Spectroscopic insights into HD 133729: Probing the physical parameters of the first blue large-amplitude pulsator (BLAP) in a binary system

Abstract

HD 133729 (TIC 75934024) remains to this day the only spectroscopically confirmed binary system in which one of the stars is a blue large-amplitude pulsator (BLAP). The known distance to the system (${D\approx 480}$ pc) reveals that the BLAP component (the secondary) is not only the brightest (${V\approx11}$ mag), but also the closest among all known BLAPs. Due to the presence of a brighter, late B-type main-sequence companion, the observed amplitude of the BLAP is significantly reduced, compared to the canonical ${0.1-0.4}$ $I$-band magnitude variation range. The brightness of this BLAP permits the acquisition of high-resolution, time-resolved spectra in which the lines of both components are visible. This, combined with the known orbital elements from the ${O-C}$ analysis of the Transiting Exoplanet Survey Satellite (TESS) observations, allows for the determination of BLAP's physical properties. We present here preliminary results from the comprehensive analysis of 30 high-resolution (${R\approx37000}$) optical/near-IR spectra of HD 133729, obtained using the High Resolution Spectrograph (HRS) mounted on the 11-m Southern African Large Telescope (SALT). Using the two-dimensional cross-correlation technique, we determine the radial velocities of both components and use them to disentangle their individual spectra. From the obtained radial velocity curves and the fitted spectral energy distribution (SED), we derive the orbital parameters and provide constraints on the masses and radii of both components.