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Title:
Oosterhoff dichotomy in the Galaxy and globular clusters in the Large Magellanic Cloud
Authors:
Bono, Giuseppe; Caputo, Filippina; Stellingwerf, Robert F.
Affiliation:
AA(Obsservatorio Astronomico di Trieste, Trieste, Italy), AB(Obsservatorio Astronomico di Trieste, Trieste, Italy), AC(Los Alamos National Lab., Los Alamos, NM, US)
Publication:
The Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 423, no. 1, p. 294-304 (ApJ Homepage)
Publication Date:
03/1994
Category:
Astrophysics
Origin:
STI
NASA/STI Keywords:
Galactic Clusters, Globular Clusters, Horizontal Branch Stars, Magellanic Clouds, Stellar Evolution, Hertzsprung-Russell Diagram, Metallicity, Stellar Motions
DOI:
10.1086/173806
Bibliographic Code:
1994ApJ...423..294B

Abstract

The long-standing problem of the Oosterhoff dichotomy is studied within the framework of up-to-date convective pulsating models and synthetic horizontal-branch (HB) computations. It is shown that the observed properties of RR Lyrae-rich Galactic globular clusters can be put in agreement with theoretical prescriptions for a constant original helium (Y(MS) is approximately 0.23), on condition that in Oosterhoff type I clusters the transition between ab- and c-type RR Lyrae variables occurs near the blue edge for fundamental mode of pulsation, whereas in Oosterhoff type II it is near the red edge for first-overtone mode. Since type I clusters have redder HB morphology than type II systems, such an evidence should support the suggestion that the RRab/RRc transition depends on the evolutionary history of the variables, as was suggested early on by van Albada & Baker (1973). The analysis of the 'Oosterhoff-intermediate' clusters in the Large Magellanic Cloud strengthens this hypothesis still more. In fact, they turn out to represent the observational counterpart of a predicted class of clusters where, as a consequence of evolutionary track morphology and hysteresis in the pulsation as well, the mean period of ab-type variables shoud vary from Oosterhoff type I to Oosterhoff type II values. On these grounds, both the Oosterhoff groups in the Galaxy and the Oosterhoff-intermediate clusters in the large Magellanic Cloud could be explained within the same theoretical scenario, the different pulsational properties being caused by a combination of HB morphology with metallicity.

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