Stellar Evolution in Real-Time: Late Thermal Pulse Evolution Models

There are not many stages of stellar evolution that can be observed in real time, so in general we rely on modeling computations and connecting the dots between observed stars in different phases of evolution. Late Thermal Pulse (LTP) stars are one of these few that can be observed in real-time. These are stars that experience a late helium pulse sometime following Asymptotic Giant Branch (AGB) departure and before reaching the planetary nebula phase. They are a cousin to Very Late Thermal Pulse (VLTP) stars, which most notably include Sakurai’s Object, V605 Aql, and Fg Sge. Because of the rapid evolutions of LTP stars, they are difficult to observe, and few LTP stars have been found. One likely case is V839 Ara (the central star of the Stingray Nebula) and one possible case is SwSt 1 (PN G001.5-06.7), although the latter may be a massive VLTP. When a star experiences an intense thermal pulse while evolving toward the PNe phase, it causes a rapid looping evolution between the AGB and PNe phases. The transient stages during this loop only last decades to centuries while increasing and decreasing in temperature, luminosity, and radius over orders of magnitude (!). Though the evolution is brief (on stellar evolution time scales) it has a significant impact on the star’s composition and final white dwarf mass. In this paper we present new evolution calculations from the Asymptotic Giant Branch (AGB) to the Planetary Nebula (PNe) phase for models between masses 0.90 M_⊙ and 2.0 M_⊙ for a range of metallicities from, Z = 10^-6 to Z = 0.03. We present our evolution tracks and report the time scales and composition changes during and following the late thermal pulse loops, and we discuss the criterion that leads to a LTP versus models that do not and ultimately evolve to become a typical DA white dwarf.