge-up of nuclear burning products. Special attention was paid to check the recent theoretical stellar evolution simulations indicating the importance of thermohaline mixing in low-mass …
We present ESO/VLT FORS2 low resolution spectroscopy of red giant branch stars in three massive, intermediate age ($\sim 1.7-2.3$ Gyr) star clusters in the Large Magellanic Cloud. We measure CH and CN index bands at 4300A, and 3883A, as well as [C/Fe] and [N/Fe] abundance ratios for 24, 21 and 12 member stars of NGC 1978, NGC 1651, NGC 1783, respectively. We find a significant intrinsic spread in CN in NGC 1978 and NGC 1651, a signal of multiple stellar populations (MPs) within the clusters. On the contrary, we report a null CN spread in NGC 1783 within our measurement precision. For NGC 1978,…
