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

TL;DR

This paper investigates low mass post-AGB stars experiencing late thermal pulses, analyzing their evolution, characteristics, and potential observational counterparts across different metallicities and masses.

Contribution

It provides detailed stellar evolution models of LTPs across various metallicities and masses, introducing a new classification based on eruption temperature and discussing potential explanations for specific stars.

Findings

LTP phases last decades to centuries with significant changes in stellar properties.

Identified a new LTP type that may explain the star FG Sge.

Presented timescales for heating, luminosity decline, and cooling phases.

Abstract

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 HR Diagram between the Asymptotic Giant Branch (AGB) and Planetary Nebula phase (PN). 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 (VLTP). 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 Planetary Nebula 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 through Z = 0.03, and for masses 0.90 $M_\odot$, 1.2 $M_\odot$, and 2.0 $M_\odot$. We focus in on our most dense series (1.2 $M_\odot$, Z = 0.015) and designate a stratification of late thermal pulse types based on at what temperature they erupt, which may hint at the progenitor mass. We discuss one type that fits neither a LTP nor VLTP, which may offer an explanation for the star FG Sge. We present the timescales during which LTP models heat up until they 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.