Synthetic spectroscopic indices for identifying multiple stellar populations in globular clusters
Emanuele Bertone, Miguel Chávez, J. César Mendoza
备注：
Accepted for publication in MNRAS main journal; 13 pages, 10 figures. The material presented in the paper is available through a dedicated web page: https://www.inaoep.mx/~modelos/gcindices.html
We present an investigation of synthetic spectroscopic indices that can plausibly help in identifying the presence of multiple stellar populations in globular clusters. The study is based on a new grid of stellar model atmospheres and high-resolution (R=500,000) synthetic spectra, that consider chemical partitions that have been singled out in Galactic globular clusters. The database is composed of 3472 model atmospheres and theoretical spectra calculated with the collection of Fortran codes DFSYNTHE, ATLAS9 and SYNTHE, developed by Robert L. Kurucz. They cover a range of effective temperature from 4300 to 7000 K, surface gravity from 2.0 to 5.0 dex and four different chemical compositions. A set of 19 spectroscopic indices were calculated from a degraded version (R=2500) of the theoretical spectra dataset. The set includes five indices previously used in the context of globular clusters analyses and 14 indices that we have newly defined by maximizing the capability of differentiating the chemical compositions. We explored the effects of atmospheric parameters on the index values and identified the optimal spectral diagnostics that allow to trace the signatures of objects of different stellar populations, located in the main sequence, the red giant branch and the horizontal branch. We found a suitable set of indices, that mostly involve molecular bands (in particular NH, but also CH and CN), that are very promising for spectroscopically identifying multiple stellar populations in globular clusters.
We review spectroscopic results concerning multiple stellar populations in globularclusters. The cluster initial mass is the most important parameter determining the fraction of second generation stars. The threshold for the onset of the multiple population phenomenon is 1-3x10^5 MSun. Nucleosynthesis is influenced by metallicity: Na/O and Mg/Al anti-correlations are more extended in metal-poor than in metal-rich clusters. Massive clusters are more complex systems than the smaller ones, with several populations characterized by different chemical compositions. The high Li abundance observed in…
It has been suggested that the oldest stellar populations in the Milky Way Galaxy are tightly bound and confined to the central regions of the Galaxy. This is one of the reasons why a handful of globular clusters located in the bulge region are thought to be remnants of the primeval formation stages of the Milky Way. The globular cluster, ESO 456-SC38 (Djorgovski~2) is one such cluster; it has a blue horizontal branch, is projected very close to the center of the Galaxy, and has an orbit confining it to the bulge/bar region. The first alpha abundances of seven stars in this heavily reddened cl…
The stars SDSS J064655.6+411620.5 and SDSS J193712.01+502455.5 are relatively bright stars that were initially observed as part of the SDSS/MARVELS pre-survey. They were selected, on the basis of their weak CH $G$-bands, along with a total of 60 others, in the range of halo globular cluster metallicities for high-resolution spectroscopic follow-up as a part of the HESP-GOMPA survey (Hanle Echelle SPectrograph -- Galactic survey Of Metal Poor stArs). The stars exhibit typical nucleosynthesis signatures expected from the so-called second-generation stars of globular clusters. The light-element a…
We present the first spectroscopic observations of the very metal-poor Milky Way globular cluster ESO280-SC06. Using spectra acquired with the 2dF/AAOmega spectrograph on the Anglo-Australian Telescope, we have identified 13 members of the cluster, and estimate from their infrared calcium triplet lines that the cluster has a metallicity of $\mathrm{[Fe/H]}=-2.48\substack{+0.06 \\ -0.11}$. This would make it the most metal-poor globular cluster known in the Milky Way. This result was verified with comparisons to three other metal-poor globular clusters that had been observed and analyzed in the…
We present an analysis of the multiple stellar populations of the globular cluster NGC 1851. We used lower resolution spectra of giant stars to measure CN, CH, and calcium H & K spectral indices, and determine elemental abundances for carbon and nitrogen. The CN and CH indices were used to confirm that there are four populations of stars in the cluster. The primordial population of stars, with the lowest CN, was found to be generally chemically distinct in elemental abundances from the second generation populations. As expected, [N/Fe] increases with increasing CN strength, but the only other …
