Star clusters globular

The interest of the astrophysical community on the evolution of the intermediate mass stars (IMS) raised in the last decades, as they have been suggested as possible responsible of the chemical anomalies which are observed in Giant and TO stars within Globular Clusters (see e.g. Gratton et al. 2004). 

Identifying stars in globular and open clusters as either old stars from the primordial explosion or new stars formed after a supernova event is based on the atomic composition of the stars. The primary way of identifying the elements in any excited state is to study the atomic spectroscopy of the stellar spectrum, such as in Figure 4.2 and identify the atoms by assigning the spectra. This becomes a complicated process for the heavier elements but is very informative even for the simple H-atom spectra. 

The most metal-deficient stars comprise field stars in the solar neighbourhood (where in some cases distances and luminosities can be found from parallaxes) and stars in globular clusters where the morphology of the HR diagram can be studied (Fig. 4.8). Such stars are of particular interest because their content of heavy elements (synthesized in still earlier generations of stars) is so low that they can... 

There are several lines of evidence that nucleosynthesis takes place in stars. The compositions of the outer envelopes of evolved low- and intermediate-mass stars show enhancements of the products of nuclear reactions (hydrogen and helium burning and s-process nucleosynthesis, as defined below). The ejecta of supemovae (stellar explosions) are highly enriched in short-lived radioactive nuclides that can only have been produced either just before or during the explosion. At the other extreme, low-mass stars in globular clusters, which apparently formed shortly after the universe formed, are deficient in metals (elements heavier than hydrogen and helium) because they formed before heavy elements were synthesized. 

Star cluster—The targets of extensive N-body computations, these are collections of several hundred (open clusters) or several hundred thousand (globular clusters) stars held together by their mutual gravity. Much has been learned about their long-term evolution from N-body computations. 

Low-mass helium-core burning stars are identified with horizontal branch (HB) stars in globular clusters. 

By way of contrast, here is one of the oldest formations in the universe, a globular star cluster. It dates back five and a half or six billion years, and every star in the cluster was formed at that time. All the gas and dust originally present had condensed at that time into these 100,000 or so individual stars. 

Prom his observations William Herschel (1738-1822) concluded that stars in the Galaxy were distributed in a disc-like strnctnre, centred on the Sun. It remained the accepted picture until early in the twentieth century, when, based on the study of globular star clusters, Harlow Shapley proposed (1912) a disc-like Milky Way surrounded by a spherical arrangement of clusters and... 

J. D. Fernie (ed.). Variable Stars in Globular Clusters and in Related Systems. Proceedings of the lAU Colloquium No. 21, held at the University of Toronto, Toronto, Canada, August 29-31,... 

Abstract. A review is presented on abundance determinations in stars of the Galactic bulge, both in the field and in globular clusters. Previous low-resolution spectroscopy results are revised. Recent high resolution and high S/N spectroscopy results based on Keck-Hires, Gemini-Phoenix and VLT-UVES data are presented. Finally, recent analyses of FLAMES data are discussed. 

The metallicity distribution of globular clusters in the Galaxy has a metal-rich peak at [Fe/H] -0.5 and a metal-poor peak at [Fe/H] -1.6 (e.g. Cote 1999), where most of the metal-rich ones are bulge clusters. Metallicities for samples of field stars were derived by McWilliam Rich (1994, hereafter MR94), Sadler et al. (1996), Ramirez et al. (2000). Zoccali et al. (2003) presented the... 

Among the sample of outer halo stars being investigated are Keck I HIRES observations of four globular clusters with [Fe/H] -2. Information regarding these clusters and comparison objects is presented in Table 1. 

In addition to results from this study, Table 1 includes two of the relatively more metal-rich globular clusters associated with the Sgr dSph. There appears to be little in common between the two metallicity groups in their < a>-abundances relative to iron. Abundances reported so far for in situ Sgr dSph field stars of comparable metallicities [4] are in accord with those of its metal-rich clusters. 

C. Sneden, I. I. Ivans, J. P. Fulbright Globular Clusters and Halo Field Stars . In Origin and Evolution of the Elements Volume 4, Carnegie Observatories Astrophysics Series, ed. by A. McWilliam, M. Rauch (Cambridge, 2004)... 

Chemical Abundance Inhomogeneities in Globular Cluster Stars... 

Fig. 1. The range of [C/Fe] (left panel) and [N/Fe] (right panel) is shown as a function of metallicity ([Fe/H]) for the globular clusters from our work on M71, M5, M13, and M15 as well as for 47 Tuc (from Briley et al 2004a). Large samples of stars, mostly subgiants, were used in each case. Each GC is represented by a horizontal line. The characteristic field star ratio, from Carretta, Gratton Sneden (2000) for C and from Henry, Edmunds Koppen (2000) for N, are indicated by vertical arrows in each panel.

Most of the stars of our sample have been selected from the H K BPS survey ( Beers, Preston Shectman [1], First, stars were selected from the weakness of their H H lines for the Balmer lines intensity on prism-objective Schmidt telescope plates. Then, the candidate stars were observed with a slit spectrograph in order to have a quantitative estimate of their metallicity. The survey has operated on about 7000 square degrees of the sky, mostly on the polar caps. It has supply a vast amount of metal-poor stars, with hundreds of them more metal-poor than the most metal-poor globular clusters. We selected from this sample stars with metallicities estimated to have [Fe/H] < -2.7. The actual metallicity histogram is given for the sample on fig. 1. 

Spectroscopic observations of globular clusters (GCs) have revealed star-to-star inhomogeneities in the light metals that are not observed in field stars. These light metal anomalies could be interpreted with a self-pollution scenario. But what about heavier (Z > 30) elements Do they also show abundance anomalies Up to now, no model has been developed for the synthesis of n-capture elements in GCs, and the self-pollution models do not explain the origin of their metallicity. In 1988, Truran suggested a test for the self-enrichment scenario [4], which could possibly explain the metallicity and the heavy metal abundances in GCs if self-enrichment occurred in GCs, even the most metal-rich clusters would show both high [a/Fe] ratios and r-process dominated heavy elements patterns, which characterize massive star ejecta as it is seen in the most metal-poor stars. 

Reid stars eJAM04 BUROO. FUL00 "MAS01.03 Globular clusters SG A TO Others... 

The sample has been cleaned of 5 suspected binaries and of 6 stars with a 215globular cluster NGC6528 [2]. The final sample contains 217 stars. The RV distribution is shown in Figure 1. The associated errors are smaller than 0.45 km.s-1, with a mean error value of 150 m.s-1. 

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