Asteroseismology of the multiperiodic field SX Phe pulsator BL Camelopardalis

Abstract

BL Camelopardalis (BL Cam) is the most metal-poor SX Phoenicis star known in the Galactic field, making it an excellent test bed for studies of Population II stellar structure and evolution, as well as for investigating the formation channels of blue straggler stars. We aim to constrain fundamental stellar parameters and pulsational properties of BL Cam and identify the nature of its observed oscillation modes. We analysed high-precision space-based data from the TESS mission and long-term ground-based observations from the Zwicky Transient Facility (ZTF) survey. The ZTF data were primarily used to derive the orbital parameters of the system, while the TESS light curves enabled us to extract a rich oscillation spectrum. We identified multiple pulsation frequencies and performed seismic modelling using a Bayesian approach. We detected numerous pulsation frequencies, including a dominant radial mode and additional non-radial components. We predicted all fitted modes in our seismic models as excited. The models reproduced the two highest amplitude-independent frequencies (i.e. the radial fundamental mode and a dipole mode) and suggested the possible presence of additional radial overtones. The inferred stellar parameters confirm the extremely low metallicity of BL~Cam. We also confirm its binary nature, with an orbital period of 144 days. BL Cam provides a valuable benchmark for testing stellar models of metal-poor pulsators. The combination of space-based photometry and Bayesian seismic modelling enables robust constraints to be placed on its internal structure and pulsation properties.