We present the abundance analysis for a sample of 18 red giant branch stars in the metal-poor globular cluster NGC 4147 based on medium and high resolution spectra. This is the first extensive spectroscopic study of this cluster. We derive abundances of C, N, O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Y, Ba, and Eu. We find a metallicity of [Fe/H]=-1.84+-0.02 and an alpha-enhancement of +0.38+-0.05 (errors on the mean), typical of halo globular clusters in this metallicity regime. A significant spread is observed in the abundances of light elements C, N, O, Na, and Al. In particular we found a Na-…
We explore the properties of selected mid-ultraviolet (1900-3200 angstrom) spectroscopic indices of simple stellar populations (SSPs). We incorporate the high resolution UVBLUE stellar spectral library into an evolutionary population synthesis code, based on the most recent Padova isochrones. We analyze the trends of UV indices with respect to age and chemical composition. As a first test against observations, we compare our results with the empirical mid-UV spectral indices of Galactic globular clusters, observed with the International Ultraviolet Explorer (IUE). We find that synthetic indice…
The visually brightest stars in globular clusters (GCs) are the ones evolving off the asymptotic giant branch (AGB) and passing through spectral types F and A--the "yellow" post-AGB (yPAGB) stars. yPAGB stars are potentially excellent "Population II" standard candles for measuring extragalactic distances. A recent survey of the Galactic GC system, using uBVI photometry to identify stars of low surface gravities with large Balmer discontinuities, discovered a candidate luminous yPAGB star in the GC M19 (NGC 6273), designated ZNG 4. The same survey also identified a bright, hotter candidate blue…
We present a spectroscopic analysis of main sequence (MS) stars in the young globular cluster NGC1818 (age~40 Myrs) in the Large Magellanic Cloud. Our photometric survey on Magellanic Clouds clusters has revealed that NGC1818, similarly to the other young objects with age 600 Myrs, displays not only an extended MS Turn-Off (eMSTO), as observed in intermediate-age clusters (age~1-2 Gyrs), 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 and a sequence of fast rotator…
We analysed red giant branch stars in 16 Galactic globular clusters, computing their atmospheric parameters both from the photometry and from excitation and ionisation balances. The spectroscopic parameters are lower than the photometric ones and this discrepancy increases decreasing the metallicity, reaching, at [Fe/H]~-2.5 dex, differences of ~350 K in effective temperature and ~1 dex in surface gravity. We demonstrate that the spectroscopic parameters are inconsistent with the position of the stars in the colour-magnitude diagram, providing too low temperatures and gravities, and predicting…
Globular clusters associated with the Galactic bulge are important tracers of stellar populations in the inner Galaxy. High resolution analysis of stars in these clusters allows us to characterize them in terms of kinematics, metallicity, and individual abundances, and to compare these fingerprints with those characterizing field populations. We present iron and element ratios for seven red giant stars in the globular cluster NGC~6723, based on high resolution spectroscopy. High resolution spectra ($R\sim48~000$) of seven K giants belonging to NGC 6723 were obtained with the FEROS spectrograph…
Spectral feature index diagrams with integrated globular clusters and simple stellar population models often show that some clusters have weak H beta, so weak that even the oldest models cannot match the observed feature depths. In this work, we rule out the possibility that abundance mixture effects are responsible for the weak indices unless such changes operate to cool the entire isochrone. We discuss this result in the context of other explanations, including horizontal branch morphology, blue straggler populations, and nebular or stellar emission fill-in, finding a preference for flaring …
Aims. We demonstrate the high multiplex advantage of crowded field 3D spectroscopy using the new integral field spectrograph MUSE by means of a spectroscopic analysis of more than 12,000 individual stars in the globular cluster NGC 6397. Methods. The stars are deblended with a PSF fitting technique, using a photometric reference catalogue from HST as prior, including relative positions and brightnesses. This catalogue is also used for a first analysis of the extracted spectra, followed by an automatic in-depth analysis using a full-spectrum fitting method based on a large grid of PHOENIX spect…
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…
Recent pieces of evidence have revealed that most, and possibly all, globular star clusters are composed of groups of stars that formed in multiple episodes with different chemical compositions. In this sense, it has also been argued that variations in the initial helium abundance ($Y$) from one population to the next are also the rule, rather than the exception. In the case of the metal-intermediate globular cluster M4 (NGC 6121), recent high-resolution spectroscopic observations of blue horizontal branch (HB) stars (i.e., HB stars hotter than the RR Lyrae instability strip) suggest that a la…
