OOSTERHOFF DICHOTOMY IN THE GALAXY AND GLOBULAR-CLUSTERS IN THE LARGE MAGELLANIC CLOUD

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) 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 should 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.