Blue Straggler masses from pulsation properties. II. Topology of the instability strip

We present a new set of nonlinear, convective radial pulsation models for main-sequence stars computed assuming three metallicities: Z = 0.0001, 0.001, and 0.008. These chemical compositions bracket the metallicity of stellar systems hosting SX Phoenicis stars (SXPs, or pulsating Blue Stragglers), namely, Galactic globular clusters and nearby dwarf spheroidals. Stellar masses and luminosities of the pulsation models are based on alpha-enhanced evolutionary tracks from the BASTI website. We are able to define the topology of the instability strip (IS) and in turn the pulsation relations for the first four pulsation modes. We found that third overtones approach a stable nonlinear limit cycle. Predicted and empirical ISs agree quite well in the case of 49 SXPs belonging to ω Cen. We used theoretical period–luminosity (PL) relations in B and V bands to identify their pulsation mode. We assumed Z = 0.001 and Z = 0.008 as mean metallicities of SXPs in ω Cen. We found respectively 13–15 fundamental, 22–6 first-overtone, and 9–4 second-overtone modes. Five are unstable in the third-overtone mode only for Z = 0.001. Using the above mode identification and applying the proper mass-dependent PL relations, we found masses ranging from ∼1.0 to 1.2 $M_{\odot }$ ($⟨M⟩$ = 1.12, $\sigma =0.04$ $M_{\odot }$) and from ∼1.2 to 1.5 $M_{\odot }$ ($⟨M⟩$ = 1.33, $\sigma =0.03$ $M_{\odot }$) for Z = 0.001 and 0.008, respectively. Our investigation supports the use of evolutionary tracks to estimate SXP masses. We will extend our analysis to higher helium content, which may have an impact on our understanding of the blue straggler stars formation scenario.