We present a new homogeneous set of metallicity estimates based on Lick indices for 245 old globular clusters of the M31 galaxy comprised in the Revised Bologna Catalog. The metallicity distribution of the M31 globular clusters is briefly discussed and compared with that of the Milky Way. Simple parametric statistics suggests that the [Fe/H] distribution is likely not unimodal. The strong correlation between metallicity and kinematics found in previous studies is confirmed. The most metal-rich GCs tend to be packed at the center of the system and share the galactic rotation as traced by the HI…
This is the first in a series of papers presenting methods and results from the Young Solar Analogs Project, which began in 2007. This project monitors both spectroscopically and photometrically a set of 31 young (300 - 1500 Myr) solar-type stars with the goal of gaining insight into the space environment of the Earth during the period when life first appeared. From our spectroscopic observations we derive the Mount Wilson $S$ chromospheric activity index ($S_{\rm MW}$), and describe the method we use to transform our instrumental indices to $S_{\rm MW}$ without the need for a color term. We i…
Stellar population studies of globular clusters have suggested that the brightest clusters in the Galaxy might actually be the remnant nuclei of dwarf spheroidal galaxies. If the present Galactic globular clusters formed within larger stellar systems, they are likely surrounded by extra-tidal halos and/or tails made up of stars that were tidally stripped from their parent systems. The stellar surroundings around globular clusters are therefore one of the best places to look for the remnants of an ancient dwarf galaxy. Here an attempt is made to search for tidal debris around the supernovae enr…
We present chemical abundances of Al and several Fe-Peak and neutron-capture elements for 13 red giant branch stars in the Galactic globular cluster NGC 6171 (M107). The abundances were determined using equivalent width and spectrum synthesis analyses of moderate resolution (R~15,000), moderate signal-to-noise ratio (<S/N>~80) spectra obtained with the WIYN telescope and Hydra multifiber spectrograph. A comparison between photometric and spectroscopic effective temperature estimates seems to indicate a reddening value of E(B-V)=0.46 may be more appropriate for this cluster than the more common…
We present simple stellar population (SSP) models with scaled-solar and alpha-element enhanced abundances. The SSP models are based on the Dartmouth Stellar Evolution Database, our library of synthetic stellar spectra, and a detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity. In order to test the relative importance of a variety of SSP model ingredients, we compare our SSP models with integrated spectra of 41 Milky Way Globular Clusters (MWGCs) from Schiavon et al. (2005). Using the Mg b and Ca4227 indices, we confirm that Mg and Ca are enhanced by abou…
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.
Globular Clusters are among the oldest objects in the Galaxy, thus their researchers are key to understanding the processes of evolution and formation that the galaxy has experienced in early stages. Spectroscopic studies allow us to carry out detailed analyzes on the chemical composition of Globular Clusters. The aim of our research is to perform a detailed analysis of chemical abundances to a sample of stars of the Bulge Globular Cluster NGC 6553, in order to determine chemical patterns that allow us to appreciate the phenomenon of Multiple Population in one of the most metal-rich Globular C…
Since the discovery of chemically peculiar stars in globular clusters in the last century, the study of multiple populations has become increasingly important, given that chemical inhomogeneity is found in almost all globular clusters. Despite various proposed theories attempting to explain this phenomenon, fitting all the observational evidence in globular clusters with one single theory remains notoriously difficult and currently unsuccessful. In order to improve existing models and motivate new ones, we are observing globular clusters at critical conditions, e.g., metal-rich end, metal-poor…
In Martell & Grebel (2010) we reported the discovery in the Sloan Digital Sky Survey-II/SEGUE spectroscopic database of a small subset of halo red giants, 2.5%, with CN and CH band strengths indicative of globular-cluster-like carbon and nitrogen abundances. Because the formation of stars with unusual light-element abundances is thought to be restricted to high-density environments like globular clusters, this result has strong implications for both cluster formation processes and the assembly history of the Galactic halo. Here we discuss two efforts to expand upon that work.
