Linear Nonadiabatic Properties of SX Phoenicis Variables

We present a detailed linear, nonadiabatic pulsational scenario foroscillating Blue Stragglers (BSs)/SX Phoenicis (SX Phe) in Galactic GlobularClusters (GGCs) and in Local Group (LG) dwarf galaxies. The sequences of modelswere constructed by adopting a wide range of input parameters and properlycover the region of the HR diagram in which these objects are expected to bepulsationally unstable. Current calculations together with more metal-richmodels already presented by Gilliland et al. suggest that the pulsationproperties of SX Phe are partially affected by metal content. Our calculationsalso suggest that the secular period change seems to be a good observable toidentify the pulsation mode of cooler SX Phe variables. The dependence ofpulsation properties on the helium envelope content is discussed and we findthat the secular period change for He-enhanced models is approximately a factorof two larger than for canonical ones. According to this evidence we suggestthat the pulsation properties of SX Phe can be soundly adopted to constrain theevolutionary history of BSs, and in turn to single out the physical mechanismswhich trigger their formation.Comment: 25 pages, 9 postscript figures, 1 latex file containing five tables, accepted for publication on Astrophysical Journa