Stellar Evolution at the Crossroads: Resolving the Nature of RV Tauri Variable Systems Using Unprecedented Observations From the Kepler and XMM-Newton Space Telescopes
Vega, Laura Daniela
RV Tauri variables are luminous pulsating supergiant stars (~10^3 Lsun) with pulsation periods between ~20--150 days and extend the period--luminosity relation of Type II Classical Cepheids. In addition to pulsational variability, a subset of RV Tauri stars (the RVb-type) exhibits a longer periodic modulation in brightness ranging between ~470 and 2800 days. It has been argued that RV Tauri variables are a subclass of post-asymptotic giant branch objects, evolved from low-to-intermediate mass stars. However, their evolution is not so simple. The majority, if not all, are in binary systems surrounded by a circumbinary disk. Though binarity has been shown to play a key role in the dynamics and evolution of these old systems, the interconnection between the various physical processes remains poorly understood. This dissertation focuses on the multiwavelength observational analysis of two archetypal RVb systems, DF Cygni and U Monocerotis. In these two systems we find evidence for disk obscuration, binary interaction, and disk evolution. We show how ultraprecise Kepler telescope observations of DF Cygni, the only RV Tauri system in Kepler's original field-of-view, demonstrates that the periodic photometric RVb phenomenon is linked to disk obscuration of the pulsating star. We also introduce observations spanning the largest coverage of the electromagnetic spectrum (X-ray to millimeter) and longest temporal baseline of the RVb system U Monocerotis, which also becomes the first RV Tauri system detected in X-rays, using XMM-Newton. This has opened new possibilities for considering X-rays in forthcoming studies to place further constraints and a deeper understanding of magnetism and accretion of these evolved binary systems.