Stellar envelopes of globular clusters embedded in dark mini-haloes

Among the many aspects of the evolution of low and intermediate mass stars, two representative topics are selected for this review: the question why stars become red giants, and the problem of the age determination of galactic globular clusters. Concerning the first topic, it is shown that this happens because of a thermal instability taking place in the stellar envelopes, and the physical nature of this instability is identified. Several ramifications of these findings are then briefly mentioned. Concerning the second topic, the Oosterhoff-Sandage effect is briefly described, together with its implications for the age estimates and the problems rised by its interpretation in terms of evolutionary models. In this connection, it is suggested that an enhanced overall abundance of the elements CNO and Ne may solve these problems, although further evolutionary calculations are required before reaching firm conclusions. It is also amphasized for both topics the important role played by the metal contribution to the radiative opacity at middle temperatures.

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The Concept of Old Stellar Populations

Highlights of Astronomy ◽

10.1017/s1539299600005001 ◽

1983 ◽

Vol 6 ◽

pp. 95-100

Author(s):

Gösta Lyngå

Keyword(s):

Globular Clusters ◽

Stellar Population ◽

Age Groups ◽

Stellar Populations ◽

Heavy Elements ◽

The Galaxy ◽

Star Formation In Galaxies ◽

Solar Abundances ◽

Central Bulge ◽

Stellar Envelopes

Almost 25 years ago Walter Baade told the Vatican conference about the meaning of stellar populations:“We also understand now why the two stellar populations, either singly or combined, are such conspicuous features in most galaxies. They are age groups which represent two significant phases of the star formation in galaxies.”There appeared a straight-forward picture with an old, metal-poor halo containing stars in elongated galactic orbits and a younger disk population, where the stars have near solar abundances and near circular orbits. The central bulge of the galaxy was considered part of the older system.New observations have made the picture more complex and also more controversial, indicating that it still might not be fully understood. Stars in the bulge of our spiral galaxy have been shown to have rather high contents of heavy elements. The gas and perhaps also the stars of the disk show a metal content that is decreasing outwards in the disk. The abundances in globular clusters differ widely between individual clusters and even between individual stars of the same cluster. It has become clear that one must discuss separately the different heavy elements; that some abundances may be considered primordial and thus characteristic of the evolution of the stellar population, whereas other abundance differences may be caused by mixing into the stellar envelopes.

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Recent Advances in High Dispersion Spectroscopy of Globular Cluster Stars

Symposium - International Astronomical Union ◽

10.1017/s0074180900035610 ◽

1988 ◽

Vol 132 ◽

pp. 525-530

Author(s):

Raffaele G. Gratton

Keyword(s):

Globular Cluster ◽

Globular Clusters ◽

High Dispersion ◽

Photographic Material ◽

Recent Advances ◽

Metal Abundances ◽

Ccd Detectors ◽

Major Progress

The use CCD detectors has allowed a major progress in abundance derivations for globular cluster stars in the last years. Abundances deduced from high dispersion spectra now correlates well with other abundance indicators. I discuss some problems concerning the derivation of accurate metal abundances for globular clusters using high dispersion spectra from both the old photographic and the most recent CCD data. The discrepant low abundances found by Cohen (1980), from photographic material for M71 giants, are found to be due to the use of too high microturbulences.

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Meridional Circulation, Turbulence, and Diffusion Processes in Stars

International Astronomical Union Colloquium ◽

10.1017/s0252921100080477 ◽

1976 ◽

Vol 32 ◽

pp. 109-116 ◽

Cited By ~ 1

Author(s):

S. Vauclair

Keyword(s):

Convection Zone ◽

Diffusion Processes ◽

Meridional Circulation ◽

Diffusion Time ◽

Work In Progress ◽

First Results ◽

Stellar Envelopes ◽

And Diffusion ◽

Turbulence And Diffusion ◽

Hr Diagram

This paper gives the first results of a work in progress, in collaboration with G. Michaud and G. Vauclair. It is a first attempt to compute the effects of meridional circulation and turbulence on diffusion processes in stellar envelopes. Computations have been made for a 2 Mʘstar, which lies in the Am - δ Scuti region of the HR diagram.Let us recall that in Am stars diffusion cannot occur between the two outer convection zones, contrary to what was assumed by Watson (1970, 1971) and Smith (1971), since they are linked by overshooting (Latour, 1972; Toomre et al., 1975). But diffusion may occur at the bottom of the second convection zone. According to Vauclair et al. (1974), the second convection zone, due to He II ionization, disappears after a time equal to the helium diffusion time, and then diffusion may happen at the bottom of the first convection zone, so that the arguments by Watson and Smith are preserved.

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