M 2 has been claimed to posses three distinct stellar components that are enhanced in iron relative to each other. We use equivalent width measurements from 14 red giant branch stars from which Yong et al. detect a $\sim$0.8 dex wide, trimodal iron distribution to redetermine the metallicity of the cluster. In contrast to Yong et al., which derive atmospheric parameters following only the classical spectroscopic approach, we perform the chemical analysis using three different methods to constrain effective temperatures and surface gravities. When atmospheric parameters are derived spectroscopi…
We introduce new color indices $cn^\prime_{\rm JWL}$ (= $Ca_{\rm CTIO} - Ca_{\rm JWL}$) and $ch_{\rm JWL}$ [=$(JWL43 - b) - (b-y)$], accurate photometric measures of the CN band at $λ$3883 and the CH G band, respectively, in the study of the multiple populations (MPs) in globular clusters (GCs). Our photometric CN--CH relation for a large number of red-giant branch (RGB) in M5 shows that the evolutions of the CN and CH between the CN-w and CN-s populations are not continuous. Armed with our new color indices, we investigate the MPs of NGC 6723, finding the RGB populational number ratio of $n$(…
We are coming to believe that stellar populations in globular clusters are not as simple as they were once thought to be. A growing amount of photometric and spectroscopic evidence shows that globular clusters host at least two different stellar populations. In our contribution to these proceedings we present the first results of a survey we are conducting to look for the presence of multiple populations in a significant number of Galactic globular clusters, using the Strömgren system. We intend to photometrically separate these populations and characterize their radial distributions and exten…
Nearly all Galactic globular clusters host stars that display characteristic abundance anti-correlations, like the O-rich/Na-poor pattern typical of field halo stars, together with O-poor/Na-rich additional components. A recent spectroscopic investigation questioned the presence of O-poor/Na-rich stars amongst a sample of asymptotic giant branch stars in the cluster M 4, at variance with the spectroscopic detection of a O-poor/Na-rich component along both the cluster red giant branch and horizontal branch. This is contrary to what is expected from the cluster horizontal branch morphology and h…
I present the latest results from our group about the multiple stellar populations in the old Milky Way globular clusters (GCs) and in the young systems both in the Magellanic Clouds and in the Milky Way. For the ancient GCs in our Galaxy I summarize the chemical properties of the stellar populations as observed on the chromosome map. Both Type I and Type II GCs are discussed. For the youngest clusters I will briefly report our latest spectroscopic analysis on the Large Magellanic Cloud cluster NGC1818 and the Galactic open cluster NGC6705 (M11), which supports the co-existence of stellar popu…
Context. Globular clusters host multiple populations of long-lived low-mass stars whose origin remains an open question. Several scenarios have been proposed to explain the associated photometric and spectroscopic peculiarities. They differ, for instance, in the maximum helium enrichment they predict for stars of the second population, which these stars can inherit at birth as the result of the internal pollution of the cluster by different types of stars of the first population. Aims. We present the distribution of helium-rich stars in present-day globular clusters as it is expected in the or…
An increasing amount of spectroscopic and photometric evidence is showing that the stellar populations of globular clusters are not as simple as they have been considered for many years. The presence of at least two different populations of stars is being discovered in a growing number of globular clusters, both in our Galaxy and in others. We have started a series of observations of Galactic globular clusters using the Strömgren photometric system in order to find the signatures of these multiple populations and establish their presence in a more complete sample of globular clusters in the Mi…
A combined effort utilizing spectroscopy and photometry has revealed the existence of a new globular cluster class. These "anomalous" clusters, which we refer to as "iron-complex" clusters, are differentiated from normal clusters by exhibiting large (>0.10 dex) intrinsic metallicity dispersions, complex sub-giant branches, and correlated [Fe/H] and s-process enhancements. In order to further investigate this phenomenon, we have measured radial velocities and chemical abundances for red giant branch stars in the massive, but scarcely studied, globular cluster NGC 6273. The velocities and abunda…
An increasing number of both photometric and spectroscopic observations over the last years have shown the existence of distinct sub-populations in many Galactic globular clusters and shattered the paradigm of globulars hosting single, simple stellar populations. These multiple populations manifest themselves in a split of different evolutionary sequences in the cluster color-magnitude diagrams and in star-to-star abundance variations. In this paper we will summarize the observational scenario.
