scholarly journals On the Interpretation of RR Lyrae Properties in Globular Clusters and in Other Population II Systems

1973 ◽  
Vol 21 ◽  
pp. 197-206
Author(s):  
V. Castellani ◽  
P. Giannone ◽  
A. Renzini
Keyword(s):  
Mass Loss ◽  
Globular Clusters ◽  
Horizontal Branch ◽  
Rr Lyrae ◽  
Horizontal Branch Stars

AbstractThe differences in observational parameters of the RR Lyrae variables and horizontal branch stars of globular clusters and other population II systems are considered. A discontinuous behaviour of some parameters is outlined. The Oosterhoff dichotomy and the HB morphology are discussed with regard to a conjecture of mass loss in the pre-HB phase.

scholarly journals Semiconvection and the RR Lyrae Variables

1973 ◽  
Vol 21 ◽  
pp. 221-228
Author(s):  
A. V. Sweigart ◽  
P. Demarque
Keyword(s):  
Time Scale ◽  
Globular Clusters ◽  
Pulsation Period ◽  
Horizontal Branch ◽  
Composition Distribution ◽  
Rates Of Change ◽  
Rr Lyrae ◽  
Horizontal Branch Stars ◽  
Convective Core ◽  
Source Models

Theoretical computations (Hoyle and Schwarzschild, 1955; Faulkner, 1966; Iben and Rood, 1970; Demarque and Mengel, 1971a, b) have identified the horizontal-branch stars in globular clusters with the evolution phase in which helium burns within a convective core and hydrogen burns in a shell outside the convective core. Most computations for such double-energy-source models have indicated that the evolution proceeds smoothly on a nuclear time scale during the horizontal-branch phase, leading to small predicted rates of change in the RR Lyrae pulsation period (Iben and Rood, 1970). Sweigart and Demarque (1972) have recently considered the effects of semiconvection on the horizontal-branch evolution of typical Population II stars and have suggested that changes in the composition distribution within the core may occur on a time scale considerably shorter than the nuclear time scale during the phase immediately preceding core-helium exhaustion. It has been found that the composition distribution generated by the growth of a semiconvective zone in the layers surrounding the convective core can become unstable when Yc, the helium abundance within the convective core, decreases below roughly 0.12. The changes in the internal structure caused by this instability result in relatively rapid movement of the models in the HR diagram and consequently produce large predicted rates of change in the RR Lyrae pulsation period. The possibility that RR Lyrae period changes may be associated with the behavior of the semiconvective zone has been previously suggested by Schwarzschild (1970). A similar instability may occur in the late core-hydrogen burning phase for stars around 10 M⊙. Percy (1970) has noted the coincidence of β Cephei stars with stellar models containing semiconvective zones. It is tempting to suggest that such an instability in the semiconvective zone could also be related to the β Cephei phenomenon.

UBV Magnitudes and Colours of 62 RR Lyrae Stars in ω Centauri (NGC 5139)

1973 ◽  
Vol 21 ◽  
pp. 88-94
Author(s):  
E. H. Geyer
Keyword(s):  
Globular Clusters ◽  
Variable Stars ◽  
Horizontal Branch ◽  
Rr Lyrae Stars ◽  
Rr Lyrae ◽  
Horizontal Branch Stars ◽  
Photoelectric Measurements ◽  
Lyrae Stars

It has been shown by the author (Geyer, 1967), and independently by Dickens and Woolley (1967), that there occur horizontal branch stars well within the RR Lyrae variable gap of the c–m-diagram of ω Centauri. Variability of these objects is not known, and presumably is unlikely, because the cluster is so well studied for variable stars. For the observationally best studied globular clusters M3, M13, M15 and M92 (Sandage, 1969), this phenomenon is not found. The author’s original published c–w-diagram of NGC 5139 showed about 10 stars with (B–V)≥0.2 and V-magnitudes comparable with those of RR Lyrae stars of the cluster. Unpublished photoelectric measurements of some of these stars, carried out in 1968 with the 40-in photometric reflector of ESO by the author, support this finding. Also the study of blue horizontal branch stars by Newell, Rodgers and Searle (1969) of the Herst-monceux catalog of ω Centauri (Woolley et al., 1966) confirms these results, the importance of which for the understanding of the horizontal branch stars and RR Lyrae variables is obvious.

scholarly journals Mass loss of different stellar populations in globular clusters

2019 ◽  
Vol 14 (S351) ◽  
pp. 341-345
Author(s):  
Marco Tailo
Keyword(s):  
Mass Loss ◽  
Globular Clusters ◽  
State Of The Art ◽  
Stellar Populations ◽  
Horizontal Branch ◽  
Horizontal Branch Stars ◽  
Colour Distribution ◽  
Chromosome Maps ◽  
First Time

AbstractOnce the age and metallicity are fixed, the colour distribution of horizontal branch stars in a globular cluster depends on few parameters: the helium abundance of the population and the mass lost during the pre-HB stages. These parameters are usually derived from the HB itself, hence they are degenerate. Breaking this degeneracy and understanding their role is a tricky and challenging problem that no study has solved yet. Combining the information obtained from the chromosome maps and the analysis of multi-band photometry with state of the art stellar evolution models, we can obtain a solid estimate of Y for the various stellar populations in a GC. We will then have, for the first time, the possibility to break the parameters’ degeneracy on the HB, understand the role of the mass loss, and lay the foundation to build another piece of the multiple populations mosaic.

scholarly journals Non-variable horizontal-branch stars

1997 ◽  
Vol 189 ◽  
pp. 363-368
Author(s):  
Robert T. Rood
Keyword(s):  
Mass Loss ◽  
Physical Basis ◽  
Horizontal Branch ◽  
Loss Process ◽  
Horizontal Branch Stars ◽  
Theoretical Results ◽  
Branch Morphology

For 25 years our ignorance of the physical basis of this mass loss process has been the barrier to progress in understanding horizontal branch morphology. I review some recent observational and theoretical results which may be giving us clues about the nature of the mass loss process.

Super-Helium-rich Populations and the Origin of Extreme Horizontal-Branch Stars in Globular Clusters

2005 ◽  
Vol 621 (1) ◽  
pp. L57-L60 ◽  
Author(s):  
Young-Wook Lee ◽  
Seok-Joo Joo ◽  
Sang-Il Han ◽  
Chul Chung ◽  
Chang H. Ree ◽  
...  
Keyword(s):  
Globular Clusters ◽  
Horizontal Branch ◽  
Horizontal Branch Stars

scholarly journals Study of the Helium Enrichment in Globular Clusters

2009 ◽  
Vol 5 (S262) ◽  
pp. 27-30
Author(s):  
Aldo A. R. Valcarce ◽  
Márcio Catelan
Keyword(s):  
High Precision ◽  
Globular Clusters ◽  
Stellar Populations ◽  
Horizontal Branch ◽  
Previous Generation ◽  
Horizontal Branch Stars

AbstractGlobular clusters (GCs) are spheroidal concentrations typically containing of the order of 105 to 106, predominantly old, stars. Historically, they have been considered as the closest counterparts of the idealized concept of “simple stellar populations.” However, some recent observations suggest than, at least in some GCs, some stars are present that have been formed with material processed by a previous generation of stars. In this sense, it has also been suggested that such material might be enriched in helium, and that blue horizontal branch stars in some GCs should accordingly be the natural progeny of such helium-enhanced stars. In this contribution we show that, at least in the case of M3 (NGC 5272), the suggested level of helium enrichment is not supported by the available, high-precision observations.

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