2001ApJ...554.1124S

We present a detailed linear, nonadiabatic pulsational scenario for oscillating blue stragglers (BSs)/SX Phe variables in Galactic globular clusters (GGCs) and in Local Group (LG) dwarf galaxies. The sequences of models were constructed by adopting a wide range of input parameters and properly cover the region of the H-R diagram in which these objects are expected to be pulsationally unstable. Current calculations together with more metal-rich models already presented by Gilliland et al. suggest that the pulsation properties of SX Phe variables are partially affected by metal content. In fact, the pulsation periods for the first three modes are marginally affected when moving from Z=0.0001 to 0.006, whereas the hot edges of the instability region move toward cooler effective temperatures by approximately 300-500 K. The inclusion of a metallicity term in the period-luminosity-color (P-L-C) relations causes a substantial decrease in the intrinsic scatter and in the individual error of the coefficients. This supports the result recently brought out by Petersen & Christensen-Dalsgaard for δ Scuti stars. Moreover, we find that the discrepancy between our relation and similar theoretical and empirical relations available in the literature is typically smaller than 5%. The comparison between theory and observations in the M_V-logP plane as well as in the luminosity amplitude-logP plane does not help to disentangle the long-standing problem of mode identification among SX Phe stars. However, our calculations suggest that the secular period change seems to be a good observable to identify the pulsation mode of cooler SX Phe variables. Together with the previous models we also constructed new sequences of models by adopting selected effective temperatures and luminosities along two evolutionary tracks characterized by the same mass value and metal content (M/M_☉=1.2, Z=0.001) but different He contents in the envelope, namely, Y=0.23 and 0.30. The He content in the latter track was artificially enhanced soon after the central H exhaustion to mimic, with a crude approximation, the collisional merging between two stars. Interestingly enough, we find that the He-enhanced models present an increase in the pulsation period and a decrease in the total kinetic energy of the order of 20% when compared with the canonical ones. At the same time, the blue edge of the fundamental mode for the He-enhanced models is approximately 1000 K cooler than for canonical ones. Moreover, we find that the secular period change for He-enhanced models is approximately a factor of 2 larger than for canonical ones. According to this evidence, we suggest that the pulsation properties of SX Phe variables can be soundly adopted to constrain the evolutionary history of BSs and in turn to single out the physical mechanisms that trigger their formation.