Theoretical models for the Late Thermal Pulse in post-AGB stars: the case of DY Cen

TL;DR

This work tackles how late thermal pulses shape the evolution of post-AGB stars and tests whether DY Cen can be produced via born-again events. The authors construct an extensive grid of VLTP/LTP models using a MESA-based framework, systematically varying core mass, hydrogen-envelope mass, and convective overshoot, then compare predictions for heating history, evolutionary tracks, and surface abundances with DY Cen. They find that none of the standard LTP or VLTP sequences reproduce DY Cen's hydrogen-rich surface or its observed heating rate, leading to the conclusion that DY Cen is unlikely to be a born-again star; the study emphasizes the sensitivity of outcomes to envelope mass and overshoot and notes potential alternative channels such as CO+He white dwarf mergers to explain some abundance signatures, albeit with remaining challenges. Overall, the work clarifies parameter dependencies in born-again evolution, constrains viable scenarios for DY Cen, and motivates exploring non-canonical formation pathways for hydrogen-rich, rapidly evolving post-AGB objects.

Abstract

We present theoretical predictions of the born-again scenario for post-asymptotic giant-branch stars. An extensive model grid for born-again objects has been constructed, particularly including models for the Very Late Thermal Pulse with and without convective overshooting, and also including models for the Late Thermal Pulse. We constructed a large parameter space to analyze the dependencies of the born-again model on core mass, hydrogen-envelope mass, and overshoot parameters, and we analyzed how changes in these parameters affect the models' evolution. We applied our grid of models to interpret observations of DY\,Cen, a star exhibiting characteristics similar to confirmed born-again stars. We compared DY\,Cen with models from multiple aspects, including heating rate, evolutionary tracks, and surface abundances. Ultimately, we concluded that none of our born-again models could match all of the observed properties of DY\,Cen, especially its surface chemistry; DY\,Cen is therefore an unlikely born-again star.

Figures (13)

• Figure 1: Top: Classical evolution of 2.3$\rm M_{\odot}$ star ($Z=0.02$) from ZAMS to WD. The main stages of evolution are identified by coloured ellipses, and sample models are labelled with total mass and age from the ZAMS. Bottom: Three different evolutionary tracks calculated for models with the same core mass ($M_{\rm c}=0.55864\hbox{,$\rm M_{\odot}$}$)
• Figure 2: Teff - log g diagram showing the theoretical evolution track for a star that experiences a Very Late Thermal Pulse (VLTP). The rapid mass-loss process is represented by a dashed line. Numbers ① to ⑥ identify the main phases of VLTP evolution. The figure also shows the positions of R CrB stars (in blue, from simon_2011) and sdO stars (in purple, from geier_2022).
• Figure 3: Stellar evolution following a VLTP, i.e. a He-shell flash while on the white dwarf cooling track, for four models with masses $M = 0.55864, 0.55875, 0.55901$, and $0.55915 \hbox{,$\rm M_{\odot}$}$. All have core mass $M_{\rm c}=0.55864\hbox{,$\rm M_{\odot}$}$. Left: tracks in the $L-T_{\rm eff}$ plane from the end of mass loss (1) to the final white dwarf stage (5). Other phases marked are (2) first red limit and maximum extent of flash-driven convection, (3) maximum radius and (4) $T_{\rm eff}=30\,000$ K. Right: the evolution of convective regions in terms of mass (shaded) and the luminosities for the H (violet) and He (red) shells for the same models, both as a function of time after the last thermal pulse.
• Figure 4: Evolutionary tracks for models having the same initial core mass ($M_{\rm c}=0.55864\hbox{,$\rm M_{\odot}$}$) and with different total masses ($M$) after mass-loss. The red line connects the start positions for the tracks. The tracks are colour-coded by total star mass, as shown in the colour scale on the right. Bottom: Hd-LTP models ranging from $M = 0.558605 - 0.558800 \hbox{,$\rm M_{\odot}$}$ from the end of the first post-AGB phase to the white dwarf stage. Middle: VLTP models for $M = 0.558730 - 0.559010 \hbox{,$\rm M_{\odot}$}$. Top: LTP models for $M= 0.558980 - 0.559150 \hbox{,$\rm M_{\odot}$}$.
• Figure 5: The helium abundance after convective mixing from all 0.559 $\rm M_{\odot}$ models, with varying hydrogen envelope masses.