Recent wide-area surveys have enabled us to study the Milky Way with unprecedented detail. Its inner regions, hidden behind dust and gas, have been partially unveiled with the arrival of near-IR photometric and spectroscopic datasets. Among recent discoveries, there is a population of low-mass globular clusters, known to be missing, especially towards the Galactic bulge. In this work, five new low-luminosity globular clusters located towards the bulge area are presented. They were discovered by searching for groups in the multi-dimensional space of coordinates, colours, and proper motions from…
We present new intermediate-band Stroemgren photometry based on more than 300 u,v,b,y images of the Galactic globular cluster Omega Cen. Optical data were supplemented with new multiband near-infrared (NIR) photometry (350 J,H,K_s images). The final optical-NIR catalog covers a region of more than 20*20 arcmin squared across the cluster center. We use different optical-NIR color-color planes together with proper motion data available in the literature to identify candidate cluster red giant (RG) stars. By adopting different Stroemgren metallicity indices we estimate the photometric metallicity…
Blue Straggler stars are present in all the properly observed Globular Clusters. They mimic a rejuvenated stellar population and their existence has been a puzzle for many years. We performed an extensive spectroscopic survey of the surface abundances and the rotational velocities of Blue Straggler stars in different Globular Clusters, by using the high-resolution spectrograph FLAMES@VLT in the UVES+GIRAFFE combined mode. In this contribution we show our main results obtained for M4, ω Centauri and M30.
According to the classical view of globular clusters, stars inside globular clusters are evolved from the same giant molecular cloud. Then their stars' chemical compositions must be the same. But recent photometric and spectroscopic studies of globular clusters reveal the presence of more-than-one stellar populations inside globular clusters. This finding challenges our classical view of globular clusters. In this work, we investigated the possibility of solving multiple stellar populations problem in globular clusters using dark matter assumptions. We showed that the presence of dark matter i…
We present FEROS--Lick/SDSS, an empirical database of Lick/SDSS spectral indices of FGK stars to be used in population synthesis projects for discriminating different stellar populations within the integrated light of galaxies and globular clusters. From about 2500 FEROS stellar spectra obtained from the ESO Science Archive Facility we computed line--strength indices for 1085 non--supergiant stars with atmospheric parameter estimates from the AMBRE project. Two samples of 312 {\it dwarfs} and of 83 {\it subgiants} with solar chemical composition and no significant $α$--element abundance enhanc…
Although numerous photometric and spectroscopic studies of stars in Galactic globular clusters suggest the existence of several stellar generations in the majority of clusters studied so far, kinematical properties of these stellar generations are still relatively poorly know. In this contribution we present first results of our study of kinematical properties of chemically-tagged stellar populations in the Galactic globular cluster 47 Tuc. Our analysis reveals the existence of three stellar generations in 47 Tuc that are different in their [Li/Na] and [Na/O] ratios. Additionally, we find that…
We present the effective temperatures, surface gravities and abundances of iron, carbon and barium of 848 giant branch stars, of which 557 also have well-defined nitrogen abundances, of the globular cluster ω Centauri. This work used photometric sources and lower resolution spectra for this abundance analysis. Spectral indices were used to estimate the oxygen abundance of the stars, leading to a determination of whether a particular star was oxygen-rich or oxygen-poor. The 557-star subset was analyzed in the context of evolutionary groups, with four broad groups identified. These groups sugges…
