Blue straggler masses from pulsation properties. II. Topology of the Instability Strip

TL;DR

This paper develops nonlinear pulsation models for Blue Straggler stars across different metallicities, defining the instability strip topology and deriving pulsation relations, which help estimate stellar masses and understand their evolution.

Contribution

It introduces new nonlinear convective pulsation models for Blue Straggler stars with detailed instability strip topology and pulsation mode identification, linking models with observations.

Findings

Predicted and empirical instability strips agree well for omega Cen SXPs.

Mass estimates for SXPs range from ~1.0 to 1.5 solar masses depending on metallicity.

Third overtones tend to approach a stable nonlinear limit cycle.

Abstract

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 IS agree quite well in the case of 49 SXPs belonging to omega Cen. We used theoretical Period-Luminosity relations in B,V bands to identify their pulsation mode. We assumed Z=0.001 and Z=0.008 as mean metallicities of SXPs in omega Cen. We found respectively 13-15 fundamental, 22-6 first 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 Period-Luminosity relations we found masses ranging from ~1.0 to 1.2 Mo (<M>=1.12, sigma=0.04 Mo) and from ~1.2 to 1.5 Mo (<M>=1.33, sigma=0.03 Mo) for Z=0.001 and 0.008 respectively. Our investigation supports the use of evolutionary tracks to estimate of SXP masses. We will extend our analysis to higher Helium content that may have an impact in our understanding of the BSS formation scenario.