A closer look at low-mass post-AGB late thermal pulses

Abstract

ABSTRACT Late thermal pulse (LTP) stellar evolution models experience a helium pulse that occurs following asymptotic giant branch (AGB) departure and causes a rapid looping evolution in the Hertzsprung–Russell (HR) diagram between the AGB and planetary nebula (PN) phase. The transient LTP phases only last decades to centuries while increasing and decreasing in temperature, luminosity, and size over orders of magnitude. LTP objects have often been described in the context of their more dramatic counterparts, very late thermal pulses (VLTPs). LTP stars do not evolve as quickly and do not become as hydrogen deficient as VLTP objects. They do not become conspicuous until after resembling a PN for thousands of years. We present stellar evolution calculations from the AGB to the PN phase for models over a range of metallicities from Z = 0.0015 to 0.03, and for masses 0.90, 1.2, and 2.0 ${\rm M_{\odot }}$. We focus in on our most dense series (1.2 ${\rm M_{\odot }}$, Z = 0.015) and designate a stratification of LTP types based on at what temperature they erupt, which may hint at the progenitor mass. We discuss one type that fits neither an LTP nor VLTP, which may offer an explanation for the star FG Sge. We present the time-scales during which LTP models heat up until reach peak helium-burning luminosity, during the rapid luminosity decline, and during the period of cooling and brightening, and we briefly discuss four LTP candidates.